Ground fault detector circuit with high current inhibit

Claim 2

A ground fault detection circuit for a motor starter, the combination comprising: a current transformer coupled to the power lines leading to the motor starter and operable to generate a current in its secondary winding which is proportional to the ground fault current in the power lines; a trip current level detector circuit coupled to the secondary winding and operable to generate a selected output voltage when the ground fault current reaches a first preselected level; a time delay circuit coupled to the trip current level detector to receive the selected output voltage and operable to impart a time delay thereto; an output circuit coupled to the time delay circuit to receive the delayed output voltage, said output circuit being coupled to the motor starter to deenergize the same when the delayed output voltage is received; and a high current level detector circuit coupled to the secondary winding and operable to generate a disabling voltage which inhibits the application of said delayed output voltage to said output circuit when the ground fault current reaches a second preset level which is substantially higher than the first preset level.

Claim 3

The ground fault detection circuit as recited in claim 1 in which the disabling voltage is applied to said time delay circuit to inhibit generation of said delayed output voltage.

Claim 4

The ground fault detection circuit as recited in claim 1 in which the output circuit is coupled to the motor starter through a circuit breaker and the current transformer secondary winding is coupled to the trip current level detector circuit and high current level detector circuit through a rectifier circuit.

Claim 5

The ground fault detection circuit as recited in claim 3 in which a pair of series connected resistors are connected across the output terminals of the rectifier circuit, said trip current level detector circuit is connected across one of said resistors, and the high current level detector circuit is connected across the other of said resistors.

Claim 6

The ground fault detection circuit as recited in claim 1 in which the second preselected current level is at least ten times greater than said first preselected current level and said current transformer is operable to generate said currents in its secondary winding proportional to the ground fault current at both preselected levels.

Claim 7

A ground fault detection circuit for a motor starter, the combination comprising: a current transformer coupled to the power lines leading to the motor starter and operable to generate a current in its secondary winding which is proportional to the ground fault current in the power lines; a rectifier circuit having inputs connected to said current transformer secondary winding and having a pair of output terminals across which a voltage proportional to ground fault current is generated; a pair of resistors connected across said rectifier circuit output terminals; a trip current level detector circuit having an input connected to one of said resistors and being operable to generate a selected output voltage when the voltage generated by said rectifier circuit reaches a first preselected level; a time delay circuit having an input connected to the trip current level detector to receive the selected output voltage therefrom and having a charging capacitor which accumulates a charge when said selected output voltage is generated and generates a voltage proportional to said accumulated charge; an output circuit having an input terminal connected to said charging capacitor and having an output coupled to the motor starter, said output circuit being operable to deenergize the motor starter when the voltage across said charging capacitor reaches a preselected trip level; and a high current level detector circuit having an input connected to the other of said resistors and an output terminal connected to said charging capacitor, said high current level detector circuit being responsive to the voltage generated by said rectifier circuit to provide a discharge path for said charging capacitor which inhibits it from reaching said preselected trip level when the voltage generated by said rectifier circuit reaches a second preset level which is substantially higher than said first preset level.

Claim 8

A ground fault detection circuit for a motor starter, the combination comprising: a current transformer coupled to the power lines leading to the motor starter and operable to generate a current in its secondary winding which is proportional to the ground fault current in the power lines; a rectifier circuit having inputs connected to said current transformer secondary winding and having a pair of output terminals across which a voltage proportional to ground fault current is generated; a pair of resistors connected across said rectifier circuit output terminals; a trip current level detector circuit having an input connected to one of said resistors and being operable to generate a selected output voltage when the voltage generated by said rectifier circuit reaches a first preselected level; a time delay circuit having an input connected to the trip current level detector to receive the selected output voltage therefrom and having a charging capacitor which accumulates a charge when said selected output voltage is generated and generates a voltage proportional to said accumulated charge; an output circuit having an input terminal connected to said charging capacitor and having an output coupled to the motor starter, said output circuit being operable to deenergize the motor starter when the voltage across said charging capacitor reaches a preselected trip level; and a high current level detector circuit having an input connected to the other of said resistors and an output terminal connected to said charging capacitor, said high current level detector circuit being responsive to the voltage generated by said rectifier circuit to provide a discharge path for said charging capacitor which inhibits it from reaching said preselected trip level when the voltage generated by said rectifier circuit reaches a second preset level which is substantially higher than said first preset level.

Claim 9

The ground fault detector circuit as recited in claim 6 in which said trip current level detector circuit is a Schmitt trigger and said output circuit includes a circuit breaker.