GB2201290A - Earth leakage circuit breaker - Google Patents

Abstract

An earth leakage circuit breaker, particularly for use in association with electricity supply switches, sockets and switched sockets, comprise a pair of contacts (1, 4) for connecting an electricity supply (9) to an output (3); and a solenoid (18) which operates, in response to the flowing of a leakage current exceeding a predetermined level, to interpose an electrically-insulating member (22) between the contacts (1, 4) to prevent current flow therethrough, the electrically- insulating member (22) including latching means (23) for retaining the electrically-insulating member (22) between the contacts (1, 4) until it is manually released. <IMAGE>

Description

EARTH LEAKAGE C-IRCUIT BREAKER This invention relates to earth leakage circuit breakers, particularly, but not exclusively, for use in association with electricity supply switches, sockets and switched sockets, such as the 13-amp sockets which are used throughout Britain.

The use of earth leakage circuit breakers is increasing, particularly in domestic premises, for protecting users of equipment connected to the electricity supply. Such circuit breakers operate by sensing any difference which may occur between the current flowing in the line conductor and the current flowing in the neutral conductor feeding the equipment.

The two current levels should be exactly equal, and any difference between the levels indicates that there is a leakage of current, possibly through the body of the user, to ground. If the leakage current reaches a predetermined level, for example 30mA, the circuit breaker opens, switching off the supply to the equipment.

Such circuit breakers commonly comprise a relay having two pairs of contacts, one pair for the line feed and the other pair for the neutral feed, which are closed by pulling-in of the armature of the relay. The armature is moved to the contact closing position manually, and is held in that position by an electromagnet. If a leakage current is detected, a winding of the relay is de-energised and the relay opens. When the leakage current reaches the predetermined level, the field from the winding is sufficient to cause the relay to drop out, opening the contacts. In order to reconnect the equipment to the supply, it is necessary to depress a resetting button, which moves the armature into the closed position.

Such a circuit breaker must be provided with safeguards. Firstly, in the event of a fault occurring which causes the relay to drop out, it must not be possible to reset the relay while the equipment is still connected to the supply. If it were possible to operate the resetting button under those circumstances, the relay would immediately attempt to drop out again due to the fault current, but the armature would be held in, at least temporarily, by the pressing of the resetting button. The operation of the relay would therefore be overridden. To overcome this problem, complicated interlock arrangements must be provided to ensure that the resetting button cannot be depressed unless the plug is removed from the socket. Secondly, it must not be possible to connect the equipment to the supply while the resetting button is depressed.This requires interlock means to ensure that the plug cannot be inserted into the socket while the resetting button is depressed.

If the relay is fitted to a switch, the relay contacts will be connected in series with the switch contacts. If the circuit breaker trips due to a fault current, the supply to the faulty equipment will be interrupted, but the switch will remain closed, unless deliberately opened by the user. This means that the user could inadvertently (or deliberately) depress the resetting button to close the relay while the equipment is still connected to the supply. As explained above, this must be prevented. Furthermore, it must not be possible for the user to turn ON the switch while the resetting button is depressed. These preventative measures involve the use of interlocks between the resetting button and the switch.

According to the invention there is provided an earth leakage circuit breaker comprising a pair of contacts for connecting an electricity supply to an output; and a solenoid which operates, in response to the flowing of a leakage current exceeding a predetermined level, to interpose an electrically-insulating member between the contacts to prevent current flow therethrough, the electrically-insulating member including latching means to retain the member between the contacts until it is released by the user.

The contacts may be contacts of a manually closed and opened switch, the latching means then being effective to retain the electrically-insulating member between the contacts until it is released by manual opening of the switch. Preferably the electrically-insulating member is a portion of a rod which is axially slidable between the contacts by operation of the solenoid, the rod also comprising an electrically-conductive portion which is normally located to interconnect the contacts when the switch is in the closed position.

Preferably the solenoid operates to interpose two of said electrically-insulating members between respective pairs of contacts arranged for double-pole operation.

The earth leakage circuit breaker may comprise, for example, part of a switch unit1 a socket, a switched socket, or an adapter for adapting any of such devices to provide earth leakage protection.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which Fig. 1 is a schematic pictorial view of part of a switched socket incorporating a circuit breaker in accordance with the invention; Fig. 2 is a schematic end view of a contact arrangement of the device of Fig. 1 in a "switch-closed" position; and Fig. 3 is a schematic end view of the contact arrangement in a "switch-open position.

Referring to Fig. 1 of the drawing, a contact assembly of a switched socket, which is very similar to that of a conventional switched socket, comprises a fixed contact 1 which is connected via a copper strip 2 to a socket terminal 3 which is arranged to receive the line connection pin of a plug (not shown). A movable contact 4 is mounted to pivot about a knife edge provided by an end 5 of a contact strip 6, under the control of a projection 7 on the underside of a switch dolly 8. The contact strip 6 is connected to a line input terminal 9 via an insulated wire 10 which passes through a sensing coil 11, comprising a winding 12 on a toroidal core 13. A contact strip 14 is connected to a neutral input terminal 15 via an insulated wire 16, which also passes through the sensing coil 11.

The winding 12 is connected to a circuit 17 which detects any imbalance between the currents flowing in the line and neutral wires 10 and 16. If such imbalance, which represents a leakage current, exceeds, say, 30mA, the circuit 17 interrupts an energising supply to a solenoid 18.

