GB605932A - Improvements in or relating to protective control systems for electric motors - Google Patents

Abstract

605,932. Protective cut-out arrangements. BROOKE, W., and BALDWIN, H. T. April 27, 1946, No. 369/48. Divided out of 605,927. [Class 38 (v)] In an earth leakage protection system for polyphase apparatus having an un-earthed neutral, a neutral point inductance is used to enable the fault current to operate a leakage relay. In the circuit shown, a motor B is controlled by a circuit-breaker C, the operating coil C4 of which is energized by a gas-filled triode J when a switch D is closed, in the manner described in Specification 568,336. A leakage relay S has its coil S<1> connected between the biassing system of the triode and a neutral point inductance A. When a balanced or unbalanced fault occurs as indicated at 12a ... 12c, a circuit is completed from the coil S1 through the inductance A, the fault path, earth, through switch D, inductance P, connection Y, and through line Z and resistance K to the coil. Contacts S2 close to shortcircuit the bias resistance K and inductance P and switch D, to increase the bias voltage, so that triode J ceases to fire, coil C4 is no longer energized and the circuit-breaker opens. The contacts S2 now open, but whilst the fault remains, the opening and reclosing of switch D will not effect the closing of the circuitbreaker C, as a parallel path still exists through the relay coil S1; inductance A and the fault path, which short-circuits the bias resistance K and prevents the triode from firing. A faultindicating flag S7 is operated by the relay S and is latched by S4, but is reset by a striker S5 when the circuit-breaker is reclosed after the removal of the fault. The neutral of the supply may be earthed as shown, through a limiting resistance G and a half wave rectifier H which permits the operation of the triode to be unaffected by this earth during the positive half-cycles.