Abstract:
An electromagnetic switch comprises at least one pair of magnetically latchable electrical contacts ( 12   a,    14   a ) operated by current flowing in an associated coil means (K 1,  K 2 ), and an electrical circuit arranged to apply a first current in a first direction through the coil means to close the contacts and subsequently to apply a second current in a second, opposite, direction through the coil means to open the contacts. In certain embodiments the coil means comprises first and second independent coils (K 1 , K 2 ) and the first and second currents flow in opposite direction in the first and second coils respectively. In other embodiments the coil means comprises a single coil (K 1 ) and the first and second currents flow in opposite directions in the single coil.

Description:
[0001]    This application is a 35 USC 371 national phase filing of International Application PCT/EP2013/052111, filed Feb. 4, 2013, which claims priority to Irish national application S2012/0150 filed Mar. 23, 2012, Irish national application S2012/0173 filed Apr. 4, 2012, and Irish national application S2012/0192 filed Apr. 17, 2012, the disclosures of which are incorporated herein by reference in their entireties. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    This invention relates to an electromagnetic switch for use with electrical equipment but which may advantageously also be used in an RCD (residual current device) socket outlet, known in the USA as a ground fault circuit interrupt (GFCI) receptacle. The terms RCD and GFCI are used interchangeably herein. 
       BACKGROUND 
       [0003]    In the present specification an electromagnetic switch is an electrical switch with mechanical contacts which are operated by a magnetic field produced by current flowing in a coil, usually a solenoid.  FIG. 1  is a diagram of a resettable electromagnetic (EM) switch of the general kind described with reference to FIG. 7 of European Patent No. 1490884 and U.S. Pat. No. 6,975,191. 
         [0004]    Switches used in RCDs can generally be divided into two types, EL types and ML types. EL types are types which require the continuous supply of electrical current through a coil to enable the contacts to be closed and remain closed, and whose contacts open automatically when the coil current falls below a certain level. In that regard they are also responsive to supply voltage conditions. ML types are types which can generally be closed and will remain closed with or without the presence of a supply current. 
         [0005]    In  FIG. 1  the electromagnetic switch has a pair of fixed contacts  12   a,    12   b  and a pair of movable contacts  14   a,    14   b  mounted on a movable contact carrier (MCC)  16  and opposing the fixed contacts  12   a,    12   b  respectively. An opening spring  18  biases the MCC  16  and moveable contacts  14   a,    14   b  upwardly (as seen in  FIG. 1 ) away from the fixed contacts  12   a,    12   b  into a first rest position. A permanent magnet  22  is retained within the MCC  16 . A fixed bobbin  24  has a solenoid coil K 1  wound on it and a ferromagnetic plunger  26  extends through the bobbin. A reset button  28  is fitted to the accessible lower end of the plunger  26 . The plunger and reset button are biased downwardly towards a first rest position by a reset spring  32 . Both the MCC  16  and plunger  26  are shown in their first positions in  FIG. 1 . 
         [0006]    When the reset button  28  is pushed upwards by manual force against the bias of the spring  32  the gap between the plunger  26  and the magnet  22  will be sufficiently reduced so as to allow the plunger to entrain the magnet. When the reset button is released the magnet  22  and MCC  16  will be drawn downwards from their first position by the greater force of the reset spring  32  in opposition to the force of the opening spring  18  until the moving contacts  14   a,    14   b  come to rest on the fixed contacts  12   a,    12   b  respectively and thereby make the electrical connections to power up a load, not shown. Here the contacts  12   a,    14   a  are assumed to be located in the live supply conductor to the load and the contacts  12   b,    14   b  are assumed to be located in the neutral supply conductor to the load. 
         [0007]    When a current above a certain release threshold is passed through the coil in a particular direction, the coil K 1  will produce an electromagnetic flux which will oppose the flux of the permanent magnet  22  and weaken it to such an extent that, provided the current persists for at least some minimum duration, the magnet  22  will release (detrain) the plunger  26  and the MCC  16  and the plunger  26  will each move back to their first positions by the action of the opening and reset springs respectively and thereby cause the contact pairs  12   a,    14   a  and  12   b,    14   b  to open. By this means the resettable EM switch can be used to connect and disconnect loads in a circuit. The switch of  FIG. 1  can be referred to as an ML type because it is held closed (latched) magnetically and does not depend on the mains supply to remain closed. It will be understood that although the contacts  12   a,    12   b  are referred to as fixed, this does not rule out their being spring mounted so that they can “give” resiliently upon engagement by the movable contacts  14   a ,  14   b.  The essential point is that they play a passive role in the operation of the device, and the term “fixed” is to be interpreted accordingly. It is also preferred that there be some degree of over-travel remaining in the reset spring  32  to ensure adequate contact pressure and to compensate for contact wear or a reduction in height of either set of contacts so that adequate contact pressure is maintained after a reasonable amount of wear and use. 
