IES80832B2 - Residual current device - Google Patents

Abstract

A residual current device (RCD) for use with a mains supply having at least one live conductor (L), a neutral conductor (N) and an earth conductor (E) includes circuit means (18) for disconnecting the mains supply in response to an earth fault current. The circuit means (18) is powered from teh mains by connection to the mains live conductor (L) and, via changeover contacts (26), selectively to the earth conductor (E) or to the neutral conductor (N). The changeover contacts are held in a first state connecting the circuit means (18) to the neutral conductor (N) while mains neutral is present but are automatically changed over, by an electromagnetic device (28), to a second state connecting the circuit means (18) to the earth conductor (E) if there is a loss of mains neutral. .

Description

RESIDUAL CURRENT DEVICE This invention relates to residual current devices.

Fig. 1 is a block functional diagram of a typical voltage dependent RCD for an AC mains supply having live L, neutral N and earth E conductors. Such devices are very well known, and therefore will only be described briefly for the purposes of the present specification.

On the load 10 side of the RCD the live and neutral conductors L and N respectively are passed through a toroidal current transformer 12 having a secondary winding 14. Under normal conditions, when contacts 16a, 16b in the live and neutral conductors are closed, the current flowing in the live conductor L from the mains supply to the load 10 will equal the current returning in the neutral conductor N from the load to the supply. There are, therefore, equal and opposite currents flowing through the transformer 12 so that the current induced into the secondary winding 14 is zero.

However, if an earth fault occurs on the load side of the RCD there will be some current flow to ground, leading to an imbalance in the currents flowing in the live and neutral conductors. This induces a non-zero current in the secondary winding 14 which is measured by electronic circuitry 18. If the current induced in the secondary winding 14 exceeds a pre-determined level, indicative of an unacceptable earth fault current, the circuitry 18 will cause a silicon controlled rectifier (SCR) 20 to be triggered (turned on).

The SCR 20 is connected across the mains line and neutral conductors L and N in series with a solenoid 22, so that when SCR 20 is triggered a current will flow though the solenoid 22 which removes power from the load 10 by opening the contacts 16a, 16b.

One drawback of such RCDs is that the electronic circuitry 18 is connected between the mains live and neutral conductors L and N and is powered by the mains. Thus, in the event of a loss of supply neutral condition, the RCD will cease to function and protection will no longer be provided by the device against a subsequent earth fault condition.

Accordingly, RCDs have been developed with various circuits which will provide protection against loss of supply neutral. All such circuits use the earth as an alternative neutral under loss of neutral conditions. The use of the earth connection in this manner is referred to as a Functional Earth (Fe) connection. This Fe arrangement is represented in Fig. 2. The supply current to the electronic circuitry 18, Is, comprises two components, IN and IE. IN flows between live and neutral, and IE flows between live and earth via the Fe circuit 24. Under a loss of neutral condition, the flow of supply current between live and earth via the Fe circuitry will maintain power to the RCD and enable it to continue to operate.

RCDs provided with an Fe connection will have one of two responses to a loss of supply neutral condition. They will either trip automatically in the event of a loss of supply neutral condition, (active response), or continue to function in their monitoring role (passive response) and be able to operate and provide protection against a subsequent earth fault current. In either event, the RCD is able to operate and provide protection to the user under such a condition. Installers and users can choose the response that best meets their needs .

Numerous loss of neutral circuits have been developed over recent years which implement an Fe connection; see, for example, Patent Specifications GB 2,224,404 and GB 2,268,011.

The common feature of all existing designs is the interposition of electronic components and circuitry between the RCD and earth as a means of sensing or detecting the loss of neutral condition and providing the alternative path for the supply current IE to the device under the loss of neutral condition.

Numerous problems arise with the use of electronic components in these arrangements. These include but are not limited to the following. 1. Need to limit IE under normal supply conditions.

Any current that flows to earth via the Fe circuit will be seen by upstream RCDs as an imbalance current. If this current is very high, it may trip the upstream RCD. This would be more likely to happen where several downstream subcircuits are fitted with RCDs with Fe connections because of the cumulative effect of their respective Fe currents. Even if these currents do not trip the upstream RCD, they are likely to affect its response to a genuine earth fault current by either increasing or decreasing the sensitivity of the upstream RCD in response to a genuine earth fault current. For these and other reasons, it is necessary to limit the level of current IE that flows to earth during normal supply conditions. 2. Cost and space problems associated with additional components .

