WO2009124338A1

WO2009124338A1 - An electrical protection device and an electrical distribution system including an electrical protection device

Info

Publication number: WO2009124338A1
Application number: PCT/AU2009/000420
Authority: WO
Inventors: Geoffrey Rubython; Wayne Callen; Walter Henry Berryman; David Paul Jankowski
Original assignee: Protectelec Pty Ltd
Priority date: 2008-04-07
Filing date: 2009-04-07
Publication date: 2009-10-15
Also published as: AU2009235938A1

Abstract

An electrical distribution system (1) for a household installation (2) includes an 12 kVA 50 Amp isolation transformer (3) having a primary winding (not explicitly shown) between terminals (5, 6) and a secondary winding (not explicitly shown) between terminals (7, 8). The primary winding is electrically connectable, via an electrical meter board (9), with a power source in the form of an MEN consumer mains supply (10). A plurality of electrical circuits - including a power circuit (11) and a lighting circuit (12) - are electrically connected in parallel to terminals (7, 8) and, hence, with the secondary winding. System (1) includes two protection devices (15, 16), one for each of circuits (11, 12). Devices (15, 16) are respectively upstream of circuits (11, 12), and are responsive to a fault voltage in respective circuits (11, 12) for isolating those respective circuits from one or more of terminals (7, 8) while allowing the other of the circuits to remain electrically connected with the secondary winding.

Description

AN ELECTRICAL PROTECTION DEVICE AND AN ELECTRICAL

DISTRIBUTION SYSTEM INCLUDING AN ELECTRICAL

PROTECTION DEVICE

FIELD OF THE INVENTION

[0001] The present invention relates to an electrical protection device and an electrical distribution system including an electrical protection device.

BACKGROUND

[0002] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

[0003] In more than the one hundred years that electrical energy has been distributed to household and industrial installations, there has been a continuing development and improvement of safety. Initially, the main safety hazard posed by an electrical distribution system was fire. As the distribution systems developed the distribution voltages also increased, and the danger of electrocution and electrical shock, particularly when AC voltages were used, became another major concern.

[0004] The electrical distribution networks in all industrialized countries, and in particular low voltage (LV) networks and household installations, are earthed for safety reasons. The intention of such "earthing" is to allow the use of electrical protection devices as part of the distribution system to protect against electric shock and electrocution. The electrical protection devices provide "electrical protection" which, in LV networks is required to perform two major functions:

• The prevention of electrocution or serious electric shock to personnel who come into direct or indirect contact with live electric conductors.

• Following the failure of any electrical insulation of equipment being supplied electrical power by the distribution system, the prevention of: damage to electrical equipment; and possible fire ignition. [0005] Given the above, various earthing systems have been developed over the last century. The three main types of earthing systems recognized internationally are:

• TT - the Neutral point is connected to Earth (Terra) = T and the Frame is connected to Earth as well = T.

• TN - the Neutral point is connected to Earth (Terra) =T and the Frame is connected to the Neutral line of the circuit. = N. (TN-C, TN-S, TN-C-S).

• IT - the Neutral point is Insulated from Earth = I and the Frame is connected to Earth = T (IT-unearthed, IT impedance earthed).

[0006] A more detailed description of these power supply systems with earth connections can be found in the EEC 60364-1 : 2005 Standard.

[0007] The terms "IT system" or "IT network" are used interchangeably within this specification, unless for any given occurrence the context clearly indicates to the contrary, to refer to an earthing methodology for an electrical distribution system or an electrical distribution system using that earthing methodology. Where reference is made in this specification to an.information technology system or network, use is made of the abbreviation "an IT&T system" or "an IT&T network".

[0008] In a number of industrialized countries the TN system is used widely. The earth path or exposed conductive parts are supplemented by connection to the distribution neutral. This is called the multiple earthed neutral system or, more usually, the MEN system. This system utilizes the supply utility neutral to improve — that is, to reduce - the earth path resistance. An earth stake included at an electrical installation is connected to the utility neutral at the installation switchboard and the utility neutral is also connected to earth at regular intervals along its path to the substation transformer. In TN systems the integrity of the earthing of the installation depends on the reliable connection of the earth stake.

[0009] An insulation fault to frame in a TN system turns into a short circuit fault, and the fault has to be removed by pre-installed protection devices. The typical protection devices used are Mains Circuit Breakers (MCBs) and residual current devices (RCDs).

[0010] While the modern approach in TN systems to personnel and equipment safety protection is to use a low resistance earth connection between the source (the supply utilities) and the load (the household installation), the often large earth resistance variation at different locations . within the distribution system and the installation provides varying levels of protection to personnel and equipment. In some cases the general mass of earth may have inherently high resistance - such as when the earth stake is in dry sandy soil or sandstone - which will inherently and undesirably increase the earth resistance.

[0011] TN (or MEN) networks are often considered a cheap and reliable mechanism for the delivery of power from a source of production to a point of consumption. However these networks are not without risk. For example, in Australia each year tens of people die from exposure to electricity. It is also the case that TN systems are prohibited in certain military and mining installations involving explosives or other volatile materials due to the risk of detonation posed by such networks. Also a TN network, due to having extremely high fault currents, is susceptible, when a fault occurs, to giving rise to fire damage or other thermal damage. Environmentally, a TN network produces more electrical noise than other networks, primarily due to the harmonics generated. It is usually the 3^rd harmonic that is most problematic, and which proves very expensive to moderate or obviate. The evidence arising out of the Australian experience with TN networks indicates that that electrical faults contribute to:

• 10% of all building fires.

• 1000's of hospital admissions from electrical shocks.

• 10,000's of damaged and destroyed electrical appliances.

[0012] There are two types of faults that are most common in TN systems. Both fault types are "earth faults" in that the fault current path is between the active conductor and earth. These two types are:

• A short circuit fault where the fault path electrical impedance is low and the resulting fault current extremely high. These high fault currents should trigger the over- current protection quickly and limit the potential for fire generation and for significant equipment damage.

• A high impedance fault where the fault path electrical impedance is high and the fault current relatively low. These low currents may be below the tripping level of any installed over-current protection and can persist for long periods of time without detection, ultimately giving rise to risks of arcing and thermal damage. [0013] The second of these, the high impedance fault, is the most hazardous type in terms of potential equipment damage, even though the fault current is lower.

^' [0014] As the great majority - in the order of 90% - of faults that cause electric shock and electrocution are "earth faults", involving a fault path between active conductor and earth. One partial solution is to make use of an IT system. Such as system is also referred, in general terms, as an unearthed system or a system having electrical separation — for example, see AS/NZS 3000:2007-7.4.1. An IT system is recognized by International and Australian standards as a possible means of providing protection against electric shock and electrocution caused by "earth faults".

[0015] An IT system prevents hazardous electric shock by removing the protective earth from the installation, hence being known as an "unearthed" system. Other common names include a floating system or an isolated system. As an "unearthed" system removes the earth connection between the source and the load, there is no closed circuit through which fault current can flow, whether electric shock current or fault over-current, to return to the source of supply. If there is no earth connection, then the fault current is very low, for the only "earth path" is through a capacitance path between the general mass of earth and the metal casing. This gives a very high impedance path and hence very low fault current. The fault current levels are typically so low as to not pose a risk of electric shock or heating and fire ignition.

[0016] Unearthed systems were used extensively in the first half of the 20^th century to gain the benefit of greater reliability of supply. This reliability arises from the ability to more robustly accommodate faults. For example, a single fault in one part of the distribution network need not shut the power down to the remainder of the network as there is no fault current to operate an over-current protection device.

[0017] More recently, IT systems are used less in distribution and more so in installations where reliability of supply is critical. Examples of such installations include industrial installation such as aluminium smelting and semiconductor fabrication plants, hospital operating theatres and certain office buildings.

[0018] The unearthed or floating system is able to be used in an installation notwithstanding it is fed from a distribution network having a TN or TT system. It does require, however, that the network and the installation are isolated by an earth free isolation transformer. Low voltage isolating transformers are referred to as Leakage Protection Devices (LPDs) and are the preferred method of protection over RCDs in operating theaters and areas outlined in AS/NZS 3003:2003 medical wiring standard.

[0019] Common IT systems suffered from a number of disadvantages. For example, if a first fault occurs, due to a high impedance path to earth, that first fault is able to exist for a long time without being detected for there is no over-current condition. If there subsequently occurs a second fault that creates a short circuit to earth- which is a likely consequence of the first fault remaining unresolved — there will be a high fault current with the same touch potential problems as occur in TT and TN system. For this reason International Standard IEC 60364-4- 41 requires IT systems to have insulation monitors for providing a visual and audible indication of the presence of the first fault.

[0020] International and Australian Standards also limit the use of IT systems to only 1 item of Class 1 equipment per isolation transformer/generator unless there is additional protection by way of automatic disconnection in case of a fault. Such additional protection includes an RCD. While RCD's have become the most popular means of protection from indirect electrical contact in TN LV electrical systems, their speeds and thresholds are variable, their reliability is questionable and they allow the full fault current to exist before isolation occurs, which has a number of disadvantages. Moreover, RCD operation generally takes 30 ms to isolate. The level at which a fault current can exist in this timeframe is often much larger than the threshold level as stated on the individual device. This has the potential to expose a person to a much greater current than the threshold before isolation takes place. It also greatly increases the potential for acing and therefore the incidence of fire in situ when a fault occurs.

[0021] In some surveys of RCDs in the UK and USA it has been found up to 15% of the RCDs were not in operating conditions when tested. Moreover, RCDs rely on having an electrical path to carry the residual current that may be generated. If there is no earth path, the RCD will not have any residual current to operate. For those applications where an earth connection is not established, or not readily available - such as portable electrical generation equipment such as generators and inverters on worksites and temporary locations - the use of RCDs may leave personnel and equipment at risk. Moreover, RCDs are unable to detect DC residual current. [0022] Given the above limitations of RCDs, when use is made of an IT system in an installation, modifications have to be made to allow the RCD to provide the required protection. In effect, this results in the placing of a low impedance protective earth path within an IT system. While some of the reliability benefits are able to be gained following the conversion, a low impedance earth path defeats the purpose of the improved safety features for protection ofpersonnel and equipment-of-having-an-IT system.

SUMMARY

[0023] It is an object of the present invention, at least in one embodiment, to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0024] According to a first aspect of the invention there is provided an electrical protection device including:

two input terminals for electrically connecting to an electrical power source that is upstream of the device;

two output terminals for electrically connecting to a load which is downstream of the device; a signal generator for providing a reference signal;

a sensor that is responsive to an electrical fault downstream of the device for providing a fault signal; and

a switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the fault signal and the reference signal for selectively electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

[0025] In an embodiment, the switching device is responsive to the fault signal and the reference signal for selectively electrically disconnecting the input terminals from the respective output terminals to prevent the supply of electrical power. [0026] In an embodiment, the supply of electrical power is provided at a first frequency/_/ and the reference signal has a second frequency /j that is different from/}. In an embodiment, one or both off i and_/j are substantially constant.

[0027] In an embodiment:

/₂ >/;; or

f₂ > \0 xf,; or f₂ > 100 xf i. [0028] In an embodiment.^? > 5 kHz and more preferably^ ≥ 10 kHz.

[0029] According to a second aspect of the invention there is provided an electrical protection device including:

two output terminals for electrically connecting to a load which is downstream of the device;

a switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the fault signal and the voltages at one or both of the output terminals for selectively electrically disconnecting the input terminals from the respective output terminals to prevent the supply of electrical power.

[0030] According to a third aspect of the invention there is provided an electrical protection device, the device including: two input terminals for electrically connecting to an electrical power source that is upstream of the device;

a sensor for connecting to a sensor element that extends downstream of the device, the sensor providing a fault signal in response to a low impedance between the sensor element and either of the output terminals; and a switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the feult signal for selectively electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

[0031] In an embodiment, the switching device is responsive to the fault signal for selectively electrically disconnecting the input terminals from the respective output terminals to prevent the supply of electrical power.

