idian Electricity Rule 116 is modified for mines. It says �In the interest of safety, appropriate switchgear with necessary protective equipment shall be suitable placed in the mine for automatically disconnecting supply to any part of the system where a fault including an earth fault occurs. The earth fault current should not be more than 750 mA in underground 550V / 1100 volts system & 50 Amps. In 6.6 / 11 KV system in open cast mine.
Operating Principle
Leakage or fault current is sensed normally by core Balance Current Transformer (CBCT). This zero-sequence current is then processed and compared with a reference voltage. The comparator output is send through a time delay circuit to prevent nonsense operation. The output of the time circuit operates a relay whose contacts are used to actual a trip device to interrupt the line. Visual or audio-visual indication can be provided if necessary.
Technical Specification
Input voltage: 550V or 110V. a.c.
Sensing Current range: 80 mA to 750 mA adjustable.
Maximum swing angle of each bucket.
Tripping time setting: 0.1 Second to 1 Second
Output: One No / One NC connection