GB2244398A

GB2244398A - Test circuit for residual current device

Info

Publication number: GB2244398A
Application number: GB9011803A
Authority: GB
Inventors: Joginder Sikka
Original assignee: MK Electric Ltd
Current assignee: Novar ED&S Ltd
Priority date: 1990-05-25
Filing date: 1990-05-25
Publication date: 1991-11-27
Anticipated expiration: 2010-05-25
Also published as: GB9011803D0; AU7868791A; WO1991019340A1; GB2244398B

Abstract

A test circuit for a residual current operated device including a current transformer (24) through which the line and neutral conductors (12, 14) connected between an a.c. source and a load are arranged to pass, comprises a test switch (96) and a test resistor (98) coupled in series between the line and neutral conductors and zero or low voltage line (44) by way of at least two turns of one winding of a bifilar or dual test winding (100) on the current transformer thereby to generate a test current on closure of the test switch. The circuit is adapted to respond to loss of neutral, loss of earth and live-neutral reversal. <IMAGE>

Description

ELECTRICAL PROTECTION DEVICES This invention relates to electrical protective devices and concerns residual current operated devices (R.C.D's) which are sometimes known as earth leakage circuit breakers (E.L.C.B's).

R.C.D's are arranged to monitor the currents flowing in the line and neutral conductors extending between an alternating current source and its load. In a correctly wired circuit the currents flowing in the line and neutral circuits should be equal and their vector sur should be zero. If there is a fault, such as an insulation fault, in the circuit such that current flcs to earth the currents in the line and neutral conductors will not balance and their vector sum will not be zero.

This imbalance or residual current is detected by the R.C.D which interrupts the a.c. supply to the load if the magnitude of the residual current exceeds a predeterined level.

In a typical R.C.D the line and neutral conductors pass through a current transformer and an imbalance of the currents in the line and neutral conductors produces in a sense winding on the transformer, an e.m.f. which is used to operate contacts via a relay or a solenoid, to open circuit the line and neutral conductors when the e.m. f.

exceeds a predetermined value.

It is desirable that the relay or the solenoid is energised with minimum delay. The R.C.D. should, therefore, be operative on both the positive and negative halfvcycles of an a.c. waveform. It is also important that the R.C.D should operate when the a.c. waveform is modulated with a pulsating d.c. component.

In Fig. 1 of the accompanying drawings there is described a residual current operated device comprising means for sensing an imbalance between the currents flowing in the line and neutral conductors connected by way of connecting means between an a.c. source and a load, control means coupled to the sensing means and arranged to provide a control signal to open said connecting means if the magnitude of said imbalance exceeds a predetermined value. The described embodiment includes a current transformer and the control means comprises a window comparator so that the R.C.D can be arranged to open the switch means on the positive-or negative-going half cycles of the a.c. waveform of the mains supply.

It is known to provide a test circuit for an R.C.D. to check that it is capable of functioning correctly in the detection of an imbalance between the currents flowing in the line and neutral conductors prior to or after the connection of a load.

According to the invention, a test circuit for an R.C.D.

including a current transformer through which the line and neutral conductors of an a.c. supply are arranged to pass, comprises a test switch and test resistor coupled in series between the line and neutral conductors and by way of at least two turns on the current transformer.

The R.C.D. may be a device as described and claimed in our co-pending Patent Applications Nos:

(32360) and

(32361).

Thus for a given number of ampere-turns required to provide sufficient flux in the current transformer to operate the R.C.D., the use of more than one turn in the test circuit reduces the magnitude of the required out-of-balance test current. This is facilitated by the use of a higher value test resistor but of lower power rating due to reduced I2R loss.

The test winding may be a bifilar winding arranged to provide two current paths, a first for the test current and a second for the neutral-earth current, such that the current in the first winding is not directly affected by the current in the second winding.

The invention will now be described by way of an example with reference to the accompanying drawings, in which: Fig.l is a circuit diagram of one embodiment of a residual current operated device according to the invention.

Fig.2 is a circuit diagram of a known R.C.D. test circuit.

Referring to Fig.l, there is shown a residual current operated device (R.C.D) 10 for monitoring the imbalance currents flowing in the line 12 and neutral 14 conductor lines connected between an alternating current source 18 and a load 20. A double-pole manually-operable switch 22, which is also operable by a solenoid 22a is connected in the line and neutral conductors as shown.

The line and neutral conductors 12, 14 pass through a current transformer 24.

