GB2276991A - Protection circuit responsive to several sensors - Google Patents

Abstract

A monitoring device for operating a switching means 40 in a power supply line 62, 64 between a supply 60 and load 50. The device comprises a plurality of sensors 10 for sensing a combination of characteristics of the power supply circuit e.g. overcurrent, residual current or cable sheath integrity and producing sensor signals in response thereto and logic means responsive to the sensors for producing an output signal for operating the switching means 40. The switching means may be placed in one 40 or both 40' of the live and neutral supply lines 62, 64. The cable sheath integrity sensor may consist of an insulating material with embedded conductors surrounding a standard 3 core mains cable (Fig. 2) where change in the resistance or capacitance of the conductors is sensed to indicate damage to the sheath. <IMAGE>

Description

Safetv Device The present invention relates to a monitoring device for electrical plugs, adaptors and the like. In particular it relates to a device for monitoring various characteristics of an electrical power supply circuit and for operating a circuit breaker in response to certain characteristics indicative of unsafe operation of an electrical appliance.

When a piece of electrical apparatus is run from a power supply such as a mains power supply, it is known to provide protection against damaging the apparatus in the event of a fault causing a short circuit in the apparatus. In addition to this, it is standard to provide protection against damaging the power supply circuit if for example, a fault causes a short circuit between a live conductor and a neutral conductor. In the above situations various types of component may be used such as fuses or residual current circuit breakers, and these are generally incorporated in the apparatus itself and in the power supply circuit.

With the above-mentioned safety devices, the emphasis is on protecting the apparatus and the power supply circuit, generator or the like. Indirectly this is done with the safety of the user in mind since to continue to supply power from a faulty or shortcircuited power supply or to a faulty piece of electrical apparatus is to run the risk of fire or of a user coming into contact with a build-up of charge or of a user completing a circuit. However, no direct protection is provided for the user. For instance, currents of up to 13 amps must generally be allowed for the supply of power to domestic appliances, but since the resistance of a human is generally much larger than that of a domestic appliance a fuse for this amount will not trip if a human is subjected to domestic mains power.

Also in some fault situations there may be no detectable earth leakage current, in which case no protection can be provided by means of known residual current detectors.

It is thus believed that greater and more direct protection can be provided for users of electrical apparatus powered from a power supply such as the mains than is done by means of fuses and circuit breakers at present. This invention is related to providing better personal protection for users of apparatus powered from an electrical supply, by incorporating circuit breakers and the like in power supply circuits whose operation is dependent on other characteristics as well as or instead of those on which the tripping of such circuits is usually dependent.

According to the present invention there is provided a monitoring device for operating a switching means in the live conductor of a power supply circuit, said device comprising a plurality of sensors for sensing a combination of characteristics of a power supply circuit and producing sensor signals in response thereto; and logic means responsive to the sensors for producing an output signal for operating the switching means.

The device of the invention may also be arranged to operate a switching means in the neutral conductor of the power supply circuit simultaneously with the switching means in the live conductor in order to provide complete, double-pole, isolation from the power supply.

The sensors may be two or more devices selected from an overcurrent detector, a residual current detector and a cable sheath-integrity detector.

Preferably the components of the monitoring device and the switching means may be housed in a plug or adaptor but with appropriate modifications to leads, connection devices and sockets, the components may be housed in sockets.

The invention will now be described by way of example with reference to the figures, in which: Figure 1 shows a logic circuit for a monitoring device with three sensors controlling the operation of a circuit breaking switch; and Figure 2 shows in cross-section a cable for supplying signals to a sheath integrity monitor being used as a sensor in the embodiment of Figure 1.

Referring to Figure 1, the monitoring device 1 comprises three sensors 10 which sense characteristics related to the supply of power to a load. For example, sensor A may be an overcurrent sensor arranged to detect a change in change in a phenomenon caused by the current flow, such as for instance a heating effect of the current or a magnetic field generated by the current.

Sensor B may be a Residual Current Detection Means arranged to measure any difference occurring between the currents flowing in the live and neutral conductors, which is known to indicate that the difference is leaking to earth. Sensor C may be a sheath integrity monitor, which will be described in more detail below.

In order to provide not only supplementary protection for a piece of apparatus 50 such as a domestic appliance supplied power by a mains power supply 60, but also protection for a user, the monitoring device 1 is arranged to provide a signal S to switching device 40 so as to break the supply of power to the apparatus 50 if any of or a combination of the sensors 10 produce signals indicative of a condition of the load or of its related circuitry in which a tripping action is required of the device 40.

As illustrated in Figure 1 the simplest form of switching device 40 provides a single switch element in the live conductor 62 of the power supply, the neutral conductor 64 being unbroken. Alternatively, as illustrated as 40' in Figure 1, switching device 40 provides simultaneous switching in the live and neutral conductors 63 and 64. This arrangement provides complete isolation of the apparatus 50 from the supply 60 in the event of a fault condition being detected.

In the monitoring device shown in Figure 1, the signals al, bl, cl from the sensors A, B, C, are supplied to a conditioning means 20. The signals may be of different types such as current signals or voltage signals and may well be ranged on scales of different orders of magnitude. The conditioning means 20 processes the sensor signals al, bl, cl as is necessary to provide conditioned signals a2, b2, c2 corresponding to the sensor signals al, bl, cl but which are compatible with each other. The conditioned signals a2, b2, c2 are supplied to a logic means 30 which is in this example an "OR" gate.If any of the conditioned signals a2, b2, c2 are above the threshold of the "OR" gate 30, an output signal S is supplied to the switching means 40 causing the switching means to trip disconnecting the appliance 50 from the power supply 60. The switching means is preferably of a type that may be reset easily once the characteristics responsible for causing the tripping signal S has been removed.