The armature 19 of the solenoid 18 is connected to a rod 20 which is axially slidable between the contacts 1 and 4. The rod comprises an electrically-conductive portion 21 and an electrically-insulating portion 22, joined end-to-end. The portion 22 has a notch 23 in its underside. The armature is normally urged towards the left in Fig. 1 by a spring 24, so that with the solenoid de-energised the non-conductive portion 22 lies between the contacts, but energisation of the solenoid 18 causes the rod to move towards the right, against the action of the spring 24, so that the electrically conductive portion 21 lies between the contacts. A flag 25, which is visible from outside a casing (not shown) containing all of the socket and circuit breaker components, is rigidly attached to the rod 20 to indicate the state of the circuit breaker.

The neutral contact strip 14 may be connected directly to a socket terminal 26 which receives a neutral connection pin of the plug. Alternatively, and preferably, the strip 14 may connect to the terminal 26 via a further movable contact and a further fixed contact, similar to the contacts 4 and 1, so that the switch is a double-pole switch. The further movable contact would be operated by the dolly 8, simultaneously with the contact 4. The rod 20 would be extended to provide another insulated portion, adjoining the conductive portion 21, and another conductive portion.

In describing the operation of the device, it will first be assumed that the device is connected to a power supply so that the solenoid is energised and the rod 20 is in the position shown in Fig. 1, but that the movable contact 4 is in the "switch open" position shown in Fig.

3. When the socket is to be used, a plug connected to equipment is inserted in the socket. The dolly 8 is operated to tilt the movable contact 4 into the "switch closed" position shown in Figs. 1 and 2. Current therefore flows to the equipment via the movable and fixed contacts and via the conductive portion 21 located therebetween. If no leakage current is detected, the solenoid 18 will remain energised.

If a fault condition occurs on the live or neutral line such that a leakage current in excess of the predetermined level flows, the solenoid 18 is de-energised and the spring 24 moves the rod towards the left in Fig. 1. The insulated portion 22 comes between the contacts 1 and 4, cutting off the supply to the equipment. The movable contact 4, still urged to tilt upwards by the dolly 8, enters the notch 23 immediately upon insertion of the portion 22 between the contacts.

Hence, the power supply to the equipment is cut off, thereby isolating the part of the circuit containing the leakage path, the rod cannot return to the right-hand position, due to the latching effect of the contact 4 in the notch 23.

In order to reset the circuit breaker it is necessary to move the dolly 8 to its opposite "switch open" position, so that the contact 4 tilts downwards into the position shown in Fig. 3, thereby releasing the rod 20. If the switch is moved to the "switch closed" position again with the faulty equipment still connected, the solenoid immediately operates again, once more cutting off the supply. It will be seen that the user cannot move the sliding rod manually into the "ON" position. The user cannot, therefore, override the operation of the circuit breaker. No special interlocks are therefore necessary to prevent such abuse of the device.

Alternative forms of the circuit breaker could be provided. For example, the device could be used in cooperation with a switch which is connected to output terminals for receiving a cable, instead of socket terminals for receiving a plug as described above.

Alternatively, the device could be used in an unswitched socket unit. In that case, one or two pairs of contacts would be provided specially within the unit, to be acted upon by the sliding rod 20. Because the contacts would not be opened and closed by a switch dolly, it would be necessary to provide a resetting button to enable the user to move the contact out of the notch 23, thereby allowing the rod to reset. Current would not immediately flow to the equipment while the resetting button is depressed, because the depressed contact would be held out of contact with the conductive portion 21 of the rod until the resetting button is released. Such an arrangement could alternatively be used in an adaptor which can be plugged into a socket and which itself provides one or more socket outlets, the latter outlets being provided with earth leakage protection by the device. Alternatively, such an arrangement could be provided in a junction box or connector unit for interconnecting input and output cables.

In the above-described embodiments the electrically conductive portion 21 normally lies between the contacts 1 and 4, and is replaced by the insulating portion 22 if a leakage current is sensed. However, it would be possible to reverse the operation, so that the insulating portion normally lies between the contacts, and the conductive portion is moved into place between the contacts provided that a non-fault current flows.

Claims (8)

1. An earth leakage circuit breaker comprising a pair of contacts for connecting an electricity supply to an output; and a solenoid which operates, in response to the flowing of a leakage current exceeding a predetermined level, to interpose an electrically-insulating member between the contacts to prevent current flow therethrough, the electrically-insulating member including latching means to retain the member between the contacts until it is released by the user.

2. An earth leakage circuit breaker according to claim 1, in which the contacts are contacts of a manually closed and opened switch, and in which the latching means are effective to retain the electrically-insulating member between the contacts until it is released by manual opening of the switch.

3. An earth leakage circuit breaker according to claim 1, in which the pair of contacts are biassed into the closed position, in which the earth leakage circuit breaker includes a reset button which is manually actuable to open the pair of contacts, and in which the latching means are effective to retain the electrically-insulating member between the contacts until it is released by manual operation of the reset button.

4. An earth leakage circuit breaker according to any one of the preceding claims, in which the electricallyinsulating member is a portion of a rod which is axially slidable between the contacts by operation of the solenoid, the rod also comprising an electricallyconductive portion which is normally located to interconnect the contacts when the contacts are in their closed position.

5. An earth leakage circuit breaker according to any one of the preceding claims, which includes two pairs of contacts arranged for double pole operation, and in which the solenoid operates to interpose two of the electrically-insulating members between their respective pairs of contacts.

6. An earth leakage circuit breaker according to any one of the preceding claims, in which the electrically insulating member is spring-biassed into position between the contacts and the solenoid acts against the spring-bias in the absence of a leakage current.

7. An earth leakage circuit breaker according to any one of the preceding claims, forming part of a switch unit, a socket, a switched socket, or an adapter for adapting any of such devices to provide earth leakage protection.

8. An earth leakage circuit breaker substantially as described with reference to the accompanying drawings.