Abstract:
A device for detecting a fault in an AC supply comprises a circuit (CT,  100 ) for detecting a particular type of fault in an AC supply to a load (LD) and providing a corresponding output ( 10 ). A relay (RLA) is responsive to said output ( 10 ) to open a set of load contacts (SW 1 ) in the AC supply to disconnect the load from the supply. Test means (TS, Rt, W 2 ) are provided for simulating a supply fault of the said type, and means (C 3,  R 5,  SCR 2,  SOL, SW 2 ) are provided for causing the load contacts (SW 1 ) to open if they do not open in response to the simulated fault within a certain period of time.

Description:
TECHNICAL FIELD  
       [0001]    This invention relates to devices for detecting a fault in an AC supply, for example residual current devices (RCDs) and arc fault detectors (AFDs). 
       BACKGROUND  
       [0002]    Residual current devices, which are also referred to as ground fault circuit interrupters (GFCIs), have been in use worldwide for over forty years, and these devices have contributed significantly to the reduction in fatal accidents arising from electric shock. The principle of operation of RCDs will be well known to those versed in the art, but detailed information can be found in the article “Demystifying RCDs”, at www.rcd.ie, which is incorporated herein by reference in its entirety. 
         [0003]    RCDs are fitted with a test device, often a manually operable button, to enable the user to verify the correct operation of the device, but if the RCD fails to trip on operation of the test device the user may be tempted to simply disregard this warning sign of failure or may delay unduly in replacing the RCD. Once the RCD has failed for any reason it ceases to provide any protection and should be replaced immediately. 
         [0004]    It is an object of the invention to provide an RCD, or other fault detecting device such as an AFD, which incorporates means to remove power from the load in the event of the device failing to trip when operated by the test device. This is sometimes referred to as “end of life” operation or an “end of life” condition. 
       SUMMARY  
       [0005]    According to the invention there is provided a device for detecting a fault in an AC supply, comprising a circuit (CT,  100 ) for detecting a particular type of fault in an AC supply to a load (LD) and providing a corresponding output ( 10 ), means (RLA) responsive to said output ( 10 ) to open a set of load contacts (SW 1 ) to disconnect the load from the supply, test means (TS, Rt, W 2 ) for simulating a supply fault of the said type, and means (C 3 , R 5 , SCR 2 , SOL, SW 2 ) for causing the load contacts (SW 1 ) to open if they do not open in response to the simulated fault within a certain period of time. 
         [0006]    Preferably the detecting circuit comprises a circuit (CT,  100 ) for detecting a differential current in the AC supply to a load (LD), the differential current having a characteristic indicative of a type of supply fault to be detected, and providing a corresponding output ( 10 ), the disconnect means comprises an electromechanical switch (RLA) controlling the load contacts (SW 1 ), the electromechanical switch being responsive to the said output ( 10 ) to disconnect the load from the supply by opening the load contacts (SW 1 ), and the test means (TS, Rt, W 2 ) simulates the supply fault by causing a differential current, having a characteristic indicative of the said type of fault, to flow in the detecting circuit in the absence of the supply fault. 
         [0007]    In the present context an electromechanical switch is an electrical switch with mechanical contacts which are operated (i.e. opened and/or closed) by a magnetic field produced by current flowing in a coil, usually a solenoid. 
         [0008]    Preferably the means for causing the load contacts (SW 1 ) to open comprises a charge storage device (C 3 ) which is connected to the supply for charging up during periods when the differential current is caused to flow in said detecting circuit by said simulating means, said certain period of time after which the load contacts (SW 1 ) are caused to open being the time taken for the voltage on the charge storage device (c 3 ) to reach a predetermined level sufficient to cause a solid state switch (SCR 2 ) to change state, the load contacts (SW 1 ) being caused to open in response to the change of state of the solid state switch (SCR 2 ). 
