Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
A MODULAR SYSTEM FOR PROTECTING AN ELECTRICAL CIRCUIT
Document Type and Number:
WIPO Patent Application WO/2018/099902
Kind Code:
A1
Abstract:
A system for protecting an electrical circuit comprises an electrical protection switching module comprising: neutral and live input terminals for electrical connection to a supply of the circuit, a neutral output terminal for electrical connection to a load of the circuit, a live output terminal, and a casing including: at least one movable contact which can be opened for disconnecting the load from the supply, an electrically controlled actuator arranged to open the movable contact, and a fault detector for causing the actuator to open the movable contact upon a detection of a fault in the circuit. An arc fault module comprising a casing including an arc fault detection circuit electrically connected to at least one of the input or output electrical terminals of the switching module for detecting an arc fault in the circuit. In case of detecting an arc fault, the arc fault module is configured to generate an electrical control signal for controlling the actuator of the switching module to open the movable contact. The system comprises an electrical connector for providing the electrical control signal to the switching module.

Inventors:
WARD, Patrick (Atreus Place Poolboy, Galway, H53 TD78 Ballinasloe, H53 TD78, IE)
Application Number:
EP2017/080632
Publication Date:
June 07, 2018
Filing Date:
November 28, 2017
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
SHAKIRA LIMITED (Atreus Place Poolboy, Galway, H53 TD78 Ballinasloe, H53 TD78, IE)
International Classes:
H02H1/00; H01H83/14; H02H3/33
Domestic Patent References:
WO2009089995A12009-07-23
Foreign References:
US20120229943A12012-09-13
US20160141123A12016-05-19
Attorney, Agent or Firm:
HANNA MOORE + CURLEY (Garryard House 25/26 Earlsfort Terrace, D02 PX51 Dublin, D02 PX51, IE)
Download PDF:
Claims:
Claims:

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

- an electrical protection switching module comprising: neutral and live input terminals for electrical connection to a supply of the electrical circuit, a neutral output terminal for electrical connection to a load of the electrical circuit, a live output terminal, and a first casing including: at least one movable contact which can be opened for disconnecting the load from the supply, an electrically controlled actuator arranged to open said at least one movable contact, and a fault detector for causing the actuator to open said at least one movable contact upon a detection of a fault in the electrical circuit; and

- an arc fault module comprising a second casing including an arc fault detection circuit electrically connected to at least one of the input or output electrical terminals of the electrical protection switching module for detecting an arc fault in the electrical circuit, and wherein, in case of detecting an arc fault, the arc fault module is configured to generate an electrical control signal for directly controlling the actuator of the electrical protection switching module to open said at least one movable contact; and

- an electrical connector for providing the electrical control signal from the arc fault module to the actuator of the electrical protection switching module.

2. The system of claim 1 , wherein the arc fault module comprises at least a live input terminal electrically connected to the live output terminal of the electrical protection switching module;

the arc fault module further comprises a live output terminal for electrical connection to the load of the electrical circuit, the live output terminal being electrically connected to the live input terminal of the arc fault module; and

the arc fault detection circuit is configured to operatively detect an arc fault by monitoring the electrical connection between the live input and output terminals of the arc fault module.

3. The system of claim 1 , wherein the neutral output terminal of the electrical protection switching module is directly connected to the load of the electrical circuit.

4. The system of claim 1 , wherein the arc fault module further comprises: - a neutral input terminal which is electrically connected to the neutral output terminal of the electrical protection switching module; and

- a neutral output terminal for electrical connection to the load of the electrical circuit and which is electrically connected to the neutral input electrical terminal of the arc fault module.

5. The system of claim 1 , wherein the electrical protection switching module is of a voltage dependent type and the electrical protection switching module shares with the arc fault module a neutral line between the supply and the load of the electric circuit.

6. The system of claim 1 , wherein the electrical protection switching module is of a voltage dependent type and:

- the actuator of the electrical protection switching module comprises a coil actuator configured for causing said at least one movable contact 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 module when turned-on; and

- the electrical control signal generated by the arc fault module is suitable for turning- on the electronical switching device.

7. The system of claim 1 , wherein the electrical protection switching module is of a voltage independent type and the actuator of the electrical protection switching module comprises a permanent magnet relay; and

- the electrical control signal generated by the arc fault module is suitable for supplying the permanent magnet relay for tripping and causing the opening of said at least one movable contact.

