Faulted circuit detector having isolated indicator

Apparatus for detecting a fault on a monitored electrical conductor, comprising:

• means for sensing a fault in the conductor, said sensing means including a temperature compensation circuit for negating the effect on said sensing means of temperature changes within the range of approximately -40 degrees C. to +85 degree C., and including a low pass filter circuit for filtering out undesirable high frequency transients induced in said sensing means;
• means for remote from said sensing means for indicating the presence of a fault in the electrical conductor;
• means for communicating the presence of a fault sensed by said sensing means to said indicating means;
• means for automatically resetting said indicating means;
• and
• means for disabling automatic operation of said resetting means unless a predetermined minimum voltage is present on the conductor;
• said communication means electrically isolating said sensing means from said indicating means;
• said resetting means continually resetting said indicating means at regular predetermined intervals unless disabled by said disabling means;
• said disabling means comprising a hold-off circuit for deactivating said resetting means when said predetermined voltage is not present on the conductor;
• and
• said temperature compensation circuit comprising:
• a current transformer having a wire coil, said wire coil having a positive temperature coefficient;
• a thermistor having a negative temperature coefficient;
• and
• a resistor connected in parallel with said thermistor, said resistor having a negligible resistance change at temperatures within the range of approximately -40 degrees C. to +85 degrees C.

Apparatus for indicating that a fault of a predetermined current magnitude has occurred in a monitored electrical conductor on the high voltage side of an electrical distribution transformer, comprising:

• a sensor for generating a signal proportional to the current in the monitored conductor;
• a low pass filter for filtering high frequency transients in the signal generated by said sensor to prevent the transients from causing misoperation of the apparatus;
• an optic transmitter for transmitting a light pulse when a fault condition has been sensed by said sensor;
• an optic receiver for receiving said light pulse and transforming said pulse into an electrical pulse;
• an indicator for indicating the presence of a fault condition in the monitored conductor upon receipt of said electrical pulse, said indicator having first and second indication states for, respectively, displaying indications of "normal" or "fault" conditions;
• a reset circuit for resetting said indicator from the second of said indication states to the first of said indication states;
• and
• a hold-off circuit for delaying the reset circuit from resetting said indicator unless a predetermined minimum voltage is present on the low voltage side of the electrical distribution transformer.

An apparatus for detecting the presence of faults on an electrical conductor, comprising:

• a sensor for sensing an overcurrent condition in the electrical conductor, said sensor comprising a temperature compensation circuit for negating the effect of temperature change on said sensing means, said temperature compensation circuit comprising:
• a current transformer having a wire coil, said wire coil having a positive temperature coefficient;
• a thermistor having a negative temperature coefficient;
• and
• a resistor connected in parallel with said thermistor, said resistor having a negligible resistance change at temperatures within the range of approximately -40 degrees C. to +85 degrees C.;
• an indicator for indicating the presence of an overcurrent condition in the electrical conductor;
• and
• a signal transmitter for communicating the presence of an overcurrent condition sensed by said sensor to said indicator.