The additional components required for detection and the provision of an alternative path for the RCD supply current add to the size and cost of the RCD. The extra components require additional space on a printed circuit board to locate the components and to ensure that adequate clearances are maintained between pads and tracks at mains voltages. Some of these components can be bulky, which compounds space problems. There are also additional costs associated with the assembly and test of the extra circuitry. 3. Susceptibility of the Fe circuit to impulse voltages between live and earth or neutral and earth, and the need for additional surge voltage protection.

It is well known that impulses of several thousand volts occur on electrical installations. These are generally caused by switching of reactive loads, large inrush currents, lightning strikes, etc. These surge voltages can be differential mode (between live and neutral) or common mode (between live or neutral and earth). Under common mode surge conditions, the Fe circuitry will be subjected to the surge voltages and will need to be able to withstand these. This often necessitates the fitting of surge protection devices such as varistors. This is shown in Fig. 3 where in addition to the varistor 30 conventionally connected between live and neutral to protect against differential mode surges a further varistor 30' is connected between neutral and earth to protect against common mode surges. This adds further to space and cost problems. 4. Stress and possible breakdown of electronic components in the Fe circuit under supply fault conditions.

Under certain fault conditions at the supply transformer, it is possible for the mains supply to rise to voltages of up to 1500 volts with respect to earth for periods of several seconds or more. Under such conditions, the Fe circuit will be subjected to these very high voltages. Surge protection devices such as varistors, etc., generally will not provide protection under such conditions as they are intended to provide protection against impulse voltages of microseconds or milliseconds duration, and are liable to break down completely under sustained high voltage conditions. Component failure in the Fe circuit is very likely to occur under such adverse conditions.

. Reliability problems associated with additional components .

In many cases, the addition of components to a product, be they electronic, electromechanical or mechanical, can result in a reduction in the overall reliability of the product. There are cases where the addition of a component will improve overall reliability, such as the fitting of varistors to snub impulse voltages that could be harmful to electronic circuitry. However, the more components that are added to a product, the greater the likelihood of reduced overall product reliability. The connection of electronic components between the RCD and earth to provide an Fe circuit is likely to reduce overall product reliability, particularly when such components are subjected to high impulse voltages or high level voltages under installation fault conditions. It is an object of the present invention to provide an improved RCD in which these disadvantages are avoided or mitigated.

Accordingly, the present invention provides a residual current device including circuit means for disconnecting a mains supply in response to an earth fault current, wherein the circuit means is powered from the mains by connection to the mains live conductor and via changeover contacts selectively to the earth conductor or to the neutral conductor, the changeover contacts being held in a first state connecting the circuit means to the neutral conductor while mains neutral is present but being automatically changed over to a second state connecting the circuit means to the earth conductor if there is a loss of mains neutral, thereby maintaining power to the circuit means.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figs. 1, 2 and 3, previously described, are block functional diagrams of known RCDs, Fig. 4 is a block functional diagram of a first embodiment of the invention, Fig. 5 is a block functional diagram of a second embodiment of the invention, and Fig. 6 is a block functional diagram of a third embodiment of the invention.

In the figures the same reference numerals have been used to indicate the same or equivalent parts.

Referring to Fig. 4, the RCD according to the first embodiment of the invention has an Fe circuit in the form of an electromagnetic device 28 connected between mains live and neutral conductors, the device 28 having associated changeover contacts 26 including a movable contact 26a which can selectively engage either one of two fixed contacts 26b and 26c. The movable contact 26a is connected to the 0 volt common line of the electronic circuitry 18 and SCR 20, the fixed contact 26b is connected to the neutral line N and the fixed contact 26c is connected to the earth conductor E. The movable contact 26a is normally biassed to engage the fixed contact 26c as shown, but in operation of the RCD with both mains live and neutral present the electromagnetic device 28 is energised to move the movable contact 26a against such bias into engagement with the fixed contact 26b.