The R.C.D 10 comprises a sense winding 26 on the current transformer having one end 26b coupled to the midpoint of a potential divider comprising resistors 28, 30, 32, 34 connected across a d.c. supply derived from the a.c.

supply by way of a conventional bridge rectifier 36, a resistor 38 and capacitor 40. The values of the resistors 28 to 34 are so selected that the datum potential at the junction of winding termination 26b with resistors 30, 32 is approximately half the voltage between the positive and zero voltage lines 42 and 44.

The other end 26a of the sense winding is coupled to te non-inverting 46a and inverting 48b inputs of two comparators 46 and 48 respectively. The inverting input 46b of comparator 46 is coupled to the junction of resistors 28 and 30. Likewise, the non-inverting input 48a of comparator 48 is coupled to the junction of resistors 32 and 34. As connected the comparators 46, 48 function as a window comparator 50 with reference levels defined by the potentials at the junctions of resistors 28, 30 and 32, 34 respectively. The comparators 46, 48 may comprise two elements of, for example, a quad op-amplifier such as that sold by National Semiconductor under type No. LM324. Connected across the sense winding 26 and in parallel with each other are a load resistor 52, capacitor 54 and oppositely poled diodes 56 and 58.

In operation, under balanced conditions, no current flows in the sense winding 26. In the event of a fault the imbalance or residual current sets up a flux in the toroidal core of the current transformer 24 to generate an e.m.f. across the winding 26 which causes a current to flow through resistor 52. The magnitude of the resulting voltage developed across resistor 52 is dependent upon the magnitude of the residual current.

The a.c. or d.c. voltage across resistor 52 is applied as an input voltage to the window comparator 50 and if it exceeds the reference voltage across resistor 30 or 32, the comparator 46 or 48 as the case may be provides an output signal by way of diode 60 or 62 and resistor 64 to the non-inverting input 66a of a comparator 66; the inverting input 66b of comparator 66 being coupled to the junction of resistors 30 and 32, to provide a reference voltage. The comparator 66 may be an element of a quad op-amplifier as aforementioned.

The signal applied to input 66a of the comparator 66 causes the voltage level at the output 66c to increase positively and if the fault leading to the imbalance current persists for a predetermined period defined by resistor 64 and the parallel combination of resistor 68 and capacitor 70, the positive-going voltage at the output 66c of the comparator 66, is coupled by way of a resistor 72 to the gate 74a of a thyristor 74, turns on the thyristor 74 to energise the solenoid 22a and thus open the switch 22 to interrupt the a.c. supply to the load 20.

Thus by the use of a window comparator 50 it is possible quickly to disconnect the a.c. supply on the positive or negative-going half cycles or pulsating d.c. components of the a.c. supply.

RCD's such as the R.C.D. of Fig.l, which use a current transformer for the detection of residual currents due to a fault condition will, of course, become inoperative in the event that the sense winding goes open circuit. In such an event the connecting means would not be actuated to disconnect the a.c. supply to the load. The user would not be aware of this potentially hazardous situation but, on the contrary, he would feel safe in his reliance on the protection provided by the R.C.D.

Referring again to Fig.l, means for detecting an open circuit in the sense winding 26 comprises a resistor 80 coupled between the positive rail 42 and the end 26a of the sense winding.

In normal operation a very small d.c. current flows from the rail 42 to the zero volt rail 44 by way of resistor 80, the parallel combination of the sense winding 26 and resistor 52, and the resistors 32, 34. As the resistance of the sense winding 26 is very much smaller than that of resistor 52, the volt drop across the parallel combination is small and well below the predetermined value to be detected by the window comparator 50.

If the sense winding goes open circuit the current supplied by way of resistor 80 flows substantially through resistor 52 and the potential developed across it is greater than the said predetermined magnitude but still less than the breakdown voltage of diode 58 and is thus detected by comparator 46 with the result that the thyristor is triggered on to energise the solenoid 22a and thus open the switch contacts 22.

Thus in the event that the sense winding 26 circuit is open-circuited the contacts 22 are automatically opened to cut-off the a.c. supply to the load.

Electronic, as opposed to electro mechanical, R.C.D.'s normally rely for their operation on an auxiliary d.c.

power supply derived from the a.c. supply to be monitored. One requirement of such R.C.D.'s is that they operate within their required specification over a wide range of magnitudes of the a.c. supply voltage, typically from the "Safety Extra Low Voltage" (SELV) as defined by the Institution of Electrical Engineers to the full mains supply voltage. The SELV is 50 volts r.m.s.

or less.