[0032] In an embodiment the supply of electrical power is provided at a first frequency // and the reference signal has a second frequency /j that is different from/_/. Preferably, one or both of// and/2 are substantially constant.

[0033] In an embodiment: /₂ >/_/; or /₂> 5 x/_/; or /₂> 10 x/_/; or /₂> 100 x/_;.

[0034] In an embodiment,/? > 5 kHz and, in another embodiment,/^ ≥ 10 kHz. In an embodiment,// < 60 Hz. [0035] In an embodiment, the source includes an isolation transformer having a secondary winding and the input terminals are connected to the secondary winding.

[0036] In an embodiment, the sensor element is a conductor.

[0037] In an embodiment, the load includes a power sink, a switched active conductor and a switched neutral conductor, and the output terminals are respectively electrically connected with the switched active conductor and a switched neutral conductor.

[0038] In an embodiment, the sensor element is bundled with the one or both of the switched active conductor and a switched neutral conductor.

[0039] In an embodiment, the sensor element is substantially coextensively bundled with the one or both of the switched active conductor and a switched neutral conductor.

[0040] According to a fourth aspect of the invention there is provided an electrical protection device, the device including:

a sensor for connecting to a sensor element that extends downstream of the device, the sensor being responsive to a voltage on the sensor element derived from one of the output terminals for providing a fault signal; and

a microprocessor controlled switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the fault signal for selectively electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

[0041] In an embodiment, the sensor is responsive to a voltage on the sensor element derived from only one of the output terminals for providing the fault signal. [0042] In an embodiment, the switching device is responsive to the fault signal for selectively electrically disconnecting the input terminals from the respective output terminals to prevent the supply of electrical power.

[0043] According to a fifth aspect of the invention there is provided an electrical protection device including:

two output terminals for electrically connecting to a load which is downstream of the device; a sensor that is responsive to an electrical fault downstream of the device for providing a fault signal;

a monitor that is responsive to one or more operational characteristics of the protection device for selectively providing a reference signal; a switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the fault signal and the reference signal for selectively electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

[0044] In an embodiment, the device includes a signal generator for providing the reference signal.

[0045] In an embodiment, the switching device is responsive to either the fault signal or the reference signal for electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

[0046] In an embodiment, the switching device is responsive to the fault signal and the reference signal for electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power. [0047] In an embodiment, the switching device is responsive to the fault signal and the reference signal for selectively electrically disconnecting the input terminals from the respective output terminals to prevent the supply of electrical power.

[0048] In an embodiment, the supply of electrical power is provided at a first frequency/_/ and the reference signal has a second frequency/_? that is different from//.

[0049] In an embodiment, one or both of/_/ and /₂ are substantially constant.

[0050] In an embodiment:

f₂ > 5 xfr, or

/? > 10 x/}; or

[0051] In an embodiment,/? > 5 kHz and, in another embodiment,/? > 10 kHz.

[0052] According to a sixth aspect of the invention there is provided an electrical distribution system including one or a combination of: one or more of the protection devices of the first aspect;

one or more of the protection devices of the second aspect;

one or more of the protection devices of the third aspect;

one or more of the protection devices of the fourth aspect; and

one or more of the protection devices of the fifth aspect.

[0053] According to a seventh aspect of the invention there is provided an electrical distribution system including:

a transformer having a primary winding and a secondary winding, wherein the primary winding is electrically connectable with a power source; a plurality of electrical circuits that are electrically connected in parallel to the secondary winding;

a protection device for each electrical circuit, wherein each protection device is upstream of the respective electrical circuit and responsive to a fault voltage in the corresponding electrical circuit for isolating that electrical circuit from the secondary winding while allowing the other circuit or circuits to remain electrically connected with the secondary winding.

[0054] In an embodiment,the system, downstream of the secondary winding, defines an IT network.

[0055] In an embodiment, the system, downstream of the secondary winding, defines a TT network.

[0056] In an embodiment, the system, upstream of the secondary winding, defines other than an IT network.

[0057] In an embodiment, the system, upstream of the secondary winding, defines a TN network.

[0058] In an embodiment, the system, upstream of the secondary winding, defines an IT network.

[0059] In an embodiment, one or more unprotected electrical circuits are electrically connected in parallel with the secondary winding.

[0060] In an embodiment, at least one of the electrical circuits includes a sub-circuit that is downstream from the respective protection device and the system includes a further protection device within the sub-circuit.

[0061] In an embodiment, the system includes a sensor element for carrying the fault voltage such that the immediately upstream protection device isolates the respective circuit from the secondary winding.

[0062] According to a eighth aspect of the invention there is provided an electrical distribution system including: a floating electrical network for drawing electrical power from a power source;

a plurality of electrical circuits that are electrically connected in parallel to the network for consuming the electrical power;

a protection device for each electrical circuit, wherein each protection device is upstream of the respective electrical circuit and responsive to a fault voltage in the corresponding electrical circuit for isolating that electrical circuit from the network while allowing the other circuit or circuits to remain electrically connected with the network.

[0063] According to a ninth aspect of the invention there is provided a method of electrical protection including the steps of: electrically connecting two input terminals of an electrical protection device to an electrical power source that is upstream of the device;

electrically connecting two output terminals of the electrical protection device to a load which is downstream of the device; providing a reference signal;

being responsive to an electrical fault downstream of the device for providing a fault signal; and

providing a switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the fault signal and the reference signal for selectively electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

[0064] According to a tenth aspect of the invention there is provided a method of electrical protection including the steps of:

electrically connecting two input terminals of a protection device to an electrical power source that is upstream of the device; electrically connecting two output terminals of the protection device to a load which is downstream of the device;

providing a switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the fault signal and the voltages at one or both of the output terminals for selectively electrically disconnecting the input terminals from the respective output terminals to prevent the supply of electrical power.

[0065] According to an eleventh aspect of the invention there is provided a method of providing electrical protection, the method including the steps of:

electrically connecting two input terminals of a protection device to an electrical power source that is upstream of the device;

electrically connecting two output terminals of the protection device to a load which is downstream of the device;

providing a sensor for connecting to a sensor element that extends downstream of the device, the sensor providing a fault signal in response to a low impedance between the sensor element and either of the output terminals; and

providing a switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the fault signal for selectively electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

[0066] According to a twelfth aspect of the invention there is provided a method of electrical protection including the steps of: _Λ

electrically connecting two input terminals of a protection device to an electrical power source that is upstream of the device; electrically connecting two output terminals of the electrical protection device to a load which is downstream of the device;

providing a sensor for connecting to a sensor element that extends downstream of the device, the sensor being responsive to a voltage on the sensor element derived from one of the output terminals for providing a fault signal; and

providing a microprocessor controlled switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the fault signal for selectively electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

[0067Ϊ According to a thirteenth aspect of the invention there is provided a method of electrical protection including the steps of:

providing a sensor that is responsive to an electrical fault downstream of the device for providing a fault signal;

providing a monitor that is responsive to one or more operational characteristics of the protection device for selectively providing a reference signal;

[0068] According to a fourteenth aspect of the invention there is provided a method of electrical distribution, the method including the steps of: provϊding an isolation transformer having a primary winding and a secondary winding, wherein the primary winding is electrically connectable with a power source;

electrically connecting a plurality of electrical circuits in parallel to the secondary winding;

providing a protection device for each electrical circuit, wherein each protection device is upstream of the respective electrical circuit and responsive to a fault voltage in the corresponding electrical circuit for isolating that electrical circuit from the secondary winding while allowing the other circuit or circuits to remain electrically connected with the secondary winding.

[0069] According to a fifteenth aspect of the invention there is provided a method of electrical distribution, the method including the steps of: drawing electrical power from a power source with a floating electrical network;

electrically connecting a plurality of electrical circuits in parallel to the network for consuming the electrical power;

≠ providing a protection device for each electrical circuit, wherein each protection device is upstream of the respective electrical circuit and responsive to a fault voltage in the corresponding electrical circuit for isolating that electrical circuit from the network while allowing the other circuit or circuits to remain electrically connected with the network.

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0071] Figure 1 is a schematic representation of an electrical distribution system according to the invention;

[0072] Figure 2 is a schematic representation of an electrical protection device according to the invention; [0073] Figure 3 is a schematic representation of an electrical distribution system according to another embodiment of the invention;

[0074] Figure 4 is a schematic representation of an electrical distribution system according to another embodiment of the invention; and

[0075] Figure 5 is a schematic representation of another embodiment of an electrical protection device according to the invention.

DETAILED DESCRIPTION

[0076] Referring to Figure 1 there is illustrated an electrical distribution system 1 for a ^J household installation 2. System 1 includes a 12 kVA 50 Amp isolation transformer 3 having a primary winding (not explicitly shown) between terminals 5 and 6 and a secondary winding (not explicitly shown) between terminals 7 and 8. The primary winding is electrically connectable, via an electrical meter board 9, with a power source in the form of an MEN consumer mains supply 10. A plurality of electrical circuits — which in this embodiment include a power circuit 11 and a lighting circuit 12 — are electrically connected in parallel to terminals 7 and 8 and, hence, with the secondary winding. System 1 includes two protection devices 15 and 16, one for each of circuits 11 and 12. Devices 15 and 16 are respectively upstream of circuits 11 and 12, and are responsive to a fault voltage in respective circuits 11 and 12 for isolating those respective circuits from one or more of terminals 1 and 8 while allowing the other of the circuits to remain electrically connected with the secondary winding.

[0077] The functionality provided by system 1 is referred to as severability of circuits 11 and 12, in that a fault in one of those circuits does not require all circuits in the installation to be electrically disconnected from terminals 7 and 8 of transformer 3. This is achieved by making use of a floating earth network downstream of transformer 3.

[0078] In other embodiments a greater number of electrical circuits are also electrically connected in parallel with terminals 7 and 8. It will be appreciated by those skilled in the art that the primary factor limiting the number of circuits is the capacity of transformer 3.

[0079] While system 1 is installed in a household installation, in other embodiments it is implemented in an industrial installation, an operating theatre installation, or one of many others. It will be appreciated that the capacity of transformer 3 will be different in different installations. For example, for a commercial installation it is usual to have a much larger capacity transformer than the 12 kVA example illustrated in Figure 1.

[0080] Supply 10 includes an active conductor 17 and a neutral conductor 18 that extend to board 9 at installation 2. Board 9 includes a service fuse 19 for detecting current overload conditions, and a service neutral link 20. An MEN link 21 is electrically connected with and extends from link 20 to a main earth point 22 for installation 2. Earth point 22 is connected to earth via an earth stake 23. In other embodiments alternative or additional earth connections are used.

[0081] A meter 24 is connected in parallel with conductors 17 and 18 downstream of ruse 19 and link 20 for gauging usage and/or cost of electricity consumption at installation 2. The active conductor 17, downstream of meter 24, is electrically connected to a circuit breaker 25 and then to terminal 5. The neutral conductor downstream of link 20 is connected to terminal 6.

[0082] A distribution board 27 includes an active conductor 29 that extends from terminal 7 and a neutral conductor 30 that extends from terminal 8. As shown in Figure 1, conductor 29 is connected to terminal 7 through a circuit breaker 31.

[0083] Device 15 includes input terminals 33 and 34 that are electrically connected to active conductor 29 and neutral conductor 30 respectively. Device 15 also includes output terminals 35 and 36 that are selectively electrically connected with respective terminals 33 and 34 for defining conductors 37 and 38 as an active conductor and a neutral conductor respectively.

[0084] Device 15 includes a sensor (not shown) for connecting with a sensor element in the form of a floating conductor 39.

[0085] Device 16 includes similar features, and corresponding features are denoted by corresponding reference numerals.

