Abstract:
A switching circuit for use with an electrical appliance, and comprising first and second normally open switch terminals and a high sensitivity impedance measuring circuit coupled to the first and second switch terminals for measuring an impedance therebetween, and producing at an output thereof a switching signal if the impendance is lower than a predetermined threshold of 500 MΩ. The appliance is coupled to the output of the impedance measuring circuit so as to be responsive to the switching signal. The circuit finds particular application for controlling and protecting electrical appliances operating from an a.c. electrical means supply, in which case one of the switch terminals is connected to a virtual ground connection which is electrically floating with respect to a ground feeder of an electrical mains supply. In the event of electrical contact between the virtual ground connection and the other switch contact, an apparent ground fault is produced, which gives raise to a small current which is detected by the impendance measuring circuit so as to produce a switching signal which may then be used to operate a suitable relay for controlling the appliance.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is the national stage under 35 U.S.C. 371 of PCT/IL98/00464, filed Sep. 25, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention is related to electrical switching and control circuits. 
     BACKGROUND OF THE INVENTION 
     Electrical and electronic devices are frequently required to be operated and controlled remotely via suitable switching and control circuits. In the case of high power devices operating on relatively high voltages, such as devices intended for operation on the main electrical distribution network, the resulting control and switching currents can be significant reaching several hundred milliamperes. When currents of such magnitudes flow through long cables, there is generated a significant voltage across the cable whose magnitude is proportional to the specific impedance, or resistivity, of the cable. This, in turn, gives rise to energy wastage and increased operating costs. The voltage across the cable can be reduced by employing lower gauge (i.e. thicker) cables whose resistivity is correspondingly lower, but this results in the control and switching cables being bulky as well as expensive. 
     Published PCT application no. WO 95/31028, in the name of present inventor, discloses a detector for monitoring the integrity of a ground connection to an electrical appliance having live and neutral terminals for feeding current to the appliance from respective live and neutral feeders of an electrical supply having a ground point for connecting to the ground terminal of the appliance. The detector comprises a differential comparator circuit for comparing a voltage at the neutral connection with a voltage at the ground terminal of the appliance and producing a fault signal if a difference therebetween exceeds a predetermined threshold. A switching device is connected in at least one of the live and neutral connections so as to be opened by a relay operatively coupled to the detector and responsive to the fault signal produced thereby. 
     WO 97/36358 published on Oct. 2, 1997, in the name of the present applicant, discloses a specific application of such a detector for use with electrical appliances which are either ungrounded or whose ground connection is impaired. Thus, in accordance with WO 97/36358 there is provided a protection device for use in conjunction with an electrical appliance having an electrically conductive outer casing and which protects against the casing becoming “live” regardless of the state of a ground connection associated with an electrical supply to which the appliance is connected, the electrical appliance having live and neutral terminals for feeding current to the appliance from respective live and neutral feeders of the electrical supply, said device comprising: 
     a virtual ground connection which is electrically floating with respect to said ground connection of the electrical supply, said virtual ground connection for electrically coupling to the casing of the electrical appliance instead of the ground connection of the electrical supply, and 
     a ground impedance measuring circuit for measuring an impedance between either the live or neutral terminals and the virtual ground connection of the appliance and producing a fault signal if said impedance falls below a predetermined threshold. 
     In effect, the protection circuit disclosed in WO 97/36358 employs the differential comparator subject of WO 95/31028 to compare the voltages between live and neutral and a floating ground connection such that any discrepancy between the measured voltages is indicative of a ground fault. There are several major advantages of the use of a floating ground connection as opposed to a regular ground connection. First, there is no danger of a person effecting electrical contact with the floating ground connection becoming electrocuted since there is no return path through actual ground for the fault current. Second, by using appropriate resistors in the differential comparator circuit, the “fault” current required to register an imbalance may be reduced to fractions of a nanoampere (i.e. less than 10 −9  ampere) as distinct from the milliamperes associated with conventional ground fault detector circuits. Yet another advantages is, of course, the lack of dependence on a reliable ground connection. 
