GB2566059A

GB2566059A - A system for protecting an electrical circuit

Info

Publication number: GB2566059A
Application number: GB1714009.6A
Authority: GB
Inventors: Ward Patrick
Original assignee: Shakira Ltd
Current assignee: Shakira Ltd
Priority date: 2017-09-01
Filing date: 2017-09-01
Publication date: 2019-03-06
Also published as: GB201714009D0

Abstract

A system 2000 for protecting an electrical circuit comprises an electrical protection switching device 1 comprising: neutral and live input terminals 2, 3 for electrical connection to a supply 30 of the electrical circuit, neutral and live output terminals 4, 5 to a load 40 of the electrical circuit, contacts 6 which can be opened for disconnecting the load 40 from the supply 30, an electrically controlled actuator 70 arranged to open the contacts 6. A fault detector 8 is provided for causing the actuator 70 to open the contacts 6 upon detection of a residual fault current. A DC detector 80 is electrically connected to the neutral and live lines for detecting a residual fault DC. In case of detecting a DC residual fault current, the DC detector 80 is configured to cause a current imbalance path 89 to simulate a residual fault current detectable by the fault detector 8 of the electrical protection switching device 1. The current imbalance path 89 is connected to the neutral and live lines at the same circuit side relative to the movable contacts 6 and at opposite sides of the fault detector 8 of the electrical protection switching device 1. In a further embodiment (figure 5), the DC detector is configured to generate an electrical control signal (1200) to directly control the actuator (70) to open the contacts (6).

Description

A System for Protecting an Electrical Circuit

Field

The present invention relates to a system for protecting an electrical circuit, in particular a system comprising at least an electrical protection switching device and a DC detector.

Background

Residual current devices (RCDs) are well known to those skilled in the art and are known as GFCIs (Ground Fault Circuit Interrupters) in North America and RCCBs (Residual Current Circuit Breakers) or RCBOs (Residual Current Breakers with Overcurrent protection) in most other countries. Such devices generally comprise a stand-alone unit with supply and load current terminals fully rated for the intended supply voltage and load current requirements of the installation in which they are installed.

Type A RCDs, based on IEC61008, are capable of detecting AC and pulsating DC residual currents. Type B RCDs, based on IEC61008 and IEC62423, are capable of detecting AC and pulsating DC as well as pure DC residual currents. However, it is widely acknowledged that Type B RCDs are very expensive in comparison to Type A RCDs, and this precludes their use in some applications, e.g. for DC fault current detection during electric vehicle (EV) charging.

US9,397,494 discloses means for detecting a DC current during EV charging, and further explains that a Type A RCD may be impaired by the presence of a DC residual current flowing through the RCD. Such impairment can arise because the CT within the RCD is adversely affected by DC residual currents above a certain magnitude, and the RCD can no longer reliably detect AC residual currents at its rated residual current operating value. Immunity to DC currents is a matter of design and appropriate component selection and responsible manufacturers will ensure that the performance of their RCDs is not impaired by DC residual currents.

Nonetheless, if a Type A ROD is of a type that can be impaired by DC residual currents, it is conceivable that in the presence of a DC residual fault current, the RCD may be impaired and that it could not reliably detect an AC residual current at its rated residual operating current. This problem can be avoided by using a Type A RCD that is not impaired by DC residual currents, and assurances of such immunity can be provided by manufacturers. Nonetheless, while some Type A RCDs may indeed be impaired by such DC currents, such assumptions should not be generalized, and many modern Type A RCDs are not impaired by DC residual currents. It is therefore assumed that US9,397,494 is intended to apply to Type A RCDs that can be impaired by DC residual currents.

EV charging stations are required to include a pilot function which is used to verify that the EV is connected to the supply and that the wiring arrangements are correct, i.e. that L, N and E connections in the EV are all present and correct. Only after satisfactory completion of such verification is the contactor allowed to close by the pilot function, and charging current can then flow to the EV via a downstream switching device such as a contactor. The DC detector in US9,397,494 causes the downstream contactor to open in the event of a DC residual fault current.

