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Patent Searching and Data

Document Type and Number:
WIPO Patent Application WO/1996/000457
Kind Code:
An electrical safety device for controlling the supply of power from a power supply such as a mains power supply to an appliance, such as an electrical powered lawnmower, is disclosed. The safety device comprises means for monitoring the impedance of the appliance circuit prior to connection of the appliance to the mains power supply, and means for allowing such connection to be made only if the impedance detected is found to be that expected of an appliance, as opposed to a human, or a short circuit for example.

Application Number:
Publication Date:
January 04, 1996
Filing Date:
June 27, 1995
Export Citation:
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International Classes:
H02H5/12; (IPC1-7): H02H5/12
Domestic Patent References:
Foreign References:
Other References:
See also references of EP 0767980A1
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1. An electrical safety device for controlling the supply of power from a power supply to an appliance circuit including an appliance, comprising an impedance monitoring circuit arranged to monitor the impedance of the appliance circuit when current at the supply voltage is not being supplied to the appliance circuit; and a switching means for allowing the supply of current at the supply voltage to the appliance circuit when the impedance is within a predetermined range.
2. A safety device according to Claim 1, wherein the impedance monitoring circuit comprises a test power source.
3. A safety device according to Claim 2, wherein the test power source is a dc power source.
4. A safety device according to Claim 2 or 3, wherein the test power source is of a substantially lower voltage than that of the power supply.
5. A safety device according to any of Claims 2, 3 and 4, wherein the test power source is powered by the power supply.
6. A safety device according to any of Claims 2, 3, 4 and 5, wherein the impedance monitoring circuit further comprises a coil of a relay.
7. A safety device according to Claim 6, wherein the switching means comprises a switch of the relay.
8. A safety device according to any of the preceding claims, wherein the impedance monitoring circuit includes solid state components.
9. A safety device according to any of the preceding claims, wherein the switching means includes solid state components.
10. A safety device according to any of the preceding claims, further comprising current monitoring means for monitoring the current at which power is supplied by the power supply.
11. A safety device according to claim 10, further comprising means for disconnecting the safety device from the power supply on detection of an overcurrent condition.
12. A safety device according to claim 10 or 11, further comprising means for disconnecting the safety device from the power supply on detection of a current imbalance condition in the power supply circuit.
13. A safety device according to any of the preceding claims, further comprising load current monitoring means for monitoring current in the appliance circuit when power is being supplied at the supply voltage to the appliance.
14. A safety device according to claim 13, further comprising appliance switching means for preventing the supply of power to the appliance when the current in the appliance circuit is outside a predetermined range.
15. A safety device according to claim 13 or 14, wherein the load current monitoring means and appliance switching means include a relay.
16. A safety device according to claim 13, 14 or 15, wherein the load current monitoring means and appliance switching means include solid state circuitry.

This invention relates to a safety switch suitable for use with electrically powered equipment (e.g. powered by mains electricity) which will allow operating power to flow only if the switch senses that it is connected to a load having an impedance within a pre-determined range.

Users of some mains powered portable electrical devices (such as an electrically powered mover or hedge trimmer) risk severing the electrical supply cable during use. The availability of protection to such users by employing a residual current circuit breaker of the earth leakage type is well known. These circuit breakers continuously monitor the current flowing in the live and neutral wires of the supply cable, allowing current to flow in these wires at balanced levels, but should the flow become unbalanced by more than a pre-determined amount, the circuit breaker trips, isolating the supply cable from the mains supply. Such an unbalancing occurs if the supply cable is severed and the live wire is earthed, either directly to ground or perhaps through the user's body. However, if both of the live and neutral wires of the severed cable are touched simultaneously, or perhaps simply allowed to lie in wet grass, the current in the wires can remain in balance and the circuit breaker may not trip, leaving a potentially hazardous situation.

The present invention seeks to eliminate this danger by allowing current at supply voltage to flow along the supply cable if it is connected to a load with an impedance within a specified range of impedance but will not allow current to flow at supply voltage if the impedance presented by the load is outside that range. The invention relies on the fact that the impedance presented by most electrical devices is small (usually

less than 200 ohms) whereas, the impedance presented by, say, a person, an animal or wet grass, is considerably higher.