     The use of a floating ground connection per se is known. Thus, EP 695 105 discloses a protection device for use with an appliance having a ground connection  33 , which might be the metal casing of the electrical appliance, which is connected via a resistor R 1  to a protection circuit comprising elements EC 2 , EC 3 , EC 4  and OC 1  so that if the leakage current flowing through R 1  exceeds a certain threshold, this protection circuit provides a trigger to a relay coil RL 1  so as to open the primary switch contacts SW 1  and SW 2 . Thus, the protection circuit produces a fault signal if the ground impedance falls below a predetermined threshold. 
     Likewise, FR 2468430 discloses a protection device wherein, as in above-described EP 695 105, the principle of operation is that, in the event of a ground fault, there will be a leakage current flowing through the ground connection and the magnitude of this leakage current is employed in order to provide a trip signal for the main current breaker(s). In the case of a regular ground fault between either the live or neutral feeders and GND, the resulting ground leakage current which flows through the virtual ground connection effects adequate protection. 
     However, in the event of a short-circuit between the live and neutral connections, there will under normal circumstances be no ground leakage current and therefore the circuits described in EP 695 105 and FR 2468430 will afford no protection. This is a very serious drawback because short-circuit faults represent a significant risk of fire. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a high sensitivity switching circuit for allowing remote switching of an electrical device via a pair of wires carrying negligible current whereby high gauge cables of minimal cross-sectional area may be employed without resulting in unacceptable ohmic losses. 
     In accordance with a broad aspect of the invention there is provided a switching circuit for use with an electrical appliance, the switching circuit comprising: 
     first and second normally open switch terminals, 
     a high sensitivity impedance measuring circuit coupled to the first and second switch terminals for measuring an impedance therebetween, said high sensitivity impedance measuring circuit producing at an output thereof a switching signal if said impedance is lower than a predetermined threshold of 500MΩ; 
     the appliance being coupled to said output so as to be responsive to said switching signal. 
     The invention finds particular application for controlling and protecting electrical appliances operating from an a.c. electrical mains supply. For such applications, one of the switch terminals is connected to a virtual ground connection which is electrically floating with respect to a ground feeder of an electrical mains supply. In the event of electrical contact between the virtual ground connection and the other switch contact, an apparent ground fault is produced. The ground fault gives rise to a small current which is detected by the impedance measuring circuit so as to produce a switching signal which may then be used to operate a suitable relay for controlling the appliance. 
     A sufficiently strong fault signal may be generated upon contact between the virtual ground connection and ground by a human being having a body resistance typically in the order of several thousand ohms. By such means, human contact with one or both of the switch terminals is sufficient to register a “ground fault” thus producing the required switching signal. If desired, one or both of the switch terminals may be connected to a metal touch plate such that momentary contact therewith by a human being results in the generation of the required switching signal. 
     Alternatively, both switch terminals can be mutually floating with respect to ground whilst exhibiting a very high contact impedance of several hundred MΩ. Shorting the switch contacts, even by means of finger contact, reduces the impedance to below the threshold of the impedance measuring circuit, thereby produce the switching signal. In this context, it is to be understood that “shorting” means lowering the contact resistance between the two switch contacts to less than the predetermined threshold of 500MΩ. The resulting current which then flows into the impedance measuring circuit may be as low as several nanoamperes. 
