AU2003201306B2

AU2003201306B2 - Residual current device

Info

Publication number: AU2003201306B2
Application number: AU2003201306A
Authority: AU
Inventors: Pat Ward
Original assignee: Shakira Ltd
Current assignee: Shakira Ltd
Priority date: 2002-07-15
Filing date: 2003-03-17
Publication date: 2007-06-07
Anticipated expiration: 2023-03-17
Also published as: GB0216396D0; GB2390944B; AU2003201306A1; GB2390944A

Description

A Residual Current Device

O

This invention relates to a residual current device (RCD).

For the purposes of this specification, an RCD is defined as any device which can detect a residual current in an A.C. or D.C. electricity supply and initiate an action in response to such detection. Such action may be the opening of contacts in the supply conductors, or the raising of an alarm.

In an RCD, it is known to charge a capacitor and, in response to the detection of a residual current, to discharge the stored charge through a solenoid winding to operate a set of contacts which disconnect the supply to a circuit protected by the RCD. Conventionally, the capacitor is initially charged via a relatively high impedance, and once the necessary charge level has been acquired, the capacitor can be discharged through the solenoid winding. It is important in such applications that the capacitor is not discharged too early during the charging process because of the risk that the charge released into the solenoid winding will not be adequate to ensure the correct and reliable operation of the solenoid. This sequencing requirement can result in an unacceptable delay between the commencement of charging of the capacitor and activation of the solenoid, precluding earlier activation of the solenoid.

It may also be necessary to operate such a circuit over a wide supply voltage range. If the capacitor is to acquire sufficient charge at the minimum operating voltage, the charge on the capacitor will be substantially higher at the maximum operating voltage. This will require an operating voltage for the capacitor to cater for the maximum voltage likely to develop across it. A possible solution to this problem would be to use a voltage regulating means such as a zener diode so as to limit the voltage across the capacitor and thereby facilitate the use of a capacitor with a lower operating voltage. However, this will result in surplus current being diverted through the zener diode once the capacitor has acquired the desired charge. This in turn will result in a power dissipation problem which will vary with the supply voltage.

A further problem can arise in that the charge on the capacitor can be drained away due to leakage factors or a need to provide power to other parts of the circuit. This in turn gives rise for a need to provide a continuous charging current to the capacitor. The need to provide for a continuous charging current to offset drainage and the need to increase the charging current to achieve the desired charge on the capacitor at minimuin operating voltage will result in increased power dissipation in the circuit, especially at the maximum operating voltage. This in turn can give rise to problems of heat dissipation, increased component size, increased costs and reduced reliability.

It is the purpose of the present invention to provide an RCD in which these problems are avoided or mitigated.

Accordingly, the present invention provides a residual current device for connection to an electricity supply, the residual current device including circuit means for detecting a residual current and discharging a capacitor in response to such detection, the device further including a charging circuit for the capacitor comprising a controlled switching device connected in series with the capacitor across a power source, and control means for turning the switching device alternately on and off in dependence upon the voltage on the capacitor in order to maintain said voltage substantially above a predetermined level.

In one embodiment the control means comprises a zener diode connected in series with a resistor across the power source, the junction between the resistor and the zener diode being connected to the control terminal of the first controlled switching device.

In a second embodiment the control means comprises a second controlled switching device connected in series with a resistor across the power source, the junction between the resistor and the second controlled switching device being connected to the control terminal of the first controlled switching device and the control terminal of the second controlled switching device being connected to a tap of a voltage divider connected across the capacitor.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a first embodiment of the invention; Fig. 2 is a circuit diagram of a second embodiment of the invention; and Fig. 3 is a circuit diagram of a third embodiment of the invention.

Fig. 1 is a circuit diagram of a first embodiment of RCD according to the invention for connection to an A.C. mains supply having live L, neutral N and earth E conductors. Except for the charging circuit for the capacitor C1, to be described in detail below, the general operation of such devices is well known and therefore will only be described briefly in the present specification.