         [0008]      FIG. 2  is a circuit diagram of a basic RCD (GFCI) circuit incorporating an electromagnetic switch of the kind shown in  FIG. 1 . In  FIG. 2  only the coil K 1  is explicitly shown, and the contacts  12   a,    14   a  and  12   b,    14   b  are shown collectively as the load contacts SW 1 . This type of circuit will be familiar to those versed in the art, but more detailed information can be found on such devices at www.westernautomation.com. 
         [0009]    Initially, the load contacts SW 1 , i.e. the contact pairs  12   a,    14   a  and  12   b,    14   b,  are manually closed by pressing and releasing the rest button  28  as previously described. For reasons which will be explained, it is significant that the latching of the load contacts SW 1  does not depend on the application of mains power to the live and neutral supply conductors L, N. The supply conductors L, N pass through the toroidal core  20  of a current transformer CT en route to a load LD and form the primary windings of the CT (the term “winding” is used in accordance with conventional terminology even though the conductors pass directly through the core rather than being wound on it). The output of the current transformer, which appears across a secondary winding W 1 , is fed to 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. The IC  100  is supplied with power via a diode D 1  and resistor R 3 . 
         [0010]    In the absence of a residual (ground fault) 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 a magnitude above a predetermined threshold, such threshold corresponding to a particular level of residual current to be detected according to the desired sensitivity of the RCD. When such a differential current is detected the IC  100  provides a high output voltage on line  10  indicating that a residual current fault has been detected, such voltage being sufficient to turn on a normally-off silicon controlled rectifier SCR 1  of an actuator circuit  200  indicated by the dashed rectangle in  FIG. 2 . 
         [0011]    The actuator circuit  200  includes SCR 1 , the coil K 1 , the diode D 1 , a resistor R 1  and a capacitor C 1 , and is powered via the diode D 1  and the resistor R 1 . The capacitor C 1  will charge up when the RCD circuit is first powered up, and if subsequently a differential current flows through the CT core having a magnitude above a predetermined threshold, the IC  100  will produce an output on line  10  which will turn on SCR 1 . This will allow C 1  to discharge and cause a current having a magnitude above the release threshold to flow through the solenoid K 1  in a direction to detrain the plunger ( 26 ) from the permanent magnet ( 22 ) and open the previously latched load contacts of SW 1  and remove power from the load LD. 
         [0012]    A key advantage of the ML arrangement of  FIG. 2  is that the RCD circuit requires minimal electrical energy for its protective function. As well as mitigating potential temperature rise problems, this can save a considerable amount of energy over the life of the product. However the ML RCD circuit of  FIG. 2  suffers a drawback in that in the event of loss of supply neutral the contacts will remain closed but the RCD will be disabled under this condition since both the RCD IC  100  and the actuator circuit  200  are powered from the supply conductors. Thus the user will have no shock protection in the event of touching an exposed live part. 
         [0013]      FIG. 2   a  shows a simple example of an RCD circuit based on an EL type electromagnetic switch operating according to the principles described with reference to FIG. 1 of Irish Patent Application No. S2011/0554 (Attorney Ref: P102912IE01 (ELM SW (WA/60))). Under normal conditions, current will flow from live L to neutral N via diode D 1 , resistor R 1 , solenoid coil K and resistor R 2 . This current will be insufficient to cause the load contacts SW 1  to close automatically. SW 2  is a manually operated switch biased to the normally open position. When SW 2  is closed, resistor R 2  will be shorted out and the current flow through coil K will be increased to a level sufficient to cause automatic closing of load contacts SW 1 . When SW 2  is released and opened, the current through coil K will fall to its original value which will be sufficient to keep the switch energised and load contacts SW 1  closed. In the event of a residual current fault SCR 1  will be turned on and coil K will be shorted out, causing the load contacts SW 1  to open. The load contacts SW 1  will also open automatically in the event of loss of either supply conductor or reduction of the mains supply below a certain level. 
         [0014]    The EL type RCD circuit depicted in  FIG. 2   a  offers the advantage of protection in the event of a reduction in the mains supply or loss of supply neutral, but has the drawback that the contacts will remain open until manually reset as described even if the neutral and the supply is restored. These drawbacks make this device unsuitable for RCDs used in the fixed installation, e.g., SRCDs. An additional drawback is that the relay based circuit of  FIG. 2   a  continuously consumes a relatively high level of current to maintain the contacts in the closed position, possibly up to twenty times the current consumption of the ML based circuit of  FIG. 2 . This can contribute to potential temperature rise problems and to relatively high energy consumption over the life of the product. 
         [0015]    The switch shown in  FIG. 1  has been used successfully in RCD circuits such as that shown in  FIG. 2  for many years, for example in RCD socket outlets. However, in recent years a problem has come to light in the USA regarding the mis-wiring of RCD (GFCI) socket outlets. This problem arose largely because such socket outlets often have a facility for “feed-through”, to supply downstream socket outlets. 
         [0016]      FIG. 3  shows a typical USA style GFCI receptacle. The socket outlet comprises an insulating housing  40  having AC supply input terminals E, N and L and AC supply feed-through (output) terminals E′, N and L′. The input and output terminals are connected by electrical supply conductors  42  within the housing  40 . The housing also contains a conventional socket outlet  44  for a three-pin plug which is connected to the L and N supply conductors. 