[0086] Circuit 11 includes three household AC power outlets 41, 42 and 43 that are connected in parallel with conductors 37 and 38 for allowing electrical power to be supplied to a load connected with the outlets. Additionally, for Class 1 equipment, there will also be a common floating link for those appliances provided by conductor 39. That is, those conductive parts of a Class 1 appliance that, in a prior art MEN system would have been connected to earth are, in this embodiment, left floating. While Class 2 appliances will not be connected to conductor 39, in the event of an insulation failure the user of the appliance is placed at a dramatically reduced risk of being electrocuted due to there only being a high impedance path to earth. That is, in the event of a fault, the fault current will be very low and, should the user contact conductor 39, that will result in a fault being detected and the appliance will be isolated.

[0087] Circuit 12 includes a load in the form of three lights 44, 45 and 46 that are connected in parallel and able to be supplied power via conductors 37 and 38. Conductor 39 is connected to all the respective metal housings of the lights to create a common floating reference point not only for conductor 39 but also for those housings. ^f

[0088] Circuits 11 and 12 are indicative of the electrical circuits used in a domestic or household installation, and are provided by way of example only. In other embodiments additional circuits are included, or circuits with loads other than those used in circuits 11 and 12.

[0089] Devices 15 and 16 are configured, as will be described in more detail below, to be responsive to one or more conditions for selectively isolating respective circuits 11 and 12 from terminals 7 and 8. Accordingly, if there is a fault in one of circuits 11 and 12, the other of those circuits need not be disconnected from terminals 7 and 8, and can continue to operate normally. The use of devices 15 and 16, together with conductor 39, eliminates the need for an RCD and allows multiple circuits to be supplied by a single transformer 3. That is, for prior art floating systems there is still a need in an MEN system to include an earth connection for the load on the secondary side of the transformer to allow an RCD to operate. Moreover, for the prior art systems to gain sufficient severability of circuits there is a need to have separate isolation transformers for the separate circuits. System 1, however, does not require an RCD, and allows the use of a single transformer for the multiple circuits.

[0090] It will be appreciated that system 1, downstream of transformer 3, provides a floating network or, more precisely, a true IT network, due to the absence of an earth connection. While in this embodiment supply 10 is an MEN-type network, in other embodiments it is a TT or IT- type network or other network-type. That is, system I is able to interact with alternatively configured supplies and still operate to protect personnel and equipment associated with the electrical circuits being powered. Moreover, in TT or IT supply networks - as opposed to the MEN supply network illustrated in Figure I - transformer 3 is able to be omitted. [0091] System 1 allows the safe use of a single isolation transformer for multiple electrical circuits and is therefore considerably more economical to commission than prior art pseudo-IT networks. More particularly, present use of IT-type networks typically allow only one transformer to be used for each piece of Class 1 electrical equipment. These networks are also often required to have each separate electrical circuit individually earthed to ensure the associated RCD will trigger in the event of a fault. The increased cost of deploying such a system — the increased cost arising from having multiple transformers and additional wiring to affect the individual earthing for each electrical circuit - restricts its use to high-cost applications where the increased cost of severability is justified.

[0092] Class 1 electrical equipment includes those appliances that have one layer of insulation over the active and neutral conductors, and where any exposed metalwork is bonded to earth.

[0093] Given the benefit of the disclosure herein, it will be appreciated by those skilled in the art that system 1 is a wiring system where all active parts or elements are either insulated from earth or one-point connected to earth through a sufficiently high impedance. The conductive parts of the electrical installation - that is, all protected circuits, socket outlets or electrical equipment — are earthed individually or collectively and are protected by a protection device such as device 15. This configuration allows the protection device typically to detect a first fault in the installation and isolate that fault quickly. For example, the circuit of Figure 2 is able to provide detection and isolation within 10 msec.

[0094] The increased ability to offer un-earthed safe operation of severable electric circuits in household, office, building and industrial installation allows the embodiments of the invention to:

• Provide even smaller installations with a more optimum continuity of supply. This is particularly attractive to the smaller consumers who, to date, have to accept that a fault in one circuit can cause all circuits to be isolated.

• Reduce disturbances of the appliances at the installation due to better isolation from electromagnetic radiation and common impedance faults. This has become increasingly important with the proliferation at smaller installations of sensitive equipment and digital communication systems such as computers, video, household and building automation controllers, and technical building management (TBM) systems and the like.

[0095] With the increasing electrical connectivity occurring between more and more components within installations, there is a greater need for the power supply system to not provide an easy path for electrical noise between the electrical circuits. System 1 achieves this by eliminating the earth connection and using a true IT network. That is, the use of floating voltages in the distribution of power to the electrical circuits reduces the risk of electrical noise from one circuit having an impact on the performance of another of the circuits. Examples of installations where this is of particular concern include commercial and industrial installations where use is made of one or more, and usually many, inductive devices together with considerable digital communications and control hardware. The plethora of different currents and frequencies in use necessitates good electrical isolation to allow effective operation in the control of a building or other infrastructure. The use of system 1 contributes to such isolation and operation by not importing EMI from external electrical systems. A further advantage of not having an earth connection downstream of transformer 3 is to allow a significant reduction in the costs of designing and wiring the installations.

[0096] System 1 also limits insulation fault currents as the reference point for detecting a fault — that is, floating conductor 39 — is not disturbed by high fault currents and harmonics.

[0097] At present the fault currents (Ia) of traditional low voltage earthing systems have the following standards:

• IT network, first fault: Id < 1 Amp. ' • TT network: I_d = 20 Amps.

• TN network: I_d = 20,000 Amps.

• IT network, second fault: U ⁼ 20,000 Amps.

[0098] While system 1 provides an IT network downstream of transformer 3, that network is a true IT network and not like conventionally installed quasi or pseudo IT networks which have an earth connection. Moreover, the use of protection devices 15 and 16 allow the detection of a fault to frame. In the Figure 1 system, not only is the fault detected but also the fault is able to be quickly isolated as it teaches 40 Volts and 5 mA. Accordingly, and unlike prior art IT networks, personnel and equipment are not exposed to the risk of a second fault occurring and, hence, not exposed to the possible high fault currents -in the order of 20,000 Amps - indicated in the table above.

[0099] The protection devices 15 and 16 in Figure 1 are illustrated as being mounted to a distribution board. In other embodiments, the devices are mounted at another location within system 1. There is considerable flexibility for such placement as the dimensions of device 15 are about 20 mm x 60 mm x 40 mm. For it is not so much the exact physical location of the protection device that is important, but the electrical placement within system 1 to provide protection to a downstream circuit or circuits.

[00100] In other embodiments the dimensions vary from those stated above. For example, high power applications make use of higher power relays, which are of a greater size typically as a result of requiring relays with higher voltage ratings. In other embodiments, device 15 is commonly packaged with other protection circuitry such as a circuit breaker, and the two are able to be packaged more effectively than could occur separately.

[00101] It will be appreciated by those skilled in the art that in other embodiments alternative dimensions are used.

[00102] Alternative placements within system 1 of protection devices as device 15 include intermediate the distribution board and the load being protected or, where there is no specific distribution board used, intermediate the meter board and the load being protected. In some embodiments where a transformer is used, the protection device is disposed adjacent to the secondary winding of the transformer. In alternative embodiments, the protection device is mounted to the interior of a wall cavity adjacent to a power outlet that is being protected. In other embodiments, the protection device is mounted directly to the power outlet. In some embodiments the power outlet includes a rear face having a formation for captively receiving the protection device. Moreover, when engaged with the formation, terminals 33, 34, 35 and 36 of the protection device are brought into contact with internal conductors of the outlet such that, when the outlet is wired into system 1 , those terminals are brought into contact with the relevant conductors of system 1 for allowing any electrical circuit downstream of the outlet to be protected. [00103] In other embodiments, use is made of an. adhesive or other retaining mechanisms for securely mounting the protection device to the power outlet.

[00104] In further embodiments a protection device of the invention is alternatively or additionally located electrically downstream of a power outlet. In one embodiment, a protection device is included within a lead of an appliance. In another embodiment, a protection device is included within an extension lead that is selectively engaged with a power outlet of system 1. In a further embodiment, a protection device is included within the housing of an appliance. In a still further embodiment, the protection device is included within an adapter, the upstream end of which is insertable into a power point, and the downstream end of which is adapted to receive a lead from an appliance. In a further embodiment, the protection device is integrally housed with an external power supply for the appliance. An example of such a power supply includes a laptop DC power supply that is able to provide a regulated DC output voltage from a range of AC input voltages.

[00105] The protection circuit of the invention is able also to be included within a TN (or MEN) network for providing electrical protection for downstream circuits. In these embodiments, the common earth of the MEN system is still used as the sensor element for the protection device.

[00106] It will be appreciated by those skilled in the art that the invention is applicable to higher voltages than those typically used in household installations. For example, in some embodiments the distribution system includes high voltage power lines for extending from a power supply in the form of a power generation station, and one or more of the protection devices are configured for operation at those higher voltages for selectively isolating downstream circuits from the power station.

[00107] Referring to Figure 2, there is illustrated one embodiment of an electrical protection device according to the invention. For ease of reference and understanding of the Figure 1 embodiment, the protection device illustrated in Figure 2 will be designated with reference numeral 15. It will be appreciated, however, that other similarly functioning but alternatively configured and constructed protection circuits are able to be used in the Figure 1 embodiment. It will also be appreciated that in the Figure 1 embodiment that device 16 is substantially identical to device 15 and, as such, will not be described separately. In other embodiments, however, device 15 and 16 are different For example, in some embodiments, the sensitivity of the respective devices to triggering in response to a fault differs. In other embodiments, the load current rating of the respective devices is different due to the nature of the operational currents the respective circuit 11 and 12 are designed to carry.

[00108] As mentioned above, device 15 includes two input terminals 33 and 34 for electrically connecting to source 10 that is upstream of device 15. The output terminals 35 and 36 are electrically connected to a load in the form of outlets 41, 42 and 43, which is downstream of device 15. Device 1 also includes: a signal generator 100 for providing an internal 10 kHz reference signal (that is indicated generally by reference numeral 111 which designates the conductor carrying the reference signal); and a sensor 112 that is responsive to an electrical fault downstream of device 15 for providing a fault signal (that is indicated generally by reference numeral 113 which designates the conductor carrying the fault signal). A switching device 114 is provided for electrically connecting input terminals 33 and 34 to respective output terminals 35 and 36 to allow a supply of electrical power from source 10 to outlets 41 , 42 and 43. Device 114 is responsive to signal 113 and signal 111 for selectively electrically disconnecting terminals 35 and 36 from respective terminals 33 and 34.

[00109] In other embodiments, device 114 is responsive to signal 113 and signal 111 for selectively electrically disconnecting only one of terminals 35 and 36 from the respective terminals 33 and 34.

[00110] For the sake of convenience, terminals 33, 34, 35 and 36 are referred to as follows:

Terminal 33 "the active terminal" or "active"

Terminal 34 "the neutral terminal" or "neutral"

Terminal 35 "the switched active terminal" or "switched active"

Terminal 36 "the switch neutral terminal" or "switched neutral"

[00111] In other words, terminals 33 and 34 are on the upstream end of device 15 and are electrically connected to the active and neutral conductors of the mains supply 10 via the intermediary components illustrated and described. Terminals 35 and 36, on the other hand, are on the downstream end of device 15 and are selectively electrically connected to terminals 33 and 34 and, similarly, selectively connected to the active and neutral conductors of supply 10. As a result of this selective electrical connection, terminals 35 and 36 are "switched" between:

• A first state where they are electrically connected with the respective input terminals and power from source 10 is able to be supplied to ekctrical circuit 11.

• A second state where they are electrically disconnected - that is, electrically isolated - from the respective input terminals and power from source 10 is not able to be supplied to circuit 11.

