GB675514A - Improvements in earth leakage protection for polyphase electrical circuits - Google Patents

Abstract

675,514. Protective cut-out arrangements. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. March 8, 1950. [Aug. 26, 1949] No. 22282/49. Class 38 (v). An earth-leakage protective arrangement has a relay and a local source of supply connected between star-connected rectifiers and earth, so that should an earth fault develop whilst the protected circuit is dead, the relay is energized through the fault path to prevent the closure of a circuit-breaker or contactor. If the neutral of the main supply is unearthed this arrangement may produce the tripping of the circuitbreaker in the event of a fault when the protected circuit is alive, but if the neutral is earthed additional protective means are necessary. Fig. 6 shows the arrangement applied for the protection of a motor M; when a fault develops with the contactor C3, open current from the transformer secondary winding 6 flows through the relay coil R, closed contacts C1, rectifiers 8, the motor and the fault. The relay R opens its contacts R1 to prevent the energization of the closing coil C of the contactor C3, so that the contactor cannot be closed until the fault is removed. Under normal conditions with the contactor C3 closed and the motor running, contacts C1 are open, so that should a fault occur, the current which flows from the phase conductors through the fault, relay coil R, and rectifiers 8 is limited by an impedance 9. Fig. 11 shows the motor M of a mining appliance supplied from a main supply having an earthed neutral and controlled by a switch 20 remote from the contactor C3. When the contactor C3 is open, the local supply source 6, relay RP and rectifiers 8 function as described above, but when the contactor is closed, the earthed supply neutral would prevent the satisfactory operation of this arrangement. Thus when the contactor is closed, contacts C1 open and in the event of a fault the relay RP is energized by the rectified output of the secondary winding of a corebalance transformer T. In either case when the relay RP is energized it opens contacts RP1 to open the circuit of the relay S, which has its coil in the remote control circuit and its contacts S1 controlling the closing coil C of the contactor C3. The core-balance transformer may operate a separate protective relay (Figs. 7 and 8, not shown) and the output of the transformer winding 6 may be rectified (Figs. 4 and 8 not shown). Specifications 605,927 and 658,823 are referred to.