Ground integrity monitor for electrical apparatus

A ground integrity monitor for electrical apparatus wherein a single phase, three-wire grounded AC power source having hot, neutral and ground terminals is coupled to the electrical apparatus for applying AC operating voltage thereto, wherein both the neutral and ground terminals are ordinarily connected to earth ground, and wherein a conductive chassis of the electrical apparatus is connected to the ground terminal, comprising:

• a pulse signal generator including a unijunction oscillator having a unijunction transistor whose base-one is connected to the ground terminal through, in the order named, first and second series resistors, said generator producing, across said resistors, sharp narrow pulses periodically recurring at a pulse repetition frequency very low relative to the commutating frequency of the AC power source;
• means for connecting the neutral/ground circuit in shunt with said second resistor to apply said pulses between the neutral and ground terminals of the AC power source;
• control signal producing means responsive to the pulsing of the neutral/ground circuit for producing a control signal effectively representing the resistance between the neutral and ground terminals,
• said control signal producing means including a light emitting diode photo coupler whose light emitting diode is connected in shunt with said first resistor and flashes when the resistance in the neutral/ground circuit is below a predetermined threshold level, thereby causing the photo resistor of said coupler to exhibit a relatively low resistance, the amplitude of said control signal being determined by the resistance of said photo resistor;
• and an integrating device, included in said control signal producing means, for rendering the monitor immune to transients that may otherwise deleteriously affect its operation.