According to one aspect of the invention an electrical safety switch incorporates an impedance- monitoring device which operates in conjunction with the switch so as not to allow current at the supply voltage to flow to a load connected to the switch unless the monitoring device senses the impedance of the load lies within a pre-determined range.

The invention may be used in conjunction with a residual current circuit breaker or other residual current safety device, or it may be used as a stand¬ alone device.

Suitably the predetermined range of impedance prior to activation is between 0 and 200 ohms, but other ranges are possible depending on circumstances. A higher or lower upper limit (say between 100 and 300 ohms) can be used and a non-zero lower limit can be desirable in some cases (say between 1 and 10 ohms).

A method of allowing current at supply voltage to flow (eg along a supply cable) to a load with an impedance which is within a specified range of impedance (measured prior to operation) but will not allow current to flow at supply voltage if the load impedance is outside that range, represents a further aspect of the invention.

A lighting or power socket incorporating a safety switch as described herewith represents a still further aspect of the invention.

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

Figure 1 shows a stand-alone version of the invention with the load not operating;

Figure 2 shows the version of Figure 1 with

the load operating;

Figure 3 shows a more sophisticated version incorporating a residual current circuit breaker;

Figure 4 shows an embodiment of the present invention which provides switching of the live and neutral lines of a 230V-AC mains supply;

Figure 5 shows an alternative current according to the present invention, suitable for providing protection in situations where bare wires in a supply cable contact each other;

Figures 6a and 6b show flow diagrams indicating the sequences of tests which may be carried out by circuits according to the present invention;

Figures 7a and 7b show a circuit for testing;

Figure 8 shows a further modified circuit according to the present invention.

Figure 9 shows a part of a further circuit according to the present invention; and

Figure 10 shows a circuit suitable as amplifier A in Figure 9.

Referring first to Figures 1 and 2, a supply source S (shown as AC but which could equally well be a DC source) connected via a cable C incorporating wires Lc and Nc to an appliance 6 (shown here as a mover) via an on-off switch 5 is provided with safety protection by the incorporation of integers 1 to 4. Integers 1, 3 and 4 represent a relay the coil 1 of which is in circuit with wire Nc and the switch 3 of which is in circuit with line Lc. Switch 3 is ganged to a further switch 4 in a low-voltage battery circuit powered by a battery 2 and connected between wires Lc and Nc. A spring 7 holds switch 3 in the normally-open position and switch 4 in the normally-closed position.

When the switch 5 of the appliance 6 is in the off position as shown in Figure 1, no AC current can flow in the wires Lc and Nc because switch 3 is open.

Closing switch 5 to actuate the appliance 6 allows current to flow through the relay coil 1 powered by the battery 2. This current is sufficient to overcome the effect of spring 7 closing switch 3 and simultaneously opening switch 4 (see the condition shown in Figure 2) . Closure of switch 3 connects the AC mains supply S to the appliance 6 and the AC supply continues to keep the relay coil 1 energised and thus switch 3 is closed. If the mower is turned off by switch 5 (ie. the load impedance in series with the coil 1 becomes infinite or alternatively if either or both of wires Lc and Nc of the cable is/are severed (suddenly reducing the current drawn - say to less than 0.2A) insufficient current flows through the relay coil 1 and the switch 3 opens disconnecting the AC supply. The switch 4 automatically reconnects the battery 2 into the low voltage supply loop in readiness for a repeat of the cycle.

The embodiment of Figures 1 and 2 operates with an impedance range between zero and the load impedance which is just enough to cause the current in the coil 1 to drop below the level at which it can hold the switch 3 against the pull of the spring 7.

Figure 3 shows a further embodiment of switch in accordance with the invention. In this case a test current is supplied from a low voltage source Vs with added series impedance, for safety reasons. In normal operation, when an appliance incorporating a motor M is connected via its on/off switch (SW), the test current sees a low impedance and as a result relay RL1 is energised, closing the contacts in the live L and neutral N lines of a supply source S and the power supply cable C to the appliance. At the same time, the source of test current may be isolated to prevent damage to it, as shown, though certain low voltage sources can be designed so as to be self-protecting.

