Page 377 - Handbook of Electrical Engineering

EARTHING AND SCREENING 365

that have high speed control of overvoltages. (See clause 411-02-02 for the actual wording and
cross-referencing to other clauses.) In general the practical significance of PELV versus SELV is
unnecessarily complicated and a suitable SELV should be chosen in preference to a PELV alternative.

13.3.3.3 Four-pole circuit breakers and isolators

Where a four-wire supply is needed in a hazardous area it is necessary to use four-pole circuit
breakers and isolators so that the neutral is completely isolated when maintenance work is required
to be undertaken. If the neutral is not electrically separated and a fault occurs elsewhere in the same
network then the neutral in the hazardous area could have its potential elevated sufficiently above zero
to cause a spark (or even an electric shock). This aspect is especially important when a switchboard
or motor control centre is supplied from more than one source such as two parallel transformers.

13.3.4 Earth Loop Impedance

A key factor in the design and choice of earth continuity conductors, e.g. cable armouring, bonding
straps, and fault current protective devices is the ‘earth loop impedance’. This is especially the case
for solidly earthed low voltage systems, whether they be three-phase, single-phase or even direct
current systems.

The earth loop impedance is the total impedance seen by the source of voltage in a faulted
circuit which involves the earthing conductors. Figure 13.10 shows the situation for a three-phase
cable supplying a load such as a motor or static load.

Figure 13.10 Earth loop impedance of a three-phase circuit.