US8,970,169 discloses a charging system comprising a charging device installed in a motor vehicle for charging a battery, and a connection device for connecting the charging device to an AC supply. The connection device comprises a type A RCD and the charging device comprises a power factor correction (PFC) filter between an input stage (comprising an input filter and a rectifier) and output stage (comprising a DC link capacitor and a DC-DC converter). The charging device further comprises a residual current detection device for detecting DC currents. In the presence of a residual DC current, the detection device is configured to disable the PFC filter for causing a change in the characteristic of the DC residual current (not detectable as such by the type A RCD). This change results in a different type of residual current which is detectable by the type A RCD which, accordingly, causes a disconnection from the AC supply. As such, the solution disclosed in US8,970,169 provides an electrical signal in response to a detection of the residual DC current, which is applied to an internal controllable component of the charging device (the PFC filter) in order to change the DC current characteristic.

EP2229685B1 considers the prospects for coupling an arc fault detection device (AFDD) and a RCD device. In one arrangement, in the presence of an arc fault, the AFDD controls the closure of a contact so as to realize a current imbalance path between the neutral and live lines of the RCD, for generating a differential current which simulates an occurrence of a residual fault current. The fault detector of the RCD detects this current and controls in response an actuator to open the RCD movable contacts.

The imbalance current path realized by closing the AFDD contact connects to the neutral and live lines at opposite sides of the RCD contacts. This means that the AFDD contact, which is open in absence of arc faults, is electrically in parallel to the RCD contacts.

Therefore, the AFDD contact is required to have at least the same isolating properties as the open RCD contacts, otherwise the isolation of the load would be compromised although the RCD contacts are open. Typically, for isolation purposes the RCD contacts have a minimum contact gap of 4 mm when they are open, and so the AFDD contact requires a similar contact gap. This adversely impacts on the size, complexity, technical performance and costs of the AFD contact as well as of the associated elements and/or components, such as corresponding driving solenoid, etc.

EP2229685B1 solves this problem by mechanically coupling an AFDD to a protection device such as an RCD or an MCB (Miniature circuit breaker) to add arc fault current protection to an installation. In the presence of an arc fault, the AFDD fault detector drives a magnetic actuator to actuate a mechanism between the RCD and AFDD. This mechanism actuates a tripping mechanism associated to the RCD contacts, so as to cause an opening thereof.

In effect, in this arrangement the AFDD provides an “add-on” module that is mechanically coupled to the protection device. A key problem with such an arrangement is that the AFDD and the protection device must be mechanically compatible to facilitate coupling. In effect, the AFDD and the protection device most likely have to be produced by the same manufacturer with the result that if an installation has protection devices provided by another manufacturer, the AFDD addon module may not be usable in such an installation with the existing protection devices.

Summary

According to a first aspect of the present invention, there is provided a system for protecting an electrical circuit according to claim 1.

In this system, in case of a DC residual fault current between the neutral and live lines, the DC detector is configured to generate an electrical signal for indirectly controlling the actuator of the electrical protection switching device to open the movable contacts associated with the live and neutral lines.

This generated electrical signal corresponds to a differential current between the live and neutral lines, which simulates the occurrence of a residual fault current detectable by the fault detector of the protection switching device.

This solution is advantageous because the DC detector can be used to verify the correct operation of the protection switching device under fault conditions, and also when DC detector is tested.

In order to generate the differential current simulating a detectable residual current fault, the DC detector is configured to realize a current imbalance path between the live and neutral lines, by closing a contact.

Advantageously, the current imbalance path is connected to the neutral line and to the live line at the same circuit side relative to the movable contacts and at opposite sides of the fault detector of the electrical protection switching device.

In this way, a simple and relatively inexpensive arrangement is provided, where the contact used to realize the current imbalance path is not electrically connected in parallel to the movable contacts of the protection switching device. This means that an electronical switching contact can be used or a mechanical contact with an isolation gap smaller than the movable contacts of the switching device, without negatively effecting the load isolation.

According to a second aspect of the present invention, there is provided a system for protecting an electrical circuit according to claim 9. In embodiments of this system, the DC detector is configured to generate an electrical control signal for directly controlling the actuator of the electrical protection switching device, so as to open the movable contacts thereof.

In this way, the system does not require a differential current to be generated in order to simulate the occurrence of a residual current fault, thus avoiding additional electrical contacts, resistors, and other electrical components or elements associated to the realization of a current imbalance path.

As such, the embodiments of the systems according to the first and second aspects are simple but add highly effective DC fault protection to any AC detecting electrical protection switching devices, especially type A RCDs but also other types of RCDs or circuit breakers, e.g. molded cases circuit breakers (MCBs), while mitigating most of the mentioned problems in the state of the art.

