Patent GB-405339-A (GB)
Title: Improvements in or relating to the protection of portable electric apparatus
IPC: H02H7/00
Filed: 1932-08-05
Abstract: 405,339. Protective cut-out systems. REYROLLE & CO., Ltd., A., Hebburn-on-Tyne, CLOTHIER, H. W., 3, Jesmond Park West, LEBEN, H., 212, Wingrove Road, both in Newcastle-on-Tyne, and BURGESS, W. A. A., Inglethorpe, Kenners Dene, Tynemouth, North Shields. Aug. 5, 1932, No. 22061. [Class 38 (v).] Electromagnetic switches ; latching and unlatching ; contacts.-In protective arrangements for portable apparatus of the kind in which the apparatus is controlled through a trailing cable by a main switch at the supply end and a twowire pilot circuit is employed for the dual purpose of earthing the frame of the apparatus and of operating the apparatus from the apparatus end of the cable, an impedance is provided in series with the pilot circuit at the apparatus end and a standard circuit containing impedance is connected across the pilot circuit at the supply end, the main switch being controlled by a differential relay which compares the current flowing in the pilot circuit with that flowing in the standard circuit and thereby prevents closure of the main switch unless the pilot circuit is healthy. A portable coalcutting apparatus A is controlled through a trailing cable A<1> by a gate-end switch C, the closing coil C<1> of which is controlled by contacts E<2> of a differential relay. A transformer B<1> B2 energizes one coil E of the relay in series with the pilot circuit D D<1> and another coil E<1> in a mimic circuit comprising an impedance F, the values of which and of an impedance H are so chosen that normally the effects of E E<1> counterbalance. When the manual control switch G is open, only E<1> is energized and accordingly E<2> are open, but closure of G causes E to be energized, whereby E<2> close. Contacts E<2> also open when a break occurs in the pilot D D<1> to de-energize E, or when the balance is upset by a partial or complete short-circuit in D D<1>, or on failure of the supply B. In order to prevent the automatic reclosure of C when a failure in the supply B occurs, the impedance H is arranged as a retaining coil for the switch G, so that the latter is released and must be manually reclosed when the supply is restored. This has the drawback that vibration may affect the hold-on coil, and there is the added drawback that E<2> are closed when the armature of E E<1> falls. Both drawbacks are obviated by the form shown in Figs. 2, 3. The impedance of the mimic circuit is in the form of a coil J with contacts J<1> in series with E<2> and interlocked at K so that J' cannot be closed if E<2> are closed. Fig. 3 shows the relay windings E, E<1>, J and contacts E<2>, J<1> mounted on armatures E<3>, J<2>, respectively. Armature E<3> when lifted engages the arm M<1> and moves the arm M, against spring-action M<2>, clear of extension J<3> on armature J<2>, so allowing the latter to rise. If the mains fail (Fig. 2), the switch C opens and latch M is moved to the operative position by spring M<2>. When the supply is restored, and if G is still closed, E<3> will remain down and prevent J<2> from rising, hence the operator has to open G to allow E3, J<2> to rise in turn, so opening E2 and closing J<1> the contacts E<2> being reclosed by subsequent reclosure of G by the operator. In another form, Fig. 6 (not shown), the two relays may be combined into a single unit having a balanced beam armature, the operating and restraining forces being derived by the coils in the mimic circuit and the pilot circuit, respectively, whereby the contacts can only be closed when the operating coil is energized and the differential coils are equally energized. A holding-coil for G is also provided.