Handbook of Electrical Installation Practice - BeKnowledge

570 Handbook of Electrical Installation Practice
sun’s rays. The percentage reduction in rating is approximately proportional to the
intensity of solar radiation. Local values of intensity depend on latitude and, above
all, clarity of the atmosphere. The intensity at or above 3000m height is about
1200W/m 2 . At sites with a clear atmosphere this is reduced to about 1000W/m 2 at
sea level at UK latitudes. In and around towns and industrial sites the intensity is
reduced to about 800W/m 2 . If the cable rating is critical, it is advisable to seek guidance
on local values of solar radiation intensity. IEC Publication 60287 provides a
method for calculating the effect of radiation. These figures apply to times of
maximum radiation, i.e. around midday in the summer. If the cable load is greatest
at other times of the day or year the effect of solar radiation is likely to be unimportant.
It is often overlooked that cables installed under transparent roofs can be subject
to solar radiation. Although direct radiation received by the cable may be reduced
by the roof material, there is an important increase in the effective ambient temperature,
not only of the air in the roof space but of the general radiative ambient
due to the surrounding surfaces of the building. Effective ambients could well reach
40°C, without taking into account the effect of heat from any processes in the building.
Such ambients introduce a reduction in rating in addition to that due to direct
solar radiation.
Utilisation
BS 7671 includes utilisation as a class of external influence and lists a number of
categories which may influence the choice of cable.
Capability of persons
This would not seem at first glance to affect the choice of cable, but if extended to
the action of persons, particularly of children, there is the problem of deliberate
damage. No system can totally resist deliberate attack, but in places where it is envisaged
that such attack may take place and where a surface installation is concerned,
there is no doubt that the most resistant system is steel conduit and that PVC/PVC
sheathed cables are the most vulnerable. MI cables have an increased resistance.
The use of flexible cords should be avoided wherever possible.
Contact of persons with earth potential
All metallic sheathed cables propagate potentials whether earth or higher. BS 7671
seeks to limit these potentials, particularly by equipotential bonding; in certain situations
of high risk, such as bathrooms, extra supplementary bonding is required of
all exposed conductive parts and extraneous conductive parts. Since the exposed
conductive parts of an installation are primarily sources of raised potentials, it seems
sensible in bathrooms and similar situations to use either PVC conduit or PVC
sheathed cables. PVC sheathed MI and PVC/SWA/PVC cables require metallic terminating
glands, although these can and should be either shrouded or contained
within a non-conducting enclosure.
Conditions of evacuation
The only consideration likely to affect the choice of a cable has been dealt with
under ‘Fire’.