Enhanced ac immunity in ground fault detection

A circuit for providing AC immunity to a ground fault detector in a central office terminal, the circuit comprising:

• a comparator having first and second inputs and providing an output;
• a reference voltage coupled to one input of the comparator;
• a capacitor, coupled to the other input of the comparator;
• a first path for charging the capacitor to a first state when the ground fault detector indicates no ground fault;
• and
• a second path for discharging the capacitor at a slower rate than the charging in the first path in response to a signal level corresponding to a fault detection;
• wherein the second path providing immunity to AC signals that alternate between levels that correspond to fault and no-fault conditions.

A method for enhancing the output of a ground fault detection circuit, the method comprising:

• detecting a signal from the ground fault detection circuit indicating a condition with no ground fault;
• rapidly charging a capacitor to a level that indicates the detected condition;
• comparing the capacitor voltage with a reference voltage;
• producing an enhanced output for the ground fault detection signal indicating the no ground fault condition;
• receiving a subsequent signal from the ground fault detector indicating a possible ground fault condition;
• and
• slowly discharging the capacitor in response to the possible ground fault condition at a rate such that the risk of the capacitor discharging to a level to change the enhanced output to indicate a ground fault is reduced when the subsequent signal is an AC induced signal.

An AC immunity circuit for a ground fault detection circuit of a central office terminal, the AC immunity circuit comprising:

• a comparator;
• a reference voltage that differentiates between a ground fault and a normal operating condition;
• and
• a pulse extender circuit, adapted to receive a signal from the ground fault detection circuit, the pulse extender adapted to extend the duration of an AC induced signal at a level indicative of a no-fault condition over a time of at least one pulse in the AC induced signal.

An AC immunity circuit for a ground fault detection circuit of a central office terminal, the AC immunity circuit comprising:

• means for generating a signal indicating a potential ground fault current;
• means for generating a reference voltage;
• means for comparing the signal indicating a potential ground fault current with the reference voltage;
• and
• means for extending pulses in the signal indicating a potential ground fault current to reduce the impact of AC induced currents on the ground fault detection.