GB521807A - Improvements in or relating to control and protective systems for electrical apparatus - Google Patents

Abstract

521,807. Protective cut-out arrangements. REYROLLE & CO., Ltd., A., and TIPLADY, J. Nov. 25, 1938, No. 34330. [Class 38 (v)] In control systems in which apparatus; such as mining machinery J, is controlled from a remote supply switch A<3> over a trailing cable B incorporating a two-conductor pilot circuit (such as conductor B' and the cable-sheath used also for earthing the apparatus), the contactor-coil A<4> maintaining the switch A' closed is controlled by the normally-closed contacts of a protective relay G and contacts of a pilot relay F which are closed when the pilot circuit is sound and energized from transformer E<1>. The relay coils are shunted by condensers which absorb inductive surges that may cause sparks and hence explosions. When the apparatus switch K is moved to the "start" position, the pilot circuit is made at contacts K', K<3>, so closing the switch A' and energizing the coil J<4> in the power-conductor, whereupon the switch K is moved to " run position in which K<1> is earthed through K<2> and contacts of J<4>, and F is energized by unidirectional current passed by the rectifiers G<3>, J<5>. If a fault to earth occurs in the pilot circuit so that J<5> is short-circuited, unidirectional current flows through the protective relay and series rectifier G<2>, so causing the main switch to open or, if already open, preventing its closure. The relay G has a low current-setting so as to respond to faults of high resistance. The relay F is de-energized by an open pilot circuit or resistance-increase therein, and by supply-voltage failure, the relay being of low resistance so as to respond to even a slight resistance-change. The pilot circuit can also be opened by earth-leakage or overload in the main line itself, and has an emergency " stop switch. The coil J<4> prevents automatic restarting if a momentary interruption in the supply occurs. For control of the apparatus J from the supply end, the link H<3> is moved to the dotted position shown and " start " switch H closed, the latter being then shunted by a back contact H<2> of the supply switch. The Specification also includes systems in which the main switch is manually closed through tripfree mechanism and retained by a latch controlled by no-volt coil in series with the relay contacts, Fig. 5 (not shown), and modifications of these arrangements in which the pilot relay is shunted by a rectifier of the same sense as that G<2>, and the relay G opens the pilot circuit instead of the trip circuit or contactorcoil circuit, Figs. 6, 7 (not shown).