8. The system of claim 1 , wherein the arc fault module comprises:

- at least a live input terminal and a neutral input terminal for direct electrical connection to the supply of the electrical circuit;

- an electrical signaling device;

- and a power control circuit operatively connected to the electrical signaling device and to the live and neutral input terminals of the arc fault module, the power control circuit being configured to apply an operating power supply to the electrical signaling device when the electrical control signal is generated by the arc fault module, and to interrupt the application of the operating power supply when said at least one movable contact of the electrical protection switching module returns to a closed position.

9. The system of claim 8, wherein the arc fault module comprises at least one output terminal for connection to the load of the electrical circuit, and the power control circuit is configured to monitor said at least one output terminal for detecting the presence of a supply voltage applied to the load.

10. The system of claim 1 , wherein the electrical connection between the electrical protection switching module and the arc fault module comprises at least one diode.

11 . A system for protecting an electrical circuit, the system comprising at least: - an electrical protection switching module 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, a neutral output terminal for electrical connection to a load of the electrical circuit, a live output terminal, and a first casing including: 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 a detection of a residual fault current between the live and neutral lines; and

- an arc fault module comprising a second casing including an arc fault detection circuit electrically connected to at least one of the input or output electrical terminals of the electrical protection switching module for detecting an arc fault in the electrical circuit; wherein, in case of detecting an arc fault, the arc fault module is configured to close a contact for realizing an electrical control signal comprising a current imbalance path generating a differential current between the live and neutral lines to simulate a residual fault current, 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 module, a portion of said current imbalance path including an electrical connector providing the electrical control signal between the arc fault module and the electrical protection switching module.

12. The system of claim 11 , wherein said contact is electronic and, in case of detecting an arc fault, the fault arc module is configured to generate an electrical signal for switching on the contact.

13. The system of claim 12, wherein said electronic contact is a silicon control rectifier (SCR). 14. The system of claim 11 , wherein said current imbalance path comprises a resistor.

15. The system of claim 11 , wherein the arc fault module comprises at least a live input terminal electrically connected to the live output terminal of the electrical protection switching module;

the arc fault module further comprises a live output terminal for electrical connection to the load of the electrical circuit, the live output terminal being electrically connected to the live input terminal of the arc fault module; and

the arc fault detection circuit is configured to operatively detect an arc fault by monitoring the electrical connection between the live input and output terminals of the arc fault module.

16. The system of claim 11 , wherein the neutral output terminal of the electrical protection switching module is directly connected to the load of the electrical circuit. 17. The system of claim 11 , wherein the electrical protection switching module is of a voltage dependent type.

18. The system of claim 11 , wherein the electrical protection switching module is of a voltage independent type.

19. The system of claim 11 , wherein the arc fault module comprises at least one output terminal for connection to the load of the electrical circuit, and the arc fault module comprises a power control circuit configured to monitor said at least one output terminal for detecting the presence of a supply voltage applied to the load.

Description:
A Modular System for Protecting an Electrical Circuit

Field The present invention relates to a modular system for protecting an electrical circuit, in particular a system comprising at least an electrical protection switching module and an arc fault module.

Background

Arc fault current detection devices are well known to those skilled in the art and are known as AFCIs (Arc Fault Circuit Interrupters) in North America and AFDDs or AFDs (Arc Fault Current Detectors) 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.

Residual current devices (RCDs) are also 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.

In some cases, arc fault technology (AFT) has been combined with residual current technology (RCT) to provide fire protection with electric shock protection. This can give rise to a range of problems such as increased complexity, increased cost, reduced reliability and even nuisance tripping. Furthermore, it is often difficult for the user to know which technology caused the protection device to trip, i.e. a residual current fault or an arc current fault.

EP2229685B1 considers the prospects for coupling an 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 a 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 could not be used in such an installation with the existing protection devices. US 2016/0141123 discloses a synthetic fault signal generator assembly remotely located on a branch circuit downstream from a circuit breaker protecting a load. The synthetic fault signal generator assembly is configured to detect an improper circuit condition that is not independently detected, detectable, or actionable by the circuit breaker such as, for example, a load or outlet receptacle specific problem that can lead to equipment damage or property damage if not mitigated. In response to the improper circuit condition being detected, the synthetic fault signal generator assembly generates a synthetic fault signal, which causes the circuit breaker to trip. The synthetic fault signal generator assembly can inject the synthetic fault signal into the branch circuit to provide the synthetic fault signal to the circuit breaker.