The flow of current through the appliance is

now detected by a relay RL2 which maintains the relay RL1 in the energised state. This is done by drawing current form the source mentioned above or an auxiliary one or from RL2 itself (which can take the form of a transformer or an extra winding on RL1, or switched connection) . If the appliance is disconnected or switch SW is turned "off", current ceases to flow through it, and this is registered by relay RL2. Consequently, relay RL1 is de-energised and the contacts in the live and neutral lines open. In addition, the source Vs of test current is once more presented to the supply cable. If this has been severed, the impedance across its ends will be relatively high (ie. much more than 200 ohms). Thus the flow of test current is impeded, and is insufficient to activate the relay RL1. As a result, the live and neutral lines (L,N) are kept isolated from the supply cable C, which remains "dead". If bare ends of the cable should touch each other, however, a very low impedance will be detected and relay RL1 will be energised, connecting the live and neutral wires to the cable C causing a large fault current to flow. It is thus advisable to include a fuse or over-current circuit breaker of some kind in the device. The optional components around relay RL3 show how over-current sensing may be added in such a way as to re-open the contacts in the light neutral lines and keep them open until the circuit is reset. Alternatively, assuming a residual current circuit breaker (RCCB) is fitted, it can be tripped so that its own live and neutral contacts are open.

If the appliance switch is turned "off" and then "on" again while its motor is still spinning, a back EMF may be developed, and it is advisable to include a suppressor Z as shown in Figure 3 to prevent this back EMF causing damage to the electrical components of the switch.

The relays RL1, RL2 and RL3 can, in practice, be solid state devices rather than electro-mechanical ones. Also relay RLl may be divided, e.g. into a sensitive device followed by a heavy-duty one. The current sensing elements of relays RL2 and RL3 may be combined if convenient, e.g. using a current transformer. The AC mains contacts have been shown in Figure 3 as operating on both the live and neutral lines, for reasons of safety (in case the two lines are transposed) . If it can be assured that the line voltage will never appear on the wrong wire, then some economy is possible by providing live contacts only.

The low voltage source Vs may be a transformer or resistive/capacitive divider powered by the AC supply, or it may even be a battery or a charge storage element of some kind.

Figure 4 shows a further and more sophisticated embodiment of switch device in accordance with this invention. This permits switching of the live lines of a 230V AC mains supply.

Referring to Figure 4, the transformer Tl produces approximately 15Vrms at its secondary, and this is routed to the potential divider R1R2 via the relay RL3. The divider gives about 3.5 Vrms, and initially this low voltage is presented to the output cable C by virtue of relay RLl being in the quiescent state, i.e. with contacts A closed and B open. If the appliance (represented by motor M and cable C) has a resistance of less than 200 ohms, when switch SW is closed a detectable current flows via the protective resistors R3R4 to the gate of a triac device Ql. Resistor R5 and the capacitor Cl prevent triggering on noise, and set a threshold current of about 8Ma. To allow for variations in triac sensitivity, R4 can be altered if necessary. Once Ql has fired, current is drawn from Tl through the winding of RLl, and after about 10 ms it changes-over.