         [0017]    The housing  40  also includes an RCD circuit including a CT having a core  20  surrounding the live L and neutral N conductors, a secondary winding W 1  and an IC  100  providing an output  10  on detection of a residual current fault, as described previously. The RCD circuit also includes an actuator circuit  200 , constructed as described for  FIG. 2 . As stated, the actuator circuit  200  is responsive to an output signal  10  from the IC  100  to open the load contacts SW 1  and remove power from the socket outlet  44  and feed-through terminals E′, L′ and N′. 
         [0018]    The mains supply is connected to the input terminals E, N and L which, when the load contacts SW 1  are closed, will feed the integrated socket outlet  44  and also feed downstream socket outlets (not shown) connected to the feed-through terminals E′, L′ and N′. When correctly wired as shown, the RCD will provide shock protection to the local socket outlet  44  and the downstream socket outlets. 
         [0019]      FIG. 4  shows how shock protection is compromised if the RCD socket outlet is mis-wired. 
         [0020]      FIG. 4  shows the RCD socket outlet of  FIG. 3  where the AC supply has been inadvertently connected to the feed-through terminals E′, N′ and L′, and the downstream sockets have been connected to the supply input terminals E, L and N. In this case, when the load contacts SW 1  are closed, all parts of the circuit will have power, and if a test button (not shown but conventionally included in such devices) is operated, the RCD circuit will trip and open the contacts SW 1 . Thus the installer will feel that the overall circuit is protected. However, it can be seen that although the downstream sockets will have power removed when the contacts SW 1  open, the internal socket  44  will not have power removed since it is located upstream of the contacts SW 1 , and a shock risk will remain on that socket outlet regardless of the state of the contacts SW 1 . 
         [0021]    UL recently introduced a new requirement for GFCI manufacturers to provide means to prevent the operation of a GFCI receptacle in the event of such mis-wiring. This problem does not apply to EL type GFCIs because they can only operate when supplied correctly. However ML types generally need to have special provision made to comply with this new requirement. Manufacturers have adopted various means to address this problem, for example the use of a separate solenoid operated switch which can only be closed when the GFCI is correctly wired, etc. In most cases the GFCI is supplied with the contacts open, and the contacts can only be closed by overriding of a lock-out means when the mains supply is connected to the supply terminals. If the mains supply is connected to the feed-through terminals with the contacts open, power will not be provided to enable deactivation of the lock-out means. 
         [0022]    As far as we are aware all of the solutions used to date with ML type devices involve the use of an additional mechanical or electromechanical means to achieve the lockout function or prevent mis-wiring. Such additional means add considerably to cost, complexity and reduced overall reliability. 
       SUMMARY 
       [0023]    It is an object of the invention to provide an improved electromagnetic switch which can be used in RCD socket outlets to address the problem of mis-wiring, but also has wider applications in electrical equipment safety. 
         [0024]    According to one aspect the present invention provides an electromagnetic switch comprising at least one pair of magnetically latchable electrical contacts ( 12   a,    14   a ) operated by current flowing in an associated coil means (K 1 , K 2 ), and an electrical circuit arranged to apply a first current in a first direction through the coil means to close the contacts and subsequently to apply a second current in a second, opposite, direction through the coil means to open the contacts. 
         [0025]    According to another aspect the present invention provides a mains socket outlet comprising a housing having a mains supply input and feed-through terminals connected by electrical supply conductors within the housing, a socket outlet connected to the supply conductors, a fault detecting circuit arranged to detect a fault in the supply conductors and to provide a corresponding output signal, and an actuator circuit including a set of load contacts in the supply conductors, the actuator circuit being responsive to a said output signal to open the load contacts and remove power from the socket outlet and feed-through terminals, wherein the actuator circuit is connected to and powered by the supply conductors and requires power from the conductors to enable closure of the load contacts, and wherein the connection of the actuator circuit to the supply conductors is made upstream of the load contacts. 
         [0026]    “Upstream” refers to the direction within the housing from the feed-through terminals to the AC supply input terminals and “downstream” refers to the direction within the housing from the AC supply input terminals to the feed-through terminals. 
         [0027]    “Load contacts” are so-called because according to their state they allow or cut off current flow to an external downstream load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0029]      FIG. 1  is a schematic diagram of a prior art electromagnetic (EM) switch. 
           [0030]      FIG. 2  is a circuit diagram of a prior art RCD circuit which may incorporate the EM switch of  FIG. 1 . 
           [0031]      FIG. 2   a  is an example of an RCD circuit based on an EL type electromagnetic switch. 
           [0032]      FIG. 3  is a diagram of a prior art RCD (GFCI) socket outlet. 
           [0033]      FIG. 4  shows the effect of mis-wiring the socket outlet of  FIG. 3 . 
           [0034]      FIG. 5  is a circuit diagram of an RCD circuit which may be used in the socket outlet of  FIG. 3  in a first embodiment of the invention. 
           [0035]      FIG. 6  is a schematic diagram of an electromagnetic (EM) switch used in embodiments of the invention. 
