GB665576A - Improvements in earth leakage protective systems - Google Patents

Abstract

665,576. Protective cut-out arrangements. BROOKE, W., and BALDWIN, H. T. Oct. 26, 1949. No. 27463/49. Class 38 (v). A remote control protective system for polyphase motors and other apparatus has a control circuit into which under fault conditions a potential is injected in opposition to a relay operating potential, so as to cause the de-energization of the relay, the injection potential being maintained so long as the fault persists to prevent the reclosure of the contactor controlled by the relay. In Fig. 1 a motor M is supplied from lines L1 ... L3 of a supply system having an earthed neutral, when the closing coil D of the contactor C is energized on the closure of the relay contacts H1, H2. The operating coil H4 of the relay H is fully energized so as to close the contacts H1, H2 when a three position switch K in the pilot and earth circuit 5, 6 is moved to a "starting" position K2, the current in this control circuit being subjected to half-wave rectification by a rectifier N. The switch K is then moved into its "run" position K3 to insert a resistance O into the control circuit, so as to reduce the excitation of the relay coil H4 to a level sufficient to maintain the relay contacts closed, but insufficient to reclose them should they have been opened by a fault condition. In the event of an earth fault, as indicated at Z, the "out-of-balance" current induced in the secondary winding Y4 of a core-balance transformer Y is rectified by a rectifier S and applied to an adjustable resistance L, so as to set up across the resistance L a D.C. potential in opposition to the D.C. operating potential applied by transformer secondary winding G2 and the rectifier N to the relay coil H4. This causes the de-energization of the relay H and the opening of the contactor C. When the contactor opens, it connects at C4 the neutral point Q of an inductance P to the centre tap 12 of a further transformer secondary winding G3 so that current flows from the winding G3 through the inductance P, conductor 3, fault Z, earth, earth continuity conductor 6, resistance L and rectifiers U, V. This current maintains a D.C. potential across resistance L in opposition to the control potential so long as the fault Z persists, and the relay H remains de-energized. To re-start the motor it is necessary to remove the fault and move the switch K into the "start" position K2, the switch then being moved back to its "run" position K3. In a modification, Fig. 2 (not shown), in which the motor is supplied from a supply system having an unearthed neutral, a transformer connected between the artificial neutral point Q of the inductance P and earth is effective to apply the opposing potential across resistance L on the occurrence of an earth fault. Specifications 605,927, 605,932 and 636,720 are referred to.