ELECTRICAL SAFETY DEVICE

Abstract: An earth leakage circuit breaker has a socket for connecting an appliance to be protected. A plug inserted in the socket depresses a plunger (34), tilting a cam (38) which acts through a striker (42) to momentarily close a relay thus resetting the circuit breaker. When the appliance is unplugged the cam returns to its normal position, by-passing the striker (42) but closing a pair of test contacts. The reset and test functions are thus carried out without the need for separate buttons, and at times when the load is disconnected, so that the circuit breaker contacts do not break load current when they open or close during resetting or testing.

Description: This invention relates to an electrical safety device of the kind commonly known as a residual current device or earth leakage circuit breaker (FLCB), which latter term vill be used herein. Earth leakage circuit breakers of different types are previously known. One rather uncomplicated type is shown in DE-A-1.515.522 and comprises a device for interrupting the current supply when an unbalances is detected in the circuit. The device is provided with a manually operated resetting means. Another type of earth leakage circuit breaker is described in US-A-4.194.231. A holding circuit maintains the relay closed so long as voltage appears at the output of the device, and a sensor arranged to sense an unbalance in current disables the holding circuit and causes the relay to open on the occurrence of any leakage current to earth. Resetting means are provided to close the relay after it has opened, or when the circuit is first switched on, and some means for testing the device is necessary to ensure that it remains operative and sufficiently sensitive to trip under fault conditions. These functions are normally provided by means of a reset button and a test button on the casing of the device. It will be noted that in the conventional earth leakage circuit breaker the contacts will usually make or break the full load current when they close on resetting or open on testing. The present invention, which is defined in the claims appended hereto, provides an earth leakage circuit breaker in which the resetting and test functions are performed automatically on plugging in or unplugging the appliance to be protected. This obviates the need for test and reset buttons, and also ensures that the testing and resetting are carried out when no load current is being drawn, thereby lengthening the life of the relay contacts, or allowing contacts of lower rating to be used. In a more elaborate version of the breaker of the invention, insertion of the plug may first re-set the breaker, then initiate a test trip, and then reset the breaker only of the initial test trip has been successful. This is achieved by a device having the caracteristics mentioned in the claim. The invention will now be described by way of example with references to the the invention; Figure 1 shows the circuitry of the earth leakage circuit breaker according to the invention Figure 2 is a diagrammatic section through a device according to the invention; Figure 3 is a plan of Figure 2; Figure 4 shows a linear representation of the cam; Figure 5A-H show the positions of cam actuators at a number of positions of the cam with the circuit breaker functioning correctly; Figures 6A-E show the positions of the cam actuators for a number of cam positions in the case in which a relay RL1 fails to trip during an initial test sequence, resulting in the cam preventing the relay from re-setting; Figure 7A shows the paths of the actuators on normal operation plug insertion; Figure 7B shows the paths of the actuators on plug removal without a trip occuring in operation; Figure 7C shows the paths of the actuators on plug removal when a trip has occured; Figure 8A shows the paths of the actuators on plug insertion with fault; and Figure 8B shown the paths of the actuators on plug removal after fault. Referring to Figure 1, a pair of input terminals 10 are provided for connection to the supply and these in turn are connected through the normally-open contacts of a changeover relay RL1 to a current balance transformer device 12 for detecting a current imbalance representing the existance of an earth leakage path. The device 12 consists of a toroidal core wound with a pair of equal and symmetrical windings connected in opposite senses, so that on flow of current through them the magnetic fields induced in the toroidal core cancel out. A sensor winding 22 detects any residual field produced by the currents becoming unbalanced, as will be described below. Output leads 14 from the device 12 are taken to the terminals of a shuttered socket, as will be described in more detail with reference to Figures 2 and 3. A flashing neon indicator bulb 16 is connected across the N.L. contacts and serves to indicate when the output contacts of the relay RL1 are open, and power is not being supplied to the protected appliances. A test contact 18 in series with a current-limiting resistor R1 is connected between the neutral lead on the output side of the device 12 and the live lead on the input side and, when closed, causes an unbalance in the currents through the toroid device 12. The magnitude of the unbalance current is determined by the currentlimiting resistor R1 and when the earth leakage circuit breaker is functioning correctly should be sufficient to cause it to trip. The transistors TR1 and TR2 form a bistable circuit using RL1, R2, R5 and R6 where TR1 will be "ON" from application of a voltage across the capacitor C2. A trip circuit 20 serves to trip the device when an unbalance is detected by the device 12. The sensor winding 22 of the toroid 12 is connected to the terminals 22A of the trip circuit and any current induced in this winding is applied to the base of a transistor TR3 and TR4, whereupon one transistor becomes conductive and removes TR1 base current generated through a resistor R2. This removes the holding current through the winding of the relay RL1, allowing the relay to open. Resistors RS1 and RS2 ensure that the sensor winding 22 of the device 12 presents a sufficient impedance to the imbalance current to enable it to produce sufficient voltage to switch TR3 or TR4. Components D3, R3, C3 and neon lamp LP1 form a relaxation oscillator causing LP1 to flash when voltage is applied to the normally closed contacts of RL1. The trip circuit 20, therefore, causes the earth leakage circuit breaker to open whenever an unbalance is detected, whether it is caused by a defective appliance connected to the terminals 14, or by the closure of the test contacts 18. Figures 2 and 3 show very diagrammatically in section and plan the mechanical arrangements of the socket part of the earth leakage circuit breaker of the invention. The causing 30 carries a standard plug socket, the apertures of which are indicated at 32 and 36, and a plunger 34. Behind the apertures there is located a cam 38 of insulating material which is angularly displaceable through an angle of a about 40°, displacement being effected by the plunger 34, when this plunger is depressed on inserting a plug into the socket. The lower part of the cam 38 carries a cam section or face 40. The cam section or face 40 engages cam followers provided at the lower end of the cam 38. Referring again to Figure 1, it will be seen that the momentary closure of the relay contacts RL1 energises the hold circuit of the relay, charging the capacitor C2 by means of the diode D1, and thereby maintaining the contacts in their closed position. Since the plunger 34 operates the cam 38 before the plug pins engage the supply contact, the relay contacts are closed before any current can flow in the load circuit. The test contacts 18 are positioned above the shutter cam 38 in such a position that they are closed by the uppermost portion of the cam section 40 when the plug is inserted. The test contacts thus close only after the load has been disconnected. Closure of the test contacts 18 unbalances the current through the toroid device 12 and cause the sensor winding to apply a current to the trip circuit 20, allowing the relay to open, as has already been described with reference to Figure 1. From the foregoing description, it will be appreciated that the earth leakage circuit breaker is first tested and then reset as the plug of the appliance to be protected is inserted into its socket, and when the plug is fully inserted the indicator bulb 16 stops flashing. When the plug is removed, the indicator bulb should start to flash, indicating that the earth leakage circuit breaker is tripping or has tripped in response to a test condition. Failure of the bulb to flash when the appliance is unplugged indicated that the breaker is not functioning correctly. The indicator bulb 16 may if desired, be replaced by an audible warning device such as a buzzer. Should the earth leakage circuit breaker trip in operation, it is reset merely by unplugging the appliance, correcting any fault that may have caused it to trip, and then plugging it in again, the act of plugging in resetting the breaker. The fact that the appliance must be unplugged in order to reset the breaker is an additional safety feature of an earth leakage circuit breaker according to the invention. Although a plunger has been employed in the above embodiment of the invention, one of the pins of the plug migth act directly on a suitably shaped part of the cam. Further, the cam need not be in the form of hinged plate; it could, for example, be a spring-loading sliding rod or a disc bearing suitable cam surfaces.