Thus, when the mains supply is available between live and neutral, the electromagnetic device 28 will be energised and the movable contact 26a will engage the fixed contact 26b, thereby isolating the RCD from earth. The supply current IS to the electronic circuitry 18 will only flow between live and neutral, and as long as the contact 26a is maintained in this position IE will be zero. However, under a loss of neutral condition the device 28 will be de-energised and the movable contact 26a will automatically move to engage the fixed contact 26c. The current IS will then be diverted to earth via the contacts 26a, 26c and thereby maintain the mains supply to the electronic circuitry 18 and enable the RCD to continue to function. If the supply neutral is restored, the device 28 will be energised again and the movable contact 26a will once again move to engage the fixed contact 26b and isolate the electronic circuitry 18 from earth.

The contact 26a may be spring biassed into contact with the contact 26c and the device 28 may be a relay or a solenoid or other electromagnetic device having a winding which can provide a magnetic force which results in movement of the contact 26a into engagement with the contact 26b against the spring bias when a current above a certain level is passed through the winding, the contact 26a reverting to engagement with the contact 26c under the action of the spring bias when the current through the winding is reduced below a second level insufficient to overcome the bias. The device 28 could alternatively comprise a permanent magnet which biases the contact 26a into engagement with the contact 26c against a spring bias tending to move the contact 26a to engage the contact 26b, and also comprise a winding which produces an opposing magnetic force to that generated by the permanent magnet when a current in excess of a certain level is passed through the winding thereby overcoming the bias of the permanent magnet and enabling the contact 26a to move to engage the contact 26b under the force of the spring bias.

On reduction of the current in the winding below a second level the opposing current ceases to be sufficient to overcome the bias of the permanent magnet with the result that the contact 26a reverts to engage the contact 26c. In either case the winding is connected between the mains live and neutral conductors such that when the winding is energised the movable contact 26a engages the fixed contact 26b whereas when the winding is de-energised the movable contact 26a engages the fixed contact 26c.

The device 28 can be an AC type or a DC type when provided with the necessary DC current to its winding. The device 28 can additionally be slugged such that there is a small delay between de-energising the device 28 and movement of the contacts 26 to the closed position. The advantage of delaying the closing of the contacts 26 is that some control can be maintained in the response to a loss of neutral condition, thereby ensuring that the contacts 26 only switch under genuine or sustained loss of neutral conditions. The response of the device 28 can be slugged mechanically or electrically. A storage capacitor can be connected across the winding of the device 28 and continue to provide the winding with sufficient current to delay the opening of the contacts 26 for a finite period of time after the loss of supply neutral. This would provide a greater degree of control over the response time to a loss of neutral condition, and overcome problems of nuisance tripping associated with active-type RCDs under a momentary loss of neutral condition. On expire of the delay, the contacts will close .

The RCD shown in Fig. 4 also has a varistor 30 connected between live and the 0 volt common line which provides protection against differential mode impulse voltages under normal conditions. As long as the movable contact 26a remains in engagement with the fixed contact 26b, the RCD will not be affected by common mode impulse voltages.

However, under a loss of neutral condition, the contact 26a will engage the fixed contact 26c and varistor 30 will be connected between live and earth and will then provide protection against common mode impulse voltages between live and earth. As a result of this design, a single varistor can be used to provide protection against common mode or differential mode impulse voltages, as compared to the two varistors 30 and 30' of Fig. 3.

A further disadvantage of existing Fe circuits is that, in general, they provide either an active or passive response to a loss of neutral condition, and are not sufficiently versatile to enable easy changeover from one mode to the other. In most cases, the Fe circuit is inflexible and cannot readily facilitate a choice of modes.

In Fig. 4, the circuit provides a passive response. Fig. 5 shows an example of the device 28 used in a circuit which can provide either an active or a passive response to a loss of neutral condition. Under normal conditions, Is flows to neutral via contacts 26a, 26b. A low impedance link is selectively connected between points X and A or between points X and P. Under a loss of neutral condition, Is will flow to earth via the contacts 26a, 26c as shown. When the link is connected between X and P, the response of the RCD will be passive. When the link is connected between X and A, however, the response will be active because Is will flow to earth via a further winding 32 on the current transformer 12. This will cause a non-zero induced current in the secondary winding 14 which will be detected by the electronic circuitry 18 which in turn will turn on the SCR 20 and cause the RCD to trip by opening the contacts 16a and 16b. Other circuit arrangements can be designed which provide for selection of active or passive mode operation of the RCD under a loss of neutral condition.