           [0036]      FIGS. 7 to 12  are further circuit diagrams of RCD circuits which may be used in further embodiments of the invention. 
           [0037]      FIG. 13  is a circuit diagram of an over- or under-voltage protection circuit which is a further embodiment of the invention. 
           [0038]      FIG. 14  is a schematic diagram of an alternative EM switch which may be used in embodiments of the invention. 
           [0039]      FIGS. 15 and 16  are schematic diagrams of improvements to the EM switch of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0040]      FIG. 5  is a diagram of an RCD circuit which can be used in the RCD socket outlet of  FIG. 3  to mitigate the problem of mis-wiring.  FIG. 5  is essentially the same as  FIG. 2  but with the addition of the following components to the actuator circuit  200 : resistors R 2  and R 4 , capacitors C 2  and C 3 , manually operable reset switch SW 2 , silicon controlled rectifier SCR 2  and a second coil K 2  on the bobbin  24 . In  FIGS. 5 and 7  to  12 , as well as in  FIG. 2 , when that circuit is incorporated in an RCD socket outlet, the load LD represents a load connected to the socket  44  or to a downstream socket connected to the terminals E′, N′ and L′ of  FIG. 3 . 
         [0041]      FIG. 6  is a diagram of the EM switch used in the actuator circuit  200  of  FIG. 5 . In  FIG. 6 , the reset button  28  of  FIG. 1  has been replaced by a retainer, for example a circlip type washer  29 , to retain the reset spring  32 . The bobbin  24  now has two independent coils, K 1  and K 2  surrounding the plunger  26 . A current passed through coil K 2  in a certain direction will generate an electromagnetic force that will increase the existing attraction between the plunger  26  and the permanent magnet  22 , and if the current through K 2  exceeds a certain threshold the magnet  22  and plunger  26  will be sufficiently drawn towards one another (the movement being primarily that of the plunger moving upwards in the bobbin) as to cause the plunger  26  to engage  the MCC  16 . In this embodiment the plunger has been reconfigured by provision of a narrow shoulder at its top end to concentrate the electromagnetic flux in the plunger to attract it into the bobbin and towards the magnet when a current is passed through coil K 2 . As in the case of  FIG. 1 , the fixed contacts  12   a,    12   b  are preferably spring mounted so that they can yield resiliently upon engagement by the movable contacts  14   a,    14   b,  and it is also preferred that there be a degree of over-travel remaining in the reset spring  32  for the reasons previously stated. 
         [0042]    K 1  represents the original coil as shown in  FIG. 1 . In the arrangement of  FIG. 5 , when the circuit is first powered up, i.e. first connected to the AC supply, C 1  charges up via R 1  and C 2  charges up via R 2 . SW 2  is a manually operable reset switch, and when this is closed a short voltage pulse will be applied to the gate of SCR 2  and cause SCR 2  to turn on and discharge C 2  through coil K 2  to cause a momentary surge of current through K 2 . An additional current will flow through K 2  via R 2 . Coil K 2  is arranged on the bobbin such that when SCR 2  turns on, the currents flowing through K 2  will generate an electromagnetic flux that will add to rather than oppose the flux of the permanent magnet  22 . The total current flowing in K 2  during the brief period when SCR 2  is turned on is sufficient to draw the plunger  26  upwardly into engagement with the MCC  16  against the bias of the spring  32  so that the plunger  26  and the magnet  22  become entrained. After the current burst stops flowing in K 2  the reset spring  32  will draw the entrained MCC  16  and plunger  26  downwards until the two sets of contacts SW 1  (i.e.  12   a,    14   a  and  12   b,    14   b ) are closed. SCR 2  will turn off at the following negative half cycle of the mains supply so the current flow through K 2  will be negligible so that even if SW 2  is held closed, automatic opening of the contacts will not be impeded by any position of SW 2 , thus ensuring trip free operation of the GCFI. When SCR 1  is turned on by an output signal  10  from the RCD IC  100  current will be drawn through K 1  to cause automatic opening of the contacts SW 1 , as before. 
         [0043]    An advantage of the modified plunger arrangement of  FIG. 6  is that by electrically drawing the plunger upwardly, entrainment can be achieved with minimal current or energy. However entrainment could also be achieved by a plunger of the kind shown in  FIG. 1  provided a sufficiently large amount of current was used to draw the magnet to entrain with the plunger. In another embodiment of the EM switch, not shown, the plunger  28  is fixed relative to the coils K 1 , K 2  and the amplitude of the current flowing in K 2  when SCR 2  turns on is sufficiently high to enable the plunger  28  to attract the magnet  22  and close the contacts  12   a,    14   a  and  12   b,    14   b  without initial movement of the plunger towards the contact carrier  16 . 
         [0044]    The RCD socket outlet shown in  FIG. 3 , incorporating the RCD circuit of  FIG. 5  rather than that of  FIG. 2 , is supplied to the user with the load contacts SW 1  open. Since the actuator circuit  200  is connected to and powered by the supply conductors L, N and requires power from those conductors to enable closure of the load contacts SW 1 , and because the connection of the actuator circuit  200  to the supply conductors is made upstream of the load contacts SW 1 , any mis-wiring of the RCD socket outlet as shown in  FIG. 4  will prevent mains power being applied to the RCD circuit and not allow the load contacts to be closed. This is because direct manual closure is not available, the EM switch and in particular the lower end of the plunger  26  not being accessible externally of the housing  40 . 
