Handbook of Electrical Installation Practice - BeKnowledge

Selection of Wiring Systems 575
Table 20.18 Cross-sectional area of protective conductor protected by BS 3036 (semienclosed)
fuses.
Type of protective conductor*
PVC insulated Protective conductor in
Fuse rating single-core PVC two-core + earth cable
(A) (mm 2 ) (mm 2 )
5s disconnecting time
5 1 1
15 1 1
20 1 1.5
30 1.5 2.5
45 2.5 4
60 4 4
100 6 10
0.4s disconnecting time
5 1 1
15 1 1
20 1 1
30 1 1.5
45 2.5 2.5
60 2.5 4
100 6 10
*The armour of XLPE/SWA/PVC cables and the sheath of heavy-duty MI cables for the above
circuits are adequate as protective conductors.
where corrosion as opposed to moisture is likely, special protective tapes must be
used.
If, because of poor installation practice, it cannot be guaranteed that the joints in
the conduit will remain sound, it is no answer to draw in a separate protective conductor
into the conduit. Even if the protective conductor is connected to every box
(which would not be possible) there is no way of bonding a length of conduit to it,
and so, while the equipment supplied by the circuit in the conduit would be protected,
lengths of conduit themselves may become and remain alive.
Attention is drawn to the minimum sizes required by BS 7671 for protective conductors
not enclosed in a wiring system and to the requirements for buried earthing
conductors.
Voltage drop
It is usually found that the choice of a cable size for any particular circuit is influenced
more by voltage drop than by any other factor and often the advantage of
the higher current ratings of some cables is lost. Information on the voltage drop