GB2178916A - Detecting abnormal neutral line - Google Patents

Abstract

Earth leakage circuit breaker utilises a transformer-action device preferably a toroidal core (20) traversed by live (10) and neutral (11) lines and having a sensing winding (21) in which net signals will appear when some of the mains signal leaks to earth. A sensing circuit (30) responds to signals from the sending winding (21) and takes its operating power supply from mains live and neutral lines via suitable circuit paths and components (24, 25; 27, 29). Means for sensing abnormal neutral line conditions includes an alternative path (70, 72) to earth for power supply return of the sensing circuit, which path, when energised, produces (via a winding 71) a signal in the sensing winding (21) detectable by the sensing circuit (30). Another winding (60) is responsive to operation of a test switch (66) and live-neutral reversal. <IMAGE>

Description

SPECIFICATION Detecting abnormal neutral This invention relates to detection of abnormal conditions in electrical circuits, particularly mains circuits.

Well-known circuits for such purpose include earth leakage circuit breakers which have long been used to protect individual circuits and apparatus having particularly high risks of faults resulting in short circuits to earth, and have further been used more generally instead of fuses for consumers' local circuits, i.e. as distributed from an input mains supply. More recently still, there has been considerable activity in relation to miniaturising such circuit breakers for incorporation into mains electrical accessories, such as socket outlets, plugs and adaptors. We have ourselves made proposals for such devices and, in doing so, have offered solutions to other fault conditions, including provision for circuit breaking action in response to wiring errors resulting in reversal of polarity, i.e. live connected to neutral and vice versa.We have also addressed ourseives to the problem of detecting abnormal neutral line conditions, including loss of neutral that would not trip a conventional earth leakage circuit breaker whether or not equipped with our reverse polarity detection system.

It is an object of this invention to provide for detection of abnormal neutral line conditions and specifically to do so in a way well-suited to incorporation into highly miniaturised circuit breakers as required for incorporation into mains electrical accessories.

Integrated semiconductor circuits are available for serving signals representing earth leakage, say via a sensing winding on a magnetic core, usually toroidal and also traversed by live and neutral lines from an a.c. mains supply in a manner such that net signals appear in the sensing winding only when some of the mains signal is leaking to earth and thus producing an imbalance between live and neutral line current. Our own previous proposals have used such circuitry and have provided for reverse polarity detection via what amounts to simulation of a condition normally detectable by the aforesaid sensing winding, actually by effectively injecting an additional signal into a further winding on the toroidal core, see our copending application no.

8430852 (P1417) where that is done utilising the same further winding as provided for testing the circuit breaker device as a whole. As will appear later, preferred embodiments of the abnormal neutral sensing provision hereof are capable of inclusion within a device also including our reverse polarity detection.

Reverting to the available integrated semiconductor circuits for signal sensing, such can take their operating power supply from mains live and neutral lines via suitable circuit paths and components, and a major problem arising in relation to detecting loss of neutral is the consequent loss of a return for the operating power supply requirements of the signal sensing circuit.

According to this invention it is proposed that, for such conditions, an alternative path be provided for such power supply return of the signal sensing circuit, for which an earth line is a generally suitable destination, such path arranged so that a sensing winding on a toroidal core traversed by live and neutral lines is energised, and so the sensing circuit, say by causing such path to traverse another winding on the core.

In the embodiments to be described relative to the drawing, such winding is additional to the injection winding by which test and/or polarity reversal are detected.

Asuitable alternative power supply return path includes a selective conduction device responsive to voltage for substantially blocking the path whenever normal neutral condition apply, i.e. assuring that the normal power supply return path is of lower resistance and thus preferred, but for offering suitable condition if that normal supply return path has an abnormal termination, i.e. ceases to be preferred. A break-down device such as a Zener diode is suitable. Thus, practical return provisions could both have normal diodes poled towards neutral and earth destinations, respectively, but with the latter further in series with a Zener diode.

Specific implementation of this invention will now be described, by way of example, with reference to the accompanying drawing showing a schematic circuit diagram.

In the drawing, live, neutral and earth lines 10, 11 and 12, respectively are shown going between supply input terminals 10S, 1 1S and 12S and local load circuit connection terminals 10C, 11C and 12C, respectively. The load line 10 has fault-break contacts 14 displaceable to break by a solenoid 15, see dashed line 15B for operative relationship. Also shown in the live line 10 is another switch 16, but same reflects only one preferred application hereof to a switched electrical connection accessory such as a socket.