         [0045]    The arrangement of  FIG. 5  shows how the resettable EM switch can be closed by the user without the need for direct manual closure of the resettable EM switch. However, the EM switch does require indirect manual closure by pressing the reset switch SW 2 . If required, fully automatic closing can be achieved by the RCD circuit shown in  FIG. 7 . 
         [0046]    The circuit of  FIG. 7  uses the EM switch shown in  FIG. 6 . When the socket outlet ( FIG. 3 ) is first powered up, capacitor C 2  will charge up at a predetermined rate as determined by its value and that of R 2 . When C 2  reaches a certain voltage level SCR 2  will turn on and this will cause C 2  to discharge through K 2  and the current through R 2  will now also flow through K 2 . The resultant current burst is of sufficient magnitude and direction as to cause the plunger  26  to move towards the magnet  22  and ensure entrainment of the MCC  16  and automatic closing of the contacts  12   a,    14   a  and  12   b,    14   b  when the plunger  26  reverts to its original position under the action of the spring  32 . SCR 2  will turn off at the next negative going half cycle of the supply and may turn on during a subsequent positive half cycle but this will have no effect on the now fully closed switch. In the event of a residual current fault SCR 1  will be turned on by an output  10  from the IC  100 , and this will discharge C 1  through K 1 , the magnitude and direction of the current flow through K 1  causing automatic opening of SW 1  contacts as previously described. 
         [0047]    In the circuit of  FIG. 7 , SW 2  is arranged as a normally closed switch, and manual opening of SW 2  will remove power from the circuit and ensure that SCR 1  and SCR 2  turn off. When SW 2  is reclosed, SW 1  contacts will automatically reclose as previously described. 
         [0048]    The RCD circuit of  FIG. 8  shows how a single coil can be used to achieve the closing and opening functions of the load contacts SW 1  using the EM switch of  FIG. 6  but with coil K 2  omitted.  FIG. 8  uses a bridge rectifier X 1  as power supply for the RCD IC  100  and the actuator circuit  200  but a single diode may be used instead of the bridge rectifier. Conversely, a bridge rectifier could be used in the embodiments which use a diode. 
         [0049]    On power up from the AC supply an initial current will flow from the supply via R 1  to charge up C 1 . Components C 2  and R 5  form a pulse generating circuit. When SW 2  is manually closed the pulse generating circuit will feed a single pulse to the gate of SCR 2 , causing SCR 2  to turn on. This will draw a current I 1  through X 1 , R 2  and K 1  for up to one half cycle of the mains supply. This current burst I 1  will be in a first direction as shown by the solid arrows and of sufficient magnitude as to cause the plunger  26  to move towards the magnet  22  and ensure entrainment of the MCC  16  and automatic closing of the contacts  12   a,    14   a  and  12   b,    14   b  when the plunger  26  reverts to its original position under the action of the spring  32 . SCR 2  will turn off at the following zero-crossover of the mains supply. C 1  will then recharge via R 1 . In the event of a residual current fault an output  10  from RCD IC  100  will turn on SCR 1  and cause C 1  to discharge via D 1  through K 1  with a second current I 2 , but this time the current I 2  will be in the opposite direction to the current I 1  as shown by the dashed arrows and will weaken the magnetic holding flux between the permanent magnet  22  and plunger  26  and cause the MCC  16  to be released with consequent automatic opening of load contacts SW 1 . Opening and reclosing of SW 2  will enable reclosing of contacts SW 1 . 
         [0050]      FIG. 9  shows an alternative arrangement for automatic closing and opening of the EM switch, again using the EM switch of  FIG. 6  but with coil K 2  omitted. 
         [0051]    On initial power up from the AC supply, capacitor C 1  charges up via X 1  and R 1 . The output of a comparator U 1  is initially low, but when the voltage on C 1  exceeds a certain threshold, U 1  output goes high. The positive going transition produces a positive going pulse which is applied to the gate of SCR 2  via C 4 . SCR 2  turns on and draws a current I 1  through R 2  and K 1 , as indicated by the solid arrows. This current burst I 1  will be in a first direction as shown by the solid arrows and of sufficient magnitude as to cause the plunger  26  to move towards the magnet  22  and ensure entrainment of the MCC  16  and automatic closing of the contacts  12   a,    14   a  and  12   b,    14   b  when the plunger  26  reverts to its original position under the action of the spring  32 . Capacitor C 3  acquires a charge via R 1  and R 4 . In the event of a residual current fault, SCR 1  will be turned on by an output  10  from the RCD IC  100  and will cause C 3  to discharge via D 1 , K 1  and SCR 1  with a second current I 2 , but this time the current I 2  will be in the opposite direction to the current I 1  as shown by the dashed arrows and will weaken the magnetic holding flux between the permanent magnet  22  and plunger  26  and cause the MCC  16  to be released with consequent automatic opening of load contacts SW 1 . 