The above problem can be aggravated if the a.c. supply is switched on by closure of switch 22 in the presence of a fault when the supply voltage is at a minimum and is further aggravated if the neutral conductor is open circuit.

Referring again to Fig. 1 there is shown means for inhibiting the supply of d.c. power from the rectifier 36 to the R.C.D 10 until the magnitude of the d.c. voltage is equal to or greater than a predetermined value sufficient to operate the device comprising a silicon controlled switch 82 having a pnp transistor 84 and an NPN transistor 86 connected as shown and a programming zener diode 88.

In operation, if switch 22 is closed to connect the a.c.

supply 18 to the load 20 in the presence of a fault when the amplitude of the a.c. is small, the d.c. supply to the R.C.D. comparator 50 is inhibited by the programmable SCS circuit 82, comprising transistors 86, 88 and zener diode 88, while the capacitor 37 charges until the voltage across it exceeds the sum of the zener breakdown voltage Vz of the diode 88 and the base-emitter voltage Vbe of transistor 84. This causes the SCS to switch on by regenerative action and supply sufficient current at a voltage level just below (less than 1 volt) the voltage across capacitor 37 for a time sufficient to allow the RCD circuit to operate and cause the solenoid 22a to open the contacts 22.

Resistors 90 and 92 provide a high degree of immunity against false triggering of the programmable SCS, due to noise. Capacitor 94 provides a similar function at high frequencies. Without the SCS "hold-off" circuit the auxiliary d.c. power supply circuit 36, 37 could be insufficient to enable the R.C.D. 10 to function properly under conditions of low a.c. supply voltage or the loss of the neutral supply or a high impedance between the supply earth and the device earth or a combination thereof. The voltage level at which the SCS is switched on can be varied by substituting a zener diode of the appropriate breakdown voltage for the diode 88.

The SCS 82 comprising discrete transistors can be replaced by a discrete silicon unilateral switch or by an SCS or SUS in the form of an integrated circuit which could be designed to switch at a predetermined voltage or to be programmable. Resistors 90, 92 and capacitor 94 are provided as shown to provide enhanced noise immunity. Resistors 90, 92 have values substantially greater than the impedance presented by the base-emitter junctions of transistors 84, 86 and noise immunity is improved because the source would have to generate a voltage across resistors 90 and 92, sufficient to exceed the base-emitter voltages Vbe of transistors 84 and 86, respectively. The capacitor 94 presents a relatively low impedance to high frequency noise, thus making it difficult for the high frequency noise to switch on transistors 84, 86.

Fig.2 shows one example of a known test circuit for an R.C.D. which comprises a test resistor 98 connected in series with a test switch 96 between the line and neutral conductors, the circuit including a single turn on the current transformer 24.

When the test switch 96 is closed the resulting current flowing from line to neutral by way of resistor 98 sets up a flux in the transformer which induces an e.m.f. in the sense winding 26. This e.m.f. is detected and a control signal is generated to fire the SCR 74 to cause the contacts 22 to open.

Referring again to Fig. 1, there is shown a test circuit according to the invention comprising a manually-operable test switch 96 and test resistor 98 coupled in series between the line 12 and neutral 14 conductors by way of a winding 100 having a plurality of turns on the current transformer 24. As aforementioned, closure of the switch 96 causes a current to flow in winding 100 and the resultant e.m.f. induced in sense winding 26, after detection causes the SCR 74 to fire and energise the solenoid 22a, thereby to open the contacts 22.

The test circuit can be further developed to facilitate the detection of one or more of the loss of the supply neutral connection, the reverse connection of the line and neutral conductors, and the loss of the supply earth connection.

The detection of the loss of supply neutral connection could be achieved by means of a resistor coupled betweenthe end 100a of test winding 100 and the earth conductor 16. However, any current flowing between neutral and earth by way of the resistor could interfere with the operation of the test circuit in the sense that it would add to or subtract from the ampere-turns determined by the resistor 98 when the test switch 96 is closed. This drawback can be alleviated or eliminated in a development of the test circuit wherein the test winding is provided as a bifilar or dual winding which provides two current paths, a first for the test current and a second for the neutral-earth current, such that the current in the second winding (neutral-earth), does not substantially interfere with the ampere-turns in the test winding.

