12.1 General

A typical colliery underground network is shown in Figure 12.1. The protection required for the medium-voltage (11 kV) network from the surface substation to the mobile transformer will require considerations about the trailing cable. However, this could be the standard protection that is followed in a normal industrial substation, taking care of short-circuit and overcurrent conditions.

Figure 12.1: Typical colliery UG network

However, 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 comprises:

• Earth-leakage protection

• Pilot wire monitors (ground continuity monitors)

• Earth fault lockout

• NERM (neutral earthing resistor monitors).

These features will now be discussed in detail point by point.

12.2 Earth-Leakage Protection

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 (see Table 12.1).