         [0052]      FIG. 10  shows an alternative arrangement for using a single coil to automatically latch and delatch (close and open) the load contacts SW 1  (in  FIGS. 10 and 11  the power supply to the RCD IC  100  is not shown). The load contacts SW 1  are initially open, and when the circuit is correctly connected as shown and AC power is applied, capacitor C 3  will acquire a charge via R 5  and D 4 , with ZD 2  clamping the voltage on C 3 . When SW 2  is manually closed, a voltage pulse will pass through C 2 , D 5 , R 2  and D 6  to the gate of SCR 1  to turn SCR 1  on. When SCR 1  turns on a current I 1  will flow from neutral N to live L via D 3 , K 1  and SCR 1  when neutral is positive with respect to supply Live. Capacitor C 4  in combination with D 5  acquires and holds a charge from the initial pulse and discharges slowly into SCR 1  gate to ensure that SCR 1  will turn on immediately or on the occurrence of the next positive going half cycle. The resultant current flow through K 1  will be in a direction which will attract the plunger and the permanent magnet in the MCC towards each other so as to reduce this gap sufficiently to cause the open contacts SW 1  to close. Although SCR 1  will turn off at the next negative going half cycle, the contacts will be held closed by the permanent magnet as previously explained. 
         [0053]    In the event of a residual current fault, the RCD IC  100  will produce an output to turn on SCR 2  which in turn will cause C 1  to cause a current I 2  to flow through coil K 1 . This current will produce a magnetic flux in opposition to that of the magnet and weaken the hold of the magnet on the plunger so as to cause its release, resulting in automatic opening of the main contacts. 
         [0054]      FIG. 11  shows an arrangement for automatically closing and opening the EM switch contacts SW 1  in response to predetermined supply conditions. 
         [0055]      FIG. 11  shows an arrangement with a window comparator comprising U 1  and U 2 , with Rx/Zx providing a reference voltage on U 1  −ve input and Ry/Zy/Cy providing a lower reference voltage on U 2  +ve input. When the supply is applied C 1  will charge up at a certain rate but the voltage on U 1  −ve input will be established almost immediately and thus hold U 1  output low initially. Cy will charge up at a slower rate than C 1  with the result that U 2  output will also be initially low. When the voltage on C 1  exceeds a certain level, U 1  output will go high which will cause SCR 2  to turn on. This in turn will draw a current through K 2  and cause the load contacts SW 1  to close as previously described. When the mains supply is removed, the voltage on C 1  will fall gradually and when it falls below the voltage level on Cy, U 2  output will go high turning on SCR 1 . When SCR 1  turns on the resultant current through coil K 1  will cause the main contacts to open as previously described. K 1  can be activated by the discharge of C 1  via D 4  or by a current drawn through R 4  or a combination of the two currents. In either case it follows that the load contacts SW 1  will open automatically in the event of loss of supply live, supply neutral, or a reduction in the supply voltage below a certain predetermined level. 
         [0056]    Under normal supply conditions, if a residual fault current occurs, the output  10  of RCD IC  100  will go high and turn on SCR 1  and thereby cause the contacts SW 1  to open. Subsequent to this event, the contacts SW 1  can be manually reclosed by operation of SW 2 . When SW 2  is closed a positive pulse will be applied to SCR 2  and turn it on and cause the main contacts to reclose. 
         [0057]    As can be seen from the foregoing, the EM switch of  FIG. 6  can be used to achieve automatic closing of a set of contacts SW 1  when a supply voltage of a predetermined level is applied on the supply side of the contacts, and prevent closing when the supply is connected on the load side of the contacts SW 1  provided the switch is contained in a housing which does not allow direct manual closure of the contacts from outside the housing. 
         [0058]    The EM switch of  FIG. 6  can be used to achieve automatic opening of the contacts SW 1  in the event of only one supply conductor being connected, thereby providing protection in the event of loss of supply live or supply neutral conductors. 
         [0059]    The EM switch of  FIG. 6  can be used to achieve automatic opening of the contacts SW 1  in the event of the supply voltage falling below a predetermined level, e.g. in the event of a brown out. 
         [0060]    The EM switch of  FIG. 6  can be used to achieve automatic opening in the event of loss of supply live or neutral or reduction of the supply voltage below a certain predetermined level and automatic reclosing in the event of restoration of the supply live, neutral or the supply voltage above a predetermined level. 
         [0061]    In all embodiments the RCD socket outlet with feed-through terminals and an actuator circuit  200  as shown in any of  FIGS. 5 and 7  to  11  will usually be supplied to the user with the load contacts in the open state and with no provision for direct manual closure of the load contacts externally of the housing. If, therefore, the installer connects the mains supply to the feed-through terminals instead of to the supply terminals it will not be possible to close the contacts, so the problem of mis-wiring is mitigated. 
         [0062]    Furthermore, with the embodiment of  FIG. 11  the RCD will open automatically in the event of loss of supply neutral and reclose automatically on restoration of the supply neutral, so the problem of loss of supply neutral with regard to an ML type RCD is also mitigated. 