so that contacts A open and B close. As a result, the test voltage from R1R2 is isolated, and the mains L and N are connected to the cable C and hence to the appliance. The current flowing in the N-return is detected by the transformer T2 (in place of RL2 in Figure 3). Using a current-transformer here has the major advantage of permitting a detection range of 0.2A to over 40A (peak) without overheating. The secondary of T2 produces short voltage pulses, which are filtered by R8C2 and then used to turn on devices Q2 and Q3, depending on the polarity of the AC waveform from Tl. As a result, trigger pulses are available to fire Ql. If the N-current is too small (or the cable cut), however, Ql is not triggered, and RLl re-opens. The delay given by R8C2 is important, since it needs to cater for pulses that might occur to early in the AC cycle for Ql to fire properly. For a heavy resistive load, the pulses are early, but for an inductive load they are late, and so the limiting resistor R6 must be low enough to ensure adequate current in RLl. If the current through the primary of T2 is excessive (say greater than 8A) the secondary voltage pulses are larger than normal, and they are detected by the over-current circuit. This employs Rll, R12, D7 and the integrating capacitor C3, whose value is chosen so as to give a delay sufficient for a load such as a large electrically-powered lawn mower to start up. The pair of devices Q4Q5 form a bistable latch, which is triggered when the voltage at the base of device Q4 exceeds a level set by R14R15 and the DC-voltage on C4. Once fired, Q4 turns on and in consequence device 05 turns on too, providing regenerative feedback via R18. The LED D9 lights up and at the same time current is fed to the coil of RL3. Its contact to R6 etc. opens, and the contact leading to Dll closes — thereby providing fully secure latching of Q4Q5. Since the current to RLl

is now effectively cut off, the contact B feeding L and N to the cable C are rapidly opened, and they remain open on account of the latch circuitry. The output cable C is therefore dead, not even receiving the test voltage (since RL3 has cut this off too). Q1Q2 remain latched (with the LED on) until the mains input is interrupted for long enough for C4 to discharge and the circuit is reset. However, in the event of the overload still being present, the circuit will trip once more as soon as C3's delay is overcome; the larger the overload, the faster C3 charges and hence the shorter this delay. The actual trip current is most conveniently adjusted by selecting an appropriate value for R14. Zener diodes Zl and Z2 suppress transient voltages that might otherwise cause damage.

An alternative version of the circuit-maker illustrated in Figure 4 is shown in Figure 5 of the accompanying drawings and this is designed to deal with, in particular, the case where bare wires in the cable may touch each other. Assume that initially the switch SW to the appliance M is open. When SW is closed, the circuit first checks the resistance presented to it via the contacts A. If the resistance is below a lower threshold Rlow, this is detected by the operational amplifier A4 which sets amplifier latch A2 via a diode D9, turning on a diode D10 and device Q2, and prevents the relay RLl from being energised. The latch A2 can be reset only by interrupting the AC supply to a transformer Tl which provides a low-tension rail V+/V- by means of rectifiers D1-D4 and capacitor Cl. If on the other hand the resistance is below an upper threshold Rhigh, as detected by amplifier A3, but above Rlow, device Q2 is turned on via R13 and hence RLl is energised, opening the contacts A and closing the contacts B. Thus the load is connected to the AC supply L and N. VR1 is a thermistor which helps to limit the

initial inrush current. The load current is sensed by a transformer T2, whose secondary feeds a rectifier- integrator Al, and if the current is deemed sufficient, a positive voltage is sent to device Q2 via Rll, thereby maintaining RLl in the energised state. However, if the current falls below a predetermined level (e.g. when SW is re-opened), the output of Al goes down to V-, and so device Q2 turns off. In the event that the load current seems too high rather than too low, this condition is sensed by a rectifier-filter D7R4C4R5C5, and latch A2 is consequently tripped by diode D8.

The operational amplifiers A4 and A3 behave as integrators, by virtue of the feedback capacitors C9 and C8. This has the advantage of providing a time delay for measuring the "short circuit" condition (i.e. resistance below Rlow) , as well as filtering out transients, pick-up and noise. Furthermore, the resistance is measured in a bridge arrangement, comprising R22 and Rload compared with R18 and R19, also with R16 and R17. Therefore variations in the low- voltage "test" source coming from R21 and V+ are compensated. In the event that the resistance measured is above Rhigh, the output of amplifier A3 remains low, and so RLl is not energised. Thus the cable continues with only the low "test" voltage applied to it — thereby rendering it safe when cut, for example.

The additional components D13D14, C10C11 and C6C7 are for protection and RF suppression.

The additional components D13D14, C10C11 and C6C7 are for protection and RF suppression.

In a variation where RLl switches only the L line, V- is effectively linked to Line N, and so the transformer T2 may be replaced by a current-sensing resistor if preferred.