On the load L side of the RCD the live and neutral conductors L and N respectively are passed through a current transformer CT having a secondary winding W. Under normal conditions, when circuit breaker contacts S1, S2 in the live and neutral conductors are closed, the current flowing in the live conductor L from the mains supply to the load L will equal the current returning in the neutral conductor N from the load to the supply. There are, therefore, equal and opposite currents flowing through the transformer CT so that the current induced into the secondary winding W is zero.

However, if an earth fault occurs on the load side of the RCD there will be some current flow to ground, leading to an imbalance in the currents flowing in the live and neutral conductors.

This induces a non-zero current in the secondary winding W which is measured by an electronic circuit EC. If the current induced in the secondary winding W exceeds a predetermined threshold, indicative of an unacceptable level of earth fault current, the circuit EC will cause a silicon controlled rectifier SCR2 to be triggered (turned on). This will discharge the capacitor C 1, which is normally maintained fully charged by a charging circuit to be described below, through a solenoid SOL. A large discharge current will now flow through the relatively low impedance winding of the solenoid, activating the solenoid. The solenoid SOL is coupled in known manner (not shown) to the contacts S 1, S2 in the live and neutral conductors L, N such that activation of the solenoid causes these contacts to open and thereby disconnect the mains supply from the load L.

The circuit for charging the capacitor C1 is powered by the live L and neutral N conductors of the A.C. mains supply. A resistor Rn drops the supply voltage to a convenient working level, and the AC mains supply is full-wave rectified by the bridge rectifier X1. The output voltage of X 1 comprises positive-going pulses which fall to zero voltage at each crossover point of the AC mains supply. The full wave rectified pulses are fed to the anode of a silicon controlled rectifier SCR1 which is connected in series with the capacitor C1 across rectifier X1. A zener diode ZD1 is connected in series with a resistor R1 across the output of X1. ZD1 limits the voltage at the gate of SCR1 to the zener breakover voltage. A current flows directly from the positive-going pulses of the supply into the gate of SCRI via the resistor R1, with the result that SCRI is turned on when the supply voltage is above a certain level. When SCRI turns on, a current flows through capacitor C1, charging it up. When the voltage on C1 exceeds the breakover voltage of ZD 1, the gate-cathode junction of SCR1 will be reverse biased and SCR1 will turn off at the next zero crossover of the AC mains supply. ZD1 therefore sets the minimum voltage level on C1 at which the charging cycles will be interrupted. It should be noted that regardless of the point in time that the voltage on C1 causes the gate cathode junction of SCR1 to become reverse biased, SCR1 will remain on until the subsequent zero crossover of the AC mains supply with the result that the voltage on C 1 will overshoot the reference level set by ZD1.

The electronic circuit EC is powered by the voltage on the capacitor C 1 via a resistor R2.

Since C1 will continue to supply current to the electronic circuit once SCR1 is turned off, the charge on C 1 will gradually fall. When the voltage on C1 falls below the zener breakover voltage, SCR1 will turn on again and repeat the charging phase. These on/off conduction cycles of SCR1 will be repeated to maintain the voltage on the capacitor C1 above the zener breakover voltage. The use of an SCR as the switching device in series with C 1 allows C 1 to acquire a voltage level higher than that of ZD1 reference level with the result that the circuit will have a degree of hysteresis which can be used to ensure substantially longer non-charging periods than charging periods, and thereby optimise power consumption. Diode D1 prevents breakdown of SCR1 that might result from reverse biasing of the gate/cathode junction. D1 could also be located between the cathode of SCR1 and C 1 to achieve the same effect.

The minimum operating voltage of the circuit is determined by R1, which limits the current flow into the gate of SCRI, the minimum gate current requirement of SCR1, and the minimum charge on C 1 required to ensure correct activation of the solenoid. Once the supply voltage exceeds a certain threshold, SCR1 will conduct and C1 will acquire a charge. It follows that at lower supply voltages, C1 will take longer to acquire a given charge than for higher supply voltages. Nonetheless, the power consumption of the circuit will be substantially the same over a wide supply voltage range.

The purpose ofX1 is to provide for rectification of the AC mains supply and to enable SCR1 to conduct on both half cycles of the AC mains supply. However, if conduction of SCR1 on both half cycles of the AC mains supply is not essential, X1 can be omitted, and SCRI will perform the dual functions of AC mains supply rectification and charging of the capacitor. In the case where X1 is omitted, the neutral conductor N is connected directly to R1 and the anode of SCR1, while the live conductor L is connected directly to ZD1 and the cathode of SCR2.