         [0009]    More preferably the electromechanical switch (RLA) may be of a type whose contacts (SW 1 ) are held normally-closed by a current flowing through the switch at least when the supply is at or above a certain minimum voltage, and the change of state of the solid state switch (SCR 2 ) causes the flow of current through the electromechanical switch (RLA) to be interrupted so as to open the load contacts (SW 1 ). 
         [0010]    In such a case the fault detecting device may include a further electromechanical switch (SOL) having normally-closed contacts (SW 2 ) in series with the first electromechanical switch (RLA), and wherein the further electromechanical switch (SOL) is responsive to the change of state of the solid state switch (SCR 2 ) to open the normally-closed contacts (SW 2 ) of the further electromechanical switch (SOL). 
         [0011]    Alternatively the fault detecting device may include a fuse (F 1 ) in series with the first electromechanical switch (RLA), and wherein change of state of the solid state switch (SCR 2 ) causes a current to flow through the fuse sufficient to blow the fuse. 
         [0012]    As used herein, the term “fuse” means any component intended to go open circuit or high impedance in response to a surge current, and includes not only conventional melting type fuses but also, for example, PTC devices. 
         [0013]    Preferably a further electromechanical switch (SOL 2  or PMR) is coupled to the same load contacts (SW 1 ) as the first electromechanical switch (SOL 1 ), and the change of state of the solid state switch (SCR 2 ) causes the further electromechanical switch (SOL 2  or PMR) to open the load contacts (SW 1 ). 
         [0014]    In such a case the change of state of the solid state switch (SCR 2 ) may cause a fuse (F 1 ) to blow, the load contacts (SW 1 ) being caused to open in response to the blown fuse. 
         [0015]    In certain embodiments the fault to be detected is a residual current fault and the test means includes a test switch (TS) having normally-open contacts (SW 3 ) and which when closed divert a portion of the supply current through the detecting circuit to cause a differential current to flow in said detecting circuit, closure of said test switch contacts (SW 3 ) also connecting the charge storage device (C 3 ) to the supply to allow the charge storage device to charge up. 
         [0016]    In other embodiments the fault to be detected is an arc fault and the test means includes a test switch (TS) having normally-open contacts (SW 3 ) and which when closed divert a portion of the supply current to power a test signal generating circuit ( 50 ) which causes said differential current to flow in said detecting circuit, closure of said test switch contacts (SW 3 ) also connecting the charge storage device (C 3 ) to the supply to allow the charge storage device to charge up. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES  
         [0017]    Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0018]      FIGS. 1 to 6  are circuit diagrams of first to sixth embodiments of the invention based on RCD circuits. 
           [0019]      FIG. 7  is a circuit diagram of a seventh embodiment of the invention based on an AFD circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    In the various figures of the drawings the same references have been used for the same or equivalent components. 
         [0021]    RCDs may be electrically latching (EL types) or mechanically latching (ML types), and the invention is applicable to both types. 
         [0022]      FIG. 1  shows an embodiment of the invention based upon the electrically latching (EL type) RCD circuit described in Patent Application PCT/EP2012/050911, which is incorporated herein by reference in its entirety. 
         [0023]    In  FIG. 1 , an AC mains supply comprising live and neutral conductors L, N is connected to a load LD via normally-open load contacts SW 1  controlled by an electromechanical relay RLA. The circuit is supplied with power via a bridge rectifier X 1 , and the relay is supplied with a DC current. The RCD circuit is built around an RCD integrated circuit (IC)  100 , which may be a type WA050 supplied by Western Automation Research &amp; Development and described in U.S. Pat. No. 7,068,047, which is incorporated herein by reference in its entirety. The IC  100  is supplied with current via a resistor R 2 . 
         [0024]    The relay RLA is known as an electrically latching relay because it needs a constant current flow through its coil to maintain the contacts SW 1  in the closed position. Thus, when a current of sufficient magnitude (known as the closing current) is passed through the coil the resultant magnetic flux causes the load contacts SW 1  to close. Thereafter, the load contacts will remain closed provided a minimum holding current, less than the closing current, continues to flow through the relay coil. However, should the current flowing in the relay coil fall below the holding current the load contacts SW 1  will automatically open and can then only be re-closed manually (if a manual reset, not shown, is provided) or by increasing the magnitude of the current through the relay at least to the closing current. This relay design is simple and well proven. 