In a further variation, the circuit of Figure 5 is preceded by a conventional earth leakage circuit

breaker (ELCB) or RCD, which can be tripped by A2, for example, using an auxiliary relay to feed an out-of- balance current to it. An effective method of coupling is to convert the LED D10 into the transmitter part of an optocoupler.

The circuit of Figure 5 can cope with most intended loads, but can have a problem with back-EMF (for example the voltage generated by a motor whilst spinning), which can arise in particular when the appliance switch is opened and reclosed before the motor has come to rest. The result is either a delay in the circuit closing RLl again, because the back-EMF opposes the test voltage coming via R21R22, or, even worse, the Rlow trip being activated because the back-EMF is negative and sufficient to overcome the test volts. These shortcomings can be overcome by employing in AC- test method, and by measuring back-EMF and/or brush- noise.

Figures 6a and 6b of the accompanying drawings are flow diagrams of sequences to compensate for these potential shortcomings. Figure 6a is a flow diagram showing the main sequence of events, and Figure 6b is a more comprehensive version that exemplifies the sort of delays engendered by testing with AC.

Figure 7a of the accompanying drawings shows schematically a circuit for AC-testing. A test signal E is low-frequency AC taken from the secondary winding of a transformer Tl, with a series resistor Rt. During test, the voltage V across the appliance is attenuated and then passed to a synchronous detector SR2 followed by a low-pass filter LP2. The test voltage E is similarly rectified and filtered by SRI and LP1. The results are compared at CMP1 and CMP2 (which can be operational amplifiers acting as comparators) using resistive dividers. The purpose of this arrangement is to enable relay RLl to turn on when the appliance

impedance is below a predetermined threshold (say 300 ohms) and yet prevent operation if it is very low (say under 6 ohms) . Positive feedback is provided at comparator CMP1 by an RC-network so RLl turns on with a definite "snap", and at comparator CMP2 by a diode so that it latches if a very low impedance is detected. In this case, device Q2 stays on, and robs device Ql of any base current so that relay RLl is disabled. Note that the resistive divider or bridge arrangement compensates for variation in the source voltage E, including the effective impedance of transformer Tl. For appliances with significant inductance as well as resistance (e.g. electric motors) , the Q-factor of the windings is generally sufficiently low that the impedance appears substantially resistive, and so the test method works satisfactory, even for induction motors on large mowers. The synchronous detectors are shown in detail in Figure 7b, and in this embodiment they use a CMOS switch followed by an operational amplifier, giving a negative output to suit the common-mode range of the LM324-type amplifiers used for CMP1, CMP2 as well as elsewhere. An alternative embodiment is described later. If a DC back-EMF is present, it gives positive and negative contributions to the detected signal, and these errors average to zero in the filter. Further, any such back- EMF is detected by filter LP3, amplifier A2 and the diode pair DPI. Once the bias on amplifier A4 is overcome, its output turns on device Ql and enables relay RLl. For back-EMF from a motor containing a bridge-rectifier, the situation is more complex, since the polarity of the EMF as seen at V appears to follow the polarity of E, rendering the method above inadequate. Therefore the sub-circuit with detector SR3 shown in Figure 7 is employed. This sub-circuit takes the synchronous-detected signal Vs and selects the peaks (e.g. the central 5ms of the 10ms half-cycle at 50Hz)

which contain a substantial amount of brush-noise whilst the motor is still spinning. The differentiator-plus- integrator A3 has a carefully chosen gain and pass-band so as to amplify this brush-noise but reject other spurious effects. The result is rectified by a diode pair DP2 at the input terminals of an amplifier A4, and so RLl is enabled as previously.

Radio-frequency interference is suppressed by a low-pass filter RuCu. For further protection, and to allow for the very large back-EMFs that can be generated by induction motors, a bi-directional over-voltage suppressor ZV1 is included; however, electrical mowers containing an induction motor normally feature a brake of some kind, and so the back-EMF is generally low enough for the circuit to operate by the time the switch is reclosed.

