GB360527A - Improvements in protective arrangements for electricity distribution systems - Google Patents

Abstract

360,527. Protective cut-out systems. DANCE, H. E., 6, Alexandra Road, Birkenhead. Aug. 21, 1930, No. 24979. [Class 38 (v).] In the protection of systems comprising movable apparatus, which must be earthed (such as mining plant having flexible supply cables) an auxiliary switch adapted when operated to cause the operation of the main circuit-breakers is located at the consuming apparatus and is restrained from operation by a winding energized only while the safety earth-connection is maintained. The system dispenses with protective pilot-wire connections. In the arrangement shown in Fig. 1 in which the source A supplies the three-phase motor D through a circuit-breaker B, the motor casing is earthed at g<1>. The circuit-breaker is closed from the motor end by means of the auxiliary circuit e<2>, e<3> containing the closing switch e<4>, the closing (and maintaining) coil b<1> of the circuit-breaker, and auxiliary source of power such as the transformer e<1> and an auxiliary switch F which opens the maintaining circuit if the frame g<1> is not efficiently earthed. The switch F is normally held closed by the current flowing from the source e<1> through the closing coil f (which is connected to the frame) and through the earth connections g<1>, g<2>. If the coil f<1> becomes deenergized due to faulty earthing, the main circuitbreaker opens. Overload trip coils for the phases are shown at b<2>. The supply for the coil f<1> may be obtained from a connection across the main supply through a resistance and the switch F, on closure due to failure of the earthing system, may connect a low resistance across the mains and bring out the circuit-breaker, Fig. 2 (not shown). As applied in conjunction with a split conductor type of protection, with transformers in the two halves c<1>, c<2>; Fig. 3, acting differentially on the trip coil e<4> of the circuit-breaker, an impedance f is included in the conductor c<2> and is short circuited by the switch F when the coil f is energized. This coil is connected between the conductor c<2> (or a tapping of one of the windings of the load) and the frame and, as the other main conductor of the supply is earthed at g<2>, the coil is energized as long as the earth circuit is complete. If the earthing becomes ineffective, the short circuit is removed from f<2> and the balance between the split conductors is upset so that the circuit-breaker is tripped. As applied to a Merz- Price current-balance system in which balance is normally maintained between current transformers e<1>, e<2>, Fig. 5, at opposite ends, a third winding f<3> is inductively related to the secondary e<2>, the trip coils e<3>, e<4> are connected to earth through a battery f<2> and to the earthed frame G through a winding f<1> which, with the earth circuit sound, holds a switch F open. When the earth circuit becomes unsound, the switch F closes and upsets the balance. Fig. 6 shows the invention applied to an intersheath earth-leakage protective system. The sheath carries a direct current from the battery f<2> and through the windings e<2>, f<1> as long as the earth circuit is sound. When it becomes unsound the switch F closes so that the sheath is connected to the conductor c<3> and, acting through the small transformer e<2>, trips the circuit-breaker.