In particular, these embodiments do not require any mechanical compatibility between the DC detector and the electrical protection switching device. This means that the electrical protection switching device and the DC detector can be placed side by side for convenience, but there is no need for mechanical coupling between them. The DC detector and the electrical protection switching device can even be placed separate from each other. This further means that the DC detector and the electrical protection switching device can be produced by different manufacturers; therefore, the DC protection functionality of the DC detector can be added to electrical protection switching devices from different manufacturers which are already installed in an electrical circuit.

Brief Description of the Drawings

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

Figure 1 illustrates a protection system comprising a ROD and a DC detector according to an exemplary arrangement;

Figure 2 illustrates a protection system comprising a RCD and a DC detector according to one embodiment of the present invention;

Figure 3 illustrates a protection system comprising a RCD and an DC detector according to another embodiment of the present invention;

Figure 4 illustrates a DC detector suitable for being used in the embodiment illustrated in Figure 3;

Figure 5 illustrates a protection system comprising a RCD and a DC detector according to another embodiment of the present invention;

Figure 6 illustrates a circuit of a voltage-dependent RCD adapted to be used in a system according to the embodiment illustrated in Figure 5;

Figure 7 illustrates a circuit of a voltage-independent RCD adapted to be used in a system according to the embodiment illustrated in Figure 5.

Description of the Embodiments

An exemplary system 1000 comprising a type A RCD 1 and a DC detector 80 installed in an electrical circuit to be protected is illustrated in Figure 1.

A neutral line N and a live line L are provided to the RCD 1 from a supply 30. In particular, the RCD 1 comprises neutral and live input terminals 2, 3 electrically connected to the supply 30 for receiving the N and L lines, and neutral and live output terminals 4, 5 for providing the N and L lines towards a load 40.

The RCD 1 comprises at least two movable contacts 6 associated with the N and L lines and which can be opened for disconnecting the load 40 from the supply 30. In practice, the opening of the contacts 6 provides an electrical isolation of the load 40 from the supply 30.

The RCD 1 further comprises an actuator 7 which can be electrically controlled to open the contacts 6. For example, the actuator 7 can cause an opening of the contacts 6 by acting on an operating mechanism 70 of the ROD 1 which is operatively connected to the contacts 6.

The RCD 1 further comprises at least a residual current detector 8 for controlling the actuator 7 to open the movable contacts 6 upon a detection of a residual fault current in the electrical circuit, in particular a residual fault AC current or a residual fault pulsating DC current.

With reference to Figures 6 and 7, the residual current detector 8 can comprise a current transformer 60 operatively coupled to the conductive path of the N and L lines through the RCD 1, so as to output a signal depending on a current imbalance between the N and L lines. In particular, the current transformer 60 is arranged on the load side of the movable contacts 6, i.e. between the contacts 6 themselves and the output neutral and live terminals 4, 5.

The residual current detector 8 can further comprise a detection circuit 61 for receiving the output signal of the current transformer 60 and detecting the presence of a residual current fault, in case a current difference between the lines N and L is above a rated residual operating current of the RCD 1.

Referring again to figure 1, the N and L lines are provided from the output neutral and live terminals 4, 5 of the RCD 1 to neutral and live input terminals 81,82 of the DC detector 80. The DC detector 80 further comprises neutral and live output terminals 83, 84 for providing the N and L lines to the load 40. As such, the terminals 81-84 of the DC detector 80 are similarly rated to the load terminals 4, 5 of the RCD 1, so as to be suitable for the rated voltage and current of the load 40.

The DC detector 80 is configured to detect pure residual fault DC currents between the N and L lines; the DC detector 80 can be configured to detect also pulsating residual fault DC currents between the N and L lines. The DC detection can be based on any known DC current detection technology.

With reference to Figure 1, the DC detector 80 can for example comprise a current transformer 85 operatively coupled to the conductive path of the N and L lines through the DC detector 80, so as to output a signal depending on a current imbalance between the N and L lines. The DC detector 80 can further comprise a detection circuit 86 for receiving the output signal of the current transformer 85 and detecting the presence of a residual DC current fault, in case a DC current difference between the N and L lines is above a rated DC residual operating current. The supply for operating the detection circuit 86 can be obtained by converting the AC voltage present between the N and L lines, as provided through the input terminals 81,82.

The DC detector 80 further comprises a relay 87 with a normally open mechanical contact 88 which can be closed to realize a current imbalance path between the L and N lines.