During the period a typical appliance is switched on and running, the current drawn through the neutral line N is sensed by a current transformer T2, rectified by a diode-bridge DB2, and amplified by an amplifier Al, which sends a signal to the base of a transistor Ql so as to keep the relay RLl on. As soon as the appliance is switched off — or the cable is cut — the voltage on capacitor C3 rapidly decays, and relay RLl is released, allowing the test cycle to begin again. The value of capacitor C3 and the gain of amplifier Al are chosen so as to give rapid pick-up on legitimate appliances, and yet reject noise and leakage effects. Protection is afforded by suppressor ZV2 and diode D3. Though transformer T2 is shown in the neutral line, it can alternatively be placed in the live line L. This offers some advantage where it is desired to keep relay RLl closed and blow the fuse in a power plug or elsewhere if the live output is shorted to earth (or to neutral). On the other hand, the circuit may be combined into a differential current sensing device

(ELCB, RCCB or RCD) . In this case, some economy may be found by using the relay RLl for both the Maker and the RCD-Breaker functions, but RLl must now be robust enough to break very large currents safely. In this hybrid arrangement, the RCD section's trip signal is fed to RLl in such a way as to disable it, and resetting is possible only by actuating a dedicated button or unplugging and reconnecting.

Figure 8 of the accompanying drawings shows an arrangement where the L 1 and N' outputs form the primary of a differential transformer T. If there is significant leakage current to earth, the L* N' fluxes are out of balance, and so a signal is developed across the secondary winding SaSb of T when large enough it trips a sense-amplifier A, and as a result device Q (which may be a transistor, thyristor or triac) is turned on and robs the relay RL of its holding current, thereby releasing the armature RA that carries the sprung contacts SCL and SCN. Resistor Rt is for testing the circuit, and it draws an out-of-balance current of some 50 mA.

A further development, shown partly in Figure 9 of the accompanying drawings, is an appliance specific method of an appliance-sensing facility to help protect the user in the event that the cable is cut or unplugged. It does not require a special cable (unlike the prior art arrangements described in EP-A-0029406 or GB-A-2200001) but is less comprehensive than the method proposed previously and is an appliance specific method, as will be described. In Figure 9, an auxiliary load Rx is added is added to the appliance, preceding its switch. Also, an extra transformer T' with windings as shown and a balancing load Ry are included, so that if the cable connected to N" and L" is cut, an appropriate signal is produced on the secondary of R' An alternative arrangement employs a current-sensing

resistor to determine the presence of Rx, but because of the wide range of load currents to be carried by the cable, the transformer method is expected to be more practical. If Rx and Ry threat T' with the same number of turns, then Rx can equal Ry, and it is preferable (though not essential) to make them of the order of 100 kilohms resistive, in order to reduce power dissipation and yet not be confused by out-of-phase loading such as cable capacitance.

The secondary winding Sa'Sb' is connected to an amplifier A' that serves four purposes:

1) it detects the current i through load Ry when it is not balanced by an equal and opposite current through load Rx.

2) it filters out or ignores back-EMFs and capacitive, inductive and noise effects.

3) it trips the amplifier A or the device Q in a conventional RCD circuit, or performs the equivalent function.

4) it survives overloads, in particular when the full appliance current is flowing through the cable, or worse still when a short-circuit occurs.

An embodiment of amplifier A' of Figure 9 is shown in Figure 10 of the accompanying drawings. The key features are as described below, but it will be appreciated that the requirements 1 to 4 mentioned above can be met by a variety of means. Also in some cases the L and N conductors are transposed but this does not materially affect the principles of operation. ZV1 in Figure 10 is a transient suppressor (varistor) for protection if the N' current is excessive. R1C1R2 form an attenuator and RF-rejection filter, and are followed by diode limiters D1D2. The signal is synchronously detected and amplified by the circuit using operational amplifiers Al and A2, with the AC reference entering via R27R28 such that Al and A2 operate alternately on the