         [0025]    The live and neutral conductors L, N pass through the toroidal core  20  of a current transformer CT en route to the load. The output of the current transformer, which appears across a secondary winding W 1 , is fed to the IC  100 . In the absence of a ground fault (residual) current, the vector sum of the currents flowing through the core  20  will be zero since the currents flowing in the L and N supply conductors will be equal and opposite; thus the voltage developed across W 1  will be zero. The function of the CT and IC  100  is to detect a differential current (i.e. a non-zero vector sum of currents) flowing through the CT core  20  having sufficient magnitude and/or duration as to be indicative of a residual current, and when such a differential current is detected to provide a high output voltage on line  10  sufficient to turn on a silicon controlled rectifier SCR 1 . 
         [0026]    In  FIG. 1  a resistor R 1  and a diode D 1  provide current to the relay RLA via a bridge rectifier X 1 . A capacitor C 1  smoothes the voltage across the relay RLA to prevent chatter. A Zener diode ZD 1  limits the voltage to a specified maximum level. For supply voltages at or above a minimum operating voltage of the RCD, C 1  will acquire a charge which will be at or above a voltage sufficient to provide a holding current through the relay RLA but insufficient to provide a closing current through the relay RLA. 
         [0027]    Thus, a current of sufficient magnitude will flow continuously through RLA coil to enable the contacts SW 1  to remain closed once they have closed. A capacitor C 2  will acquire a charge via a resistor R 3 , and for supply voltages at or above the minimum operating voltage of the RCD, C 2  will acquire a charge which will be at or above a voltage sufficient to provide a closing current through the relay RLA, although clamped by a Zener diode ZD 2  at a safe level. The charge on C 2  is supplied via the resistor R 3 , but this current flow is limited to a relatively low value so as to minimise power dissipation in R 3 . When a reset button MR is closed by manual means, the voltage on C 2  will be applied to the RLA coil and the momentary application of this higher voltage will cause the relay RLA to close its contacts SW 1 . The voltage applied from C 2  will quickly collapse but RLA will be held in the closed state by the holding current supplied via R 1  and C 1 . 
         [0028]    In the event of a residual current fault, as evidenced by an output on line  10  of the IC  100 , the SCR 1  will be turned on and effectively short out the coil of the relay RLA. 
         [0029]    The resultant collapse in RLA voltage will cause the load contacts SW 1  to open and remove power to the load LD. The SCR 1  will turn off and C 1  will charge up to its previous voltage again but RLA will not automatically reclose until the reset button MR is closed again. 
         [0030]    The RCD also includes a test switch comprising a manually operable test button TS which, when pressed, bridges normally-open contacts SW 3 . Pressing the test button TS diverts a portion of the supply current through a winding W 2  on the core  20 , via a resistor Rt. The current diverted through the core  20  will produce a differential current (i.e. a non-zero vector sum of currents) flowing through the CT core  20 , and the magnitude of the diverted current is selected such that the differential current so produced simulates a residual current. Accordingly, provided the RCD is operating correctly, the CT winding W 1  will produce an output which will be detected by the IC  100  which will, in turn, produce an output  10  to turn on SCR 1  and effectively short out the relay coil just as in the case of an actual residual current. Windings W 1  and W 2  may be separate windings or formed from a bifilar winding. 