     According to a preferred embodiment of the invention, the impedance measuring circuit comprises a differential comparator circuit for comparing a fraction of the voltage between the live and neutral connections of the electrical supply with a voltage at the virtual ground connection. A fault signal is produced if a difference between the two voltages exceeds a predetermined threshold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, there will now be described a switching circuit for connection to an electrical supply feeder, by way of non-limiting example only and with reference to the accompanying drawings, in which: 
     FIG. 1 shows schematically the principles of the invention; 
     FIG. 2 is a schematic representation of a switching circuit connected to an incoming electrical supply for remote switching of an electrical appliance; 
     FIG. 3 shows schematically a detail of a differential comparator circuit used within the switching circuit; 
     FIG. 4 is a schematic representation of a high-sensitivity switching circuit according to an alternative embodiment of the invention; and 
     FIG. 5 is a schematic representation of an application using low cost surface wiring for use with the switching circuit according to the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a system  10  comprising an electrical supply  11  having live and neutral supply rails phases  12  and  13 , respectively and a ground rail, GND. Connected to the live rail  12  is a voltage regulator  14  for producing at an output  15  thereof a regulated low voltage d.c. supply. The output  15  of the regulator  14  thus constitutes a high d.c. rail, whilst the neutral supply rail  13  constitutes a low d.c. rail and will be referred to thereby in the subsequent description. 
     Connected across the high and low d.c. rails is a first voltage divider comprising a pair of resistors  16  and  17  whose common junction  18  is connected to a first output  20  of a differential voltage comparator  21 . A second input  22  of the differential voltage comparator  21  is connected to a common junction  23  of a second voltage divider comprising a pair of resistors  24  and  25 . A free end  26  of the resistor  25  is connected to a touch plate and constitutes a switch terminal. 
     An output  27  of the differential voltage comparator  21  is connected to the base of an NPN bipolar junction transistor  28  whose emitter is connected to the low d.c. rail and whose collector is connected to the high d.c. rail  15  via a suitable switching element K which may be an electromagnetic or solid-state relay. An appliance  29  has power connections which are energized via a normally open switch contact  30  which is closed under control of the switching element K in known manner. Thus, the switching element K is analogous to known contactors which allow high voltage devices to be switched via low voltage control or switching circuits. 
     A person  31  who touches the switch contact  26  whilst standing on ground, GND allows a small electric current to flow through the second pair of resistors  23  and  24  through him to OND. As a result, there is produced an imbalance between the voltages at the two inputs  20  and  22  of the differential voltage comparator  21  whose output  27  thus goes high. When this happens, the bipolar junction transistor conducts thereby energizing the relay K and closing the normally open switch contact  30  thus supplying current to the appliance  29 . It will, of course, be appreciated that the same principle can equally well be applied to interrupting the current to the appliance  29  by substituting a normally closed switch contact for the normally open switch contact  30 . 
     Within the context of WO 97/36358, the person  31  causes a ground fault and the switch contact  26  which is electrically floating with respect to GND constitutes a virtual ground connection. These terms will become clearer from the following description of a specific arrangement. 
     By Ohm&#39;s Law, the impedance of an electrical device is equal to the voltage across the device divided by the current flowing therethrough. Thus, if there is no earth leakage to the switch contact  26 , then the impedance between the switch contact  26  and the live supply rail  12  is extremely high, assuming that the resistors  23  and  24  have suitably high value resistances. Specifically, if the phase voltage is 220 V and the maximum permitted safe leakage current is 0.9 mA, then the impedance is typically in excess of 250 KΩ. However, in the event of an apparent ground fault, such that the leakage current to the switch contact  26  rises, the impedance drops accordingly. The impedance between the high d.c. rail and the junction  23  thus serves as a measure of whether there is an earth leakage to the switch contact  26 . 
     Since the switch contact  26  is floating relative to GND, the integrity or lack of integrity of the ground connection to the neutral feeder  13  of the electrical supply  11  is no longer significant. Specifically, the provision of a sound ground connection GND or the lack of such provision is not relevant; in either case fast operation of the relay K is ensured within no more than several milli-seconds. It will also be appreciated that by using suitably high value components for the resistors  16 ,  17 ,  23  and  24 , an apparent ground fault may be produced even when the “ground” leakage current is negligible: for example in the order of nanoamperes. This order of sensitivity is hardly applicable when the detector circuit is used as a regular ground fault protection device as taught in WO 97/36358. However, it is admirably suitable to the application of a high sensitivity touch switch, and the resulting negligible current flow allows high gauge control cables having very small cross-sectional areas to be employed thus reducing the cost of such cables as well as their bulk. 