negative and positive halves of the cycle. The outputs are combined by R15R16 and low-pass filtered by R17C5. The sense of the wires from Sa'Sb' is such that the voltage at R18 remains low when current i flows through both loads Rx and Ry, but it shifts upwards when only Ry draws current. Note that the current through the appliance is by-and-large in phase with that through load Rx. Amplifier A3 acts as a comparator or threshold-detector, with its reference (at its inverting input) derived from the circuit's DC supply voltage and therefore proportional to mains voltage and hence to i. Thus a valid detection of the cable being cut can be made, with the output of A3 going high, and being latched high by D8. The output from A3 going high, and being latched high by D8. The output from R21 can be used to drive a device Q in the conventional circuit (see Figure 8), or an alternating signal from R26 can be fed to A; in either case, the relay RL is tripped, and the contacts SCL,SCN fly open. Alternatively, one of these signals can be fed to a circuit that disables some other kind of contactor, power-relay, etc. A typical RCD trips within 30 ms at 30 mA, whereas the novel circuit described above has different requirements, e.g. to trip at 2 or 3mA (preferably using a similar type of transformer) but with a response time that can be extended to around 250ms. In certain cases, it may be possible to make Rx and Ry of lower value, say 10 kilohms, and so the circuit may be simplified. It is assumed when the cable is cut that any leakage is substantially less than the specified value i. Therefore the response time has to be less than the time taken by someone to pick up a severed cable.

The method and circuits described in Figures 5 and 10 utilise common operating principles to those discussed in relation to Figures 1 to 4. It will be appreciated that parts of the circuits shown can be

replaced by alternative devices achieving broadly the same results. For example, most of the analogue signal processing can be substituted by digital signal processing. A specific example of this is in Figure 7a, where the voltages E, V, Vi etc. are polled by analogue- to-digital converter(s) , and the decisions shown in the flow-diagram of Figure 6b are taken by a microprocessor with some form of embedded software, or a hard-wired- digital-logic circuit, or a programmable-logic circuit, or an application-specific integrated circuit or a combination of any of these. This argument applied similarly to the other methods and diagrams described.

It should be appreciated that the invention can be employed equally well for different supply voltages other than the main supply and for a variety of different types of load.

The invention also has applications generally where the full supply current from a source is only required to flow when the correct appliance is attached to the source. Thus, for example, it can be arranged that appliances having a load impedance less than the specified amount cannot be operated from a particular outlet socket, e.g. so that for instance a motor mower may be used but not a transistor radio. Alternatively, a device in accordance with the invention can be used to prevent appliances with a too low load impedance being operated from a particular outlet (e.g. a razor outlet) designed only for appliances drawing low levels of power from the source.

The invention can also be used to protect sockets such as light bulb sockets. When a light bulb is removed from a socket there is a potential danger presented as the contacts at mains voltage can be accessed by fingers. If a safety switch in accordance with the invention is incorporated within the socket, it

would automatically turn off the mains voltage when the bulb was removed thus rendering the socket safe. The impedance-monitoring device of a switch in accordance with the invention may be built into a socket or a plug or it can be provided as a separate intermediate adaptor for insertion into the socket. It is not ruled out that other arrangements can also be used.

A safety device according to the invention could be fine tuned to only accept loads within a certain power range. For instance if tuned to lkW ± 500 W it would trip 1.5kW but not empower appliances rated at less than l.OkW.

A device according to the invention would be an appropriate safety feature for fitting to all electrical fixtures such as lighting on mains distribution sockets which are prone to getting wet, particularly those exposed to the weather. Unless a load was applied to a socket protected in this way, the socket would never become live and thus would be safe from shocks due to damp. Such an arrangement would be particularly useful for extra safety when changing external light bulbs. If such light bulbs should become broken (of which there is a high chance in external conditions) the broken light bulb and socket would automatically be rendered safe to touch.

A device according to the invention would be appropriate for fitting to illuminated traffic bollards and signs which are frequently damaged. Conventionally when this happens a large notice and box have to be placed around the broken bollard to warn of live parts. If a device according to the invention were to be fitted, the live parts would automatically be made safe.

In marine applications (e.g. on a ship) where spray and damp are continuous hazards, a device according to the invention would be a valuable asset.