Circuit for measuring the ground resistance of an ungrounded power circuit

Claim 1

A circuit arrangement for measuring the ground resistance of an ungrounded power circuit comprising: a. a series circuit containing an ac voltage generator, a measuring resistor and a coupling member, coupled between the ungrounded power circuit and ground; and b. an evaluating circuit, having inputs coupled respectively to the point at which said series circuit is coupled to said ungrounded power circuit and across said measuring resistor to provide first and second voltage inputs proportional respectively to the voltage at said ungrounded power circuit and the current through said measuring resistor, for determining the ratio of the components of said voltage and current which are in phase with each other, said evaluating circuit comprising: 1. an absolute magnitude circuit having as its input said first measured voltage; 2. a mean forming circuit having the output of said absolute value circuit as an input; 3. a phase sensitive rectifier circuit having as its input said second measured voltage, said rectifier circuit also coupled to said first measured voltage and providing at its output the portion of said second measured voltage which is in phase therewith; 4. a second mean forming circuit having at its input the output of said phase sensitive rectifier circuit; and 5. a divider having as inputs the outputs of said first and second mean forming circuits and providing an output which is one of the ground resistance and ground conductance of the circuit being measured.