High impedance fault detection apparatus and method

Apparatus for detecting a high impedance fault on an electrical circuit having alternating current flowing therethrough, and having a circuit breaker for disconnecting the circuit from a source of electrical power, which comprises:

• a transducer coupled to the electrical circuit for providing a voltage signal indicative of the magnitude and waveform of the current in the circuit;
• a filter coupled to the transducer, for substantially removing from the voltage signal frequency components below a preselected frequency;
• means for sampling the filtered voltage signal a plurality of times during a cycle of the alternating current, and converting each voltage sample to a digital data word representative of the voltage sample;
• a microcomputer system, including interconnected central processing unit (CPU) and memory means, coupled to said sampling and converting means,
• said memory storing a fault detection program routine comprising an instruction set for instructing predetermined computation processing by the central processing unit using the voltage samples digital data, to determine whether the magnitude of the high frequency components in a cycle of the alternating current in the electric circuit is at least a prescribed level, and after determining that said components are above the prescribed level, to further determine whether the magnitude of the high frequency components of the alternating current in the electric circuit remains at least the prescribed level for at least a prescribed minimum number of cycles during a prescribed time interval, thereby determining a fault condition on the electrical circuit;
• and
• means coupled to the central processing unit and actuated thereby upon a determination of a fault condition on the electrical circuit, for actuating the circuit breaker to disconnect the electrical circuit.

A method of detecting high impedance faults in an electrical circuit having alternating current flowing therein, comprising the steps of:

• (a) producing a signal representative of the magnitude and waveform of the alternating current;
• (b) filtering the signal to substantially remove frequency components below a prescribed frequency;
• (c) sampling the amplitude of the filtered signal a plurality of time during a present cycle of the alternating current;
• (d) computing the energy level of the filtered signal during a present cycle based on the samples taken during the cycle;
• (e) comparing the computed energy level of the present cycle to an average of the computed energy levels over a preceding time interval spanning a plurality of preceding cycles of the alternating current;
• (f) if the computed energy level of the present cycle is at least a prescribed amount greater than the average energy level, computing the energy level of the filtered signal during each of a predetermined number of the successive following cycles of the alternating current;
• (g) comparing the computed energy level of each cycle to said computed energy level average;
• (h) counting the number of cycles wherein the computed energy level exceeded said computed energy level average;
• (i) comparing said count of cycles to a predetermined count value;
• and
• (j) producing a fault indication if said count of cycles equals or exceeds the predetermined count value.

A method of detecting high impedance faults in an electrical circuit having electrical current flowing therein, comprising the steps of:

• generating a signal indicative of the magnitude and waveform of the current in the circuit;
• removing from said generated signal the portion indicative of frequency components below a preselected frequency, so as to provide a signal indicative of the high frequency components of the current in the circuit;
• monitoring the high frequency components of the electrical current to measure the energy level thereof;
• detecting an increase in high frequency component energy level above an average energy level existing over a preceding prescribed time interval;
• and
• determining the existence of an increased high frequency components energy level over a prescribed period of time, thereby determining the existance of a high impedance fault condition on the electrical circuit.