GB980415A - Improvements relating to earth leakage protective systems - Google Patents

Abstract

980,415. Protective arrangements. BALDWIN & FRANCIS Ltd. May 20, 1963 [Feb. 23, 1962], No. 7188/62. Heading H2K. [Also in Division H3] An earth-leakage protective system for a polyphase apparatus M comprises a corebalance transformer CBT having an auxiliary primary winding 11 connected to an A.C. source 13 having a frequency which is relatively high with respect to the mains frequency, the secondary winding 12 of the transformer CBT controlling a transistor T which controls the current in the primary winding of an auxiliary transformer 21 having a secondary winding 22 connected to a relay R controlling a main contactor 4, 5. In the absence of an earth fault, the coupling between the windings 11, 12 provides a high frequency control current to the transistor T which provides a high frequency output. This output is amplified at 20, transformed at 21, rectified at 23 and energizes the relay R so that the contactor energizing circuit is closed by the contacts 6 of the relay R. When an earth fault occurs, a current of main frequency is induced in the secondary winding 12 of the transformer CBT and this current causes the transistor to be alternatively cut-off and overdriven so suppressing the high frequency component from the output of the transistor T. The auxiliary transformer 21 presents a much lower impedance to the mains frequency current and its output voltage comes insufficient to maintain the relay R fully energized so that contacts 6 open to open the contactor. The Provisional Specification refers to the provision of lock-out protection when the contactor is open, by injecting a small A.C. current through the fault path, the injection circuit including a further auxiliary winding on the core-balance transformer.