     Referring now to FIG. 2, there is shown a circuit diagram of a touch switch  35  based on the above-described principles. Connected across the live and neutral feeders  12  and  13  of a mains electricity supply  11  is an isolation transformer  36  having a tapped secondary winding  37  for stepping down a primary voltage of 110 or 220 V approximately 10 V across respective taps. A d.c. bridge rectifier  38  is connected to an appropriate one of the tapped secondary windings via a selector switch  39  and produces an output voltage of approximately 12 V d.c. across a high voltage rail  40  and a low voltage rail  41  constituting, respectively, supply and ground rails. 
     The live and neutral inlets  12  and  13  are also connected via corresponding switches  42  and  43  to live and neutral socket outlets  44  and  45 , respectively to which the appliance  29  shown in FIG. 1 is connected As shown, there is also provided a ground socket under  46  to which an outer metallic casing of the appliance  29  may be connected, if required. Such connection is not, however, mandatory since unlike the circuit described in WO 97/36358, the touch switch  35  is not used to safeguard against actual ground faults but rather is used for remote switching of the appliance  29 . 
     The switches  42  and  43  constitute switching devices which are simultaneously operated by a relay coil  48  (constituting a main feeder switching means which may be a contactor) connected between ground and the collector of an NPN bipolar junction transistor  49 . An emitter of the NPN bipolar junction transistor  49  is connected to ground and a base thereof is connected via a resistor  50  to a first normally open switch contact  51   a  of a two-pole changeover switch operated by a relay coil  52 . The relay coil  52  constitutes an “actuator” having a first pole  53   a  and a first normally closed switch contact  54   a  and having a second pole  53   b  and corresponding second normally open and closed switch contacts  51   b  and  54   b,  respectively. 
     The first pole  53   a  of the changeover switch is connected to the positive low voltage d.c. supply rail  40  whilst the first normally closed switch contact  54   a  is connected to one end of a red indication lamp  55  whose other end is connected to ground. Likewise a green indication lamp  56  is connected between ground and the first normally open switch contact  51   a.  The second pole  53   b  of the changeover switch is connected via a rectifier diode  57  to the one end of the relay coil  52 , a second end of which is connected to the positive low voltage d.c. supply rail  40  via a detector circuit depicted generally as  58  and described in greater detail below with reference to FIG. 3 of the drawings. 
     A filter capacitor  59  is connected across the relay coil  52 . Likewise, for the sake of completeness, a filter capacitor  60  is shown connected between the base of the bipolar junction transistor  49  and ground. Connected between the live and neutral inlets  12  and  13  is a voltage divider comprising a pair of resistors  61  and  62  whose common junction is coupled via a current limiting resistor  63  to one input of a differential comparator within the detector  58  and whose second input is connected to ground. Thus, the differential comparator compares a fraction of the voltage between the live and neutral inlets  12  and  13  to the voltage on the virtual ground connection  46 . 
     Referring to FIG. 3 it is seen that the detector  58  comprises a low voltage d.c. rail  61  connected to the d.c. supply rail  40  shown in FIG. 2. A 6 V regulator  62  is connected between the low d.c. voltage rail  61  and ground such that there exists a regulated 6 V d.c. voltage between a stabilized voltage rail  63  of the regulator  62  and ground. 
     A first voltage divider comprising resistors  64  and  65  is connected between the stabilized voltage rail  63  and ground and has a common junction connected to a first inverting input (pin  2 ) of a dual comparator  66  so as to provide a reference voltage signal which is offset from the ground potential by a fixed amount. The common junction of the voltage divider shown in FIG. 2 is also connected via a resistor  67  to the first non-inverting input (pin  3 ) of the comparator  66 . A 6 V Zener diode  68  is connected in series with a current limiting resistor  69  between the stabilized voltage rail  63  and ground, between which is connected a smoothing capacitor  70 . 
