GB758551A - Improvements in and relating to protective systems for electric induction furnace installations - Google Patents

Abstract

758,551. Protective cut-out systems. METROPOLITAN - VICKERS ELECTRICAL CO., Ltd. March 10, 1955 [March 17, 1954], No. 7792/54. Class 38(5) In a protective system for an electric conduction furnace comprising means 7 for earthing the furnace charge 6 and a protective relay 11 which is connected either directly or through a transformer 20 between earth and the furnace circuit in series, or as shown in parallel through an impedance 19, with an auxiliary sources of current different from that of the furnace, so that an earth fault on the furnace circuit provides a circuit determining the operation of the relay, the relay circuit is connected with the auxiliary source by a circuit including current dependent resistances L, M so that the voltage applied to the relay circuit will be subject to a proportionally lesser variation than that to which the auxiliary source may be subject. The auxiliary source may be a direct current source or an alternating current source of a frequency different from that of the furnace circuit. The resistance L may be a metal filament lamp with a positive temperature coefficient, working at a voltage less than its rated voltage and causes the current flowing through it to vary less than the auxiliary voltage derived at a transformer 21. The resistance element M may be of a material having a negative resistance current characteristic to cause the voltage across the relay 11 to vary less than the current through the lamp L. The invention is described in its application to the various arrangements of Specification 506,665 in which the relay 11 may be energized by the auxiliary source through the fault path or in which the current through the relay is reduced on the occurrence of an earth fault as in the -arrangement shown.