Ground monitoring system and method

A continuity monitoring system for a machine having a power cable for supplying electrical voltage and current to the machine from a power supply, the cable having power conductors and a ground conductor which is grounded to the machine, said system comprising;

• transmitter means for generating a monitor signal having a predetermined amplitude and a predetermined frequency different from the frequency of said current supplied by said power supply, first coupling means located between the power supply and the machine for coupling the monitor signal to a conductor of the cable, second coupling means at the machine for receiving the monitor signal from the conductor and for coupling the signal to a ground on the machine so that the signal is conducted to and transmitted along the ground conductor, receiver means, third coupling means remote from the machine for coupling the monitor signal from the ground conductor to said receiver means, said receiver means comprising a discriminator means comprised of a pair of phase locked loop means connected in parallel, each of said phase locked loop means having the same amplitude and frequency characteristic and each being responsive to said received monitor signal for producing a predetermined output only when said received monitor signal is above a predetermined level and within a predetermined frequency range, power switch means having two states and being connected between said power supply and said cable for supplying power from said supply to said cable when in a first state and for interrupting said power when in a second state, and circuit means connected between said phase locked loop means and said power switch means for causing said power switch means to switch to said second state to interrupt the power to said cable in response to the absence of said predetermined output from either one of said phase locked loop means, so that said switch means interrupts the power to the cable when the improper monitor signal is received even if one said phase locked loop means is defective, said circuit means comprising two switch means connected in series, a different one of said phase locked loop means being connected to each of said switch means, each of said switch means comprising means for becoming electrically an open circuit responsive to the absence of said predetermined output from the phase locked loop means to which it is connected, and said series combination of switch means being connected between a source of voltage and said power switch means so that current supply to said power switch means is interrupted when said predetermined output is absent from either one or both of said phase locked loop means.