The bifilar winding may be so wound that the start of the windings is connected to the neutral conductor and the two ends are coupled to the test circuit and the loss of neutral and/or earth detector respectively.

Referring again to Fig. 1 there is shown such an arrangement in which the winding 100 is shown as a bifilar winding having the start 100b of the windings coupled to the neutral conductor 14, a first end 100a connected to the test resistor 98 as previously described and a second end 100c coupled through a resistor 102 and diodes 104, 106 to the earth conductor 16.

In operation, and under normal operating conditions, current from the auxiliary d.c. power supply obtained from the line voltage by way of resistor 38, capacitor 40 and the bridge rectifier 36 returns to the neutral conductor. However, if the supply neutral connection goes open circuit, the current is routed by way of the winding 100b - 100c, the resistor 102 and diodes 104, 106 to earth. The magnitude of this current multiplied by the number of turns in the winding, generates sufficient flux to induce in sense winding 26 a voltage high enough to be detected by the window comparator 50 with the result that the thyristor 74 is triggered on, to energise the solenoid 22a and thus open the contacts 22 as hereinbefore described.

To block the flow of current to earth under normal conditions and also to reduce the possibility of "nuisance tripping", up to a pre-determined level, when the neutral potential could be substantially higher than the earth potential, zener diodes 104, 106 coupled back to back and in series with resistor 102 are included in the circuit. However, if the neutral potential with respect to earth exceeds the said predetermined level, the solenoid 22a is automatically triggered to open the switch contacts 22, thus providing protection against a dangerously high potential difference between neutral and earth conductors.

The circuit is also capable of detecting if the line and neutral connections are reversed. If the connections are reversed, the potential of the neutral conductor will be greater than the blocking potential of the diodes 104, 106 to cause a current to flow through winding loob 100c, resistor 102 and diodes 104, 106 to earth. The magnitude of this current is arranged to be sufficient to develop a voltage across the sense winding 26, greater than the predetermined value necessary to cause the R.C.D. 10 to open the contacts 22 as hereinbefore described.

A further development of the invention includes the provision of a bleed resistor 108 between the line and earth conductors.

Under normal operating conditions a small current, determined by the value of resistor 108 is allowed to flow from line to earth. However, if the supply earth connection is lost, the small current from the line conductor is routed by way of diodes 106, 104, resistor 102 and winding 100c - 100b to the neutral conductor.

Again, the ampere-turns set up in the winding 100c - 100b are sufficient to induce a voltage across the sense winding 26 which is greater than the predetermined value to operate the R.C.D. 10 as hereinbefore described to open the connection 22.

In the circuit as described the loss of neutral and the loss of earth detectors are also operative in the event that the supply line and neutral connections are reversed.

Claims

1. A test circuit for a residual current operated device including a current transformer through which the line and neutral conductors connected between an a.c.

source and a load are arranged to pass, the test circuit comprising a test switch and a test resistor arranged to be coupled in series between the line and neutral conductors and by way of at least two turns on the current transformer thereby to generate a test current on closure of said test switch.

2. A test circuit according to claim 1, wherein said test switch is manually operable.

3. A residual current operated device comprising means for sensing an imbalance between the currents flowing in the line and neutral conductors connected by way of connecting means between an a.c. source and a load, control means coupled to the sensing means and arranged to provide a control signal to open said connecting means if the magnitude of said imbalance exceeds a predetermined value, said means for sensing comprising a current transformer through which said conductors are arranged to pass, and a sense winding on said transformer for providing an output signal dependent upon the imbalance of the current flowing in the line and neutral conductors in combination with a test circuit according to claim 1 or 2 wherein closure of said test switch results in a test current of such a magnitude as to induce a potential difference across said sense winding to cause the control means to open said connecting means.

4. A device according to claim 1, 2 or 3, wherein the test winding is a bifilar or dual winding, each having at least two turns arranged to provide two current paths, a first for the test current and a second for neutral-earth current, the arrangement being such that the test current through the first winding is substantially unaffected by current through the second winding.

5. A device according to claim 4, comprises means for deriving an auxiliary d.c. power supply from said a.c.

source and the test circuit comprises means for detecting an open-circuit in the neutral conductor and/or a reversal of the line and neutral conductors, wherein said second winding is coupled between the neutral conductor and an earth conductor whereby an open circuit in the neutral conductor causes a.c. current from said auxiliary power supply to flow through said second winding to earth, the a.c. current causing a potential difference to be induced in said sense winding sufficient to cause said control means to open said connecting means.

