GB2255456A - Residual current operated device - Google Patents

Abstract

A residual current operated device (10) comprises means (24, 26) for sensing an imbalance between currents flowing in line and neutral conductors (12, 14) between an a.c. source (18) and a load (20), comparators (46, 48, 66) coupled to the sensing means and arranged to provide a control signal (66c) if the magnitude of said imbalance exceeds a predetermined value in either direction, and a gated oscillator (75) responsive to the control signal to provide pulses at a frequency substantially lower than that of the a.c. source, e.g. at about 1 Hz for a 50 Hz supply for causing a thyristor (74) to actuate the switch (22). Under certain conditions, such as reversal of the line and neutral connections to the device, when the device would otherwise be activated at the supply frequency, the reduced frequency may avoid destruction of the thyristor and/or the switch, and warn the user. The gated oscillator comprises potential divider (77, 77a) cooperating with positive feedback resistor (79) to form the non-inverting input of op-amp (75), the inverting input of which is coupled via a resistor to a reference potential and via series connected resistor (79a) and diode (82) to the op-amp output.

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 sum should be zero. If there is a fault, such as an insulation fault, in the circuit such that current flows 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 predetermined level.

Another fault which should be detected by a R.C.D. is the reversal of the line and neutral connections. In such a situation if the R.C.D. has only a single-pole switch arranged, on detection of a fault, to break the line supply to a load the reversal of the line and neutral conductors would result in the neutral conductor being opened by the switch.

This results in a potentially hazardous situation in the sense that the user may feel secure in the belief that operation of the R.C.D. has disconnected the line supply to the load when, in fact, the neutral supply has been disconnected and the load neutral is at line potential with respect to earth.

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 conductor 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 half cycles of an a.c. waveform. It is also important that the R.C.D. should operate when residual current flows as a result of loads giving rise to pulsating d.c. superimposed onto a low level of smooth d.c.

According to the invention there is provided 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 control means comprising a gated oscillator arranged to provide said control signal in the form of pulses at a pulse-repetition frequency substantially lower than the frequency of said a.c. source.

Preferably, the control means is arranged to generate the first pulse substantially instantaneously with the sensing means detecting that the said imbalance exceeds the predetermined value. If the source frequency is 50Hz the control signal frequency may be less than 10Hz and preferably about 1 Hz. The connecting means may be an electro-mechanical switch such as a solenoid or relay operated device energised by a semi-conductor switch, such as a thyristor, having a control electrode arranged to receive said control signal. The device may include a current transformer and the control means may further comprise 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.

The R.C.D. may be a device as described and claimed in our co-pending British Patent Applications Nos: 9011799.5 (our file 32360), 9011795.3 (our file 32361) and 9011803.5 (our file 32362) filed on 25th May 1990.

One embodiment of a residual current operated device according to the invention will now be described by way of an example with reference to the accompanying drawing.

Referring to the drawing, 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 single-pole manually-operable switch 22, which is also operable independently by a solenoid 22a is connected in the line conductor 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 half-wave rectifier 36, a voltage-dropping resistor 38, smoothing capacitor 40, and a zener diode 41 to stabilise the d.c. voltage at 16 volts. 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 the 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 the parallel combination of resistor 52 and capacitor 54. 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.

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, could be used as a control signal to open the switch 22. For example, the ouput 66c could be coupled by way of a resistor 72 to the gate 74a of a thyristor 74, to turn on the thyristor 74 and to energise the solenoid 22a and thus open the switch 22 to interrupt the a.c. line supply to the load 20.

This would work quite satisfactorily in most applications of the R.C.D. but can cause problems if the output of the comparator 66 remains high, for example if the line and neutral connectors are reversed. In such a case the high output of comparator 66 would cause the thyristor 74 to conduct during each positive half cycle of the a.c. supply.

This would make the short duty cycle rated solenoid typically used in this application operate at 50 Hz, resulting in excessive power dissipation therein and the eventual destruction of the solenoid winding and the thyristor.

Referring again to the drawing, this problem is alleviated by coupling the output of the comparator 66 to the non-inverting input 75a of a fourth comparator 75, configured as a gated oscillator by way of a relatively high value resistor 77.

The comparator 75 may be the fourth element of a quad opamplifier as aforementioned. The output 75c of comparator 75 is coupled to its non-inverting input 75a by way of a relatively lower value resistor 79 to provide hysteresis.

The output 75c of the comparator 75- is coupled by way of resistor 72 to the gate electrode of the thyristor 74.

At switch on of the supply to the R.C.D. circuit, the state of the input offset voltages of the op-amps could be such that the output 75c of the comparator 75 would be high even without the presence of any residual current. This would result in a gate drive signal to the thyristor 74 which would energise the solenoid 22a and open the switch 22 when the a.c. circuit is in fact performing normally.