[00112] Device 114 includes an active relay in the form of a HF-115/160-1 HS3A relay 115 and a neutral relay in the form of a HF-115/160-1 HS3A relay 116 that have their respective coils directly connected to each other in series. This configuration ensures that both relays switch at Jhe same time in response to a fault. This fault could be an external fault — as in, an electrical fault in the load — or an internal fault - as in, a fault within device 15. Relay 115 is also connected in series to a switch actuator in the form of a ZVP 1320F p-channel metal-oxide- semiconductor field-effect transistor (MOSFET) 117. The source of MOSFET 117 is connected to a 500 kΩ resistor 118 on one side and the gate of MOSFET 117 is connected to resistor 118 on the other side. A 1 nF capacitor 119 is connected in parallel with resistor 118. The gate of MOSFET 117 is also connected to a 1 nF capacitor 120 and the other side of capacitor 120 is connected to conductor 30. Also connected in parallel with resistor 118 and capacitor 119 are two diodes in the form of BAV99 diodes 122 and 123 that are directly connected in series by a conductor 124. This conductor is connected to another 1 nF capacitor 125 and the other side of capacitor 125 is connected to the collector of a dual optocoupler in the form of a TCLT1600 optocoupler 130.

[00113] Relay 116 is also connected in series to a switch actuator in the form of a ZVN3320FTA n-channel metal-oxide-semiconductor field-effect transistor (MOSFET) 131. The source of MOSFET 131 is connected to conductor 30 and the gate of MOSFET 131 is connected to a 1 nF capacitor 132. The other side of capacitor 132 is a 33 Volt voltage source 133. The gate of MOSFET 131 is connected to a 500 kΩ resistor 134 on one side and the source of MOSFET 131 is connected to resistor 134 on the other side. A 1 nF capacitor 135 is connected in parallel with resistor 134. Also connected in parallel with resistor 134 and capacitor 135 are two diodes in the form of BAV99 diodes 136 and 137 that are directly connected in series by a conductor 138. This conductor is connected to another 1 nF capacitor 139 and the other side of capacitor 139 is connected to the collector of optocoupler 130. The gate of MOSFET 131 is also connected to the collector of optocoupler 130 via a diode in the form of a BASl 6 diode 140.

[00114] The collector of optocoupler 130 is connected via a 12 kΩ resistor 141 to voltage source 133. Also connected to voltage source 133 is a zener diode in the form of a BZX84C33 zener diode 142 and the other side of zener diode 142 is connected to conductor 30. A 100 nF capacitor 143 is connected in parallel to zener diode 142.

[00115] The emitter of optocoupler 130 is connected to conductor 30.

[00116] Generator 100 includes a Schmidt gate in the form of a NC7SZ14 Schmidt gate 150, which generally drives the production of reference signal 111. Contacts 2 and 3 of Schmidt gate 150 are connected to either side of a capacitor 151. Contacts 2 and 4 of Schmidt gate 150 are connected to either side of a resistor 152. The combination of capacitor 151 and resistor 152 that result in signal 111 having a frequency of about 10 kHz. In other embodiments, capacitor 151 and resistor 152 are selected to provide signal 111 with an alternative frequency. Preferably, the selected output frequency is greater than the frequency of the supply voltage provided by source 10, and at least five times the frequency of that supply voltage. In this embodiment, supply 10 provides a voltage to system 1 of 50 Hz and, as such, the frequency of the output signal of generator 100 is about 200 times that of the supply voltage.

[00117] In this embodiment, the frequency of signal 111 is substantially constant. However, in other embodiments the frequency varies across a range of frequencies, or is selectable from a set of specific frequencies. In those embodiments where there is more than one frequency for signal 111, it is preferred that the lowest of those frequencies is greater than the frequency of the supply voltage being distributed to the electrical circuits or circuits being protected by device 15.

[00118] Contact 5 of Schmidt gate 150 is connected to a 120 Volt voltage source 153. Contacts 3 and 5 of Schmidt gate 150 are connected to either side of a 100 nF capacitor 154. Capacitor 154 is also connected in parallel to a zener diode in the form of a BZX84C5V1 zener diode 155. [00119] Contact 4 of Schmidt gate 150 is connected to an optocoupfer in the form of a TCLT1006 optocoupler 156, at a contact 157, via a 1.2 kΩ resistor 158. Another contact 159 of optocoupler 156 is connected to contact 3 of Schmidt gate 150. The collector of optocoupler 156 is connected via a 150 Ω resistor 160 to a PNP bipolar transistor in the form of a BC857B transistor 161 at the emitter. The emitter of optocoupler 156 is connected the base of transistor 161. The collector and emitter of optocoupler 156 are also connected to either side of two directly serially connected diodes in the form of BAV99_1 diodes 162 and 163. The collector of optocoupler 156 is also connected via a 3.3 uF capacitor 164 to conductor 30. The base of transistor 161 is connected via a 20 kΩ resistor 165 to conductor 30. A diode in the form of a BAS16 diode 166 is connected, on one side, to the collector of optocoupler 156 and, on the other side, to a conductor 167. A l kΩ resistor 168 is connected on one side to wire 167 and on the other side to a conductor 37, which is a "switched active" conductor, via a 27 nF capacitor 170. Conductor 167 is connected to conductor 30 via a 33 V zener diode 171. The collector of transistor 161 is connected to a first contact 172 of optocoupler 130.

[00120] Contact 172 is connected to a diode in the form of a BAS 16 diode 173 that is in turn serially connected to a resistor 174 that is in turn connected to optocoupler 130 at another contact 175. Contact 172 is also connected to a resistor 176 that is in turn serially connected to a diode in the formofa BAS16 diode 177 that is in turn connected to contact 175. Another 300 Ω resistor 178 is connected on one side to contact 172 and on the other side to: a zener diode in the form of a BZX84C4V7 zener diode 179; conductor 39, via a 1.5 kΩ resistor 181; and a 10 kΩ resistor 182. Zener diode 179 is connected in back-to-back with another zener diode in the form of a BZX84C4V7 zener diode 183 that is connected to contact 175. Contact 175 is connected to conductor 38, which is a "switched neutral" conductor. Resistor 181 is connected via a switch 185 to conductor 37.

[00121] Switch 185 provides a means for testing the operation of device 15, by allowing a user to apply a fault voltage to the device. Accordingly, upon manual depression of switch 185, device 185 should progress from an operative state - in which the relay coils are energized - to an isolated or triggered state - in which the relay coils are de-energized.

[00122] Back to back diodes 179 and 183 provide over-voltage protection for sensor 112. [00123] Device 15 includes a regulator 186 having an n-channel metal-oxide-semiconductor field-effect transistor (MOSFET) in the form of a FQDIN60C MOSFET 187. The gate of MOSFET 187 is connected to neutral conductor 30 via a 22 nF capacitor 188. The gate and drain of MOSFET 187 are each respectively connected to either side of a 47 kΩ resistor 189. The gate of MOSFET 187 is connected to a zener diode in the form of a BZX84-C62 zener diode 190 which is in turn serially connected to another zener- diode in the form-of a BZX84- - C62 zener diode 191 which is in turn connected to conductor 30. The drain of MOSFET 187 is also connected to a diode 192 which, in turn, is connected to active conductor 29 via a 100 Ω resistor 193. A varistor capacitor 194 extends between the anode of diode 192 to conductor 30. The source of MOSFET 187 is connected to a diode in the form of a IN4007 diode 195 which is in turn connected to both source 153 and conductor 30, via a 10 uF capacitor 196. Source 153 is connected to a 30 kΩ resistor 197 which is in turn connected to source 133.

[00124] Device 15 includes a fault detector 200 having a NPN bipolar transistor in the form of a BC847B transistor 201. The collector of transistor 201 is connected to the collector of optocoupler 130. The base and emitter of transistor 201 are connected to either sides of a 15 kΩ resistor 202. The base of transistor 101 is also connected to a 400 kΩ resistor 203 which in turn is connected to the conductor 30 via a 1 uF capacitor 204. Finally the base of transistor 201 is connected to a diode in the form of a BAV99 1 diode 205 that is serially connected by wire 206 to another diode in the form of a BAV99_1 diode 207 and in turn connected to conductor 30. Wire 206 is connected to the source of MOSFET 117 via a 100 nF capacitor 208.

[00125] Reference signal 111 is a constant and relatively high frequency signal of about 10 kHz that is fed into optocoupler 130. Signal 111 is a 2-volt peak-to-peak square wave signal with a 50% duty cycle. That is, signal 111 is, in effect, a ±1 -volt square wave with a 1 -volt DC offset. It is appreciated that in other embodiments signal 111 is other than 10 kHz and other than a 2-volt peak-to-peak signal with a 1 -volt DC offset.

[00126] Regulator 186 draws current from the active conductor 29 upstream of device 15, and provides a regulated DC voltage of about 120 Volts at 153. It will be appreciated that while this voltage includes some ripple it is not, during normal operation of device 15, problematic. In fact, advantage is taken of this ripple to aid the operation of device 15. [00127] Generator 100 draws current from regulator 186, and is tuned to provide signal 111. It will be appreciated that while the frequency of signal 111 is sourced from Schmitt gate 150, . transistor 161 provides current gain for that signal to ensure it is able to adequately drive optocoupler 130 such that a 10 kHz signal derived from signal 111 appears at the collector of optocoupler 130.

[00128] Optocouplers 130 and 156 divide device 15 into two sections, one that is electrically isolated from any fault voltage on conductor 39, and another that is exposed to that fault voltage. As shown in Figure 2, those components to the left of the optocouplers are electrically isolated from a fault voltage, and those to the right are not. While there is electrical isolation between those two sections of the circuit, the 10 kHz signal exists in both sections.

[00129] Switching device 114, during normal operation, has transistors 117 and 131 turned ON, and the coils of relays 115 and 116 energized. As the relays are normally open relays, the energizing of the coils results in active conductor 29 being electrically connected to switched active conductor 37, and neutral conductor 30 being electrically connected to switched neutral conductor 38. It will be appreciated by those skilled in the art that should the 10 kHz signal no longer appear at the collector of optocoupler 130, transistors 117 and 131 will be switched OFF, the coils of relays 115 and 116 de-energised, the above conductors will become electrically disconnected from each other, and any downstream load will be isolated from both active conductor 29 and neutral conductor 30. That is, an absence of the 10 kHz signal at the collector of optocoupler 130 results in a triggering of device 15. It will also be appreciated that, in this embodiment, this triggering of device 15 is due to one or either of: the detection of an electrical fault downstream of device 15; or the detection of an internal fault within device 15. That is, signal 111 is a reference signal used by device 15 to self-monitor or self-diagnose the internal performance of device 15, as well as to allow detection of a fault in the downstream circuit.

[00130] Device 114 is responsive to an absence of signal 111 for electrically isolating the load from source 10. Other embodiments work in an opposite sense, in that device 114 is responsive to the presence of that reference signal for electrically isolating the load from source 10. In the context of this specification, the above two alternatives are considered functional equivalents. [00131] Under normal operation of device 15, signal 111, and other signals derived from signal 111, exists within the circuitry of device 15. More particularly, such signals are present on both of the electrically isolated sections of the circuitry. This reference signal 111 is therefore an active signal that is constantly monitoring, during normal operation of device 15, for faults. In other embodiments a periodic detection is used.

[00132] Sensor 112 is configured to detect a voltage on conductor 39 and, in response to that voltage being large enough, generating a fault signal. In this embodiment, the fault signal is a voltage applied to contact 172 of optocoupler 130. As has been mentioned above, this is also the same contact where signal 111 is applied. Both signal 111 and the fault signal are summed and optocoupler 130 is driven hard ON if the summed voltage is sufficiently high, or hard OFF if the summed voltage is sufficiently low. In practice, the voltage on conductor 39 need only be ±3 Volts instantaneous to result in optocoupler 130 being driven hard ON or hard OFF. While the frequency of the mains voltage - and hence the frequency of the voltage on. conductor 39 during a fault — is 50 Hz, that is much less than the frequency of signal 111 and is, in terms relative to signal 111, a DC signal. Accordingly, once the mains frequency voltage on conductor 39 rises above about 3 Volts instantaneous, a fault signal is generated by sensor 112, the high frequency signal — that is, signal 111 or a signal derived from signal 111 - no longer appears at the collector of optocoupler 130, relays 115 and 116 open, and the downstream circuit is isolated from source 10.

