ndian 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.
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.
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