Enhanced AC immunity in ground fault detection
View Patent ↗A method for reducing the occurrence of false ground fault detections in a central office terminal is provided. The method includes 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.
1. A central office terminal, comprising:
at least one communication circuit adapted to communicate signals with at least one remote unit over at least one communication medium;
a power supply adapted to provide power to the remote terminal over the at least one communication medium;
a ground fault detection circuit, coupled to the power supply to detect ground fault current;
an AC immunity circuit, coupled to the ground fault detection circuit, that is adapted to modify the output of the ground fault detection circuit to reduce the occurrence of false ground fault detections due to AC induction; and
a processor, coupled to the power supply and the AC immunity circuit, that is adapted to determine when a ground fault occurs based on the signal from the AC immunity circuit.
2. The central office terminal of claim 1 , wherein the ground fault detection circuit comprises:
a resistor coupled to a power supply;
a bipolar junction transistor coupled between the resistor and chassis ground; and
an optocoupler, coupled to the base of the bipolar junction transistor and to the resistor.
3. The central office terminal of claim 1 , wherein the AC immunity circuit comprises:
a voltage reference;
a comparator with one input coupled to the voltage reference;
a pulse extender circuit, coupled to another input of the comparator, the pulse extender circuit coupled to receive a signal from the ground fault detection circuit, and to extend the duration of pulses in AC induced signals in the signal from the ground fault detection circuit; and
wherein the comparator is adapted to produce an enhanced ground fault detection signal based on a comparison of the voltage reference and the output of the pulse extender circuit.
4. The central officer terminal of claim 1 , wherein the processor is adapted to implement a software filter to filter a signal from the AC immunity circuit.
5. The central office terminal of claim 1 , wherein the at least one communication medium comprises two twisted copper pairs.