[00133] The resistance of resistors 174 and 176 differs to account for the effective DC offset of the reference signal, signal 111. That is, the reference signal introduces into the detection an asymmetry, which sensor 112 compensates for. It will be appreciated that the asymmetry of signal 111 has the advantage of allowing simplicity in the remainder of the circuitry within device 15, and the method for compensation is able to be achieved simply. In other words, the minor overhead involved in compensation for the asymmetry is justifiable in the context of the device as a whole.

[00134] Device 15 includes three resistors that are specifically tailored at the time of manufacture to a very high tolerance. These are resistors 160 in generator 100,^' and resistors 174 and 176 in sensor 112. More particularly, these resistors are all screen-printed and fired resistors which, as part of the manufacture process, are laser trimmed to an accuracy of at least ± 2%. This trimming occurs during the application of one or more, test voltages and/or currents to relevant parts of the circuit. In other embodiments alternative tolerances are used, for example, of about ± 1%. In yet further embodiments, the resistors are laser trimmed to an accuracy of ± 0%

[00135] In other embodiments resistors 160, 174 and 176 are surface mounted resistors or variable resistors that are calibrated as part of the manufacturing process.

[00136] Resistor 160 is trimmed to bias transistor 161 to provide a predetermined current gain. This gain is such that signal 111 is sufficient to drive optcoupler 130 to ensure a 10 kHz signal appears at the collector of that optocoupler in the absence of a fault signal. The typical minimum resistance of resistor 160 is 150 Ω.

[00137] The use of resistors .to calibrate the gain is because the characteristics of optocouplers vary considerably between notionally like devices. Two of the characteristics of the optocoupler that often vary, and which are accommodated by the present protection circuit, include one or more of: the current gain of the optocoupler; and the different bias required for the two back-to-back LEDs in optocoupler 130.

[00138] Once resistor 160 is trimmed to provide the required current for signal 111 - and with device 15 operative in that the relays are now energized - a 4 mA positive current flow is established through resistor 174. While device 15 remains active and the positive current flow continues, resistor 174 is laser trimmed until the relays 115 and 116 turn OFF — that is, until the relay coils are de-energized. Resistor 174 prior to trimming has a resistance of about 1.2 kΩ and, hence, after trimming, will have a resistance of at least that value.

[00139] Following from the trimming of resistor 174, the positive current flow is stopped and device 15 reset. Thereafter, a negative current flow of 4 mA is established through resistor 176 and that resistor laser trimmed until the relays 115 and 116 turn OFF. Resistor 176 prior to trimming has a resistance of about 300 Ω and, hence, after trimming, will have a resistance of at least that value.

[00140] Due to signal 111 having an effective DC-offset and the fact that this signal is added with the fault signal, the resistance of resistor 174 will be greater than the resistance of resistor 176 to provide the same response characteristics to both a positive or a negative voltage on conductor 39. [00141] This symmetry of response is also provided notwithstanding an asymmetry provided by the two LEDs in optocoupler 130. In some embodiments, the characteristics of those LEDs vary by a factor of two. However, the specific tailoring of the device 15 allows these variation to be accommodated and for the device to detect a fault, on either a positive or a negative half cycle, when the fault current is only about 4 mA. It will be appreciated that the specification for the embodiments is a 5 mA triggering current, with the safety factor of I mA being included to accommodate temperature variation across the operational temperature range. It will be appreciated by those skilled in the art that the temperature characteristics of device 15 are relatively stable as the components upon which the device is primarily calibrated are passive components. In particular, the components are resistors 160 and 174, which are thick-film resistors.

[00142] In other embodiments, sensor 112 is configured to be less sensitive and to require greater than 4 mA to result in a fault signal being generated. However, in other embodiments where electrical protection is being provided for a load that includes extremely sensitive electronic equipment, sensor 112 is configured to provide a feult signal when less than 4 mA is drawn by either of resistors 174 or 176.

[00143] As sensor 112 is responsive to both positive and negative voltages appearing on conductor 39, it is able to act extremely quickly to isolate a fault. Moreover, the sign of the voltage does not impact upon the speed at which that fault is detected or isolated.

[00144] It will be appreciated that device 15 is responsive to a voltage on conductor 39 regardless of whether that voltage is induced from the switched active conductor or the switched neutral conductor.

[00145] Detector 200 is responsive to the supply voltage provided by regulator 186 for selectively generating a reference signal at the base of transistor 201. More particularly, detector 200 is responsive to the ripple on the supply voltage provided by regulator 186 during normal operation and, should that ripple increase beyond a predetermined amount, the reference signal is generated. This has the effect of removing the 10 kHz signal from switching device 114, as the collector of optocoupler 130 is held low due to transistor 201 being hard ON. The ripple on the supply voltage often increases when capacitor 196 is approaching the end of its serviceable life. Accordingly, detector 200 is an internal monitor of an operational characteristic of device 15 and, should that characteristic vary beyond predetermined parameters, device 15 will be disabled, and any downstream load isolated from supply 10. It will be appreciated by those skilled in the art that an increased ripple is, in some instance, indicative of other characteristics of device 15, such as a shorting of transistor 187. Moreover, in other embodiments, alternative or additional characteristics are monitored by one or more additional detectors (not shown). - - -

[00146] Device 15 makes use of an internally generated and propagated relatively high frequency signal - which in this embodiment includes signal 111 or a signal derived from signal 111 — for providing internal monitoring of device 15. Switching device 114 is responsive not only to a fault signal provided by sensor 112, but also to a "monitoring" signal such as that provided by signal 111, for selectively isolating the downstream load from supply 10. This self-checking and failsafe operation makes device 15 particularly effective in providing electrical protection to not only the loads being protected, but also to personnel and other property.

[00147] When power is initially applied to conductors 29 and 30, device 15 is OFF, in that the relay coils are de-energized. However, upon power being applied to those conductors, and regulator 186 generating the 120 Volt supply, switching device 114 acts to latch the relays to an ON state. This latching need only persist until reference signal 111 is established, which is in the order of milliseconds. The latching is performed by the combination of diodes 123 and 124 and capacitor 119 using the start-up ramping up of the 120 Volt supply to latch transistor 117 ON. Similarly, diodes 137, 138 and capacitor act similarly to latch transistor 131 ON. Once both transistors are ON, the switched active conductor 37 and the switched neutral conductor 38 will be connected to supply 10 and generator 100 will operate to establish signal 111. This, in turn, will drive optocoupler 130 such that the 10 kHz signal appears at the collector of that optocoupler, and transistors 117 and 131 will be kept ON.

[00148] If, during the above described start-up procedure a fault exists, the 10 kHz signal will not be established at the collector of optocoupler 130 and transistors 117 and 113 will exhaust the available energy in the associated capacitors and turn OFF. [00149] It will be appreciated by those skilled in the art, that during normal operation the ripple voltage is much smaller than the ramping up of voltage that occurs at start-up, and that the ripple will not be sufficient to sustain a latching of transistors 117 and 131.

[00150] The major advantages of device 15 are delivered through:

• Use of an internal reference signal that has a frequency that is relatively high in comparison to the frequency of the signal being supplied to the load being protected.

• Failsafe operation, in that if a fault is detected, or abnormalities occur, the result will be that the load being protected is isolated from the power supply.

• Electrical components that are separated into at least two distinct circuits that are electrically isolated from each other. In this embodiment the isolation is achieved through the use of two optocouplers. In other embodiments a different number of the same or other transducers are used.

• Having key performance parameters that are able to be easily calibrated at the time of manufacture for reliable and consistent operation over the life of the device. For example, only three resistors - that is, components that are able to be accurately set, and which have low thermal sensitivity — are set to accurately configure device 15.

• The use of more than one frequency within device 15.

[00151] All resistors are printed, fired onto a ceramic circuit board & later laser trimmed. Other components of device 15 - for example semiconductors and capacitors - are surface mounted to a circuit board that is combined with the ceramic circuit board. The combined board is encapsulated in an epoxy compound for protection against electrical, physical and chemical interference.

[00152] In other embodiments use is made of alternative or additional substrates to the ceramic and circuit boards referred to above. For example, in the Figure 2 embodiment, capacitors 170 and 188 and resistor 189 are mounted to a ceramic substrate and the other components are mounted to a circuit board that is, in turn, mounted to the substrate. A further example includes varistor 194, that is in some embodiments soldered manually to a board. A further example is an embodiment that makes use of a single circuit board only. [00153] Where an embodiment includes a circuit board and is intended for high voltage applications, use is often made of silicone encapsulants to provide electrical isolation. However, in other embodiments, alternative isolation packaging is used.

[00154] While device 15 has been described in detail, it will be appreciated by those skilled in the art that the functionalities provided are able to be delivered by alternative circuits.

[00155] Reference is now made to Figure 5 where there is illustrated a schematic representation of a further embodiment of device 15 and where corresponding features are denoted with corresponding reference numerals. The actual components used in the illustrated configuration are set out in the tables below, and are identified by the reference indicia used in Figure 5.

[00156] The asterisk (*) in the above tables indicates that the respective components are on a separate substrate or on the PCB to which the relays are mounted. In other embodiments, different combinations of substrates and/or PCBs are used.

[00157] The active trim process for this embodiment includes:

• The trimming of Rl 2 until Q5 turns off with 4.7 mA of positive current. • Followed by a check of the negative current turn off = 4.7 ± 0.2 mA.

[00158] The circuit of Figure 5 includes three main differences over the circuit of Figure 2. Particularly, in the circuit of Figure 5:

• The regulator has been redesigned to increase its efficiency. This allows the regulator to generate less heat and, hence, run cooler. This is achieved by the addition of transistor Q9 and resistors R3 and R24 that allows the regulation capacitor Cl to only be charged during a limited supply voltage window. This prevents high dissipation in the FET package when connected to a 240 Volt AC line.

• The optocoupler Q4 in Figure 2 is running in the linear mode. To improve the temperature performance in Figure 5, the gain of the circuit has been increased by the addition of transistor Q2. Accordingly, a full 24 volt swing at 10 kHz (which is the monitoring, testing and latching frequency) exists on the collector of Q2. When a DC or 50 Hz fault current appears on the sensor the 10 kHz AC is quenched and the relays are turned off rapidly through Q8 and Q5 and more slowly through Q6.

• The high voltage breakdown performance is enhanced by the addition of Varistor Vl and V2 and the addition of the resistor R22 and Varistor V3 in the sense line. The capacitor C2 has been added to avoid ialse turn on caused by, for example, long extension cords or other capacitive sources.

[00159] A further embodiment of the invention, in the form of an electrical distribution system 300, is illustrated in Figure 3 where corresponding features are denoted with corresponding reference numerals. System 300 is included within an installation that is a commercial building 301 having a building management system 302. System 302 allows the manager of the building to monitor and control various fiinctions within the building, including security and access control, HVAC, power supply to different parts of the building, and the like.

[00160] System 300 includes two protection devices 15 and 16 that are disposed within building 302 for protecting electrical loads in the form of respective AC electric motors (not shown) for automatically opening doors (also not shown). While these protection devices are number identically with those shown in Figure 2, it will be appreciated that the correspondence in number refers to similarities in the protection functionality provided rather than exact replication of components and design. Particularly, devices 15 and 16, in this embodiment, are programmable devices and each include a microprocessor, memory, software contained within the memory, hardware interfaces, and other electronic components. Additionally, building 302 includes many other electrical circuits (not shown) for facilitating the operation of the building control functions, and these circuits are also protected by devices functioning like device 15 and 16.

[00161] In this embodiment, each of devices 15 and 16 are individually securely packaged and commonly mounted to distribution board 27 for ease of manual access and inspection by service personnel. Board 27 is mounted within a secure cabinet to reduce the risk of unauthorized access to the protection circuitry mounted to that board.

[00162] Also mounted to board 27 are separate wireless communication interfaces 305 and 306 for allowing respective devices 15 and 16 to communicate wirelessly with a central controller 307 via a further wireless interface 308 that is located elsewhere within building 301. In other embodiments, protection devices commonly mounted to a distribution board make use of a single common communications interface to communicate with controller 307.

[00163] It will be appreciated that controller 307 is a combination of hardware and software, typically implemented over a number of servers, located within building 301. These servers will include processors, memory, interfaces, communications ports and other components to function, as will be understood by those skilled in the art.