The current imbalance path comprises a resistor 89 in series with the contact 88, and extends between a connection point 90 to the N line and a connection point 91 to the L line. The connection points 90 and 91 are, respectively, downstream and upstream relative to the current transformer 60 of the residual current detector 8 as well as relative to the movable contacts 6 of the RCD 1.

In case of a residual fault DC current, or in case of a test, the detection circuit 86 is configured to generate an electrical signal for causing the relay 87 to close the contact 88. The closure of the contact 88 causes an imbalance current to flow between the connection points 90 and 91. This imbalance current causes a difference between the current flowing in the N line and the current flowing in the L line downstream to the connection point 91. This difference is detectable by the current transformer 60 of the residual current detector 8 of the RCD 1.

The current difference value between the N and L lines depends on the value of the imbalance current, which in turn is set by the resistance value of the resistor 89. Thus, by choosing an appropriate resistance value the differential current generated between N and L lines can be greater than the rated residual operating current of the RCD 1.

In this way, the generated differential current can simulate an occurrence of a residual current fault which is detectable by the residual current detector 8. In response to the detection, the detector 8 causes the actuator 7 to drive the operating mechanism 70 for opening the contacts 6.

It is important to note that whilst the current causing the tripping of the ROD 1 is a differential current, it is not flowing to ground and is therefore not a residual current in the commonly accepted sense and is in reality an additional load current in the installation.

A drawback of this exemplary solution illustrated in Figure 1 is that the imbalance current path realized by closing the contact 88 connects to the N and L lines at opposite sides of the contacts 6 of the RCD 1. Thus, the contact 88, which is normally open in absence of DC current residual faults, is electrically in parallel to the contacts 6 of the RCD 1.

Therefore, the contact 88 is required to have at least the same isolating properties as the open contacts 6, otherwise the isolation of the load 40 would be compromised although the contacts 6 are open. Thus, size and cost of the contact 88 and associated relay 87 can make the arrangement illustrated in Figure 1 impractical for use in some applications.

Replacing the contact 88 with a solid-state relay, e.g. a silicon controlled rectifier (SCR), would also compromise the isolating properties of the RCD 1, and may not be acceptable where isolation is of importance.

An exemplary system 2000 according to the present invention is illustrated in Figure 2, wherein elements and/or components in common with the exemplary system 100 illustrated in Figure 1 are indicated with same reference numerals.

The system 2000 differs from the system 1000 in the way in which the current imbalance path between the L and N lines is realized for simulating a residual fault current detectable by the residual current detector 8 of the RCD 1.

In particular, the DC detector 80 illustrated in Figure 2 comprises an SCR 95 which acts as an electric contact which can be switched on by the detection circuit 86 to realize the current imbalance path.

The connection points 90, 91 of the current imbalance path to the N and L lines are, respectively, downstream and upstream relative to the current transformer 60 of the residual current detector 8 of the ROD 1, but differently than the arrangement illustrated in Figure 1 the connection point 91 is downstream of the movable contact 6 of the L line (between the current transformer 60 and the movable contact 6 itself). Such an arrangement requires electrical access to a conductor within the RCD 1, but this can easily be facilitated during manufacture.

Operatively, in absence of DC residual current fault conditions the SCR 95 remains switched-off. In case of DC residual current fault conditions, or in case of a test, the detection circuit 86 is configured to generate an electrical signal for switching on the SCR 95.

The switching-on of the SCR 95 causes an imbalance current to flow between the connection points 90 and 91. This imbalance current causes a difference between the current flowing in the N line and the current flowing in the L line downstream to the connection point 91. This difference is detectable by the current transformer 60 of the residual current detector 8.

The current difference value between the N and L lines depends on the value of the imbalance current, which in turn is set by the resistance value of the resistor 89 in series with the SCR 95. Thus, by choosing an appropriate resistance value the differential current generated between N and L lines will be greater than the rated residual operating current of the RCD 1.

In this way, the generated differential current can simulate an occurrence of a residual current fault which is detectable by the detector 8. In response to the detection, the detector 8 causes the actuator 7 to drive the operating mechanism 70 for opening the contacts 6.

In the arrangement illustrated in Figure 2 both the connection points 90, 91 are advantageously downstream of the movable contacts 6. Thus, the SCR 95 is not in parallel to the movable contacts 6 and, in case of the contacts 6 opening, the SCR 95 remains connected only to the load circuit portion disconnected from the supply 30.