When C1 discharges through the solenoid due to SCR2 being turned on, the supply voltage to EC will be removed. Activation of the solenoid will cause the circuit breaker contacts S1, S2 to open and removal of the supply to the load L. However, the circuit can also be used as a residual current monitor (RCM) or in similar applications. When used as an RCM, SOL can be replaced by an audio or visual alarm to indicate the presence of a residual current above a predetermined level. In this type of application, SCR2 will be turned on as before and C1 will now discharge through the indicator causing activation of the indicator. Once C 1 has discharged, SCR1 will revert to the charging condition and the EC will be supplied with power again. If the EC still has an input signal above the predetermined threshold, SCR2 will be turned on again after a delay time set by EC, and the cycling process will be repeated for as long as the EC input signal remains above the predetermined threshold. In this application, the alarm indicator will be activated intermittently in line with the charging and discharging cycles of the circuit. The value of Rn can be chosen to optimise the charging cycles.

Figure 2 shows an arrangement whereby the supply to EC is not removed immediately on discharge of C1. In this arrangement, a diode D2 and a capacitor C2 are added to the circuit as shown in Fig. 2. Capacitor C2 will acquire a charge via R2 and D2, and D2 will prevent any of the charge on C2 from flowing into the solenoid when SCR2 is turned on. C2 acts as a reservoir capacitor and should be of sufficiently large value to maintain the supply to the electronic circuit until C 1 has acquired sufficient charge during subsequent charging periods.

Alternatively, the electronic circuit EC could be supplied directly from the AC mains supply so as to reduce the drain on C1, if desired.

Figure 3 shows another embodiment of the circuit, whereby a bipolar transistor TR1 is used to control the conduction cycles of SCR1. Transistor TR1 is connected in series with the resistor R1 across the power supply, in parallel with SCR1 and C1, such that the junction between R1 and TRI is connected to the gate of SCR1. The base of the transistor is connected to the tap of a voltage divider R3/R4 connected across the capacitor C1. When the voltage on C1 reaches a certain threshold, TR1 will turn on due to the voltage on its base provided by R3/R4.

When TRI turns on, the gate of SCR1 will be pulled low and SCRI will turn off at the next zero crossover of the supply. When the voltage on C1 falls below a certain reference level, TR1 will turn off and allow SCR1 to revert to the charging cycle, thereby maintaining the voltage on the capacitor above the reference level.

The foregoing circuits are normally supplied via the live and neutral conductors of the A.C.

mains supply. However, it may be desirable to ensure the continued operation of the circuit in the event of a loss of supply neutral. The mains earth is normally at the same potential as neutral and can therefore act as an alternative neutral in the event of a loss of neutral. To provide for this condition a connection is made to the earth terminal via resistor Re. The values of Rn and Re are chosen to encourage the supply current to flow to the capacitor C 1 via the neutral rather than the earth when the neutral is present. In the case of the earth circuit being connected via Re, a possible problem might arise whereby the live and neutral connections may inadvertently be reversed. Whilst the circuit would still function correctly, the reverse L N condition would cause a current to flow through Rn and Re, which might cause them to burn out if their combined impedance was relatively low. A PTC (positive temperature coefficient) device is therefore connected in series with the resistor Re in the earth circuit to overcome such a problem. The characteristic of the PTC device is such that it has a relatively low impedance at ambient temperature, but the impedance increases substantially with increasing temperature. If the temperature of the PTC increases due to the current flow through it or due to an increase in local temperature caused by a temperature rise in nearby components, the impedance of the PTC will increase such as to reduce the current flow in the circuit. The reduced current flow will ensure that component burn-out is avoided.

Refinements can be made to the above circuits without undermining their basic functionality.

SCR1 could be replaced by a transistor so as to provide more precise control of the voltage level on C1 and reduce overshoot and hysteresis. One or both of Rn and Re could be replaced by PTCs.

