GB830018A - Protective device for electrical circuits and apparatus - Google Patents

Abstract

830,018. Protective cut-out systems. GERRARD, M. C. March 11, 1958 [March 13, 1957], No. 8236/57. Class 38 (5). A protective system includes a relay or breaker B having contacts which may isolate a protected apparatus 27 from the supply and an operating coil 17 energized by a potential derived from the voltages across two pairs of conductors, one conductor being an earth conductor, so that if the sum of the voltages is substantially below its correct value the relay or circuit-breaker will remain open. Thus, in Fig. 1, a transformer T has two primary windings 8, 9 connected respectively between the neutral and line conductors 2, 4 and the earth and line conductors 3, 4. The transformer secondary winding 10 is connected to the operating winding 17 through earth connections E<SP>2</SP>, E<SP>1</SP>, the metal frame or casing 29 of the protected apparatus 27, auxiliary contacts 20 of a circuit-breaker A and contacts 21 of a differential current relay C. When the operating push-button 23 of the circuit-breaker A is fully depressed, the contacts 20 are closed and coil 17 is energized to close the circuitbreaker B. On the release of the push-button 23, a spring 24 returns the moving parts of the circuit-breaker A to a position in which the contacts 20 are open but the main contacts are held closed by a mechanical linkage (indicated at M) extending from the circuit-breaker B, the circuit for the coil 17 being maintained through the earth and neutral conductors 3, 2, back to the point at which the supply neutral is earthed. If there should be a break in any of the main conductors or if these conductors are not correctly connected, either or both of the windings 8, 9 are not energized and both circuit-breakers A, B open and/or circuitbreaker B cannot be closed. If any part of the circuit between the winding 10 and the coil 17 is broken, or if an earth leakage fault causes the operation of the relay C then the coil 17 is de-energized and the circuit-breakers open. To protect against overheating, the insulation 28 of the appliance 27 may be such that its resistance falls quickly above a predetermined temperature, the resulting current leakage to earth causing the relay C to open. A fullwave rectifier may be connected between the secondary winding 10 and the circuit it supplies to prevent incorrect operation due to stray A.C. potentials between neutral and earth, Fig. 3 (not shown). The circuit-breakers may be opened by the operation of a push-button switch 13 which interrupts the supply to the windings 8 and 9 and has a mechanical link for resetting the relay C.