US 6,477,022 discloses a miniature circuit breaker incorporating ground fault protection and arc fault protection including main separable contacts. An operating mechanism actuated by a trip mechanism opens the main separable contacts in response to predetermined current conditions. Auxiliary separable contacts are disposed in series with the main separable contacts. A solenoid having a movable plunger opens and closes the auxiliary separable contacts in response to a remote external signal. A ground fault trip circuit detects a ground fault, and an arc fault trip circuit detects an arc fault. An actuator mechanism, energizable by the ground fault trip circuit and the arc fault trip circuit, actuates the plunger to open the auxiliary separable contacts.

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 embodiments of this system, the arc fault module is configured to generate an electrical control signal for directly controlling the actuator of the electrical protection switching module, so as to open the movable contacts thereof.

According to some embodiments of the system, the electrical protection switching module is of a voltage dependent type and the electrical protection switching module shares with the arc fault module a neutral line between the supply and the load of the electric circuit. In this way, an electrical control signal generated by the fault detector of the electrical protection switching module and the electrical control signal generated by the arc fault module advantageously have a same reference value and therefore they are fully compatible for controlling the same actuator.

According to some embodiments of the system, the arc fault module comprises:

- at least a live input terminal and a neutral input terminal for direct electrical connection to the supply of the electrical circuit;

- an electrical signaling device;

- and a power control circuit operatively connected to the electrical signaling device and to the live and neutral input terminals of the arc fault module, the power control circuit being configured to apply an operating power supply to the electrical signaling device when the electrical control signal is generated by the arc fault module, and to interrupt the application of the operating power supply when said at least one movable contact of the electrical protection switching module returns to a closed position.

In this way, the signaling device is advantageously driven by the power control circuit so as to indicate that the movable contacts of the electrical protection switching module are open due to an arc fault detection rather than a detection of another electrical fault by the fault detector of the electrical protection switching module.

According to some embodiments, the arc fault module comprises at least one output terminal for connection to the load of the electrical circuit, and the power control circuit is configured to monitor said at least one output terminal for detecting the presence of a supply voltage applied to the load.

In this way, the power control circuit can detect when said at least one movable contact of the electrical protection switching module has returned in the closed position.

The system according to the first aspect of the present invention is simple but adds highly effective arc fault protection to any electrical protection switching devices, especially RCDs but also circuit breakers, e.g. molded cases circuit breakers (MCBs), while mitigating most of the mentioned problems in the state of the art. In particular, the direct provision through the electrical connection of the electrical control signal from the arc fault module to the actuator of the electrical protection switching module does not require any mechanical compatibility between the arc fault module and the electrical protection switching module.

This means that the electrical protection switching module and the arc fault module can be placed side by side for convenience, but there is no need for mechanical coupling between them. The arc fault module and the electrical protection switching module can even be placed separate from each other. This further means that the arc fault module and the electrical protection switching module can be produced by different manufacturers; therefore, the arc fault protection functionality of the arc fault module can be added to electrical protection switching devices from different manufacturers which are already installed in an electrical circuit. Furthermore, 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. According to a second aspect of the present invention, there is provided a system for protecting an electrical circuit according to claim 12.

In this system, in case of an arc fault, the arc fault module is configured to generate an electrical signal for indirectly controlling the actuator of the electrical protection switching module 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 module.

This solution is advantageous because the arc fault module can be used to verify the correct operation of the protection switching module under fault conditions, and also when the arc fault module is tested. In order to generate the differential current simulating a residual current fault, the arc fault module 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 module. 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 module. 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 module, without negatively effecting the load isolation.

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 RCD and an AFD according to an embodiment of the present invention;

Figure 2 illustrates a circuit of a voltage-dependent RCD adapted to be used in a system according to one embodiment of the present invention;

Figure 3 illustrates a circuit of a voltage-independent RCD adapted to be used in a system according to another embodiment of the present invention; and

Figure 4 illustrates a protection system comprising a RCD and an AFD according to another embodiment of the present invention.