6. A test circuit according to claim 5, wherein the second winding has one end coupled to the neutral conductor and the other end coupled to the earth conductor by way of a resistor and first and second diodes connected back to back.

7. A device according to claim 6, wherein the diodes are zener diodes.

8. A device according to claim 6 or 7, wherein the test circuit includes means for detecting an open circuit in the earth conductor, between the source and load comprising a resistor coupled between the line and earth conductors whereby in normal operation a small, known current flows between line and earth and in the event that the earth connection is open circuit, said current flows through said second winding by way of said diodes and resistor, thereby to induce a potential difference across said sense winding sufficient to cause said control means to open said connecting means.

9. A device according to any one of claims 3 to 8 including a d.c. power supply and means for inhibiting the supply of d.c. power to the operational part of the device until the magnitude of the d.c. voltage is equal to or greater than a predetermined value.

10. A device according to claim 9 wherein the means for inhibiting comprises a serial electronic switch means coupled between the d.c. source and the device, the switch being arranged to close when the d.c. voltage applied to it is equal to or exceeds said predetermined value.

11. A device according to claim 10, wherein said switch means is programmable selectively to control said predetermined value.

12. A device according to claim 10 or 11, wherein said switch means is a silicon controlled switch.

13. A device according to claim 10, 11 or 12, wherein the switch means comprises a first PNP transisitor having its emitter coupled to said d.c. source, its collector coupled to said device and its base coupled to said device by way of a zener diode whereby the said predetermined value is determined by the sum of the potential differences across the base-emitter junction (Vbe) of the first transistor and the zener diode.

14. A device according to claim 13, further comprising a second, NPN transistor having its base and collector coupled to the collector and base respectively of said first transistor and its emitter coupled to said device, first and second resistors coupled between the base and emitter of said first and second transistors respectively, the values of the resistance of said resistors being lower than the base-emitter impedance of their respective transistors in the non-conducting state.

15. A device according to claim 13 or 14, further comprising capacitive means coupled between the base and emitter of said first transistor.

16. A device according to claim 10 or 11, wherein said switch means comprises a silicon unilateral switch.

17. A device according to any one of claims 3 to 16 comprising means for detecting an open circuit in the sense winding circuit and for opening said connecting means in consequence thereof.

18. A device according to claim 17, wherein said means for detecting comprises means for generating a potential difference across said sense winding substantially independent of the load current flowing between said source and said load, and said potential difference increasing from a relatively low, first value when the sense winding is continuous to a relatively high, second value if the sense winding is open circuit, the second value being applied to open said connecting means.

19. A device according to claim 18, wherein said potential difference is coupled to said control means.

20. A device according to claim 18 or 19, wherein the means for generating a potential difference comprises at least two resistors connected in series across a source of a d.c. potential, said sense winding being coupled in parallel with one of said resistors, the magnitude of the resistance of said one resistor being substantially greater than that of said sense winding.

21. A device according to any one of claims 3 to 20, in which said control means is arranged to provide said control signal if the magnitude of said imbalance signal exceeds said predetermined value in a positive or a negative-going direction.

22. A device according to any one of claims 3 to 21, in which the control means comprises a window comparator.

22. A device according to any one of claims 3 to 21 in which the sense means is arranged to provide a sense signal which alternates about a datum level, the control means comprises first and second comparators in the form of operational amplifiers coupled across a source of a d.c. supply, the inverting input of a first operational amplifier being coupled to a first d.c. level at a predetermined magnitude greater than said datum level, the non-inverting input of a second operational amplifier being coupled to a second d.c. level at a predetermined magnitude less than said datum level, the sense signal being coupled to the non-inverting and inverting inputs respectively of said first and second operational amplifiers.

23. A device according to claim 22, wherein the outputs of said first and second operational amplifiers are coupled by way of delay means to said connecting means thereby to delay operation to open said connecting means until said imbalance has persisted for a predetermined period.

24. A device according to claim 22 or 23, wherein said connecting means comprises switch contacts connected in said conductors and electrically operable from a closed to an open condition on the energisation of an inductive device coupled in series with a gated semiconductor switch, the gate electrode of the switch being coupled to the outputs of said first and second operational amplifiers.

25. A device according to claim 24, wherein said connecting means is a solenoid-operated device and said gated semiconductor device is a thyristor.

26. A test circuit substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

27. A residual current operated device according to claim 3, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.