9.2.5. Protective Earthing

In protective earthing, all of the conductive parts not belonging to the service circuit are connected with the protective earthing device by means of the protective conductor. Since the fault current is conducted through the earthing resistance, no impermissibly high contact voltage must occur in the form of voltage drop in this resistance until the breaking current is reached. Therefore, it is necessary to realise a sufficiently small earthing resistance (see formula 9.1.)

where:

UB perm	maximum permissible contact voltage (e.g. 65 V)
Ia	breaking current
RS	earthing resistance

Fig. 9.5. shows an example of a fault-current circuit

Fig. 9.5. Fault circuit involved in the protective measure called protective earthing

I_F - fault current
R_S - earthing resistance
R_B - operational earthing resistance

Depending on the fuse used, the breaking current must be selected in such a way that it is higher by the factor k than the rated current of the fuse (formula 9.2.)

I_a = k I_n

where:

Ia	breaking current
k	switching off factor
In	rated current of the fuse

In this way it is to be achieved that, in case of a fault, the circuit is interrupted within a adequately short time. High values of k ensure an increased safety due to a quicker response of the fuse but in many cases they cannot be realised by an economically justifiable expense. The minimum values of k are specified, in special regulations in dependence of the type of fuse connected.

For the return flow of the fault current, the water pipes may be used if permission is given. But this is rarely used today because no-metallic water pipes are increasingly employed.