GB209831A - Improvements in or relating to the earthing of electrical apparatus - Google Patents

Abstract

209,831. Williams, A. W., and Rowley, F. W. Oct. 18, 1922. Protective arrangements; earthing.-In the protection of electrical apparatus, for example portable coal-cutting machines, by earthing the switch casing and the frame of the motor or machine, the main switch can be closed and maintained closed only when a trip device, energized by a current other than the working current and located in the earth circuit, is operative. As shown in Fig. 1, the switch a is connected with the machine b by an armoured multicore cable c' through plug-and-socket connections. One of the cores g constitutes an earthing lead and is connected through the plug-and-socket at one end to the frame of the machine at k and, at the other end, to the frame of the switch at o through the trip coil m and through the secondary n of a low-tension transformer the primary of which is connected across the supply through the auxiliary switch r. If the earthing connection becomes unsound while the main switch is closed, the reduced current in the coil n allows the switch to open. If the connection remains unsound the switch cannot be again closed. If the voltage of the supply falls off owing to fault &c., the switch cannot be closed if open, and open if closed. The last connection to be made and the fuse to be broken in the plug and socket couplings is the earth connection and, in consequence, the switch cannot be closed until the couplings are fully home, and, if the coupling is disconnected, the earth circuit of the trip coil is broken first, so that the supply circuit is interrupted at the switch and not at the plug and socket contacts. An entirely separate local source of suppy, for example a battery, may be provided for the earthing 0 circuit.