Claims: An earth leakage circuit breaker device for interrupting a mains supply current when a leakage current to earth exceeds a threshold, the device comprising an output socket and normally open relay (RL1) arranged to interrupt the supply of power to the terminals of the socket, a hold circuit for the relay, means (20) for tripping the relay on occurance of an unbalance current and actuating means (38, 42, 43) for effecting resetting and testing of the function of the device, characterized in that the actuating means comprises a cam (38), a reset and test-actuator (42 and 43, resp.) cooperating with the cam to automatically accomplish a test of the device when a plug belonging to an appliance to be protected is inserted into the socket, whereby the cam (38) is moved to a first position (Fig. 5B) in which the cam influences the reset actuator (42) to close the relay in order to supply power to the terminals, that further movement of the plug into the socket brings the cam mechanism (38) to a second position (Fig. 5C) in which the test actuator is momentarily influenced so that an unbalance current is created which means either A) that if the test does not find any fault the relay is opened and the reset actuator (42) is returned to its original position (Fig. 5D), that further movement of the plug into the socket brings the cam mechanism to a third position (Fig. 5F) in which it again influences the reset activator (42) to close the relay thereby supplying current to the terminal and that further movement of the plug into the socket brings the cam mechanism (38) to a fourth position (Fig. 5H) in which the reset actuator is free to return to its original position when an unbalanance is created, or B) that if the test indicates a fault by not opening the relay when the cam mechanism (38) is in the third position (Fig. 6) the reset activator is maintained in its non original position and cooperates with stop means on the cam mechanism (38) to prevent further movements of the cam mechanism thereby inhibiting further insertion of the plug into the socket.

Inventors: TURNER, COLIN
Applicants: FLYMO LIMITED
Application year: 1987
Main classification: H01R13
IPC codes: H02H3, H01R13