Abstract: A ground wire monitoring circuit particularly adapted for use in connection with mining machinery that is supplied with alternating current through a flexible cable from a power source remote from the machine. The cable contains phase conductors, a ground wire and a pilot wire. The ground wire is energized through the pilot wire from a voltage derived from the alternating current power supply at the remote source. Any failure of the pilot wire or ground wire is detected by circuit means adjacent the remote power source and in the event of such failure the power circuit leading to the machine is immediately opened. The power circuit is opened in the event of failure of the ground wire even though there may be low resistance parallel ground paths between the machine frame and the remote power source.

Abstract: A surge arrester of the multi-gap type made up of a series of gap units and a non-linear resistance disposed in a conventional porcelain housing. The gap units comprise a series of metal electrodes in the form of thin discs or plates arranged in a stack and spaced by ceramic insulating studs or posts; a surface of at least one of the electrodes in each gap unit, except in the area where the sparkover is to take place, is insulated to prevent wandering of the arcs over the surfaces of the plates to the edges of the plates where sparkover might take place along or across the edges of the whole series of plates making up the arrester. Such an action would greatly reduce the voltage of the multi-gap almost to the voltage of a single gap instead of an entire series of gaps. By eliminating this danger of wandering, the diameter of the plates can be substantially reduced, thus reducing the size and cost of the entire arrester assembly.