Ground fault circuit interrupter utilizing a single transformer
First Claim
1. A ground fault circuit interrupting device for interrupting an electrical power circuit including at least one line conductor and a neutral conductor, grounded at the source, in the event of a ground fault on the line conductor, said device including, in combination:
- A. circuit interrupting contacts for connection in series with at least the line conductor of the power circuit;
B. a differential current transformer having a first primary winding for connection in series with the line conductor, a second primary winding for connection in series with the neutral conductor, and a secondary winding;
C. an active load connected across said secondary winding;
D. means operating during a secondary current sampling period to repeatedly switch said active load between extremely low and extremely high impedance states to alternately operate said secondary winding into essentially a short-circuit and an open-circuit, each short-circuiting of said secondary winding creating a sampling interval during which the current flowing in said active load is a function of the current flowing in said secondary winding;
E. means for developing a signal indicative of the integral of the current flowing in said active load over a plurality of sampling intervals within said sampling period; and
F. means for initiating opening of said interrupter contacts when said signal exceeds a predetermined threshold.
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Accused Products
Abstract
A single transformer is alternately operated as a differential current transformer with its secondary winding connected as the input to a low impedance differential amplifier with large negative feedback during a ground fault current sampling period and then as a coupled resonant circuit operating into a negative resistance network during a neutral-to-ground resistance sampling period. During the fault current sampling period, the amplifier is gated ON and OFF to switch the transformer secondary loading between essentially short-circuit and open-circuit conditions. The resulting amplifier feedback current pulses, indicative of transformer secondary current, are integrated either directly or indirectly, to develop a DC signal proportional to any ground fault current.
62 Citations
20 Claims
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1. A ground fault circuit interrupting device for interrupting an electrical power circuit including at least one line conductor and a neutral conductor, grounded at the source, in the event of a ground fault on the line conductor, said device including, in combination:
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A. circuit interrupting contacts for connection in series with at least the line conductor of the power circuit; B. a differential current transformer having a first primary winding for connection in series with the line conductor, a second primary winding for connection in series with the neutral conductor, and a secondary winding; C. an active load connected across said secondary winding; D. means operating during a secondary current sampling period to repeatedly switch said active load between extremely low and extremely high impedance states to alternately operate said secondary winding into essentially a short-circuit and an open-circuit, each short-circuiting of said secondary winding creating a sampling interval during which the current flowing in said active load is a function of the current flowing in said secondary winding; E. means for developing a signal indicative of the integral of the current flowing in said active load over a plurality of sampling intervals within said sampling period; and F. means for initiating opening of said interrupter contacts when said signal exceeds a predetermined threshold. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. A ground fault circuit interrupting device for interrupting an electrical power circuit, including at least one line conductor and a neutral conductor grounded at the source, in the event of a ground fault on the line conductor, said device including, in combination:
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A. circuit interrupting contacts for connection in series with at least the line conductor of the power circuit; B. a differential current transformer having a first primary winding for connection in series with the line conductor, a second primary winding for connection in series with the neutral conductor, and a secondary winding; C. a differential amplifier having (1) an inverting input connected directly to one side of said secondary winding, (2) a non-inverting input connected directly to the other side of said secondary winding, and (3) an output connected back to said inverting input as a negative feedback path; D. gating means for gating said amplifier ON to essentially short circuit said secondary winding and OFF to essentially open circuit said secondary winding; E. an oscillator synchronized to the power circuit line frequency and controlling said gating means to repeatedly and periodically gate said amplifier ON and OFF over a sampling period during which the slope of the current waveform in said secondary winding is uniformly of one polarity, each gating ON of said amplifier providing a secondary current sampling interval during which a current pulse flows in said amplifier feedback path in response to current flow in said secondary winding, each said current pulse being proportional to the change in the current flowing in the short-circuited secondary winding during the associated sampling interval; F. a capacitor connected in said amplifier feedback path for integrating said feedback current pulses over each sampling period; and G. threshold detector means operating to initiate opening of said interrupter contacts when the voltage appearing across said capacitor during a sampling period exceeds a predetermined threshold level. - View Dependent Claims (9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20)
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Specification