GB535446A - Improvements in and relating to earth leakage protective systems - Google Patents

Abstract

535,446. Protective cut-out arrangements. GENERAL ELECTRIC CO., Ltd., TURNER, H. C., and EDSALL, E. A. M. Jan. 15, 1940, No. 857. [Class 38 (v)] Electrical apparatus 1, such as a cooker with its element 2 fed. through circuit-breaker 5 from a supply having its neutral 4 earthed only at the feeding-point 11, is connected over normallyclosed test key contact to the junction point of oppositely-wound trip coils 6, 7 for the breaker 5, the other ends of the trip coils being connected to the neutral 4 and an earth connection 10, respectively. When the line, which is assumed to be long, carries a heavy load, the alternating potential-to-earth of the neutral 4 at the apparatus 1 sends a current through the coils 6, 7 but without effect; by suitably proportioning the turns, however, tripping may be produced when, due to a break in the neutral between the breaker 5 and the supply, a high neutral potential is set up. Tripping is produced when currents flow in opposite directions through the trip coils due to a leak between the element 2 and apparatus-frame. If the frame 1 is independently earthed at a distance from electrode 10 so as to shunt some of the current from coil 6 away from the coil 7, the latter must have more turns. A high frame-potential can be simulated for tripping the breaker by depressing test key 9 and connecting the phase-wire 3 to the coils over contact 12 and resistance 13. The windings may be shunted by radio-interference suppressing condensers (not shown) and may comprise two separate coils or a doublewound coil. In order normally to restrict the frame-potential relative to that of the neutral, the resistance of the frame-to-electrode 10 circuit is equal to, or less than, the frame-toneutral circuit, and the winding in the latter may have an impedance in series with it.