The live and neutral lines 10 and 11 are shown traversing a transformer-action device, specifically going through a toroidal core 20 in a self-cancelling manner in relation to flux generation in the core 20 under normal circuit conditions. In the event of abnormal circuit conditions giving rise to leakage of current, there will be imbalance of such flux generation in the core 20, and sensing winding 21 is provided thereon to detect same and supply signals over lines 22,23 to integrated circuit unit 30 as said sensing circuit.

A suitable power supply to the integrated circuit 30 is shown taken via a resistor 24 in branch line 25 from line 26 connected between the live line 10 and the solenoid actuator coil 15, and returned to the neutral line 11 via line 27 and its branches 28 and 29, the latter including a diode 31. Basic current flow is, of course, set by the resistor 24 in line 25, it being assumed further that the required voltage conditioning is assured by on-chip voltage regulation circuitry of the chip 30. Basic smoothing of the supply voltage to the circuit unit 30 is via capacitor 34 connected between line 25 and line 27 at 35 after the resistor 24.

The integrated circuit unit 30 is operative in response to signals at inputs connected to lines 22, 23 and in accordance with a trip level set at another input connected by line 36 from the circuit junction 35 via variable resistor 37. Signals at 22, 23 above the trip level are integrated using another capacitor 39 in line 38 from the chip to return line 27. The capacitor 39 serves to control response time versus noise rejection and will be kept discharged by the circuit unit 30 under normal (no fault) conditions, i.e.

no signal at lines 22, 23. When integration reaches a preset threshold, the circuit unit 30 applies an output to line 41 that turns ON silicon controlled rectifier 40 (via its gate) in line 42 connecting the return line 27 to the solenoid actuator coil 15.

Otherwise, of course, the circuit unit 30 holds line 41 low and thus silicon controlled rectifier 40 OFF.

When the silicon controlled rectifier 40 is turned ON, it effectively connects the solenoid actuator coil 15 to the neutral line 10 via the diode 21 to place virtually the half wave supply across the solenoid actuator coil 15, which will thus be driven hard and break contacts 14. Then, of course, loss of mains power results in release of the circuit unit 30 and the silicon controlled rectifier going OFF, i.e. ready for resetting of the break contacts 14.

Two capacitors 43 and 44 are connected from input and output sides, respectively, of the silicon controlled rectifier 40 to the return line 27 and are both concerned with preventing spurious triggering thereof. Capacitor 43 snubs out fast transients to the anode of the silicon controlled rectifier 40, and capacitor 44 prevents gate pick-up by the silicon controlled rectifier 40.

There is, of course, advantage to be gained from improving immunity to noise from the load circuit via the winding 21 and into lines 22, 23. Accordingly, noise filter capacitors 45 and 46 are shown connected between lines 22 and 23 and between lines 23 and 27, respectively. A.C. coupling capacitor 50 in line 23 is to pass a.c. fault signals from the winding 21 but to keep from the circuit unit terminal connected to line 22 free of any d.c. bias at its terminal connected to line 23.

Afurther protection feature comprises a voltage dependent resistor 51, such as a Zener device connected between the lines 26 and 29 before the solenoid 15 in line 26. The normally high resistance of device 51 will drop to a low value for high voltage transients in the live and neutral lines 10 and 11, and so protectthesilicon controlled rectifier40from voltage breakdown that could result in the solenoid 15 being energised spuriously for long enough to trip the contacts 14.

It will be appreciated that variable resistor 37 enables the fault trip level to be adjusted over a considerable range, and fixed resistor 52 in series therewith serves to protect the circuit 30, i.e. set a minimum resistance seen thereby. Also, appropriate selection of the capacitor 39 sets desired fault current integration time and thus noise rejection, and good protection against nuisance tripping is readily achieved via other illustrated capacitors and voltage dependent resistor.

A further winding 60 on the toroidal core 20 is connected at one end via line 61 to the neutral line 11 at 62, to which line 27 is also connected via diode 31, and at its other end to a junction point 63 between two series connected resistors 64 and 65.

The resistor 64 is in series with test contacts 66 connected to the live line 10, and resistor 65 is in series with a capacitor 67 to the earth line 12. There is another resistor 68 paralleling the resistor connection 65/67, which will be of high value and serve only in assuring capacitor discharge (and could, of course, simply be across the capacitor 65).

