The present invention relates to electromagnetic actuator arrangements and relates particularly but not exclusively to residual current circuit breakers.

It is necessary for residual current circuit breakers in particular to trip very quickly, e.g. in less than 20 milliseconds. In order to meet this requirement, it has been necessary to provide a very powerful biasing spring which trips the actuator when the actuator is de-energised. A correspondingly powerful and bulky electromagnet has been reauired to overcome the spring bias in the energised condition.

German patent number 532028 discloses a single-pole switching circuit breaker with a switch 12 biased by a spring 14 towards its open position but, in use, held closed by an armature 18 which is held against legs 16 of an electromagnetic core 15. The circuit breaker is for use with D.C. sources only, and if any excess current is detected, a coil 22 reduces the magnetic strength of the core 15. The spring 14 is then able to pull the armature 18 away from the core 15, thereby opening the switch 12.

U.S. patent number 4,063,299 discloses a ground fault circuit interrupter in which a torroidal sensing coil 22 is used to detect any disparity between the live 12 and neutral 14 power lines in order to indicate a ground fault. In normal operating conditions, no disparity is detected and so the remanent magnetic latching relay 80 remains unmagnetised. The armature is biassed by a spring away from the latching relay pole 90 and holds switches 86,88 closed. As soon as the sensing coil 22 detects a ground fault, the latching relay is activated, pulling the armature 92 and opening the switches 86,88.

An object of the present invention is to provide an electromagnetic actuator arrangement which is faster-acting and/or requires a less powerful electromagnet than comparable conventional actuator arrangements.

According to the present invention, an electromagnetic actuator arrangement comprises an electromagnetic actuator, including at least one operating member of magnetically soft material which cooperates with biasing means and is magnetised when said electromagnetic actuator is energised characterised in that the actuator arrangement further comprises means for reversing the magnetising force acting on said operating member when the said electromagnetic actuator is de-energised.

The invention is applicable particularly but not exclusively to circuit breakers and relays, which incorporate two such operating members, namely a fixed yoke and a movable armature. By reversing the magnetising force on de-energising an electromagnetic actuator of this type, the residual magnetic flux in the armature is opposed by the reversed magnetic field and the normal tendency of the armature to stick to the yoke immediately after the winding has been de-energised is avoided.

In fact during the brief period in which the residual magnetic flux in the armature maintains its original polarity, the armature is repelled by the reversed magnetic field arising from the reversed magnetising force and thus the biasing force is briefly augmented at the instant of de-energising the winding. Accordingly the invention is also applicable for example to solenoids and other devices which incorporate only a single "operating member" of magnetically soft material.

In typical preferred embodiments, particularly of relay and circuit-breaker arrangements, the armature will be removed from the magnetic field of the winding by the biasing means by the time that the residual magnetic flux in the armature has been reversed by the reversal of magnetising force on de-energising the actuator. The resulting force of attraction acting on the armature will accordingly be very weak in comparison with the opposing biasing force and the tripping time will not be adversely affected.

A particularly convenient way of limiting the reverse magnetisation is to reverse the magnetising force by means of a capacitor (which may for example by connected in parallel with the winding) which is arranged to discharge through the winding on de-energisation of the actuator. Preferably the discharge is oscillatory so that the residual magnetisation of the armature is thereby reduced to a very low level.

In a preferred embodiment the winding of the electromagnetic actuator is energised from a current force (preferably a substantially constant current source) and a gate-controlled semi-conductor switching device is arranged both to bypass current from said current source in response to a control signal applied to the gate of said device on de-energising said electromagnetic actuator, and to conduct the current which is discharged by said capacitor through said winding.

A preferred embodiment of the invention is described below by way of example only with reference to the accompanying drawing, Figure 1, which is a circuit diagram of a residual current circuit breaker in accordance with the invention.

The circuit shown in Figure 1 comprises live (L) and neutral (N) mains input terminals which are connected via a two-pole switch SW1 to mains output terminals, which may in turn be connected to an electrical appliance (not shown). Switch SW1 is controlled by an electromagnetic actuator comprising a movable armature AR1 and a yoke Y1 which is provided with a winding L1. Armature AR1 is mechanically linked to switch SWl and the latter is biased open by a tension spring SP which acts on the armature. Armature AR1 and yoke Y1 are composed of magnetically soft iron and contact one another when switch SW1 is closed.

In use, switch SW1 is closed manually and energised by a rectified constant current source comprising a series-connected diode D1, resistors R3, R4 and R5 and R6 and capacitor C6 (which are connected between the mains conductors) and resistor R2 (which is connected between the junction of R3 and C6 and one end of winding L1).

The other end of winding L1 is connected to pin 6 of an RA3783 integrated circuit IC1 ad thence via pin 4 thereof to the free terminal of smoothing capacitor C6. Accordingly winding L1 is energised and maintains switch SW1 ON.

A ferrite-core transformer T1 is coupled to the mains conductors and its output is fed to pins 2 and 3 of IC1.
The potential difference across pins 2 and 3 is monitored by IC1 and in the event that it exceeds a predetermined threshold value (as a result of an inbalance in the forward and return currents in the mains conductors due to a potentially dangereous leakage of current to earth) an output voltage is generated at pin 5. A series-corrected discharge capacitator C1 and thyristor SCR1 are corrected in parallel with winding L1 and the gate of SCR1 is connected to pin 5. The cathode of thyristor SCR1 is also connected to the negative terminal of capacitor C6 so that thyristor SCR1 bypasses the output current of the current source as well as providing a discharge path for capacitor C1 through winding L1. The current from R2 is sufficient to hold SCR1 ON whilst capacitor C1 is discharging.

Accordingly, when an earth leakage current is detected, a voltage appears at terminal 5 of IC1 which fires thyristor SCR1, thereby bypassing the forward current from resistor R2 of the current source and allowing capacitor C1 to send a brief reverse discharge current through winding L1 which generates a reverse magnetising force which opposes the residual flux in armature AR1 and yoke Y1. Accordingly, armature AR1 is immediately repelled from yoke Y1 and is rapidly removed from the yoke by biasing spring SP. By this time, the current in winding L1 has fallen to zero. Thus switch SW1 breaks the mains circuit very rapidly.

When safe conditions have been restored, the circuit may be re-set by depressing switch SW1 manually.

A pair of test contacts TC are connected in series with a resistor R7 between the neutral output terminal and a live input terminal of the mains conductors to enable a residual current to be generated artificially in order to test the circuit.

The sensitivity of the circuit can be adjusted by altering the value of a resistor R8 which is connected across the winding of T1 or the value of resistor R1 which is connected between terminals 1 and 7 of IC1. It is by-passed by a noise suppression capacitor C4 and an additional noise-suppression capacitor C3 is connected between pins 1 and 8 of IC1. Resistor R8 is by-passed by a noise-suppression capacitor C5 and a noise-suppression capacitor C2 is also connected between the gate and cathode of thyristor SCR1.

It should be noted that the circuit automatically switches switch SW1 OFF in the event of loss of mains supply.