Fault detection apparatus and method

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.