GB2251741A - Rapid response ground fault circuit interrupter - Google Patents

Abstract

A circuit interrupter has a single zero-phase current transformer 14 having a toroidal core coupled to supply conductors 11, 12 to sense out-of-balance supply currents. The core has a centre-tapped secondary winding forming two secondary windings 15, 15'. The trip circuit uses a solenoid 13 for isolating the supply. The operating circuit for the solenoid comprises a full wave rectifier bridge 17 and two thyristors 16, 16' with their gates connected directly to the respective secondary windings 15, 15' of the core such that the signals applied to the gates are 180 DEG out of phase. An auxiliary electrical supply 25-29 imposes a voltage on the gate circuit to increase the sensitivity of the interrupter. A test circuit 19, 20 is provided, and may include a constant current arrangement (Fig. 4) or an additional winding (Fig. 5). <IMAGE>

Description

GROUND FAULT CIRCUIT INTERRUPTER This invention relates to ground fault circuit interrupters and in particular to "ALCI's".

Such devices are already known especially to isolate the power supply to a distribution system such as in a domestic dwelling or building. Certain safety regulations or guidelines exist and are proposed to require interrupters at local general power outlets and in some cases for each individual appliance that is to be used. At present interrupters are either too bulky and/or too costly so that they are cumbersome or represent a significant cost as compared to the cost of the appliance itself. This is particularly true for hair dryers and the like.

Presently, ground fault interrupters use a circuit based on an integrated circuit (I.C.) such as Raytheon's "RB 4145". A typical circuit diagram for such an interrupter is shown in Figure 1. From this circuit, it can be seen that two coils 81 & 82 are required as well as the I.C. 83. As can be seen, the circuit contains a large number of components which all add to the expense and size of the interrupter. It should be noted that this circuit does not show a surge compressor circuit or a test circuit facility.

In the applicant's co-pending patent application, GB9018948.1, a ground fault circuit interrupter was proposed which had a typical circuit diagram as shown in Figure 2. From this diagram, it can be seen that only one coil 14 is used to sense the out of balance current flow. The trip solenoid of the line contactor is operated by a thyrister 16 which is switched on by the current which flows in the secondary winding 15 of the coil 14 in an out of balance current flow situation.

This circuit has a test circuit, a snubber circuit and a biasing circuit. Although the circuit works extremely well in most situations especially on 110 volt systems because it uses only a single thyrister 16 to trigger the trip solenoid, the response time may be as long as one cycle in a worst case situation. This response time is considered to be slightly too long for 240 volt systems.

A ground fault circuit interrupter for an electrical supply, comprising at least two supply conductors, contact means for electrically isolating the supply conductors, electrical operating means for operating the contact means, switch means for supplying power from the supply conductors to the operating means, a zero-phase current transformer having a toroidal type core with first and second secondary windings, primary windings being formed on the toroidal-type core by looping the supply conductors through the core to generate a net magnetic flux whenever there is an imbalance in the supply current causing a voltage to be generated in the secondary windings, and the secondary windings being connected to the switch means, wherein the switch means comprises first and second thyristors, the first thyristor having its gate connected to the first secondary winding and the second thyristor having its gate connected to the second secondary winding such that the inputs to the respective gates are phase shifted by 1800.

The conductors are preferably looped around the core at least two or more times.

An auxiliary electrical supply may be provided to bias the switch means towards an operative condition.

Optionally, a switched test circuit, connected between the conductors, is included to simulate a fault condition when the switch of the test circuit is operated.

The test circuit may be a resistor in series with a switch providing a current path between the conductors which by-passes one of the primary windings to simulate on out of balance load. Preferably the test circuit includes a winding on the toroidal-type core arranged to generate in use additional magnetic flux to simulate a fault condition.

The contact means may include movable spring loaded contacts biased towards an open position with respect to co-operating fixed contacts, and a latch to hold each movable contact in the closed position, the latch being mechanically connected to and arranged to be released by the operating means.

The earth fault circuit interrupter may be housed in a plug for an appliance or in an outlet socket of an electrical supply distribution system. The interrupter may also be housed in a plug or socket of an extension cable so that the interrupter can be used for a number of different appliances. The interrupter can then be used beyond the life of the appliances which is not the case if the interrupter is housed in a sealed plug of the appliance. Alternatively, the interrupter could be housed in the distribution board and be used to protect one or more circuits.

