Mine underground distribution protection

It is important to pay particular attention to the protection of the low-voltage “front-end electrics” especially at the coalface where most activity takes place, increasing the possibility of electrical faults occurring. In coal mining, the normal protection found in the flameproof gate-end boxes earth-leakage protection, pilot wire monitors (ground continuity monitors), earth fault lockout, and NERM (neutral earthing resistor monitors). Earth-leakage protection is primarily employed to protect life. It must therefore detect and isolate faulty equipment as soon as possible to protect the rest of the system and to minimize fault damage. Consequently, it needs to be as sensitive and as fast as possible. However, ultra-sensitivity and high speeds can lead to nuisance tripping so a compromise is necessary. Generally, one only needs to consider protecting against indirect contact. This is considered justified, as only qualified persons should have access to live terminals, equipment, and interlocks being designed accordingly. The chapter explains the neutral earthing resistor monitor (NERM). The final element in the protection system is the relay, which ensures the integrity of the neutral earthing resistor. If this device opens or short-circuits, the NERM will operate to either alarm or trip.