         [0063]    The EM switch of  FIG. 6  does not require any current flow through its coil to retain the contacts in the closed state, and thereby provides the benefits of an EL type switch whilst also providing the benefits of an ML switch whilst mitigating the drawbacks of EL and ML type switches. 
         [0064]    Refinements may be made to the circuit without departing materially from the scope of the invention. For example, the EM switch of  FIG. 6  could be used on AC or DC systems, in electric vehicles or solar panels, etc. For example,  FIG. 12  shows a DC application of the EM switch of  FIG. 6 . 
         [0065]      FIG. 12  is based on a DC supply, but operates largely on the same principle as  FIG. 11  for an AC system. When the DC supply voltage reaches a predetermined level, U 1  output goes high and a positive going pulse is applied to transistor TR 1  to turn it on momentarily. The resultant current through coil K 2  will cause the main contacts to close, as previously described. In the event of a reduction in the supply voltage below a certain level, due for example to a broken supply conductor, U 2  output will go high and cause a positive going pulse to be fed to the gate of SCR 1  via D 2  and C 3  and turn SCR 1  on, resulting in automatic opening of the load contacts SW 1 . Restoration of the DC supply will result in automatic reclosing of the load contacts. In the event of a residual current fault, the DC residual current detecting circuit  110  will go high and a positive going pulse  10  will be fed to the gate of SCR 1  via D 3  and C 3 . The circuit 110 operates according to the principles described in Patent Application PCT/EP2011/066450 (Attorney Ref: P98463pc00 (Ydo (WA/49)). The pulse  10  will cause SCR 1  to turn on and discharge capacitor C 1  through coil K 1 , resulting in automatic opening of the load contacts. SCR 1  will remain turned on as long as the supply is present. SW 2  is a normally closed switch, and manual opening of this switch will remove the supply and force SCR 1  to turn off. Reclosing of SW 2  will restore the supply and cause the main contacts to automatically reclose. 
         [0066]      FIG. 13  shows an arrangement for detection of undervoltage and overvoltage conditions. 
         [0067]      FIG. 13  is similar to  FIG. 11  except that an overvoltage detection circuit has been added. Comparator U 3  output has a reference on its −ve input which is higher than the voltage on its +ve input derived from the mains supply via potential divider R 6  and R 7  under normal supply conditions. Capacitor C 5  provides smoothing and a certain time delay before the voltage on U 3  +ve input can go high. Thus, under normal supply conditions U 3  output will remain low, and the main contacts can close automatically and open under low supply conditions and open under a residual fault condition as before. However, in the event of an abnormally high supply voltage, which could happen if the circuit was connected to a 240V supply when intended for operation on a 110V supply, the voltage at U 3  output will go high and the resultant positive output will cause SCR 1  to turn on and open the contacts, thus providing protection against a sustained overvoltage condition. 
         [0068]      FIG. 14  shows an alternative and more efficient embodiment of the EM switch which can be used in the various circuits described herein in the place of the EM switch of  FIG. 6 . In  FIG. 14  the same references have been used for components equivalent to those of  FIG. 6 . 
         [0069]    The switch comprises a bobbin  50  which is fitted to a ferromagnetic pole piece  52  fixedly mounted on a ferromagnetic frame  53 . The frame and pole piece could also be formed from a single piece of ferromagnetic material. A solenoid coil K 1  is wound on the bobbin  50 , surrounding the pole piece  52 . A pivoting ferromagnetic armature  54  is fitted to the top of the frame  53  and is biased into a first, open position (as shown in  FIG. 14 ) by a spring  56 . The free (left hand) end of the armature  54  cooperates with a movable contact  14   a  which is independently mounted on a spring carrier  58 . A “fixed” contact  12   a  opposes the movable contact  14   a.    
         [0070]    A permanent magnet  22  is located on the frame  53  and induces a flux into the pole piece  52 , frame  53  and armature  54  but due to the gap between the armature and pole piece this flux is not strong enough to draw the armature  54  towards the top of the pole piece  52 . When a first current I 1  of a certain magnitude is passed in a certain direction through the coil K 1 , the free end of the armature  54  is drawn towards and engages the top of the pole piece  52  and thereby creates a closed magnetic circuit. Since the magnetic circuit is closed, the flux from the permanent magnet  22  alone is sufficient to hold the armature  54  in the closed position on termination of the first current I 1 . In moving to the closed position, the armature  54  resiliently deflects the moving contact  14   a  downwards to press against the fixed contact  12   a.  The closed contacts  12   a,    14   a  provide power to the load LD as before (it is to be understood that fixed and movable contacts  12   b,    14   b  are also present but not shown, and are opened and closed by the same armature  54  simultaneously with the contacts  12   a,    14   a.    
         [0071]    When a second current I 2  of sufficient magnitude is passed through coil K 1  in the opposite direction to that of the first current I 1 , the magnetic flux will be sufficiently weakened as to release the armature  54  and enable the armature  54  and the moving contact  14   a  to revert to their open states under the action of the spring  56 . 