A preferred ground fault circuit interrupter according to the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of a typical prior art circuit interrupter; Figure 2 is a circuit diagram of the applicant's similar circuit interrupter; Figure 3 is a circuit diagram of the preferred circuit interrupter according to the present invention; Figure 4 is an alternative test circuit used to test the operation of the circuit interrupter; and Figure 5 is a circuit diagram similar to that of Figure 3 with a modified test circuit.

From the circuit diagram of Figure 3, we can see that the ground fault circuit interrupter has line in and line out terminals. The line in terminals are directly connected to contact means in the form of a line contactor 8 having contacts 9 and 10. The contacts 9 and 10 are connected to the line out terminals by supply conductors 11 and 12. The opening of the contactor 8 is controlled by electrical operating means in the form of a trip solenoid 13.

The contactor 8 is described as being "normally closed" as during normal operation it is in the closed position.

The preferred contactor is of the type which includes movable spring loaded contacts biased towards an open position with respect to co-operating fixed contacts, and a latch to hold each movable contact in the closed position, the latch being mechanically connected to and arranged to be released by the trip solenoid 13.

A zero-phase current transformer 114 is provided which has a toroidal-type core 14 through which the supply conductors 11 and 12 are looped to form a primary winding. Any imbalance in the supply current, as occurs for example, when there is an earth fault or earth leakage will cause a net magnetic flux to be generated within the toroidal core. Two secondary windings 15 and 15', which may be in practice produced by a single centre-tapped winding, are formed on the core 14 with a large number of turns. The net flux generated in the core generates an electrical signal by inducing a voltage across the windings 15 and 15'.

The secondary windings 15 and 15' are connected to switch means for supplying power to the solenoid 13.

The switch means is connected in series with the solenoid across the supply conductors on the output side of the contactor 8 but before the zero-phase current transformer. The switch means comprises four diodes 17 providing a full wave rectifier bridge. Across the bridge are two thyristors or silicon controlled rectifiers (SCR's) 16 and 16'. The gate of the first SCR 16 is connected to one side of secondary winding 15.

The gate of the second SCR 16' is connected to one side of secondary winding 15'. The gate of one SCR would be connected to the north side of one secondary winding while the gate of the other SCR would be connected to the south side of the other winding such that the signal supplied to the respective gates are 1800 out of phase.

The other side of the secondary windings are joined and may be connected directly to the cathode or negative terminal of the full wave bridge.

Alternatively, as shown in Figure 3, the common output of the secondary windings may be connected to a biasing circuit comprising resistors 25, 26, diode 27, capacitor 28 and zenner diode 29. This biasing circuit inserts a biasing voltage into the triggering circuit so that the gate voltage or trip voltage of the SCR's is approached in the non-fault situation. In this way, the sensitivity of the ground fault circuit interrupter can be increasing by reducing the voltage required to be generated in the secondary windings to generate a trip condition.

For example, if the gate voltage of the SCR is 0.8 volts and the biasing circuit is set to provide 0.6 volts, a voltage of 0.2 volts only is required to be generated in the secondary windings rather than 0.8 volts as would be required if the windings were connected directly to the cathode terminal.

Resistor 25 limits the current flow through the biasing circuit. Zenner diode 29 ensures that the output voltage is maintained at a substantially constant voltage in use. Resistor 26 is used as a current controller or limiter and diode 27 provides the forward voltage drop of 0.6 volts to maintain the constant voltage increase of the biasing circuit at 0.6 volts.

Capacitor 28 is provided to suppress noise in the circuit. As will be appreciated, the effect of noise in such a circuit will increase the risk of false trips due to an electrically noisy supply or environment. Thus accidental actuation due to noise in the biasing circuit is reduced by the inclusion of a noise suppressor such as capacitor 28. Similarly, capacitors 18 and 18' absurb low intensity high frequency excursions in the power supply generated as a result of noise in the power supplied by the conductors 11 and 12, again, helping to reduce the risk of false trips due to a noisy supply.