To prevent this from happening, the negative pin 81 of the comparator integrated circuit is coupled to the negative rail 44 by way of a resistor 83 and the inverting input 75c of the comparator 75 is coupled by way of a capacitor 78a to the junction of the pin 81 with the resistor 83. When the a.c.

supply is switched on the voltage developed across the resistor 83 due to the supply current taken by the comparator integrated circuit is sufficient to maintain the inverting input 75b of the comparator 75 at a level about 50mV above that of the non-inverting input 75a, to prevent the output 75c from going high and switching on the thyristor 74. This also prevents regenerative action due to the feedback resistor 79 which would have caused the non-inverting input 75a to be pulled up by the output, which would, in turn, cause the output 75c to go high and latch on the thyristor 74.

However, when a current is induced in the sense winding 26 of the current transformer 24, it is detected by the window comparator 50 as hereinbefore described. The output of comparator 50 causes the voltage level on the non-inverting input 66a of comparator 66 to go higher than that of the reference voltage on input 66b and causes the output 66c to go high. The output 66c coupled to the non-inverting input 75a of comparator 75 causes the voltage level on input 75a to exceed that of input 75b. The output 75c thus goes high to provide a gate drive signal to the gate 74a to turn-on the thyristor 74, energise the solenoid 22a and open the switch 22 in the line conductor.

As aforementioned it is important to ensure that the thyristor 74 is not continuously triggered on as this could cause the eventual destruction of both the thyristor and the solenoid 22a. This is achieved by configuring the comparator 75 to operate under these conditions as a gated oscillator which is arranged to operate such that when the output level at the output 66c of comparator 66 goes high in response to a detected fault, the output 75c of comparator 75 follows substantially simultaneously to trigger thyristor 74 with minimal delay. This is important to avoid any substantial delay between the sensing of a residual current in the sense winding and the consequent disconnection of the supply.Thereafter, if the fault persists, such as in the case of the device having been reverse wired (line and neutral reversed) to the supply, the comparator 75 operates as an oscillator to gate the thyristor 74 at an acceptably low frequency of, say, 1Hz. The mark-space ratio is determined by the charging and discharging rate of the capacitor 78a; the charge-up time constant of which is dependent upon the values of capacitor 78a and the resistors 79a and 80 (ignoring the forward resistance of the diode 82) whereas the discharge time constant is dependent upon the values of capacitor 78a and resistor 80 (ignoring any leakage current through the diode 82).

The values of the resistors 79a, 80 is such that the discharge time constant is much longer than that of the charge time constant resulting in a small mark-space ratio to provide a short pulse of about 25m sec. duration about once every 800m secs. This has two effects, firstly to open switch 22 substantially instantaneously if the device is latched on again and secondly to facilitate a low repetition rate audible noise to bring to the attention of the user/installer that there is something wrong with the connections of the R.C.D.

The switch 22 cannot be made to remain closed under these conditions. The low repetition rate pulses could also be used to provide a visual indication of a fault.

The d.c. coupling first described between the output 66c of comparator 66 and the gate of the thyristor 74 could be replaced by an a.c. coupling but due to the sinusoidal nature of the a.c. supply it would be difficult and expensive to achieve reliable operation due to synchronization problems.

The use of the window comparator 50 makes it 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.

Claims (12)

1. 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 control means comprising a gated oscillator arranged to provide said control signal in the form of pulses at a pulse-repetition frequency substantially lower than the frequency of said a.c. source.

2. A device according to claim 1 wherein the control means is arranged to generate a first said pulse substantially at the instant when the said imbalance exceeds the predetermined value.

3. A device according to claim 1 or claim 2 wherein the control signal frequency is less than 10Hz.

4. A device according to claim 3 wherein the control signal frequency is about 1 Hz.

5. A device according to any preceding claim wherein the connecting means is an electro-mechanical switch energised by a semi-conductor switch having a control electrode arranged to receive said control signal.

6. A device according to any preceding claim wherein the means for sensing a current imbalance includes a current transformer.

7. A device according to any preceding claim wherein the control means further comprises a window comparator coupled to the output of the sensing means.

8. A device according to any one of claims 1 to 6 wherein the gated oscillator comprises an operational amplifier having a non-inverting input coupled to the sensing means via a first resistor, an inverting input and an output, a second, feedback, resistor coupled between the said output and said non-inverting input, a third resistor coupled between said non-inverting input and a source of reference potential, a fourth resistor and a diode connected in series between said output and said inverting input, and a capacitor coupled between said inverting input and the source of reference potential.

9. A device according to claim 8 wherein the control means further comprises a window comparator coupled between the output of the sensing means and the first resistor.

10. A device according to claim 9, wherein the window comparator comprises a plurality of op-amps in the same package as the op-amp of the gated oscillator.

11. A device according to any preceding claim arranged so that the switch means can be opened on the positive and negative-going half cycles of the a.c. waveform of the mains supply.

12. A residual current operated device substantially as hereinbefore described with reference to the accompanying drawings.