[00164] The communications between the protection devices and controller 307 is established under a predetermined wireless protocol. In this embodiment the communications are secure and two-way, in that data is able to be sent and received by both the protection devices and the controller and that encoding and decoding of the data occurs at the interfaces. The data sent by the protection devices is indicative of the timing and nature of a detected fault in the electrical circuit being protected, the results of any internal monitoring or diagnosis from within the protection device, supplying the controller with requested contents of the memory, or confirmation of one or more commands provided by the controller to the protection device. The central controller is in communication with the protection devices primarily to receive the above data and to provide^'updates to the software embedded with the microprocessor or held within memory. However, for some configurations, additional data is communicated. For example, protection device 15 is responsive to the voltage on'conductor 39 for detecting a fault. If a fault is detected this will be a "fault to frame" in an IT network and, as such, there will be low fault currents, if any. Data indicative of the detection of the fault is communicated wirelessly to controller 307 together with a timestamp and an identifier of device 15. Controller 307 includes within memory a database table of identifiers and other properties or flags and, upon receipt of a fault indication, accesses the table to determine the response to the fault. For the specific instance being examined, the table includes for device 15 a flag that is set to "automatically isolate". Accordingly, controller 307 communicates with device 15, via interface 27, with a command to isolate the circuit being protected. In some embodiments, this command is present and held in memory at device 15. Controller 307 is responsive to other events within building 301 to set the same flag to "operator determined". This occurs, for example, when controller 307 detects the triggering of a fire alarm in building 307 in the same area as the doors that are being powered by the motor that is protected by device 15. Accordingly, if data is communicated from device 15 indicative of a fault, controller 307 accesses the table as before and determines the flag status. The controller is responsive to this status to drive a user interface, in the form of a touch screen monitor 309, to alert an operator of the fault. The operator is able to choose between isolating the fault - and hence totally disabling the circuit being protected — and allowing the circuit to remain connected. Clearly, the circuit will only be allowed to remain connected if: the fault is above 40 V and 5 mA; the- functionality provided by the circuit is required in response to the fire alarm; and if the likelihood of persons being in the area is low. In this embodiment, monitor 309 provides the operator with a flashing display seeking input. Controller 309 is responsive to the input being provided for communicating to device 15 a command corresponding to the choice made by the operator. If input is not provided by the operator within a predetermined period, controller 307 either seeks input from another operator (interface not shown) or is configured with a default action for such an occurrence.

100165] . In other embodiments the communications interfaces communicate with each other through dedicated cabling that extends within building 301. In further embodiments, the interfaces communicate via one or more of the conductors used to supply power to the loads in the building. In further embodiments, the interfaces communicate with each other via conductor 39, which is common to all electrical equipment and loads within building 301. In even further embodiments, one or more of the communications interfaces is able to select for a given communication which communication path is used. For example, where a protection device is to communicate with the controller, the default communication path is conductor 39. If this communication fails, or if this communications path or "communications bus" is being used for other transmissions to or from other protection devices to the controller, use is then made of the wireless communications path.

[00166] In other embodiments, the communications path is via one or more of the conductors carrying power. In the Figure 1 example, that would include one or both of conductors 29, 30, 37 and 38.

[00167] In some embodiments, one or more of the communications interfaces also selectively establishes communication with a remote interface of another party. For example, in an embodiment the remote interface is of an electricity distribution authority that provides supply 10. Accordingly, in the event of a fault, the authority is alerted. This functionality is typically reserved for those protection devices that protect a large circuit. An example of such a protection device is provided in building 301, which includes a top-level protection device (not shown) through which all current to the circuits in building 301 is supplied. If this protection device triggers it is usually indicative of a major fault of which the distribution authority will have an interest in understanding.

[00168] A further embodiment of the invention, in the form of an electrical distribution system 310, is illustrated in Figure 4 where corresponding features are denoted with corresponding reference numerals. System 310 includes two nested protection devices 15 and 16 for protecting electrical loads 311 and 312. While these protection devices are number identically with those shown in Figure 2 and Figure 3, it will be appreciated that the correspondence in number refers to similarities in the protection functionality provided rather than exact replication of components and design.

[00169] In the Figure 4 embodiment, the input terminals 33 and 34 of device 15 are connected, via conductors 29 and 30, to the secondary winding of a transformer. Terminals 35 and 36 of device 15 are connected to conductors 37 and 38 to allow electrical power from the secondary winding to be available to a load connected to those conductors. As illustrated, load 311 - which is a Class 1 electrical appliance - includes two terminals, an active terminal 315 and a neutral terminal 316, that are connected with the active conductor 37 and the neutral conductor 38 respectively. Load 311 includes a metal casing that is electrically connected to conductor 39 by a further conductor 317. It will be appreciated that conductors 315, 316 and 317 are separately sheathed in respective individual insulating sleeves and bundled together within an outer insulating sleeve to define an electrical lead that extends from load 311. The connection of the lead to conductor 37, 38 and 39 is via a power outlet (not shown). The individual insulating sleeves are preferentially color-coded to aid identification of the conductors by personnel involved in maintenance or installation of system 1.

100170] In the event of an insulation fault or other fault at load 311 that establishes a sufficient voltage on conductor 39, device 15 will detect the fault and isolate terminals 35 and 36 from terminals 33 and 34. Hence, both loads 311 and 312 will be isolated from the secondary winding of the transformer.

[00171] The input terminals 33 and 34 of device 16 are connected to active conductor 37 and neutral conductor 38 respectively. The output terminals 35 and 36 of device 16 are connected to active conductor 319 and neutral conductor 320 respectively. Load 312 — which is also a Class 1 electrical appliance - includes two terminals, an active terminal 321 and a neutral terminal 322, that are connected with the active conductor 319 and the neutral conductor 320 respectively. Load 312 includes a metal casing that is electrically connected to conductor 39 by a further conductor 323. It will be appreciated that conductors 321, 322 and 323 are bundled together to define an electrical lead that extends from load 312. The connection of the lead to conductor 319, 320 and 321 is via a further power outlet (not shown). In the absence of device 16 detecting a fault- that is, when terminals 35 and 36 of device 16 are respectively connected with terminals 33 and 34 - electrical power is able to flow from conductors 319 and 320 to load 312.

[00172J In the event of an insulation fault or other fault at load 312 that establishes a sufficient voltage on conductor 39, device 16 will detect the fault and isolate terminals 35 and 36 of device 16 from terminals 33 and 34 of device 16. Hence, load 312 will be isolated from the secondary winding of the transformer while load 311 will remain connected to the secondary winding.

[00173] It has been found for the nested configurations of like protection devices, such as that illustrated in Figure 4, that the protection device immediately upstream of a fault will in substantially all instances detect and isolate a fault prior to a further upstream protection device detecting the same fault. In some instances it has been found that a protection device downstream of a fault will first detect the fault and isolate the circuit downstream of that device. However, as the fault still persists, the immediately upstream device also triggers. It will be appreciated that the delay between the triggering of the downstream and then the upstream protection devices is very small, and in the order of 1 msec.

[00174] It is understood that the advantageous timing of the detection and isolation by the protection device that is immediately upstream of the fault is a result of using a relatively small voltage measurement to detect the fault. For as the voltage is small, and the required current to trigger the protection circuits low - in the order of 4 mA for household and commercial installations - the resistance of the conductor 39 will have some effect on the current that flows into each of the protection devices from conductor 39. Accordingly, the closest protection device, be it upstream or downstream, will more usually trigger first. And even in the event that the downstream protection device triggers first, the immediately upstream device will then offer the next lowest resistance current path.

[00175] • The ability of the protection circuits of the invention to facilitate nesting without tuning is a substantial advantage over prior art systems that make use of RCDs. These prior art device require considerable effort to correctly set to ensure that a fault will only trigger a downstream device, and not an upstream device. Moreover, due to environmental sensitivity, reliable ongoing operation is compromised, even with careful and regular maintenance and testing.

[00176] The embodiments described above detect a fault by detecting a voltage on a conductor that, during normal operation, is floating. The use of a voltage detection, as opposed to prior art current detection, allows considerably greater sensitivity to be achieved, together with fast fault detection and isolation. Moreover, the embodiments of the invention are able to be used as Line Insulation Monitors (LIM) or Leakage Protection Devices (LPD).

[00177] In the above-described embodiments the protection device includes in a single location or package the required components to both detect a fault in a downstream load and to isolate that load from the upstream power supply, and isolation. In other embodiments, however, the fault detection function and the isolation function are performed by different circuits, and those circuits are physically spaced apart. In the Figure 4 embodiment, for example, the detection function is able to be performed by a sensor located at an outlet, and the isolation function by a switching device located at a distribution board. In this case, there is a need for communication of a fault signal from the sensor either directly to the switching device or via an intermediate device such as the central controller.

[00178] Some advantages of the embodiments include:

• No reliance placed upon having an earth path to allow the protection device to operate.

• High speed of operation also assists in limiting fault duration and risk of damage from I²R losses.

• Reduces significantly over an RCD the voltage transients experienced when isolating the load from the power source.

• Is able to trigger for extremely low fault current magnitudes. That is, there is no need for any load current to be flowing for a fault to be detected.

• Operates on both AC & DC supply voltages.

[00179] The protection devices of the above embodiments operate on an entirely different principle to a "safety switch" or prior art residual current device (RCD). The embodiments provide tripping characteristics that are equal to or better than those of the standard RCD while also providing for tripping in response to faults that would not activate an RCD. Accordingly, the above described embodiments provide a wider range of protection for personnel.

[00180] The embodiments of the invention are able to provide fault detection and isolate the circuit experiencing the fault extremely quickly. With detection and isolation times in the order of 10 msec being achieved, it will be appreciated that the embodiments described above will outperform most commercially available electrical safety devices.

[00181] As presently contemplated by the inventors, the improved performance of the embodiments arises from the use of voltage sensing to detect a fault. The prior art relies upon the measurement and/or comparison of load currents or other currents. This current measurement is typically very dependent upon the quality of the material used in the protection device, and any external influences. This is particularly problematic when inductive devices are used in the measurement process. For example, current monitors often rely on magnetic field coupling in a coil to detect fault current and this coupling is not always ideal. The field is also susceptible to interference from stray magnetic fields, and spurious tripping of RCDs is a known problem. Voltage measurement on the other hand is much more precise and less prone to external factors. This allows for an increased sensitivity of measurement and subsequent processing of the information. AU in all, the prior art measurement techniques are susceptible to considerable variation in quality, with contributes to considerable inconsistency in tripping characteristics for RCDs. The use of voltage sensing allows the protection devices of the above embodiments to be more finely and accurately tuned for repeatable and consistent operation.

[00182] Accordingly, the embodiments of the invention are able to offer not only earlier detection of a fault (notwithstanding use is made of an isolated or true IT network) but is able to operate extremely quickly to isolate that fault. While the above embodiments have been described with reference to AC loads, other embodiments are suitable for protecting DC loads — for example, electrical equipment or appliances that are powered by DC voltages. It will be appreciated by those skilled in the art that for DC loads use is made of a differential voltage measurement for fault detection.

[00183] Due to the use of a common floating conductor 39 in place of an earth conductor, the embodiments of the invention are able to monitor the voltage on exposed metal surfaces of electrical equipment. That is, so long as conductor 39 is electrically connected with the exposed surfaces - that is, the surfaces personnel are most likely to come into contact with - the personnel and other persons will be protected against a fault such as the active conductor coming into contact with that surface. When a voltage is detected on an exposed conductor or metal surface this voltage triggers the protection device of the invention. Unlike an RCD, this trigger is not dependent upon the load current being high or rapidly rising. Rather, and in the extreme case, the load current can be zero and the only power being supplied through device 15 at the time of the trigger may be that sufficient to have that device trigger. More usually, however, there will be some load current being drawn. Once the fault occurs and is detected, device 15 requires very little energy to both detect and then isolate the downstream electrical circuit. Accordingly, the total current drawn into the electrical circuit will be substantively only the load current at the time. Moreover, the quick detection of the fault and the subsequent expeditious isolation of the circuit prevent any rise in the current being drawn by the circuit due to the fault occurring. In the above embodiments the vohage required on conductor 39 to trigger a fault is about 3 volts AC or DC. In other embodiments different trigger voltages are used. Is some embodiments used with 240 Volt AC mains supply, the protection device is configured to for a trigger voltage of about 1 Volt AC or DC on conductor 39.

