Handbook of Electrical Installation Practice - BeKnowledge

Selection of Wiring Systems 573
concealed) and the customer is not prepared to pay for a long life or rewireability,
PVC/PVC cables are the only choice, hence the widespread use in domestic and
small offices. It indicates that a single circuit of cable in steel conduit is marginally
more expensive than a two-core PVC/SWA/PVC. It also indicates that two circuits
of cable in steel conduit are cheaper than two PVC/SWA/PVC cables.
Table 20.16 takes no account of the ability of conduit (and trunking) to span wider
gaps than any other system, and its ability (in certain circumstances) to support
luminaires. Nor does it take account of the neatness of MI cables or screened cables,
of the relatively shallow chases needed (if at all) for MI cable, or of its long life. It
does not take account of the ability of PVC/SWA/PVC to stand a certain amount
of vibration and movement. Neither does it take into account the third factor affecting
the selection of cable, namely thermal considerations.
Thermal considerations
The choice of a cable to carry a given current is determined from the tables in
Appendix 4 of BS 7671 applying where necessary the relevant correction factors for
grouping and ambient temperature and the presence of thermal insulation. It should
be noted that these factors are cumulative only where more than one condition
calling for a factor is present; so that if a cable is grouped in one place, is in a high
ambient in another and in thermal insulation at yet another, each factor is applied
once and the cable is selected to meet the worst condition. For large cables, jointing
of different sizes of cable may be worthwhile; the cost of jointing must be
assessed against savings in cable and installation costs.
A cable protected against overload is also protected against short-circuit, but the
size of the protective conductor incorporated in a cable may not be adequate to
withstand the thermal effect of the current flowing through it at the time of a fault.
Of the wiring systems under consideration the following form the protective
conductors:
PVC/PVC cable Copper protective conductor incorporated
PVC cables in metal conduit Conduit
PVC cables in PVC conduit Separate conductor drawn in (size to suit)
MI cable Sheath
PVC/SWA/PVC cable Armouring
Screened cable Copper protective conductor incorporated
Flexible cables and cords Copper protective conductor incorporated
Tables 20.17 and 20.18 indicate that in the range of cable sizes considered, the protective
conductor incorporated in a cable, be it a separate copper conductor, cable
sheath or armouring, has a cross-sectional area which is adequate for the energy letthrough
demands of semi-enclosed fuses to BS 3036, which will also protect the
circuit conductors against overload.
BS 88 fuses may at first sight appear marginally to fail to protect the protective
conductors of PVC twin and earth cables; before rejecting this combination of fuse
and cable the actual disconnecting time should be calculated. The circuit impedance
is often limited by voltage drop, and a small reduction in impedance could reduce