Description of the Embodiments

With reference to Figure 1 , an exemplary system 100 according to the present invention comprises a RCD 1 and an AFD 10 installed in an electrical circuit to be protected. Although schematically illustrated, each of the RCD 1 and AFD 10 comprises its own case 101 , 102 including at least the illustrated components.

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 and a neutral output terminal 4 directly electrically connected to a load 40. (In practice, the RCD 1 and the AFD 10 share the same N line for supplying the load 40 to be protected). A RCD live output terminal 5 would normally be electrically connected to the load 40, but in the present embodiment, this is connected to a suitably rated live input terminal 13 of the AFD 10, with the associated line L passing unbroken within the AFD 10 to a live output terminal 14 for providing the live supply to the load 40. The live terminals 13 and 14 of the AFD 10 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 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 RCD 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 current fault in the electrical circuit. With reference to Figures 2 and 3, 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. The residual current detector 8 further comprises 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 fault threshold. The AFD 10 comprises an arc fault detection circuit 11 which is operatively connected to the L line passing through the AFD 10 between the live input and output terminals 13, 14. In particular, the arc fault detection circuit 11 is configured for detecting an arc fault in the protected electrical circuit by monitoring the L line between the terminals 13, 14. For example, the arc fault detection circuit 11 can detect series and parallel arc fault currents flowing in the L line; the circuit 11 can also detect parallel arc fault current between the L line and ground, even if in this case, the RCD 1 will most likely trip before the circuit 11 can detect the fault. The arc fault detection circuit 11 can be based on any known AFD technology, but it is preferably based on the AFD technology described in patent application GB1601425.0.

The AFD 10 further comprises a neutral input terminal 15 and a live input terminal 16 directly electrically connected to the supply 30. In this way, the supply voltage between the neutral and live lines can be used to provide an operating supply to the arc fault detection circuit 11 .

In the event of an arc fault current being detected in the L line, the arc fault detection circuit 11 is configured to generate an electrical control signal 120 for directly controlling the actuator 7 of the RCD 1 to open the movable contacts 6.

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 can be a voltage dependent (VD) type and an exemplary circuit of a VD- RCD 1 is illustrated in figure 2, 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 silicon controlled rectifier (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 to 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 a 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 arc fault detection circuit 11 is configured such that the generated electrical control signal 120 can also be used for turning-on the SCR 21 . In the exemplary embodiment illustrated in Figure 2, the signal line providing the signal 120 from the AFD 10 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 11 .

Since the RCD 1 and the AFD 10 share the neutral line N, the electrical control signal generated by the residual current detector 8 and the electrical control signal 120 generated by the arc fault detection circuit 11 have a common reference value and therefore they are fully compatible for controlling the same SCR 21 .

In an alternative embodiment, RCD 1 can be of a voltage independent (VI) type and an exemplary circuit of a VI-RCD 1 is illustrated in figure 3, 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 120 generated by the arc fault detection circuit 11 of the AFD 10 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 arc fault condition. In particular, the arc fault detection circuit 11 is configured to generate the electrical control signal 120 such that this signal 120 can be used to provide the supply required by the PMR 22 to open the contacts 6. Referring back to Figure 1 , the AFD 10 can further comprise an electronic lighting device 50, such as a LED, and a power control circuit 51 operatively connected to this device 50 and to the live and neutral input terminals 15, 16.

The power control circuit 51 is configured to apply an operating power supply to the lighting device 50 when the electrical control signal 120 is generated by the arc fault detection circuit 11 . In this way, in the event of arc fault detection, the lighting device 50 will light and it will remain lit because the supply to the AFD 10 provided through the line and neutral input terminals 15, 16 is not disconnected by the opening of the movable contacts 6 of the RCD 1 . The lighting device 50 can thus indicate that the RCD 1 opened in response to an arc fault current rather than a residual current fault.

The power control circuit 51 can be further configured to interrupt the application of the operating power supply to the lighting device 50, when the movable contacts 6 of the RCD 1 return to a closed position. For example, the power control circuit 51 can be configured to monitor the live terminal 14 of the AFD 10 for detecting the presence of a supply voltage applied to the load 40.

The invention is not limited to the embodiments illustrated in Figures 1 -3 and refinements or changes may be made without departing from the essence of the invention.

