Sensitive peak current detector for ground fault protection circuits

Claim 2

In a ground fault protection circuit including a power circuit fault current sensor and a current detector operatively coupled thereto that produces an output indication of abovethreshold currents sensed by said fault current sensor, the improvement wherein said current detector comprises at least one transistor that conducts except during the intervals the instantaneous current sensed by said fault current sensor is above a predetermined threshold magnitude, bias circuit means for continuously supplying base current to maintain said transistor conducting, current-diverting means responsive to the sensing of instantaneous above-threshold currents by said fault current sensor for diverting a sufficient amount of the base current to turn off said transistor, filter means rendered operative by the turning off of said transistor for reducing the detector sensitivity to abovethreshold high-frequency transient currents, and output indication means actuated by said filter means for producing a valid low-power frequency fault current indication.

Claim 3

A circuit according to claim 1 wherein said current diverting means is a second transistor.

Claim 4

A circuit according to claim 2 wherein said filter means includes a first filter circuit comprising a series connected first resistor and capacitor, and said output indication means includes a semiconductor switch operatively connected with said first capacitor to be rendered conductive upon the charging of said first capacitor to a predetermined voltage.

Claim 5

A circuit according to claim 3 wherein said filter means further includes a second filter circuit comprising a series connected second resistor and charging capacitor connected to an electrode of said first-mentioned transistor, and means including a rectifier for delaying the operation of said first filter circuit until said second capacitor charges to a predetermined voltage.

Claim 6

A circuit according to claim 1 wherein said current diverting means comprises another transistor connected in mirror-image fashion such that the base electrodes of both transistor are effectively coupled together, and wherein said filter means includes a series connected resistor and capacitor, and said output indication means is actuated by the charging of said capacitor to a predetermined voltage.

Claim 7

A ground fault protection circuit comprising a peak current detectoR circuit energized by a source of unidirectional voltage and including a first and second transistors connected in mirror-image fashion with their respective base electrodes effectively coupled together, a bias circuit for continuously supplying base current to at least said first transistor to maintain said first transistor conducting, a power circuit fault current sensor operatively coupled with said transistors to initiate the diversion of sufficient base current from said first transistor to said second transistor, in response to the sensing of an instantaneous above-threshold current of one polarity by said fault current sensor, to turn off said first transistor, a filter network rendered operative by the turning off of said first transistor for filtering above-threshold high-frequency transient currents to prevent producing a false output indication, a detector output circuit actuated by said filter network for producing a valid low power frequency fault current indication, and a power circuit interrupter connected to be opened by the output indication from said detector output circuit.

Claim 8

A ground fault protection circuit comprising a peak current detectoR circuit energized by a source of unidirectional voltage and including a first and second transistors connected in mirror-image fashion with their respective base electrodes effectively coupled together, a bias circuit for continuously supplying base current to at least said first transistor to maintain said first transistor conducting, a power circuit fault current sensor operatively coupled with said transistors to initiate the diversion of sufficient base current from said first transistor to said second transistor, in response to the sensing of an instantaneous above-threshold current of one polarity by said fault current sensor, to turn off said first transistor, a filter network rendered operative by the turning off of said first transistor for filtering above-threshold high-frequency transient currents to prevent producing a false output indication, a detector output circuit actuated by said filter network for producing a valid low power frequency fault current indication, and a power circuit interrupter connected to be opened by the output indication from said detector output circuit.

Claim 9

A circuit according to claim 6 wherein said filter network includes a first filter circuit comprising a series connected first resistor and capacitor, and said detector output circuit comprises a voltage-sensitive semiconductor switch operatively connected to be rendered conductive by the charging of said first capacitor to a predetermined voltage.

Claim 10

A circuit according to claim 7 wherein said fault current sensor is a differential current transformer, and said detector output circuit further includes a thyristor having a gating circuit coupled to said voltage-sensitive semiconductor switch, said thyristor and power circuit interrupter being connected to open said interrupter upon the turning on of said thyristor in response to the output indication generated by current flow through said voltage-sensitive semiconductor switch.

Claim 11

A circuit according to claim 6 wherein said bias circuit also continuously supplies base current to said second transistor, and said fault current sensor is effectively connected between the respective base electrodes of said first and second electrodes so that, additionally, base current is diverted from said second transistor to said first transistor in response to the sensing of an instantaneous above-threshold current of the other polarity by said fault current sensor.

Claim 12

A circuit according to claim 9 wherein said filter network includes a first filter circuit comprising a series connected first resistor, first capacitor, and third transistor that is effectively connected to both said first and second transistors to be rendered conductive by the turning off of either of said first and second transistors, and said detector output circuit includes a voltage-sensitive semiconductor switch connected in parallel circuit relationship with said first capacitor to be rendered conductive by the charging of said first capacitor to a predetermined voltage.

Claim 13

A circuit according to claim 10 further including a fast capacitor discharge circuit connected in parallel with said first capacitor and to said third transistor to effect discharge of said first capacitor in response to the turning off said third transistor.

Claim 14

A circuit according to claim 11 wherein said filter network further includes second filter circuits including a series connected second resistor and capacitor and series connected third resistor and capacitor respectively coupled to the corresponding electrodes of said first and second transistors, and a pair of diodes respectively coupled between said third transistor and said second and third capacitors to delay the turning on of said third transistor until said second and third capacitors respectively charge to a predetermined voltage.