Although the invention has been described above in connection with a typical single phase mains with grounded neutral, some installations use a multi-phase supply and the present invention can be used equally effectively on such installations. Fig. 6 shows an embodiment in which the three phase (live) lines are shown at Ll, L2 and L3 and the electronic circuitry 18, solenoid 22 and electromagnetic device 28 are connected to each phase line through a respective diode DI, D2 and D3 in known manner. Otherwise, the operation of Fig. 6 is essentially the same as Fig. 4.

The embodiments described above result in an RCD which can provide protection in the event of loss of neutral conditions and which overcomes many of the problems associated with electronic Fe circuits. The problems highlighted above are overcome as follows. 1. Need to limit IE under normal supply conditions.

As there is no physical connection between the RCD and earth under normal supply condition, no current can flow to earth, thereby overcoming problems associated with the flow of IE under normal conditions. 2. Cost and space problems associated with additional components .

The electromagnetic device may be designed to be very compact and low cost which overcomes many of the problems of space and costs associated with electronic Fe circuits. In addition to its electronic components, the conventional Fe electronic circuit often requires a surge protection device to safeguard the circuit from surge voltage between the mains supply and earth. Such protection devices add considerably to space and cost problems. The invention can use a single varistor to provide protection against common mode or differential mode impulse voltages, and thereby reduces space and cost requirements. 3. Susceptibility of the Fe circuit to impulse voltages between live and earth or neutral and earth, and the need for additional surge voltage protection.

As the RCD is isolated from earth under normal supply conditions, there is no requirement for additional surge voltage protection for the Fe circuit. When the Fe circuit is connected to earth by closing the contacts 26, surge protection is provided by the varistor. 4. Stress and possible breakdown of electronic components in the Fe circuit under supply fault conditions.

By using contacts to isolate the RCD from earth, the Fe circuit is virtually immune to sustained high voltages between the mains supply and earth.

. Reliability problems associated with additional components .

The simplicity of the embodiments and the isolation of the 5 RCD from earth overcomes many of the problems and stresses caused by high voltages to earth which tend to undermine the reliability of Fe circuits comprising of electronic components . 6. Fixed response to loss of neutral condition (passive or active only).

The new Fe arrangement of the second embodiment can facilitate the use of active or passive responses to a loss of neutral condition, and such responses can be delayed by use of a delayed response electromagnetic device.

Claims (6)

1. A residual current device (RCD) for use with a mains supply having at least one live conductor, a neutral conductor and an earth conductor, the RCD including circuit means for disconnecting the mains supply in response to an earth fault current, wherein the circuit means is powered from the mains by connection to the mains live conductor(s) and, via changeover contacts, selectively to the earth conductor or to the neutral conductor, the changeover contacts being held in a first state connecting the circuit means to the neutral conductor while mains neutral is present but being automatically changed over to a second state connecting the circuit means to the earth conductor if there is a loss of mains neutral.

2. An RCD as claimed in claim 1, wherein an electromagnetic device is connected between mains live and neutral conductors which device maintains the contacts in the first state as long as it remains energised, a loss of mains neutral causing loss of energisation of the device and consequent changeover of the contacts to the second state.

3. An RCD as claimed in claim 2, further including means providing a delay between loss of energisation of the electromagnetic device and the changeover of the contacts such that changeover of the contacts only occurs in respect of a sustained loss of mains neutral.

4. An RCD as claimed in claim 2 or 3, wherein the changeover contacts are spring biased into the second state, and wherein the electromagnetic device comprises a winding which when energised holds the changeover contacts in the first state against the spring bias.

5. An RCD as claimed in claim 2 or 3, wherein the 5 changeover contacts are spring biased into the first state, and wherein the electromagnetic device comprises a permanent magnet which biases the changeover contacts into the second state against the spring bias and a winding which when energised opposes the bias of the permanent magnet

6. 10 sufficiently to allow the changeover contacts to be spring biassed into the first state.