[00184] The embodiments of the invention described above provide an IT network that allows fast detection of a dangerous fault to frame. Accordingly, the significant risk that comes with a second fault hi an IT network is eliminated. That is, the protection device of the preferred embodiments offers the advantages of low fault currents and quick detection and, if required, isolation. Moreover, the advantage of severability of electrical circuits is still able to be achieved.

[00185] The protection device 15 in Figure 2 also functions as an insulation monitoring device by sensing voltage leakage to frame and quickly isolating that fault before it reaches a dangerous level. While device 15 is able to detect and isolate a fault within about 5 ms, in other embodiments alternative sensitivities are used. Accordingly, the need for additional insulating monitoring devices, on circuits protected by the device, is obviated.

[00186] By using an earth free environment downstream of the isolation transformer, the protection devices of the embodiments limit fault currents to extremely low levels. This, in effect, removes hazardous touch potentials that would otherwise exist in the presence of a fault to frame. Moreover, low fault currents, and fault currents of a short duration, will considerably reduce the risk of thermal damage in the event of a fault.

[00187] The electrical distribution system and protection devices of the preferred embodiments allow for the detection and, where required, the rapid isolation of a dangerous fault in a floating network. While the fault to frame in such a prior network may not, by itself, present immediate danger, it does create a pre-condition for a second fault to occur, which is extremely dangerous.

[00188] In addition to detecting a second fault to frame > 40 Volts and 5 mA in an IT network, system 1 allows for that fault to be isolated. In some embodiments a user having a predetermined authority is alerted to the detection of the first fault and provided an opportunity to determine whether or not to isolate the fault. An example of includes an installation such as a hospital with an operating theatre, where the user is a surgeon performing an operation in the theatre. However, this circumstance is envisaged for specialized installations rather than household installations.

[00189] Conventional IT networks are usually configured to detect and isolate a second fault by switching a mains circuit breaker. Unlike the electrical distribution system 1, however, these circuit breakers isolate most if not all circuits from the source of power, and not just the circuit in which the fault occurred. This is a considerable inconvenience - in that more circuits are isolated then needed — but also because it makes tracing the fault more difficult.

[00190] The single protection device of the Figure 2 embodiment not only rapidly and automatically detects a fault, but also rapidly and automatically isolates the fault. In conventional IT networks these functions are required to be separately performed or performed manually.

[00191] Another advantage of the embodiments of the invention is to allow severability of the electrical circuits. Accordingly, a fault is able to be quickly detected and the relevant circuit isolated while the remaining circuit or circuits continue to normally operate.

[00192] The use of the protection devices according to the invention provides for low fault currents (less than 5 mA) and for low transient currents that occur when switching in the event of a fault. That is, the use of a voltage measurement to sense a fault condition allows the protection device to detect a fault without a large fault current having to be flowing. Accordingly, the switching or triggering of the protection circuit to isolate the fault occurs when very little fault current is present, and the transient currents and voltages often associated with the switching of protection devices are substantially reduced. For device 15 of Figure 2, the fault current - that is, the current that flows in addition to the load current at the time — is restricted to less than 5 mA.

[00193] A further advantage of the preferred embodiments is that the fault detection mechanism relies upon a simple measure of the magnitude of a single low frequency voltage. There is no need to measure a variety of inputs at varying frequencies to ascertain whether or not a fault has occurred, or where that fauh has occurred. This reduces the need for complexity in the measurement and the associated detection circuitry, and increases the speed of operation. [00194] Moreover, the voltage is measured at a floating reference point where there should not be a voltage and, as such, as stable reference is provided to assess a small change in the voltage. By comparison, prior art devices often have to analyze a current to ascertain the likely presence of a fault, where that current is the sum of a load current and a fault current. Accordingly, until such time as the sum of the fault current and the load current has a greater magnitude than the maximum allowable current, an accurate determination of a fault-is less reliably and consistently achieved. If, in the event of a fault, there is no load current, the fault current is able to be dangerously high before any action will be taken to detect, let alone isolate, the fault.

[00195] The protection devices of the preferred embodiments include internal fault detection and monitoring. Accordingly, in addition to accurately detecting the presence of a fault in the circuit being monitored, the devices are configured to isolate the circuit if the protection device itself is assessed as not operating within the required parameters. This is intended to provide a fail-safe operation, in that any irregularity will result in detection and isolation. The internal monitoring undertaken in the embodiments is continuous. In some embodiments, use is made of periodic testing. In any event, the inclusion of this functionality within the protection devices obviates the need for manual testing.

4

[00196] In some embodiments an assessment of inadequate self-performance results in an alarm signal being raised and the decision of whether or not to isolate the associated circuit is made by a separate controller or a suitably authorized operator.

[00197] The electrical distribution system and electrical protection devices of the invention are applicable to most if not all new installations, and can be cost effectively included within the design and construction phases. The distribution system and protection devices are also suitable for retrofitting into existing installations, for example:

• Existing IT network applications, where the prior earthing is omitted. This includes, for example: o Hospital operating theatres, o Defence installations, o Industrial installations. r-

- 51 - o IT&T installations, o Shipping installations, o Port installations. o Other transport installations such as aircraft o Portable/remote power systems and renewable energy. o Mining installations.

• Existing MEN installations, which are converted to an IT network such as that offered by the present invention, where the costs of such a conversion are justified to gain the benefit of the additional functionality and safety. This includes: o Hospitals/medical installations. o Industrial & commercial installations which are due for rewiring. o Telecommunications installations, including television transmitters, commercial and public radio transmitters, film studio installations, aircraft control centre installations, emergency services communication centres, and the like. o Publicly used installation such as shopping centres, swimming centres, and community and government buildings. o Installations for childcare facilities, aged care, and education facilities.

• National electricity distribution grids used to provide electrical energy from electricity generators, through any intermediaries, to the ultimate consumer installations. For example those grids presently using an IT network for distribution - for example, Norway — but where at the consumer installation use is made of an MEN or TN network. Other distribution grids operate as TT networks, which are equally suited for use with the invention.

[00198] The distribution system of the invention is able to be used with an upstream power supply of the same or different network type. For example, the IT network system of the invention is able to be used with an upstream supply of an TT, TN or IT network-type. [00199] The preferred embodiments have failsafe operation to provide the best possible electrical protection for the downstream load. In the above embodiments, the failsafe operation is provided by internal checking within the protection device that ensures that if the device is not operating properly it will not operate at all other than the isolate the downstream circuit from the supply. That is, if an internal failure is detected, the circuit trips and isolates the downstream circuit from the supply. This is in sharp contrast to prior art protection devices such as RCDs, the malfunctions of which typically remain undetected, leaving the downstream circuit unprotected.

[00200] In some embodiments, the protection device includes a first protection circuit and a second protection circuit, where during normal operation the protection for the downstream circuit is provided by the first protection circuit, while the second protection circuit remains in active. Upon detection of an internal fault, however, the first protection circuit is decommissioned and the second protection circuit activated to protect the downstream circuit. Additionally, an alarm signal is raised by the protection device to alert the system to a need to repair or replace the first protection circuit.

[00201] Because of the floating nature of system 1, it is possible for the protection devices, such as devices 15 and 16, to determine whether or not they are able to detect a fault. That is, if there is a high resistance between the reference point - that is, conductor 39 — and any of the live conductors - that is, conductors 29 and 30 - it will be known that the protection device is able to operate. The prior art protection devices such as RCDs cannot do this, for they have to assume a low impedance earth path to earth exists.

[00202] In alternative embodiments, the circuit that provides the failsafe functionality for the protection device of the invention is physically spaced apart from the other components of the protection device.

[002031 The electrical distribution system of the invention is suitable for use with one or more of a variety of electrical power sources, whether they be AC or DC sources. Examples include: mains power sources; battery powered sources; generators; inverters; fuel cells; solar cells or panels; wind generated electricity; and others.

[00204] The use of a protection device of the invention allows for: • The better management of risk of injury to personnel and damage to equipment, particularly as the risk of incorrect or faulty insulation is, in effect, continually monitored by the protection device.

• Improved productivity, particularly in the use of portable generators and inverters and other portable electrical equipment, as earth staking is no longer required to allow safe operation.

• Improved safety, in that the protection device 15 operates approximately four times fester than a commonly available safety switch, and responds to a broader range of faults.

• Increased simplicity in design, installation and maintenance.

[00205] The protection devices of the preferred embodiments are compatible with most existing upstream protection devices such as fuses, circuit breakers, RCDs and the like. Moreover, the protection devices of the embodiment are able to be configured with additional protection features such as one or more of:

• Overload protection to prevent the electrical circuit (and any ultimate load being supplied) from drawing more than a predetermined amount of power and/or current.

• Surge protection to protect against any transients from the mains supply feeding through to the load.

• An integral mains circuit breaker.

• A residual current device.

[00206] In some embodiments the additional protection features are provided as a separate device that works in combination with the protection device of the invention. In other embodiments, however, the additional features are combined within the protection device of the invention to provide a single protection device embodying all the protection functionalities.

[00207] Other types of protection will be known to those skilled in the art.

[00208] Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "processing", "processing system", "computing", "calculating", "determining", "analyzing" or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulate and/or transform data represented as physical, such as electronic, quantities into other data similarly represented as physical quantities, hi a similar manner, the term "processor" may refer to any device or portion of a device that processes electronic data, for example, from registers and/or memory to transform that electronic data into other electronic data that, for example, may be stored in registers and/or memory. A "computer" or a "computing machine" or a "computing platform" may include one or more processors.

[00209] The methodologies described herein are, in one embodiment, performable by one or more processors that accept computer-readable (also called machine-readable) code containing a set of instructions that when executed by one or more of the processors carry out at least one of the methods described herein. Any processor capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken are included. Thus, one example is a typical processing system that includes one or more processors. Each processor may include one or more of a CPU, a graphics processing unit, and a programmable DSP unit. The processing system further may include a memory subsystem including main RAM and/or a static RAM, and/or ROM. A bus subsystem may be included for communicating between the components. The processing system further may be a distributed processing system with processors coupled by a network. If the processing system requires a display, such a display may be included, for example, a liquid crystal display (LCD) or a cathode ray tube (CRT) display. If manual data entry is required, the processing system also includes an input device such as one or more of an alphanumeric input unit such as a keyboard, a pointing control device such as a mouse, and so forth.

[00210] The term "memory unit" or "memory" as used herein, if clear from the context and unless explicitly stated otherwise, also encompasses a storage system such as a disk drive unit. The processing system in some configurations may include a sound output device, and a network interface device, for example. The memory subsystem thus includes a computer- readable carrier medium that carries computer-readable code (for example, software) including a set of instructions to cause performing, when executed by one or more processors, one of more of the methods described herein. Note that when the method includes several elements, for example, several steps, no ordering of such elements is implied, unless specifically stated. The software may reside in the hard disk, or may also reside, completely or at least partially, within the RAM and/or within the processor during execution thereof by the computer system. Thus, the memory and the processor also constitute computer-readable carrier medium carrying computer-readable code.

[00211 J Furthermore, a computer-readable carrier medium may form, or be includes in a computer program product.

[00212] In alternative embodiments, the one or more processors operate as a standalone device or may be connected, for example, by being networked to another processor or other processors. In such a networked deployment, the one or more processors may operate in the capacity of a server or a user machine in a server-user network environment, or as a peer machine in a peer-to-peer or distributed network environment. The one or more processors may form a personal computer (PC), a tablet PC, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine.