     An output  71  (pin  7 ) of the comparator  66  is connected to the base of an NPN bipolar junction transistor  72  (constituting a “normally open switching circuit”) whose emitter is connected to ground and whose collector is connected to the second normally open switch contact  51   b  shown in FIG.  2 . The output  71  is also connected via a resistor  73  (shown in FIG. 2) to ground. 
     The rectifier diode  57  which is connected across the relay coil  52  shunts any high back e.m.f. generated by the coil  52  and thus avoids damage to the bipolar junction transistor  72 . 
     In order to increase the reliability of the detector  58 , the comparator  66  comprises dual comparators sharing a common output and connected in an analogous manner to the arrangement described above. The dual comparator may be constituted by an integrated circuit such as National Semiconductor&#39;s LM193 series. 
     The operation of the detector  58  is as follows. Under normal conditions, there is a large imbalance between the voltage at the virtual ground connection  46  and the fractional feeder voltage which are respectively fed to the input pins of the comparator  66 . Consequently, the output voltage of the comparator  66  is high so that the base voltage of the bipolar junction transistor  72  is high and the bipolar junction transistor  72  conducts. In this condition, the bipolar junction transistor  72  functions as short circuit between its emitter and collector such that the normally open switch contact  51   b  is connected to ground. 
     The high voltage rail  40  is connected via the resistor  50  to the base of the bipolar junction transistor  49 . The bipolar junction transistor  49  thus conducts, allowing current to flow through the relay coil  48  which energizes and closes the switches  42  and  43 , thereby connecting the inlet terminals  12  and  13  of the supply  11  to the corresponding socket outlets  44  and  45 . At the same time, the high voltage rail  40  is connected to the green indication lamp  56  which thus illuminates and provides a visible indication that the appliance  29  is energized. 
     If now an operator touches the virtual ground connection  46  constituting the touch plate of the switch contact  26  shown in FIG. 1, an apparent ground fault is produced such that the differential voltage seen by the comparator  66  falls and its output  71  goes low. The transistor  72  is thus cuttoff and its collector is no longer connected to ground. Consequently, the voltage applied to the normally open switch contact  51   b  (which is closed when the appliance  29  is energized) is no longer at ground potential and so the relay coil  52  de-energizes and the changeover switch contacts revert to the “fault” state shown in FIG.  2 . In this state, the high voltage rail  40  is connected to the red indication lamp  55  which illuminates and provides a visual indication that the appliance  29  is now de-energized. At the same time, since the switch contact  51   a  is no longer connected to the high voltage rail  40 , the voltage applied to the base of the bipolar junction transistor  49  goes low and the bipolar junction transistor  49  is cutoff. This de-energizes the relay coil  48  thereby tripping the supply between the inlet terminals  12  and  13  of the adapter  20  and the corresponding outlet terminals  44  and  45  thereof. 
     There is thus provided in accordance with the invention a highly sensitive touch switch which is responsive to negligible current flow for allowing remote operation and control of an electrical appliance. 
     Furthermore, since the ground connection of the installation, if present, is in any case isolated from the appliance  29  in that it is quite distinct from the virtual ground connection employed thereby, there is no longer any danger of electric shock if the actual ground of the installation becomes live owing to a breakdown in insulation between the live and ground feeders. 
     It will also be appreciated that, since the impedance of the appliance is negligible in comparison with the nominal impedance of the virtual ground loop (whose threshold is set to be in the order of 250,000MΩ), the impedance of the virtual ground loop can be measured between the virtual ground connection and either the live or neutral feeders. Assuming a supply voltage of 230 V, such calibration causes a leakage current as low as 0.009 μA to cause operation of the switch  35 . 