         [0031]    The embodiment of  FIG. 1  further includes circuitry to disable the RCD in the event of the RCD failing to trip (i.e. the contacts SW 1  failing to open) on operation of the test button TS. Such circuitry comprises the components resistors R 4  and R 5 , capacitor C 3 , silicon controlled rectifier SCR 2  and a solenoid SOL having normally-closed contacts SW 2  in series with the relay RLA. Whereas the relay RLA is electrically latching and its contacts SW 1  are only closed when a holding current is flowing through its coil, the solenoid SOL is mechanically latched and its contacts will open when a current is passed through its coil (SOL could alternatively be a permanent magnet relay (PMR) which is held closed by a permanent magnet and opens when a current is passed through the PMR, or any convenient switching means intended to cause permanent removal of the supply from the load). 
         [0032]    When the test button TS is operated, the RCD will normally trip (i.e. the contacts SW 1  will normally open) within about 40 mS. During this period a current will flow through the closed test switch and the resistor R 4  to charge up the capacitor C 3 . If the RCD trips in response to operation of the test button within its specified time, the current flow to C 3  will cease. However, if the RCD fails to trip on operation of the test button, the capacitor C 3  will continue to charge and eventually, after a certain time longer than the normal response time of the RCD, the voltage on the capacitor C 3  will rise to a level sufficient to turn on SCR 2  via resistor R 5 . At this point capacitor C 3  will discharge through the solenoid SOL. Activation of SOL will cause its normally closed contacts SW 2  to open and remove the supply to RLA, causing its contacts SW 1  to open in turn and remove power from the protected load LD. Preferably the RCD will be disabled with this arrangement to the extent that the RCD would need to be repaired or corrected before it could be successfully tested and operate normally again. More usually, however, the RCD would simply be replaced. A bleed resistor (not shown) may be placed across C 3  to ensure its discharge after each operation of the test circuit. 
         [0033]    Instead of a silicon controlled rectifier, the solid state switch SCR 1  and/or SCR 2  may be a bipolar transistor, MOSFET or other solid state device which changes between high and low impedance states under the control of a signal applied at a control terminal. 
         [0034]      FIG. 2  shows a second embodiment of the invention which is similar to that of  FIG. 1  except that SOL and its contacts SW 2  have been replaced by a fuse F 1  in series with the relay RLA and which is suitably rated for normal operation of the RCD circuit. If the RCD fails to trip on operation of the test button TS, capacitor C 3  will charge up as before and cause SCR 2  to turn on after a certain period of time, and the resultant current flow via resistor R 6 , SCR 2  and fuse F 1  will cause the fuse to blow due to the excessive current flow through it. The relay RLA will therefore de-energise and its contacts SW 1  will open as before, and the RCD will be disabled. 
         [0035]    The arrangement of  FIG. 2  ensures end of life operation of the RCD in the event of failure of any of the key components including but not necessarily limited to X 1 , CT, Rt, WA050, TR 1 , R 2 , etc. 
         [0036]      FIG. 3  is a circuit diagram showing a simplified version of a mechanically latched (ML type) RCD circuit embodying the invention. In this case the load contacts SW 1  are normally mechanically latched closed but can be opened by a sufficient current flowing through an associated solenoid SOL 1 . As in the previous embodiments, if there is a differential current flowing through the CT core having a magnitude and/or duration characteristic of a residual current, whether produced by an actual residual current or by the test circuit on pressing the test button TS, the IC  100  will produce an output  10  which will turn on the SCR 1 . This will allow supply current to flow through a solenoid SOL 1  which will open its mechanically latched contacts SW 1  and remove power to the load LD. 
         [0037]    SOL 1  typically comprises a plunger which is biased towards a first position by a spring and which is moved to a second position during energisation of the solenoid so as to cause the load contacts to open, the plunger reverting to the first position on de-energisation of the solenoid and thereby facilitating manual reclosing of the contacts SW 1 . 
         [0038]    The embodiment of  FIG. 3  further includes circuitry to disable the RCD if the RCD contacts SW 1  fail to open when the test button TS is operated. Such circuitry comprises diode D 2 , resistors R 2  and R 3 , capacitor C 1 , silicon controlled rectifier SCR 2  and solenoid SOL 2 . The solenoid SOL 2  is coupled to the same mechanically latched load contacts SW 1  as the solenoid SOL 1 . 
