GB2150374A - Fault detection circuitry - Google Patents

Abstract

An arrangement for detecting both live/neutral reversal and some other fault-typically earth leakage - in an a.c. circuit uses a transformer (20) and an integrated circuit (30) operative via a sensing winding (21). Polarity reversal of the lines (10, 11) is detected by a circuit (67, 68) from neutral to earth (12) via further injection winding (60) on the transformer and operates in a wattless manner. The injection winding (60) is also in another circuit from live to neutral including test contacts (64). High immunity to noise and adjustability of threshold/response time is provided. <IMAGE>

Description

SPECIFICATION Fault detection circuitry The invention concerns improvements in and relating to circuitry for detecting abnormal conditions in electrical circuits, typically earth leakage in a.c.

mains driven-electric circuits or, more generally, reverse current detection.

The invention has particular, but not exclusive, application to compact provision of safety trip-out in electrical connection accessories, i.e. plugs and sockets on which we have done much recent work, see our cofiled patent applications Nos. 8430850 (P1190) and 8430851 (P1258).

In arriving at the present invention, we have studied various existing approaches whether or not specifically directed at achievement of the aforesaid desired compact in-accessory objective. In general, we find that reliance is placed on transformer-action to cancel out whatever effects might otherwise arise from normal circuit conditions. Suitably small and effective transformer-action devices are available via the well-known toroidal cores made of ferrite, and it appears to be a normal practice to use two such cores, one specifically for sensing fault currents and the other for neutral/earth sensing.Basic further provisions, of course, comprise circuit break contacts and an actuator therefor, conveniently a solenoid device, and a set of test contacts making of which will set up circuit conditions that force the break contacts to be broken even in normal conditions. The detection circuitry associated with such components can be characterised by the requirements to present a predetermined threshold to signals sensed via some furthertoroid winding or windings in response to abnormal circuit conditions, to enable a drive circuit for the contacts actuator when above-threshold conditions are detected, and similarly to respond to test conditions whether as like imbalance sensed signals via the further winding or windings or as signals otherwise derived.

Microelectronic integrated semiconductor circuit units, or chips, are well known and readily available for sensing thresholds and providing corresponding outputs. It is intended herein to use such a circuit unit in a generally advantageous way, preferably relative to a single transformer-action device such as a toroidal core traversed by live and neutral supply lines and further having both of an abnormal circuit condition sense winding and another winding serving to introduce an imbalance sensable by the abnormal circuit condition sense winding both in response to test contact closure and in response to live/neutral reversal.Basic provision for live/neutral reversal sensing is, of course, the subject of our copending application No. 8404351 (Pill 9), and even the dual-purpose winding hereof is obviously capable of application in systems using more than one toroidal core.

Hence, essentially, this invention provides means operative both in response to closure of test contacts and in response to reverse of live and neutral connections, for injection into transformer-action means of electrical signals sufficient to be sensed by a sensing winding present on the transformer-action means for other purposes.

Said injection may be via a winding of said transformer-action means, preferably a single toroidal core, and such winding will be in two circuit-connections, one including test contacts, typically connected between live and neutral lines and the other between neutral and earth, typically including reactance signal passing means.

Preferred said signal passing means is of a non-power-consuming kind, for example capacitive acting in a wattless manner, and, as such as applicable generally to circuit condition detection circuitry.

Preferred integrated circuit units afford a substantial range of adjustment of their effective threshold level, typically via a variable resistor; and/or operation on the basis of fault current integration with ready adjustability of time to reach threshold, typically via a capacitor; and/or afford output directly to operate a silicon controlled rectifier then used in driving actuator for break contacts, typically by effectively shorting a power supply diode bridge for the integrated circuit unit to convert to a diode switch for mains supply to the actuator; and/or ready association with noise rejection circuitry, typically suppression of noise pulses that could cause tripping of the aforesaid silicon controlled rectifier.

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 105, 115 and 125 and local load circuit connection terminals 10C, 1 1C 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, self-evidently 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.

A suitable power supply to the circuit unit 30 is shown taken via a full-wave rectifier shown as diode bridge 25, supplied over line 26 from live line 10 via the solenoid actuator 15 with return via line 27 to the neutral line 11, and supplying the circuit unit 30 over line 31 via resistor 32 with return via line 29 and branch 33. Supply current flow is, of course, set in line 28, it being assumed that the required voltage drop 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 29 and line 31 at 35 after the resistor 32.

The 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 to line 36 from circuit junction 35 via variable resistor 37. Signals at 22,23 above the trip level are integrated according to capacitor 39 in line 38. 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 connected between lines 28 and 29 on the bridge (25) side of the current flow control elements 31,32 (if any).Otherwise, of course, the circuit unit 30 holds line 41 low and thus silicon controlled rectifier OFF.

When the silicon controlled rectifier 40 is turned ON, it effectively shorts out the positive and negative outputs of the bridge 25 so that the latter acts as a diode switch then placing virtually the full a.c. supply across the solenoid actuator 15, which will thus be driven hard and will 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 will be noted associated with the silicon controlled rectifier 40, both concerned with preventing spurious triggering thereof. Capacitor 43 in line 44 between lines 41 and 29 is to prevent gate pick-up by the silicon controlled rectifier 40, and capacitor 45 in line 46 between lines 28 and 29 on the bridge side of line 42 snubs out fast transients to the anode of the silicon controlled rectifier 40.

If desired, power supply to the circuit unit 30 could be taken from line 28 via a resistor serving the same purpose as that referenced 32, usually with a diode in series and poled for flow towards the circuit unit 30. Then, it would be arranged that power supply requirements of the circuit unit 30 correspond to current flow through the solenoid actuator 15 far below the latter's requirement for operation to break the contacts 14.

