This invention relates generally to electrical distribution apparatus, and more particularly to apparatus for indicating the existence of a fault in any of a plurality of conductors in a multi-conductor electrical distribution system.
Fault indicating apparatus, and more particularly ground fault sensing and protection devices, have generally comprised current transformers inserted into each conductor in a multi-conductor distribution system, and which were connected to associated means which obtained unidirectional output current varying with the ground current in the system being protected. The associated means compare the unidirectional current signal with a predetermined signal level, and whenever the unidirectional current signal exceeded the predetermined signal level, the apparatus would indicate the existence of a ground fault, most notably by causing a circuit breaker to trip. U.S. Pat. Nos. 3,660,722 and 3,697,810 illustrates a typical example of this type of fault indicating apparatus.
Other types of fault indicating apparatus utilize sensing means electrically coupled to the conductors to be protected, and contain independent power sources for powering the fault indicating apparatus. A problem associated with both types of priority devices is that they are continuously operative. Extraneous signals or interactions between the current transformers may produce a fault indication when none is present. In these instances, the indicating and protective means may become operational when in reality a fault does not exist.