Patent ID: 7095598

Claim:
An apparatus for producing a trip signal to activate a circuit breaker in an alternating current power line, said apparatus comprising: a sensing circuit operable to sense values representing: a phase difference between current and voltage in said power line; and a current amplitude in said power line; a comparison circuit operable to compare said sensed values of phase difference and current amplitude with a trip characteristic defining fault conditions and non-fault conditions and to generate a fault indication signal when a fault condition arises; a fault discriminator operable in response to said fault indication signal to generate a fault identifying signal discriminating between; a first class of fault in which a mean current value after said fault indication increases relative to a mean current value before said fault indication; and a second class of fault in which a mean current value after said fault indication does not increase relative to a mean current value before said fault indication; and a trip signal generator responsive to said fault identifying signal and operable to generate said trip signal; wherein, when said fault identifying signal identifies that said fault condition is due to said second class of fault occurring, said fault discriminator generates a state indication signal discriminating between a second class of fault caused by: a first state in which there is loss of current for a period following said fault indication followed by restoration of current to a level corresponding to the current level before said fault indication; and a second state in which there is loss of current for a period following said fault indication followed by a current at a level lower than the current level before said fault indication; said trip signal generator generates a trip signal when said state indication signal identifies that said fault indication is due to said second state occurring; and said second condition is an increase in the resistance of a circuit comprising said power line.