Apparatus for detecting faults in electric cables

For use in combination with an electric power distribution cable and the normal source of voltage supply to said cable, the cable being connected between said normal source of voltage supply and at least one load normally applied to said cable, apparatus for use in selectively detecting faults in said electric power cable, which apparatus comprises a solid state switch unit comprising two thyristors arranged in an inverse-parallel configuration and continuously conducting current up to a pre-set maximum and becoming non-conductive after conducting a current in excess of the pre-set maximum for a pre-set period, a metering unit connected to said solid state switch unit for indicating the size of a current flowing through said cable, said metering unit including means for measuring the current in the solid state switch unit, and a trigger circuit connected to said solid state switch unit and metering unit which includes a manually operated trigger switch so connected to said solid state switch unit that when the trigger switch is actuated the trigger circuit will render the switch unit conductive, period measuring means connected to said solid state switch unit maintaining said switch unit conductive for said pre-set period, and switch terminating means rendering the solid state switch non-conductive prior to the end of the pre-set period if the metering unit indicates a test current larger than that which can be safely conducted during the pre-set interval, said apparatus in use being connected in series between said cable and said normal source of voltage supply to the cable when said cable is on load, whereby the trigger circuit will maintain said switch unit electrically conductive for a pre-set period long enough to allow the metering unit to indicate the size of the current flowing through said cable and hence the existence or otherwise of a fault in said cable and, when the metering unit indicates a current larger than that which can be safely conducted for the whole of the pre-set period without damage to the apparatus, the cable under test and the normal source of voltage supply to the cable, will render the switch unit non-conductive before the end of the pre-set period.