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 48 and 49 are shown connected between lines 22 and 23 and between lines 23 and 29, 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's output connected to line 22 free of d.c. bias at its output connected to line 23, as applied to one preferred integrated circuit unit, namely LM 1851 from National semiconductor.

Afurther protection feature comprises a voltage dependent resistor 51, such as a Zener device, connected between the lines 26 and 27 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 protect the bridge 25 and silicon controlled rectifier 40 from 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, fixed resistor 52 in series therewith serving to protect the circuit unit 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 capacitors 34,43,45,48, 49 and voltage dependent resistor 50, 51.

Turning now to further winding 60 on the toroidal core 20, it will be noted that same is connected at one end via line 61 to the neutral line 11 and at its other end to junction point 62 in turn connected, on the one hand, via line 63 containing test contacts 64 and a resistor 65 to the live line 10 and, on the other hand, via line 66 containing a capacitor 67 and a resistor 68 to the earth line 12. In effect, then, there are two circuits including the winding 60, one from the live line 10 to the neutral line 11 via the test contacts 64 and resistor 65, and the other from the neutral line 11 to the earth line via the capacitor 67 and the resistor 68. The former serves to inject imbalance or disturbance into the core 20 for test purposes at operation of the test contacts 64. 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 68 serving to limit current pulses during normal conditions so as to avoid spurious tripping out. Itwill, 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.

Operation of the circuit of Figure 1 should now be evident. In normal conditions, the circuit unit 30 is powered up but the break contacts 14 remain ciosed with comprehesive protection against noise and transients causing spurious operation. On detection of abnormal conditions, most usually earth leakage where 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 lotto 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.

Claims (22)

1. Circuitry for detecting abnormal conditions in alternating current electrical signals comprising transformer-action means operative relative to nominally live and neutral lines to sense certain abnormal conditions via a sensing winding, and means operative both in response to closure of a test circuit and in response to reversal of said nominally live and neutral lines for injecting into said transformer-action means electrical signals sufficient to be sensed via said sensing winding.

2. A.C. signal condition detection circuitry according to claim 1, wherein said means for injecting comprises a further winding of said transformer-action means, such further winding being in two circuit connections, one including closable test contacts and the other between said neutral line and an earth line.

3. A.C. signal condition detection circuitry according to claim 2, wherein said one circuit connected is between said live and neutral lines.

4. A.C. signal condition detection circuitry according to claim 3, wherein said one circuit connection further includes signal level setting means therefor.

5. A.C. signal condition detection circuitry according to claim 4, wherein said signal level setting means is resistive.

6. A.C. signal condition detection circuitry according to claim 5, wherein the resistive means is a resistor in series with said test contacts.

7. A.C. signal condition detection circuitry according to any one of claims 2 to 6, wherein said latter circuit connection includes signal level setting means.

8. A.C. signal condition detection circuitry according to claim 7, wherein said other circuit connection includes an electrical resistance for setting the level of said sufficient signals.

9. A.C. signal condition detection circuitry according to claim 8, wherein said reactance is capacitive.

10. A.C. signal condition detection circuitry according to claim 9, wherein said reactance comprises a capacitor operating in a wattless manner and a series resistorfor limiting effects of current pulses against spurious production of said sufficient signals.

11. A.C. signal condition detection circuitry according to any one of claims 2 to 10, wherein said transformer-action means is a toroidal coil similarly traversed by said live and neutral lines and having therein said sensing winding and said further winding.

12. A.C. signal condition detection circuitry according to any preceding claim, further comprising a microelectronic semiconductor integrated circuit unit for processing signals from said sense winding to produce a control signal for its sensing of said abnormal conditions and said injected sufficient signals, break contact means for interrupting said a.c. electrical signals at least in said live line, break actuator means for said break contact means, and an electrical drive circuit for said break actuator means and enabled by said control signal.

13. A.C. signal condition detection circuitry according to claim 12, wherein said circuit unit is operative to produce said control signal by integrator action according to the value of an external capacitance means.

14. A.C. signal condition detection circuitry according to claim 12 or claim 13, wherein said circuit unit has external variable resistance means for setting its threshold or trip level.

15. A.C. signal condition detection circuitry according to claim 14, wherein said variable resistance means is branched from power supply to said circuit unit.

16. A.C. signal condition detection circuitry according to any one of claims 12 to 15, wherein power supply to said circuit unit is via rectifier means connected to said live and neutral lines.

17. A.C. signal condition detection circuitry according to claim 16, wherein connection from said live line to said rectifier means is via a solenoid of said actuator means.

18. A.C. signal condition detection circuitry according to claim 16, comprising voltage dependent resistance means for shorting out said rectifier means at high transients in said a.c. electrical signals.

19. A.C. signal condition detection circuitry according to claim 15 or claim 16, wherein said rectifier means comprises a diode bridge affording supply and return lines to said circuit unit with gate-controlled high-low semiconductor resistance means connected between those supply and return lines for response to said control signal for shorting those lines and converting the diode bridge to a diode switch then for switching said a.c. electrical signals to drive said solenoid actuator.

20. A.C. signal condition detection circuitry according to claim 19, comprising capacitors connected between said return line and both gate and anode of said high-low resistance means for suppressing noise and transients.

21. A.C. signal condition detection circuitry according to any one of claims 12 to 20, comprising filter capacitors connected between lines from said circuit unit to said sensing winding and therefrom to power supply return of said circuit unit.

22. A.C. signal condition detection circuitry arranged and adapted to operate substantially as herein described with reference to and as shown in the accompanying drawing.