Furthermore, since the connection points 90, 91 are connected downstream of the contacts 6, the imbalance current ceases to flow when the contacts 6 open.

As such, the arrangement illustrated in Figure 2 overcomes the problems mentioned for the exemplary arrangement illustrated in Figure 1.

The SCR 95 may be connected to produce either an AC differential current or a pulsating DC current - both can cause the required imbalance to trigger the detector circuit 8. A bridge rectifier may be connected before the SCR 95 so as to provide for a full wave residual current when the SCR 95 is turned on.

The system 2000 illustrated in Figure 2 may use a voltage dependent RCD 1 or a voltage independent RCD 1.

Further, the DC detector 80 illustrated in Figure 2 may be mechanically coupled to the RCD 1 or placed adjacent to it, as convenient.

The contact used to realize the current imbalance path can be different than the SCR 95, e.g. another suitable electronic contact, such as a transistor, or a mechanical contact, regardless its level of isolation when open.

Another exemplary system 3000 according the present invention is illustrated in Figure 3, wherein elements and/or components in common with the exemplary system 2000 illustrated in Figure 2 are indicated with same reference numerals.

The system 3000 differs from the system 2000 in that it provides an arrangement where fully rated supply and load terminals are not necessary for the DC detector 80.

In particular, with reference to Figure 4, in this arrangement the DC detector 80 can be provided with an aperture 960 to facilitate passing through the detector 80 of the N and L lines as provided by the RCD 1 through its output terminals 4 and 5. In this case, the transformer 85 is arranged proximal to the aperture 960, in order to be operatively electrically coupled to the N and L lines passing through the aperture 960. Examples of such types of DC detectors are the RCM14 or the RCM20 current monitoring devices supplied by Western Automation Ireland.

With reference to Figure 3, the DC detector 80 further comprises at least a first supply input terminal 96 and a second supply input terminal 97 (which are also viewable in Figure 4) for providing an operating supply to the detection circuit 86.

The terminals 96 and 97 are connected respectively to the live output terminal 5 and the neutral output terminal 4 of the RCD 1 through shunt connectors 98 and 99. As such, only a portion of the current of the N and L lines flows through the shunt connections 98, 99, and the electrical rating of the supply input terminals 96, 97 can be advantageously less than the electrical rating of the output terminals 4, 5 of the RCD 1.

The SCR 95 is electrically connected to the supply input terminal 97. In this way, as illustrated in Figure 3, the current imbalance path realized by switching on the SCR 95 connects through the shunt connection 99 to the N line at a point 90 downstream the contacts 6 of the RCD 1.

Thus, also in the arrangement illustrated in Figure 3 both the connection points 90, 91 of the current imbalance path are advantageously provided downstream the movable contacts 6, so as the SCR 95 is not in parallel to such contacts 6.

A further exemplary system 4000 of the present invention is illustrated in Figure 5, wherein elements and/or components in common with the above disclosed systems 1000, 2000, 3000 are indicated with same reference numerals.

The system 4000 differs from the systems 1000, 2000, 3000 in that, in the event of a residual fault DC current being detected by the detection circuit 86 of the DC detector 80, the circuit 86 is configured to generate an electrical control signal 1200 for directly controlling the actuator 7 of the RCD 1 to open the movable contacts 6. The control signal 1200 can be provided to the RCD 1 through an electrical connector 1300.

In order to ensure reliable operation, the electrical control signal 120 is compatible with the type of RCD 1 used in the system 100.

The RCD 1 of the system 4000 can be a voltage dependent (VD) type and an exemplary circuit of a VD-RCD 1 is illustrated in figure 6, where the detection circuit 61 of the residual current detector 8 uses the voltage present between the L and N lines for its supply.

The actuator 7 comprise at least one coil actuator 20 configured for opening the movable contacts 6 when supplied with a predetermined voltage. For example, the application of the predetermined voltage can cause a release of a movable element, e.g. a plunger, of the coil actuator 20, which can act on the operating mechanism of the movable contacts 6.

The actuator 7 further comprise a SCR 21 which is arranged so as to connect the coil actuator 20 between the L and N lines when it is turned-on. In this way, the turning-on of the SCR 21 provides the coil actuator 20 with the energy required to open the movable contacts 6.