         [0039]    When the test button TS is operated, the RCD contacts SW 1  will normally open within about 40 mS. During this period a current will flow through the closed test switch and D 2  and R 2  to charge up the capacitor C 1 . If the RCD trips in response to operation of the test button within its specified time, the current flow to C 1  will cease. However, if the RCD fails to trip on operation of the test button, the capacitor C 1  will continue to charge and eventually, after a certain time longer than the normal response time of the RCD, the voltage on the capacitor C 1  will rise to a level sufficient to turn on SCR 2  via resistor R 3 . This will allow supply current to flow through the solenoid SOL 2  which will open the mechanically latched contacts SW 1  and remove power to the load LD. 
         [0040]    In this case the plunger in SOL 2  may be a latching type which when moved from its first position to a second position remains in the second position so as to prevent manual reclosing of the contacts, and in this way prevent restoration of supply to the protected circuit. A bleed resistor (not shown) may be placed across C 1  to ensure its discharge after each operation of the test circuit. 
         [0041]    A permanent magnet relay (PMR) would also be suitable for this application, as shown in  FIG. 4 . In the arrangement of  FIG. 4 , a permanent magnet relay (PMR) is used instead of the solenoid SOL 2 . When the voltage on C 1  reaches a certain level SCR 2  will turn on and cause C 1  to discharge through the PMR which is turn will cause the contacts SW 1  to open. The user will not have access to the PMR and it will not be possible to reset it and to reclose the contacts. The PMR has the advantage over the solenoid arrangement of being isolated from the mains supply which reduces the risk of SCR 2  being inadvertently turned on, for example by voltage spikes on the mains supply. 
         [0042]    Provision could be made to facilitate resetting of SOL 2  or the PMR by removing the RCD from its installation and manually resetting the SOL or PMR opening means, for example by providing access from the back or the side of the RCD. However, this facility would preferably not be available to the user once the RCD has been installed and would only be intended for use by an experienced installer. If the SOL or PMR was reset, the RCD could be reinstalled, reclosed as normal and tested again by operation of the test button. Failure to trip as intended would again result in disabling of the RCD. 
         [0043]      FIG. 5  shows another embodiment of an ML RCD, which uses a fuse to disable the RCD. 
         [0044]    In the arrangement of  FIG. 5  there is an end of life circuit comprising the components solenoid SOL 2 , diode D 3 , resistor R 4 , silicon controlled rectifier SCR 3  and fuse F 1 . SCR 3  is held in a non-conducting state because its gate is tied firmly to ground by fuse F 1 . If the RCD fails to open on operation of the test button, C 1  will charge up and cause SCR 2  to turn on, as previously described. The resultant current flow through R 5 , SCR 2  and F 1  will cause fuse F 1  to blow (the term “blow” includes the case of, for example, a PTC device going high impedance). SCR 3  will be turned on by R 4  pulling its gate high, at which stage supply current will flow through SOL 2 , causing the RCD contacts SW 1  (which are connected in common to SOL 1  and SOL 2 , as before) to open. Each time the RCD is subsequently reset it will autotrip if a conventional fuse is used for F 1 . If a PTC device is used, it will revert to its original low impedance when it has cooled down, but on subsequent operation of the test button it will force the RCD to trip as previously described. 
         [0045]    The arrangement of  FIG. 5  ensures end of life operation of the RCD in the event of failure of any of the key components including but not necessarily limited to, D 1 , CT, Rt, SOL 1 , R 1 , IC, SCR 1 , etc. 
         [0046]    An LED with a current limiting resistor may advantageously be placed in parallel with F 1  or SOL 2 . This LED will light up momentarily when SCR 3  turns on, thus providing indication of an end of life operation. 