     It will also be appreciated that, whilst in the preferred embodiment the touch switch serves to de-energize the appliance, it can equally well be employed to energize the appliance from an initial de-energized state. This can easily be achieved by reversing the initial states of the switch contacts  53   a  and  53   b  or using a contactor having normally closed contacts which open when the bipolar junction transistor  49  becomes saturated. Alternatively, a pair of touch switches can be provided: one for energizing and the other for de-energizing the appliance. Yet a further possibility is to replace one of the bipolar junction transistors  49  or  72  by a bistable multivibrator (flip-flop). Each time the touch switch  35  is touched (thus creating an apparent ground fault), an output of the bistable multivibrator changes stage thus alternatively energizing and de-energizing either one of the relays  48  or  52 . 
     It is also to be understood that use of the virtual ground connection  46  as the ground socket outlet as shown in FIG. 2 is illustrative only. In fact, this is not a desirable implementation for a grounded appliance since no effective ground protection is then afforded. When ground protection is itself an important consideration, the circuit must have much lower sensitivity so as to operate with a leakage current in the order of 0.9 mA. This can best be achieved by using two circuits: one having a virtual ground serving as a ground connection for the appliance and being calibrated for ground fault connection; and the other having very high sensitivity and serving as a touch switch as explained above. 
     Reference is now made to FIG. 4 showing a touch switch depicted generally as  35  comprising normally open first and second switch contacts  81  and  82 . The first switch contact  81  is connected to an incoming neutral feeder  83  of an a.c. main supply whose live feeder  84  is connected to an input  85  of a d.c. comparator  86 , as described above with reference to FIGS. 1 and 3 of the drawings. Connected to an output of the comparator  86  is the base of an NPN bipolar junction transistor  87  whose emitter is connected to the d.c. zero voltage rail and whose collector is coupled to a relay  88 , which may be a contactor for interrupting current to an a.c. appliance. A capacitor  89  is connected at the output of the comparator  86  for smoothing the comparator output voltage which serves as the base bias voltage of the transistor  87 . 
     The touch switch  35  operates as follows. Under normal circumstances, the impedance between the two switch contacts  81  and  82  exceeds the threshold of 500MΩ and the switch is “open circuit”. When the impedance across the two switch contacts falls below this value, a small a.c. current in the order of several nanoamperes flows into the input  85  of the comparator  86  causing it to oscillate. That is, the output voltage across the capacitor  89  goes HIGH and LOW The circuit is calibrated so that, when this happens, the net voltage across the capacitor  89  is sufficient to push the transistor  87  into saturation, thereby operating the relay  88 . 
     It should also be noted that, in practice, the first switch contact  81  may be connected to a virtual ground connection as explained above such that finger contact with the second switch contact  82  causes an effective “short” between the two switch contacts, within the context of the invention. That is to say, the impedance between the two switch contacts will fall from several hundred MΩ to less than 1MΩ. 
     FIG. 5 is a schematic representation of an application using low cost surface wiring for use with the touch switch shown in FIG.  4 . Thus there is shown a touch switch  35  for remotely operating an electrical appliance  29  as explained above with reference to FIG. 4 of the drawings. The touch switch  35  is connected to a switch contact  92  via an electrically conductive track  93 . Touching the switch current  92  causes a “ground fault” as explained above, thereby operating the touch switch  35  and interrupting current to the electrical appliance  29 . The electrically conductive track  93  may be a graphite track and may even be hand drawn using a pencil on the exterior surface of a wall (or any other surface) on which the touch switch  35  is mounted. Alternatively, wallpaper (constituting a surface-covering) may have printed on a surface thereof a conductive track so that by abutting several pieces of wallpaper with the electrical tracks in mutual contact, the effective length of the conductive track can be extended. Preferably, the conductive track is printed on the backside of the wallpaper and the switch contact  92  may be push-fitted though the wallpaper at a desired location so as to effect electrical contact with the hidden track. In similar manner, several switch contacts may be connected to the same track, so as to allow operation of the touch switch  35  from more than one location via a suitably modified wallpaper, carpet or other surface-covering. 
     The invention also contemplates that the switching circuit is an adapter for coupling the appliance to an electrical socket outlet. Likewise, it may be formed integral with an electrical socket outlet or an electrical plug or with the appliance.