In effect, there are two circuits including the winding 60, one from the live line 10 to the neutral line 11 via the resistor 64 and test contacts 66, and the other from the neutral line 11 at 62 to the earth line via the resistor 65 and the capacitor 67. The former serves to inject imbalance or disturbance into the core 20 for test purposes at operation of the test contacts 66.

The other serves to inject imbalance or disturbance into the core 20 if the neutral line 11 is too high, i.e.

detecting live/neutral reversal. In both cases, current flow is effective to cause detection via the fault sense winding 21. For test purposes, the current flow is set by the resistor 65. For line reversal, the current is set by the reactance of the capacitor 67, which has the advantage of operating wattless and so not dissipating power, the resistor 65 serving to limit current pulses during normal conditions so as to void spurious tripping out. It will, of course, be appreciated that, in normal conditions, i.e. no line reversal, the neutral line 11 will be low and there will be little voltage difference from the earth line and thus only small current flow insufficient for tripping purposes.

In addition, the drawing shows a connection 70 from the integrated circuit unit's return supply line 27 to the earth line 12 via a further winding 71 on the toroidal core 20. That connection 70 includes a series connection of a diode 72 poled towards the earth line and a voltage break-down device shown as a Zener diode 73. It will be appreciated that the Zener diode 73 results in a greater impedance in the path 70 to earth than is represented by the diode 31 in the normal return to neutral. Accordingly, the path 70 will not be operative unless there is a departure from normal neutral line conditions, usually, of course, loss of that neutral whether by a break in the line or a bad connection.The advantage of using a breakdown device, such as the Zener diode 73, is, of course, that normal conditions do not result in any appreciable current flow to earth via the path 70, and thus avoids spurious operation.

Plainly, however, if the Zener diode 73 breaks down, the core winding 71 will be energised and there will be a resulting flux imbalance in the core that will be detected by the circuit unit 30 so that the solenoid 15 is tripped out, but the circuit unit 30 itself stays operative with adequate supply current.

Operation of the circuit of the drawing should now be evident. In normal conditions, the circuit unit 30 is powered up but the break contacts 14 remain closed with comprehensive protection against noise and transients causing spurious operation. On detection of abnormal conditions giving rise to earth leakage, there will be relatively increased current/ flow in the live line 10 producing imbalance of flux in the core 20. Corresponding signals from the sense winding 21 will be processed by the circuit unit 10 result in a control signal on line 41 turning ON the high-low resistance device 40, specifically silicon controlled rectifier, and causing energisation of the solenoid 15 to break the contacts 14. The same result will occur if the test contacts 44 are closed causing injection of sufficient current via the further said winding 60 to get the abnormal condition response via the sense winding 21. Also, should live and neutral lines 10 and 11 be reversed, the further winding 60 will immediately get an injection of sufficient current for abnormal condition response via the sense winding 21 and the contacts 14 will break after only a few cycles of a.c. current.

Additionally, should abnormal conditions arise in the neutral line, specifically loss of neutral, the Zener diode 73 will break down and energise the winding 71, also resulting in response via the sense winding 21 and breaking of the contacts 14.

Claims (8)

1. Earth leakage circuit breaker utilising a transformer-action device, preferably a toroidal core, traversed by live and neutral lines and having a sensing winding in which net signals will appear when some of the mains signal leaks to earth, a sensing circuit for signals from said sensing winding which sensing circuit takes its operating power supply from mains live and neutral lines via suitable circuit paths and components; and means for sensing abnormal neutral live conditions which means includes, operative for such conditions, an alternative path for power supply return of the sensing circuit which path, when energised, causes a signal in said sensing winding detectable by said sensing circuit.

2. Earth leakage circuit breaker according to claim 1, wherein said path goes to an earth line.

3. Earth leakage circuit breaker according to claim 1 or claim 2, wherein said path traverses a further winding on said transformer-action device.

4. Earth leakage circuit breaker according to claim 3, wherein said further winding is additional to a winding on said transformer-action device by which test and/or polarity reversal is/are detected.

5. Earth leakage circuit breaker according to any preceding claim, wherein said path includes a selective conductor device responsive to voltage for substantially blocking said path whenever normal neutral condition applies, but for offering suitable condition of the normal return to neutral for said sensing circuit lower supply has an abnormal termination.

6. Earth leakage circuit breaker according to claim 5, wherein said device in said path is as a branch from said normal return but presents higher resistance than said normal return.

7. Earth leakage circuit breaker according to claim 5 or claim 6, wherein said device is a Zener diode.

8. Earth leakage circuit breaker substantially as herein described with reference to and as shown in the accompanying drawing.