The detection circuit 61 is configured to generate an electrical control signal upon detection of a residual current fault and this is provided to SCR 21 via a diode 18 suitable for turning-on the SCR 21.

The detection circuit 86 of the DC detector 80 is configured such that the generated electrical control signal 1200 can also be used for turning-on the SCR 21. In the exemplary embodiment illustrated in Figure 6, the signal line providing the signal 1200 from the DC detector 80 to the RCD 1 comprise a diode 17 with each diode 17, 18 presenting a high impedance to the signal provided from other of the detection circuits 61 and 86.

Since the RCD 1 and the DC detector 80 share the same neutral line N, the electrical control signal generated by the residual current detector 8 and the electrical control signal 1200 generated by the detection circuit 86 of the DC detector 80 have a common reference value and, therefore, they are fully compatible for controlling the same SCR 21.

In an alternative embodiment, the RCD 1 of the system 4000 can be of a voltage independent (VI) type and an exemplary circuit of a VI-RCD 1 is illustrated in figure 7, where the supply for the operation of the detection circuit 61 is provided by the output signal generated by the current transformer 60.

The actuator 7 can comprise at least one permanent magnet relay (PMR) 22 and the detection circuit 61 is configured to generate an electrical signal for supplying the PMR 22 in the presence of a residual current fault. In particular, the PMR supply signal is such as to cause the PMR 22 to open the contacts 6. For example, the PMR 22 can comprise a movable element kept in a retracted position by a permanent magnet and released through the energy provided by the electrical signal generated by the detection circuit 61.

In practice, in the VI configuration, the detection circuit 61 and the PMR 22 float with respect to the supply voltage present between the L and N lines and it is therefore possible to couple to them without compromising their performance.

Hence, the electrical control signal 1200 generated by the detection circuit 86 of the DC detector 80 can be fed to the PMR 22 via a diode 17 to cause an opening of the movable contacts 6 in the event of an DC residual fault condition. In particular, the detection circuit 86 is configured to generate the electrical control signal 1200 such that this signal 1200 can be used to provide the supply required by the PMR 22 to open the contacts 6.

Note that in the arrangements of figures 6 and 7, the issue of RCD impairment does not arise because the signal generated by the DC detector is used to activate the tripping means within the RCD and not to generate a residual current. In such cases any Type A RCD may be used.

The invention is not limited to the embodiments illustrated in Figures 2-7 and refinements or changes may be made without departing from the essence of the invention.

For example, the above arrangements could be applied to two-phase or three phase supplies by the use of two or three DC detectors 80 or by using a single DC detector 80 for such applications.

For example, the electrical connection 1300 providing the electrical control signal 1200 from the DC detector 80 to the RCD 1 can comprise any means for electrical signal transmission between the RCD 1 and DC detector 80 placed side by side or separated from each other. For example, the electrical connection 1300 can comprise an electrical cable or wire.

The RCD 1 and the DC detector 80 may be placed side by side or one above the other as convenient, but there is no need for mechanical coupling between them.

Although the DC detector 80 illustrated in Figure 5 comprises fully rated input and output terminals 81-84 as in the DC detector 80 illustrated in Figures 1 and 2, it is to be understood that the DC detector 80 used in the arrangement of Figure 5 can instead be provided with the input supply terminals 96, 97 according to the arrangement above disclosed with reference to Figure 3.

Finally, the above arrangements could comprise protection devices other than a type A RCD 1, for example a circuit breaker having a shunt trip coil, or a contactor, etc. The systems 2000, 3000, 4000 could in fact be used for detection of AC, DC and pulsating DC currents in accordance with the Type A and Type B product standards or in compliance with other RCD product standards as applicable to EV charging, etc.

For example, although it is well known that Type A and AC type RCDs are intended to detect residual currents at 50 or 60Hz with a tolerance of about 10% on these values, and that they generally cannot be used for detecting residual current at higher frequencies, the systems 2000, 3000, 4000 described above could include a switching device for protection against AC residual currents over the range DC - 1 KHz as required.

The DC detector 80 can comprise an AC/DC converter 94 operatively connected to the supply input terminals 96, 97 so as to convert the AC voltage therebetween to a DC voltage suitable to operate the detection circuitry 86, or the mains supply derived via lines 98 & 99 may be fed to an external transformer to be reduced to a substantially 5 lower ac voltage level to power up DC detector 80. This reduced ac voltage may further be rectified externally so as to provide a DC supply to operate the DC detector 80, as convenient.