         [0072]    It will be seen that one difference between  FIG. 6  and  FIG. 14  is that in the former the movable contacts  14   a,    14   b  are mounted on the contact carrier  16 , whereas in  FIG. 14  the movable contact  14   a  is independently mounted and resiliently deflected by the armature onto the fixed contact  12   a.  It will be understood that in  FIG. 6  the movable contacts could likewise be independently mounted and resiliently deflected into engagement with the fixed contacts. Conversely, the movable contacts  14   a,    14   b  of  FIG. 14  could be mounted on the armature, similarly to  FIG. 6 . 
         [0073]    The arrangement of  FIG. 14  is more efficient than that of  FIG. 6 . It differs from that of  FIG. 6  in that the permanent magnet does not move, and it uses a closed magnetic circuit. Nonetheless it operates essentially on the same principle of using a first current through a coil to close a set of contacts and using a permanent magnet to hold the contacts closed after expiry of the first current, and using a second current in the opposite direction to the first current to open the contacts. 
         [0074]    The present invention describes a simple, reliable and cost effective technique for use of a resettable EM switch to mitigate the problem of mis-wiring in a socket outlet with feed-through terminals. Furthermore, the solution is effective each time the device is wired up and thereby facilitates removal and rewiring of the device without subsequent risk of mis-wiring. However, the switch has a wider application, as described above. For example, the invention may be used in portable devices and in panel mounted devices, and may be used in DC systems or in TN, TT or IT AC systems. 
         [0075]    It will be seen that as embodiments of the present invention do not require a mechanical reset button such as the button  28  of  FIG. 1  to close the contacts SW 1 , the housing  40  can in general be sealed. Whereas in conventional RCD or GFCI devices, a button is used to reset the device, in embodiments of the present invention, the reset switch SW 2  of  FIGS. 5 ,  7 ,  8  and  10 - 13  as well as any test switch (not shown) can be implemented with, for example, a membrane keypad affixed to the external surface of the housing and connected to the remainder of the circuitry by, for example, a flexible tape passing through a slot of minimal dimensions in the housing. Using such a membrane means that the device can be operated even by users who may find difficulty accessing and operating within the limited space typically afforded to RCD/GFCI devices in panels. It also means that no space needs to be allowed for mechanical movement of a reset button so providing for greater flexibility in the overall design of the housing, 
         [0076]    The arrangement of  FIG. 6  can be modified as shown in  FIGS. 15 and 16  to improve the performance and efficiency of the switch. 
         [0077]    In the arrangement of  FIG. 15 , the pole piece  26  is fixed and retained in position by the retainer  29 . When a current of sufficient magnitude is passed through coil K 2 , an electromagnetic flux will be induced into the pole piece and will be of such polarity or direction as to attract the permanent magnet  22  towards the pole piece. When the current in K 2  is of sufficient magnitude the permanent magnet will become magnetically entrained to the pole piece such that when the current in K 2  is removed the permanent magnet  22  and the pole  26  piece will remain entrained due to the flux of the permanent magnet. During this process the MCC  16  and its associated contacts  14   a , 14   b  will move towards the fixed contacts  12   a , 12   b.  In this embodiment, the MCC contacts are fitted with biasing springs  15   a,    15   b  so as to bias them towards the fixed contacts. When the moving and fixed contacts touch, the MCC contacts will be deflected upwards until the permanent magnet  22  and the pole piece  26  become entrained, at which state the biasing springs  15   a , 15   b  will ensure adequate pressure between the fixed  12   a,    12   b  and moving contacts  14   a , 14   b  to ensure reliable operation under the required operating current and voltage conditions. The biasing springs  15   a , 15   b  also ensure that there will be adequate contact pressure even after a certain amount of wear on either the fixed or moving contacts. When a current of a certain magnitude and direction is passed through coil K 1 , the holding flux of the permanent magnet will be sufficiently reduced so as to cause the permanent magnet  22  and MCC  16  to move to their open position due to the force of the opening spring. The arrangement of  FIG. 15  simplifies the solenoid design and assembly and provides more optimal contact pressure on each set of contacts. 
         [0078]    The arrangement of  FIG. 15  can be further improved by the arrangement of  FIG. 16 . In the arrangement of  FIG. 16 , the pole piece  26 ′ now comprises a U shaped part rather than a rectangular or cylindrical part, the U shaped part having two ends  26   a,    26   b  facing a permanent magnet. When a current of a certain polarity and magnitude is passed through coil K 2 , the permanent magnet will be drawn towards and will entrain with the pole piece and the two sets of contacts will close as described for  FIG. 15 . However, the arrangement of  FIG. 16  provides a closed magnetic circuit which ensures that virtually all of the permanent magnet flux is harnessed to provide the holding force and contact pressure, leading to improved performance and efficiency. In the case of a dual coil system, the coils K 1 , K 2  may be placed on a single arm as shown, or placed on the separate arms if preferred. The bobbin  24  of  FIG. 15  has been omitted from  FIG. 16  because it is not an essential requirement. 
         [0079]    The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.