For example, the above arrangement could be applied to two-phase or three phase supplies by the use of two or three AFD modules 10 or by using a single AFD module for such applications. For example, the N line can be routed from the RCD 1 to the AFD 10. In particular, in this case the AFD 10 can comprise a suitably rated neutral input terminal connected to the neutral output terminal 4 of the RCD1 , and a neutral output terminal electrically connected to such neutral input terminal and the load 40. The neutral terminal of the AFD 10 would be similarly rated to the load terminals 4, 5 of the RCD 1 .

In this way, the N line passing through the AFD 10 can be used, together with the L line, for power supply purposes, e.g. for providing operating supply to the arc fault detection circuit 11 and/or to the lighting electronic device 50. The N line passing through the AFD 10 can also be monitored by the arc fault detection circuit 11 in addition or in alternative to the L line.

The electrical connection providing the electrical control signal 120 from the ADF 10 to the RCD 1 can comprise any means for electrical signal transmission between the RCD 1 and AFD 10 placed side by side or separated from each other. For example, the electrical connection can comprise an electrical cable or wire.

The electrical control signal 120 generated by the arc fault detection circuit 11 could also be applied to protection devices other than an RCD 1 , for example a circuit breaker having a shunt trip coil, or a contactor, etc.

Afurther exemplary system 200 according to a second aspect of the present invention is illustrated in Figure 4, wherein elements and/or components in common with the exemplary system 100 illustrated in Figure 1 are indicated with same reference numerals.

In this system 200, the current transformer 60 of the residual current detector 8 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 connected to the load 40.

The output live terminal 5 of the RCD 1 is connected to the input live terminal 13 of the AFD 10, which in turn is connected to the output live terminal 14. In this way, the live line L passes through the AFD 10 and can be monitored by the fault detection circuit 11 in order to detect arc fault conditions.

The AFD 10 further comprises a silicon control rectifier (SCR) 201 which acts as an electric contact which can be switched on/off by the fault detection circuit 11 to realize/interrupt a current imbalance path between the L and N lines.

The current imbalance path comprises a resistor 202 in series with the SCR 201 , and extends between a connection point 203 to the L and a connection point 204 to the N line. The connection points 203 and 204 are, respectively, upstream and downstream relative to the current transformer 60; in particular, the connection point 203 is between the current transformer 60 and the movable contact 6 of the L line.

Thus, both the connection points 203, 204 are at the load side of the movable contacts 6. In this way, the SCR 201 is not in parallel to the movable contacts 6 and, in case of opening of the contacts 6, the SCR 201 remains connected only to the load circuit portion disconnected from the supply 30.

In the example illustrated in Figure 4, the detection circuit 11 of the AFD 10 is connected, at point 205, to the electrical line between the SCR 201 and the connection point 204 to the N line. In this way, the circuit 11 can be supplied with the power present between the N line and the L line passing monitored by the circuit 11 itself.

In absence of arc fault conditions the SCR 201 remains switched-off. In case of an arc fault, or in case of a test, the fault detection circuit 11 is configured to generate an electrical signal for switching on the SCR 201 .

The switching-on of the SCR 201 causes an imbalance current to flow between the connection points 203 and 204. 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 203. This difference is detectable by the current transformer 60.

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 202. Thus, by choosing an appropriate resistance value the differential current generated between N and L lines will be greater that the fault threshold used by the detection circuit 61 . In this way, the generated differential current can simulate an occurrence of a residual current fault which is detectable by the circuit 61 . In response to the detection, the circuit 61 cause the actuator 7 to drive the operating mechanism 70 for opening the contacts 6. The SCR 201 may be connected to produce either an AC differential current or a pulsating DC current - both can cause the required imbalance to trigger detection circuit 61 .

The system 200 illustrated in Figure 4 may use a voltage dependent RCD 1 , such as that illustrated in Figure 2, or a voltage independent RCD 1 , such as that illustrated in Figure 3.

Further, the AFD 10 illustrated in Figure 4 may be mechanically coupled to the RCD 10 or placed adjacent to it, as convenient.

The contact used to realize the current imbalance path can be different than the SCR 201 , e.g. another suitable electronic contact, e.g. a transistor, or a mechanical contact, regardless its level of isolation when open. As in the embodiment of Figure 1 , the embodiment of Figure 4 can be applied to two- phase or three phase supplies by the use of two or three AFD modules 10 or by using a single AFD module for such applications.