Although not shown in any of the figures herein, as indicated above, EV charging 10 systems typically include a pilot function and a downstream switching device such as a contactor. However, because the present application relates specifically to operation of the RCD and not to the downstream switching device, the pilot function and the switching device are omitted from the figures for convenience.

Claims

Claims:

1. A system for protecting an electrical circuit, the system comprising at least:

- an electrical protection switching device comprising: neutral and live input terminals for electrical connection to a neutral line and a live line provided by a supply of the electrical circuit, neutral and live output terminals for providing the live and neutral lines to a load of the electrical circuit, a movable contact associated with the neutral line and a movable contact associated with the live line which can be opened for disconnecting the load from the supply, an electrically controlled actuator arranged to open said movable contacts, and a fault detector for causing the actuator to open said movable contacts upon detection of a residual fault current between the live and neutral lines; and

- a DC detector electrically connected to the live and neutral lines for detecting a residual fault DC current between the neutral and live lines;

wherein, in case of detecting a residual fault DC current, the DC detector is configured to close a contact for realizing a current imbalance path generating a differential current between the live and neutral lines to simulate a residual fault current detectable by the fault detector of the electrical protection switching device, the current imbalance path being connected to the neutral line and to the live line at the same circuit side relative to the movable contacts and at opposite sides of the fault detector of the electrical protection switching device.

2. The system of claim 1, wherein said fault detector is downstream the movable contacts and the current imbalance path is connected to the neutral line and to the live line downstream the movable contacts.

3. The system of claim 1, wherein said DC detector comprises a first supply input terminal and a second supply input terminal which are connected respectively to the neutral line and the live line through shunt connectors, the electrical rating of the first and second supply input terminals being less than the electrical rating of the live and neutral terminals of the electrical protection switching device; and the DC detector comprises an aperture for passing therethrough of the live and neutral lines.

4. The system of claim 1, wherein said contact is electronic and, in case of detecting a residual fault DC current, the DC detector is configured to generate an electrical signal for switching on the contact.

5. The system of claim 1, wherein said electronic contact is a silicon controlled rectifier (SCR).

6. The system of claim 1, wherein said current imbalance path comprises a resistor.

7. The system of claim 1, wherein the electrical protection switching device is of a voltage dependent type.

8. The system of claim 1, wherein the electrical protection switching device is of a voltage independent type.

9. A system for protecting an electrical circuit, the system comprising at least:

- an electrical protection switching device comprising: neutral and live input terminals for electrical connection to a neutral line and a live line provided by a supply of the electrical circuit, neutral and live output terminals for providing the live and natural lines to a load of the electrical circuit, a movable contact associated with the neutral line and a movable contact associated with the live line which can be opened for disconnecting the load from the supply, an electrically controlled actuator arranged to open said movable contacts, and a fault detector for causing the actuator to open said movable contacts upon detection of a fault in the electrical circuit; and

- a DC detector electrically connected to the live and neutral lines for detecting a DC residual fault current between the neutral and live lines, and wherein, in case of detecting a residual fault DC current, the DC detector is configured to generate an electrical control signal for directly controlling the actuator of the electrical protection switching device to open said movable contacts.

10. The system of claim 9, further comprising an electrical connector for providing the electrical control signal from the DC detector to the actuator of the electrical protection switching device.

11. The system of claim 9, wherein said DC detector comprises a first supply input terminal and a second supply input terminal which are connected respectively to the neutral line and the live line through shunt connectors, the electrical rating of the first and second supply input terminals being less than the electrical rating of the live and neutral terminals of the electrical protection switching device; and the DC detector comprises an aperture for passing therethrough of the live and neutral lines.

12. The system of claim 9, wherein the electrical protection switching device is of a voltage dependent type and:

- the actuator of the electrical protection switching device comprise a coil actuator configured for causing said movable contacts to open when supplied with a predetermined voltage, and an electronic switching device which is arranged so as to connect the coil actuator to the live and neutral terminals of the electrical protection switching device when turned-on; and

- the electrical control signal generated by the DC detector is suitable for turning-on the electronical switching device.

13. The system of claim 9, wherein the electrical protection switching device is of a voltage independent type and the actuator of the electrical protection switching device comprises a permanent magnet relay; and

- the electrical control signal generated by the DC detector is suitable for supplying the permanent magnet relay for tripping and causing the opening of said movable contacts.