[00213J Where a figure only illustrates a single processor and/or a single memory that carries the computer-readable code, those in the art will understand that many of the components described above are included, but not explicitly shown or described to reduce the risk of obscuring the inventive aspect. For example, while only a single machine is illustrated, the term "machine" shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein.

[00214] Thus, one embodiment of each of the methods described herein is in the form of a computer-readable carrier medium carrying a set of instructions - for example, a computer program - that are for execution on one or more processors. For example, the one or more processors that are part of a protection device. Thus, as will be appreciated by those skilled in the art, embodiments of the present invention may be embodied as a method, an apparatus such as a special purpose apparatus, an apparatus such as a data processing system, or a computer- readable carrier medium, for example, a computer program product. The computer-readable carrier medium carries computer readable code including a set of instructions that when executed on one or more processors cause a processor or processors to implement a method. Accordingly, aspects of the present invention may take the form of a method, an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present invention may take the form of carrier medium (for example, a computer program product on a computer-readable storage medium) carrying computer-readable program code embodied in the medium

{00215] The software may further be transmitted or received over a network via a network interface device. While the carrier medium is shown in an exemplary embodiment to be a single medium, the term "carrier medium" should be taken to include a single medium or multiple media (for example, a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term "carrier medium" shall also be taken to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by one or more of the processors and that cause the one or more processors to perform any one or more of the methodologies of the present invention. A carrier medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical, magnetic disks, and magneto -optical disks. Volatile media includes dynamic memory, such as main memory. Transmission media includes coaxial cables, copper wire and fibre optics, including the wires that comprise a bus subsystem. Transmission media also may also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications. For example, the term "carrier medium" shall accordingly be taken to included, but not be limited to, solid-state memories, a computer product embodied in optical and magnetic media, a medium bearing a propagated signal detectable by at least one processor of one or more processors and representing a set of instructions that when executed implement a method, a carrier wave bearing a propagated signal detectable by at least one processor of the one or more processors and representing the set of instructions a propagated signal and representing the set of instructions, and a transmission medium in a network bearing a propagated signal detectable by at least one processor of the one or more processors and representing the set of instructions.

[00216] It will be understood that the steps of methods discussed are performed in one embodiment by an appropriate processor (or processors) of a processing - that is, a computer - system executing instructions - that is, computer-readable code - stored in storage. It will also be understood that the invention is not limited to any particular implementation or programming technique and that the invention may be implemented using any appropriate techniques for implementing the functionality described herein. The invention is not limited to any particular programming language or operating system.

[00217] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" or "in another embodiment" or "in an alternative embodiment" or similar phases in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[00218] Similarly it should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in^'a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.

[00219] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art For example, in the following claims, any of the claimed embodiments can be used in any combination as would be understood by a skilled addressee given the benefit of the teaching herein.

[00220] Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus or system embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.

[00221] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[00222] As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

[00223] In the claims below and the description herein, any one of the terms comprising, comprised of or which comprises is an open term that means including at least the elements/features that follow, but not excluding others. Thus, the term comprising, when used in the claims, should not be interpreted as being limitative to the means or elements or steps listed thereafter. For example, the scope of the expression a device comprising A and B should not be limited to devices consisting only of elements A and B. Any one of the terms including or which includes or that includes as used herein is also an open term that also means including at least the elements/features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.

[00224] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall whhin the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.

[00225] Similarly, it is to be noticed that the term electrically connected, when used in the claims, should not be interpreted as being limitative to direct electrical connections only. The terrns "coupled" and "connected," along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A connected to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Connected" may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other. It will also be appreciated from the context of each sentence whether "connected" is reference to an electrical connection or otherwise.

[00226] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and farther modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.

Claims

1. An electrical protection device including:

a signal generator for providing a reference signal;

2. An electrical protection device according to claim 1 wherein the switching device is responsive to the fault signal and the reference signal for selectively electrically disconnecting the input terminals from the respective output terminals to prevent the supply of electrical power.

3. An electrical protection device according to claim 1 or claim 2 wherein the supply of electrical power is provided at a first frequency// and the reference signal has a second frequency /2 that is different from//.

4. An electrical protection device according to claim 3 wherein one or both of// and /₂ are substantially constant.

5. An electrical protection device according to claim 3 or claim 4 wherein:

/₂ >/y; or

/₂ > 10 x/;; or

6. An electrical protection device according to claim 5 wherein^} > 5 kHz.

7. An electrical protection device according to claim 5 wherein^ > 10 kHz.

8. An electrical protection device including:

two output terminals for electrically connecting to a load which is downstream of the device; a sensor that is responsive to an electrical fault downstream of the device for providing a fault signal; and

9. An electrical protection device, the device including:

a sensor for connecting to a sensor element that extends downstream of the device, the sensor providing a fault signal in response to a low impedance between the sensor element and either of the output terminals; and a switching device for electrically connecting the input terminals to respective output terminals to allow a supply, of electrical power from the source to the load, wherein the switching device is responsive to the fault signal for selectively electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

10. An electrical protection device according to claim 9 wherein the switching device is responsive to the fault signal for selectively electrically disconnecting the input terminals from the respective output terminals to prevent the supply of electrical power.

11. An electrical protection device according to claim 9 or claim 10 wherein the supply of electrical power is provided at a first frequency// and the reference signal has a second frequency,/? that is different from/).

12. An electrical protection device according to claim 11 wherein one or both of// andyϊ are substantially constant.

13. An electrical protection device according to claim 11 or claim 12 wherein:

/; >/_/, or

/₂ > 10x/_/; or

14. An electrical protection device according to claim 13 wherein/2 > 5 kHz.

15. An electrical protection device according to claim 14 wherein/2 ≥ 10 kHz.

16. An electrical protection device according to claim 14 wherein// < 60 Hz.

17. An electrical protection device according to any one of claims 9 to 16 wherein the source includes an isolation transformer having a secondary winding and the input terminals are connected to the secondary winding.

18. An electrical protection device according to any one of claims 9 to claim 17 wherein the sensor element is a conductor.

19. An electrical protection device according to any one of claims 9 to claim 18 wherein the load includes a power sink, a switched active conductor and a switched neutral conductor, and the output terminals are respectively electrically connected with the switched active conductor and a switched neutral conductor.

20. An electrical protection device according to claim 19 wherein the sensor element is bundled with the one or both of the switched active conductor and a switched neutral conductor.

21. An electrical protection device according to claim 20 wherein the sensor element is substantially coextensively bundled with the one or both of the switched active conductor and a switched neutral conductor.

22. An electrical protection device, the device including:

23. A protection device according to claim 22 wherein the sensor is responsive to a voltage on the sensor element derived from only one of the output terminals for providing the fault signal.

24. A protection device according to claim 22 wherein the switching device is responsive to the fault signal for selectively electrically disconnecting the input terminals from the respective output terminals to prevent the supply of electrical power.

25. An electrical protection device including:

a sensor that is responsive to an electrical fault downstream of the device for providing a fault signal; a monitor that is responsive to one or more operational characteristics of the protection device for selectively providing a reference signal;

26. A protection device according to claim 25 including a signal generator for providing the reference signal.

27. A protection device according to claim 25 or claim 26 wherein the switching device is responsive to either the fault signal or the reference signal for electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

28. A protection device according to claim 25 or claim 26 wherein the switching device is responsive to the fault signal and the reference signal for electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

29. An electrical protection device according to claim 25 wherein the switching device is responsive to the fault signal and the reference signal for selectively electrically disconnecting the input terminals from the respective output terminals to prevent the supply of electrical power.

30. An electrical protection device according to any one of claims 25 to 29 wherein the supply of electrical power is provided at a first frequency/_/ and the reference signal has a second frequency _/? that is different from//.

31. An electrical protection device according to claim 30 wherein one or both of/} and/2 are substantially constant.

32. An electrical protection device according to claim 30 or claim 31 wherein:

33. An electrical protection device according to claim 32 wherein/2 ≥ 5 kHz.

34. An electrical protection device according to claim 32 wherein/2 ≥ 10 kHz.

35. An electrical distribution system including one or a combination of: one or more of the protection devices of claims 1 to 7; one or more of the protection devices of claim 8; one or more of the protection devices of claims 9 to 21; one or more of the protection devices of claims 22 to 24; and one or more of the protection devices of claims 25 to 34.

36. An electrical distribution system including: a transformer having a primary winding and a secondary winding, wherein the primary winding is electrically connectable with a power source;

a plurality of electrical circuits that are electrically connected in parallel to the secondary winding;

37. An electrical distribution system according to claim 26 that, downstream of the secondary winding, defines an IT network.

38. An electrical distribution system according to claim 26 that, downstream of the secondary winding, defines a TT network.

39. An electrical distribution system according to claim 26 that, upstream of the secondary winding, defines other than an IT network.

40. An electrical distribution system according to claim 29 that, upstream of the secondary winding, defines a TN network.

41. An electrical distribution system according to claim 26 that, upstream of the secondary winding, defines an IT network.

42. An electrical distribution system according to any one of claims 26 to 31 including one or more unprotected electrical circuits that are electrically connected in parallel with the secondary winding.

43. An electrical distribution system according to any one of claims 26 to 32 wherein at least one of the electrical circuits includes a sub-circuit that is downstream from the respective protection device and the system includes a further protection device within the sub-circuit.

44. An electrical distribution system according to any one of claims 26 to 33 including a sensor element for carrying the fault voltage such that the immediately upstream protection device isolates the respective circuit from the secondary winding.

45. An electrical distribution system including:

a floating electrical network for drawing electrical power from a power source;

46. A method of electrical protection including the steps of:

electrically connecting two input terminals of an electrical protection device to an electrical power source that is upstream of the device; electrically connecting two output terminals of the electrical protection device to a load which is downstream of the device; providing a reference signal;

47. A method of electrical protection including the steps of:

electrically connecting two output terminals of the protection device to a load which is downstream of the device; being responsive to an electrical fault downstream of the device for providing a fault signal; and

providing a switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the fault signal and the voltages at one or both of the output terminals for selectively electrically disconnecting the input terminals from- the respective output terminals to prevent the supply of electrical power.

48. A method of providing electrical protection, the method including the steps of:

providing a switching device for electrically, connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein theswitching device is responsive to the fault signal for selectively electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

49. A method of electrical protection including the steps of:

electrically connecting two output terminals of the electrical protection device to a load which is downstream of the device; providing a sensor for connecting to a sensor element that extends downstream of the device, the sensor being responsive to a voltage on the sensor element derived from one of the output terminals for providing a fault signal; and

50. A method of electrical protection including the steps of: electrically connecting two input terminals of a protection device to an electrical power source that is upstream of the device;

electrically connecting two output terminals of the protection device to a load which is downstream of the device; providing a sensor that is responsive to an electrical fault downstream of the device for providing a fault signal;

providing a monitor that is responsive to one or more operational characteristics of the protection device for selectively providing a reference signal; providing a switching device for electrically connecting the input terminals to respective output terminals to allow a supply of electrical power from the source to the load, wherein the switching device is responsive to the feult signal and the reference signal for selectively electrically disconnecting at least one of the input terminals from the respective output terminal to prevent the supply of electrical power.

51. A method of electrical distribution, the method including the steps of:

providing an isolation transformer having a primary winding and a secondary winding, wherein the primary winding is electrically connectable with a power source;

electrically connecting a plurality of electrical circuits in parallel to the secondary winding; providing a protection device for each electrical circuit, wherein each protection device is upstream of the respective electrical circuit and responsive to a fault voltage in the corresponding electrical circuit for isolating that electrical circuit from the secondary winding while allowing the other circuit or circuits to remain electrically connected with the secondary winding.

52. A method of electrical distribution, the method including the steps of:

drawing electrical power from a power source with a floating electrical network;

electrically connecting a plurality of electrical circuits in parallel to the network for consuming the electrical power; providing a protection device for each electrical circuit, wherein each protection device is upstream of the respective electrical circuit and responsive to a fault voltage in the corresponding electrical circuit for isolating that electrical circuit from the network while allowing the other circuit or circuits to remain electrically connected with the network.