Patent GB-813748-A (GB)
Title: Improvements in and relating to protective systems for transportable electrical equipment
Inventor: SEAWARD THOMAS STEPHEN GEORGE
IPC: H02H11/00
Filed: 1956-02-29
Abstract: 813,748. Protective cut-out arrangements. IMPERIAL CHEMICAL INDUSTRIES Ltd. Feb. 27, 1957 [Feb. 29, 1956; Aug. 10, 1956), Nos. 6303/56 and 24564/56. Class 38 (5). A protective system for a transportable equipment M3 comprising a contactor or circuitbreaker B and a monitoring circuit containing relay means H and two earth conductors P, Q is arranged so that (a) the monitoring circuit is supplied with current from a safe supply when starting the equipment to prove the continuity of the monitoring circuit; (b) the monitoring circuit is then supplied with current from a safe supply within the equipment enclosure to operate or permit the operation of the contactor or circuit-breaker,; and (c) the monitoring circuit is supplied with current from a safe supply within the equipment enclosure to maintain, through the relay means H, the supply to the equipment. The earthing conductors are connected independently to the enclosure of the equipment and to an earthed member, e.g. the enclosure, of the fixed part of the system and are arranged so that each conductor forms a separate path for earth fault current. These conductors and components in series with them are arranged to have a low impedance and a high current-carrying capacity which is not less than that of fuses T1, T2, in the main supply circuit. In the arrangement of Fig. 2, the closure of an isolator I causes a current transformer D to supply current to the monitoring circuit to prove its continuity. On the closure of contacts L1 of a starting switch, the potential produced by a transformer E/F, which is energized by the current in the monitoring circuit, is applied to the main supply conductors R1, R2. This causes the energization of a relay C1 which closes its contacts C2 to energize the closing coil C of the contactor B. Contactor B now changes over its main contacts B1, B2 to energize the equipment motor M3, closes contacts B3 to prepare a holding circuit and opens contacts B4 to disconnect the transformer D. The transformer E/F is now energized from the supply conductors R1, R2 and supplies current to the monitoring circuit to cause the relay H to close its contacts H1, or to maintain these contacts closed, so as to maintain the closing coil C energized after the delayed opening of the contacts C2. In Fig. 3, on the operation of the start switch L1, the relay H is energized from a dry cell G and closes its contacts H1 to energize the coil C and close the contactor B. Thereafter the contactor is maintained closed by current in the monitoring circuit provided by a rectifier F1 and a transformer F connected to the main supply conductors. A double-wound A.C. relay K opens the contactor in the event of earth fault currents in the monitoring circuit. Various modifications are referred to, these modifications including the use of pilot wires to provide a safe supply from the fixed enclosure to the equipment enclosure where it may be converted before application to the monitoring circuit, the use of a self-contained supply source in the equipment enclosure to perform function (a), (b) and (c) and the provision of the safe supply from a hand generator or a transistor oscillator powered by a small battery.