In the above example, it is clear that the characteristics monitored by the device 1 may be selected from a variety of characteristics related to the appliance, the power supply or the circuit. These characteristics need not be electrical. The sheath integrity monitor described as sensor C in the above example is sensing a physical characteristic of a part of the electrical supply circuit, this characteristic being the condition of the insulation material around the conductors of the supply circuit.

Figure 2 shows an embodiment of a cable for supplying signals to a sheath integrity monitor being used as a sensor in the embodiment of Figure 1. The cable 100 comprises conductors which in the example of a UK mains cable are a live (L), a neutral (N) and an earth (E) conductor insulated from each other by an insulating region 110. A sheath of insulating material 120 surrounds the conductors L, E, N and the insulating region 110. The sheath contains lengths of conducting material 130 arranged in the sheath around the mains conductors such that any object slicing or penetrating the cable will cut these outer conductors before it comes into contact with the mains conductors.An arrangement achieving this may be described by defining that the lengths of conducting material 130 are arranged such that, in a cross-section of the cable, a straight line drawn between any two adjacent conductors 130 does not pass through the mains conductors but passes above and outside of them.

The conducting material 130 is arranged so that any break in it is sensed by one of the sensors 10 in Figure 1 which would cause the monitoring device to supply a signal S to the switching means 40 causing it to trip. This is preferably achieved by connecting all the lengths of conducting material 130 in series, with the signal conditioning device 20 being arranged such that any change in the resistance of the loop of series connected conductors causes the device to supply a signal to the switching means 40 causing it to trip.

Such a change in resistance could be either a drop caused by a short circuit or a change to an open circuit condition caused by a clean cut.

The required operation could also be achieved by other various arrangements of the conducting material 130 within the sheath 120. For example, the conducting material 130 may be a single wire running the length of the cable 100, preferably several times so as to appear at several positions within the cross-section as shown in Figure 2. Alternatively, it may spiral around the insulating area 110. The conducting material 130 need not be a single wire; it may be a set of individual wires each running the length of the cable once with sufficient sensing means to detect a break in any of the wires. Other embodiments may be envisaged in which it is not necessary for a length of conductor material 130 to be cut in order for a sensor to detect that damage to the sheath 120 has occurred.For example the resistance or capacitance between two spaced apart lengths of the conducting material 130 could be monitored.

Other physical characteristics that could usefully be monitored include the temperature of the cable, which may indicate a loss of safety if it rises due to power dissipation in the conductors or if it rises as a result of coming into contact with a hot object such as a household iron or a soldering iron.

Amongst other characteristics, magnetic fields around the conductors could be monitored. Various characteristics of the sheath material itself could be monitored without the need for lengths of the conducting material therein.

The signal conditioning means 20 may simply relay signals to the logic means or act as a buffer. It may be required to amplify signals by different amounts in order that they may reach the logic means as signals on a common scale, or it may convert some signals from analogue to digital. It may be required to act as a transducer for some signals, converting them from a variety of types into a common type, or it may be required to hold signals for a sampling period until each sensor has supplied a signal. These or other types of conditioning means would be chosen according to individual requirements.

The logic means 30 could simply be an "OR" gate as described above, or may be a comparator. It may process the signals from the conditioning means according to an algorithin, or may simply be a junction of wires. There is, of course, no limit to the number of sensors 10 which may supply signals via the conditioning means 20 to a single logic means 30, but according to requirements it may be necessary for there to be a plurality of logic means each supplied with a plurality of conditioned signals, with each logic means providing a signal S to the same switching means 40 or to separate switching means 40.

The components of the monitoring device and switching means may be incorporated into a standardsized plug or an adaptor which may be produced in association with a cable surrounded by a sheath suitable for use with a sheath-integrity monitor. The plug or adaptor may be produced and sold integrally with such a cable, which may be produced and sold separately from an electrical appliance or integrally, as a part of the appliance.

The components may alternatively be housed in a socket suitable for receiving a standard plug or a plug modified so as to convey information such as sheath-integrity information from the cable to the device in the socket. Other embodiments may also be envisaged in which the components are housed in other parts of the power supply circuitry, for example, in a housing at a point in a length of a power cable itself.

Such a cable would not need to be attached to or dedicated to any one particular piece of apparatus or any one plug, socket or adaptor.

Claims (8)

CLAIMS:

1. A monitoring device for operating a switching means in the live conductor of a power supply circuit, said device comprising a plurality of sensors for sensing a combination of characteristics of a power supply circuit and producing sensor signals in response thereto; and logic means responsive to the sensors for producing an output signal for operating the switching means.

2. A monitoring device according to claim 1, arranged also to operate a switching means in the neutral conductor of the power supply circuit simultaneously with the switching means in the live conductor.

3. A monitoring device according to claim 1 or 2, wherein the sensors include at least two devices selected from an overcurrent detector, a residual current detector and a cable sheath-integrity detector.

4. A monitoring device according to claim 1, 2 or 3, wherein the monitoring device is housed in plug or an adaptor.

5. A monitoring device according to claim 1, 2 or 3, wherein the monitoring device is housed in a socket.

6. A monitoring device according to claim 1, 2 or 3, wherein the monitoring device is housed in an appliance.

7. A monitoring device according to claim 1, 2 or 3, wherein the monitoring device is housed in a power line.

8. A monitoring device substantially as hereinbefore described with reference to the accompanying drawings.