Also, in the embodiments above the power source for charging the capacitor C1 is the A.C.

mains, either after rectification in X1 or, in the alternative embodiment referred to where the rectifier X1 is omitted, the A.C. mains itself. However, it is possible for the power source for charging the capacitor C 1 to be an auxiliary, independent power source, such as an independent A.C. or D.C. supply, since it is not uncommon for an RCD to be powered from a supply other than the circuit being protected due to a need to maintain supply to the RCD after the fault has been cleared, for example, for alarm or fault indicating purposes. Replacement of SCR1 with a transistor, for example, would also enable the circuit to be operated from an auxiliary DC supply.

Finally, it will be understood that the utility of the invention is not limited to A.C.

applications, but may also be used in the case of DC applications. For example, in an ungrounded, battery-powered D.C. circuit a Hall effect or giant magneto resistive (GMR) sensor can monitor both the outgoing positive and returning negative lines. If a point in the P OPER\GCP003201306 rspns doc21/05/2007 -8circuit is shorted to ground a DC residual current is detected by the sensor and this detection can be used to trigger SCR2.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims

1. A residual current device for connection to an electricity supply, the residual current device including circuit means for detecting a residual current and discharging a capacitor in response to such detection, the device further including a charging circuit for the capacitor comprising a controlled switching device connected in series with the capacitor across a power source, and control means for turning the switching device alternately on and off in dependence upon the voltage on the capacitor in order to maintain said voltage substantially above a predetermined level.

2. A residual current device as claimed in claim 1, wherein the power source is, or is derived from, the electricity supply.

3. A residual current device as claimed in claim 1 or 2, wherein the controlled switching device is a silicon controlled rectifier.

4. A residual current device as claimed in claim 1, 2 or 3, wherein the control means comprises a zener diode connected in series with a resistor across the power source, the junction between the resistor and the zener diode being connected to the control terminal of the first controlled switching device.

A residual current device as claimed in claim 1, 2 or 3, wherein the control means comprises a second controlled switching device connected in series with a resistor across the power source, the junction between the resistor and the second controlled switching device being connected to the control terminal of the first controlled switching device and the control terminal of the second controlled switching device being connected to a tap of a voltage divider connected across the capacitor.

6. A residual current device as claimed in claim 5, wherein the second controlled switching device is a transistor.

7. A residual current device as claimed in any preceding claim, wherein the electricity supply is an A.C. supply having live, neutral and earth conductors.

8. A residual current device as claimed in claim 7 when directly or indirectly dependent on claim 2, further including means for rectifying the A.C. supply, the said power source being the output of the rectifying means.

9. A residual current device as claimed in claim 7 when directly or indirectly dependent on claim 2, wherein the controlled switching device rectifies the AC supply.

A residual current device as claimed in claim 7, 8 or 9, wherein the earth conductor is connected to the neutral conductor via an impedance which preferentially encourages charging current to flow via the neutral conductor.

11. A residual current device as claimed in claim 10, further including a positive temperature coefficient device in series with said impedance.

12. A residual current device as claimed in any preceding claim, wherein the capacitor is connected in parallel with a series connection of a solenoid and a further controlled switching device, the circuit means turning the further controlled switching device on in response to detection of a residual current whereby the capacitor discharges through the solenoid.

13. A residual current device as claimed in any preceding claim, wherein the circuit means is powered by the voltage on the capacitor such that the power supply to the circuit means is removed upon discharge of the capacitor.

14. A residual current device as claimed in claim 13, wherein discharge of the capacitor activates an alarm, the electronic circuit being arranged to repeatedly discharge the capacitor in the case of a persistent residual current so that recurrent activation of the alarm may be effected. 11 A residual current device as claimed in claim 13, wherein a second capacitor is connected via a diode to the first controlled switching device such that the second capacitor is charged up simultaneously with the first capacitor, and wherein the circuit means is powered by the voltage on the second capacitor such that the power supply to the electronic circuit is not removed upon discharge of the first capacitor.

P \OPERkGCPU003201306 do..2MA5flOO7 12

16. A residual current device substantially as hereinbefore described with reference to and/or as shown in any one of Figure 1, Figure 2 and Figure 3.