Enhanced AC immunity in ground fault detection

1. A method for reducing the occurrence of false ground fault detections in a central office terminal, the method comprising:

generating a no-fault signal when no ground current is detected;

delaying generation of a fault signal when ground current is detected at least for the duration of an expected pulse in AC induced signal; and

when the ground current persists for a sufficient period, generating a signal indicating a fault condition.

2. The method of claim 1 , wherein generating a no-fault signal when no ground current is detected comprises generating a logical high signal.

3. The method of claim 1 , wherein delaying generation of a fault signal comprises delaying generation of the fault signal while discharging a capacitor through a resistor in a signal path with a higher resistance value that a resistor in a signal path used to charge the capacitor.

4. 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.

5. The circuit of claim 4 , wherein the first path includes a first resistor in series with a diode.

6. The circuit of claim 5 , wherein the second path includes a second resistor with a resistance value that is substantially larger than the first resistor.

7. The circuit of claim 5 , wherein the first resistor is coupled between a voltage source and the output of the ground fault detector.

8. The circuit of claim 7 , wherein the second resistor is coupled between the capacitor and the output of the ground fault detector.

9. The circuit of claim 7 , wherein the reference voltage comprises a resistor divider coupled between the voltage source and ground.

10. 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.

11. 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.

12. The AC immunity circuit of claim 11 , wherein the reference voltage comprises the output of a voltage divider circuit with the voltage set at a level that produces a ground fault detection when the ground current exceeds a selected level.

13. The AC immunity circuit of claim 11 , wherein the pulse extender circuit comprises an RC circuit with a time constant that leaves the ground fault detection signal substantially unchanged during a portion of the AC induced signal that would otherwise cause a ground fault detection.

14. 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.

15. A method for reducing the occurrence of false ground fault detections in a central office terminal, the method comprising:

generating a no-fault signal when no ground current is detected in said central office terminal;

delaying generation of a fault signal when ground current is detected at least for the duration of an expected pulse in AC induced signal; and

when the ground current persists for a sufficient period, generating a signal indicating a fault condition in said central office terminal.