A snubber circuit 60, comprising resistor 61 and capacitor 62 is shown connected across the anode and cathode of the SCR's. The snubber circuit helps to alleviate the occurrence of false trips during surging of the supply voltage. False trips due to surging becomes more of a problem as the sensitivity of the ground fault circuit interrupter is increased e.g. by adding a biasing circuit as previously discussed.

A metal oxide varistor (MOV) 24 protects the electrical components against high voltage surges which may otherwise damage them and which occur due to interferrence in the power supply or possibly at switch on of the appliance e.g. a hair dryer.

A switched test circuit comprising switch 19 and resistor 20 is connected across the supply conductors 11 and 12. One end of the test circuit, e.g. the resistor end, is connected to conductor 12 before the toroidal core and the other end is connected to conductor 11 after the toroidal core. This provides a current circuit with only one leg passing through the current transformer to generate an out of balance current flowing through the current transformer when the switch 19 is operated. The current through the test circuit is limited by resistor 20. The value of resistor 20 is chosen depending upon the intended voltage on which the circuit interrupter is to be operated and the required test current.

A test circuit as shown in Figure 4 may be used in which the out of balance test current i2 is constant regardless of the supply voltage as long as it is above a certain threshold limit. Diodes 22 and 23 in anti-parallel provide a 0.6 volts forward bias voltage drop with the out of balance i2 current being limited by resistor 21.

However, the total current through the test circuit i1 will vary depending on the actual voltage between the supply conductors at the time of operating the test switch 19.

Alternatively, the test circuit may be configured as shown in Figure 5. In this configuration the test circuit also has a winding 115 on the toroidal core to increase the sensitivity of the test circuit. Thus a lower test current may be used to simulate a larger actual fault current so that the actual test current may be kept below a desired or prescribed maximum limit such as the 9mA test current limit required by Underwriters' Laboratories.

The 2 SCRs 16 and 16' are connected to the two secondary windings 15 and 15' respectively such that the voltage supplied to the gates of the respective SCRs are 180 out of phase. In this arrangement, if a fault occurs during the positive half cycle the fault will be detected by one of the SCRs. If the fault occurs during the negative half cycle, the other SCR will detect the fault and trip the contactor 8. In this way, the trip time may be reduced in the worst case situation to half that of a single SCR trip circuit.

Claims (10)

1. A ground fault circuit interrupter for an electrical supply, comprising at least two supply conductors, contact means for electrically isolating the supply conductors, electrical operating means for operating the contact means, switch means for supplying power from the supply conductors to the operating means, a zero-phase current transformer having a toroidaltype core with first and second secondary windings, primary windings being formed on the toroidal-type core by looping the supply conductors through the core to generate a net magnetic flux whenever there is an imbalance in the supply current causing a voltage to be generated in the secondary windings, and the secondary windings being connected to the switch means, wherein the switch means comprises first and second thyristors, the first thyristor having its gate connected to the first secondary winding and the second thyristor having its gate connected to the second secondary winding such that the inputs to the respective gates are phase shifted by 1800.

2. A ground fault circuit interrupter according to claim 1 in which the switch mean includes a snubber connected in parallel with the thyristor.

3. A ground fault circuit interrupter as defined in claim 2 wherein the snubber comprises a resistor and a capacitor connected in series.

4. A ground fault circuit interrupter according to any one of the preceding claims in which the conductors are looped through the core at least two times.

5. A ground fault circuit interrupter as defined in any one of the preceding claims in which an auxilary electrical supply is provided to bias the switch means towards an operative condition.

6. A ground fault circuit interrupter according to any one of the preceding claims in which the electrical operating means for operating the contact means is a solenoid connected in series with the switch means across the supply conductors.

7. A ground fault circuit interrupter according to any one of the preceding claims including a switched test circuit connected between the conductors to simulate a fault condition when a switch of the test circuit is operated.

8. A ground fault circuit interrupter according to claim 7, wherein the switched test circuit includes a winding on the toroidal-type core arranged in use to generate magnetic flux to simulate a fault condition.

9. A ground fault circuit interrupter as substantially hereinbefore described with reference to figures 3 to 5 of the accompanying drawings.

10. An electrical plug having housed therein, a ground fault circuit interrupter as defined in any one of claims 1 to 9.