         [0047]    The solenoids SOL 1  and SOL 2  in  FIG. 5  could be combined such that the combination comprises of a single solenoid device with two windings, SOL 1  and SOL 2 . Winding SOL 2  would only be operated by SCR 3  in the event of failure of the RCD to open on operation of the test button. Likewise with  FIG. 4 , a single PMR could be used instead of a solenoid and a PMR. 
         [0048]      FIG. 6  shows an alternative version of  FIG. 5 . In the arrangement of  FIG. 6 , a single solenoid SOL 1  is used. This can be energised under normal conditions by SCR 1 , or under end of life conditions by SCR 3 , as previously described. 
         [0049]      FIG. 7  shows an embodiment of the invention based upon the arc fault detector (AFD) circuit described with respect to  FIG. 8  of Patent Application PCT/EP2011/058754, which is incorporated herein by reference in its entirety. 
         [0050]    A single phase AC mains supply to a load LD comprises live L and neutral N conductors. In the absence of an arc fault condition, the full load current will flow in the conductors. A series or parallel arc fault condition will result in an arcing current flow in the circuit with a broad spectrum of frequencies. 
         [0051]    In this embodiment, for the detection of arc faults a current transformer CT has a core  20  which surrounds just one of the supply conductors, in this case the neutral conductor N. The design of the CT is such that it has minimal response to slowly rising or sustained load currents at the mains supply frequency but is highly responsive to current pulses with very fast rise times which would be generated by arcing. 
         [0052]    Arc fault current pulses in the neutral conductor N induce voltage spikes across the secondary winding W 1 . When these are above a certain threshold the IC  100 , here configured as an arc detector, will produce an output to an actuator  40 . In response, the actuator  40  opens associated load contacts SW 1  to disconnect the mains supply from the load LD. The details of the actuator  40  are not shown, but the actuator  40  may include a relay, solenoid or PMR, together with associated circuitry, as described for previous embodiments. In the present case it will be assumed that the actuator  40  includes a solenoid SOL 1  arranged as shown and described with reference to  FIG. 6 . 
         [0053]    When a test button TS is operated a test signal generator  50  is powered up from the mains supply and produces a series of pulses which will flow through a further winding W 2  on the CT core  20 . These pulses are designed to produce a differential current in the CT having characteristics which simulate the differential current produced by an actual arc fault (actually, due to the neutral conductor N passing though the core  20 , there will always be a non-zero vector sum of currents flowing through the CT, but the characteristics of the detection circuitry are designed not to respond to it). These pulses will be detected by the winding W 1  and fed to the IC  100  and cause the actuator  40  to open the load contacts SW 1  as before, typically within about 50 mS. The contacts can be reclosed by manual operation. 
         [0054]      FIG. 7  also includes an “end of life” circuit comprising diode D 2 , resistors R 2 , R 3 , R 4  and R 5 , capacitor C 3 , silicon controlled rectifiers SCR 2  and SCR 3 , and fuse F 1 . These operate in the same way as the like referenced components in the end of life circuit of  FIG. 5 . 
         [0055]    When the test button TS is pressed to bridge the contacts SW 3 , capacitor C 1  will start to charge up via R 2  and D 2 . However, the AFD would normally trip (i.e. load contacts SW 1  open) within a certain time, e.g. 50 mS, in which case power would be removed from the circuit and all activity would cease. However in the event of the AFD failing to trip within the allotted time, C 1  would continue to charge up, and after a certain period, e.g. 200 mS, the voltage on C 1  would rise to a value sufficient to turn on SCR 2  via R 3 . Once SCR 2  turns on a relatively large current will flow via R 5  through SCR 2  and fuse F 1 . This current will be well in excess of the rating of the fuse F 1  so as to cause it to blow. When F 1  blows SCR 3  gate will be pulled high by R 4  which will cause SCR 3  to turn on and activate the solenoid SOL 1  in the actuator  40 . On each subsequent occasion when the AFD load contacts SW 1  are manually closed, the device will automatically trip because of SCR 3  being turned on and it will no longer be possible to use the AFD, thus indicating that it has reached the end of life state. 
         [0056]    The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.