bandpass filter 62 to block 60 hz and harmonic components of the 1 khz signal and is coupled through the primary winding of current sampling transformer t 2 to pilot wire 18 of cable 10 this sensing or monitoring signal circulates in the ground monitoring circuit comprised of pilot wire 18 terminating resistor rt and ground conductor 16 for simplicity only a single ground conductor 16 is illustrated in fig3 in practice two ground conductors and the power conductor shields that are illustrated in fig1 are connected in parallel voltage transients are suppressed by means of a voltage breakdown device 56 such as a gas discharge tube that is connected between ground wire 16 and pilot wire 18 for the same purpose a bipolar semiconductor voltage breakdown device 57 such as a back to back zener diode device also is connected between the ground and pilot wires choke coil 58 in series with pilot wire 18 also is employed for transient suppression a sample of the 1 khz current flowing in the ground monitoring circuit is taken by transformer t2 and a voltage sample of the 1 khz signal in the ground monitoring circuit is taken at junction point 63 the sampled current signal is coupled from the secondary winding of transformer t2 to the lead 67 is amplified in preamplifier 69 filtered in active bandpass filter 71 and is converted to a corresponding d c signal by full wave rectifier and filter 73 the voltage signal that is sampled at point 63 is filtered in active bandpass filter 65 is amplified in amplifier 77 again.