Handbook of Electrical Installation Practice - BeKnowledge

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Special Installations or Locations 257
Wetness/wet skin
Proximity of earthed metalwork
Arduous conditions and the possibility of physical damage
Temporary nature of connections.
It is important to remember that temporary installations on construction sites,
demolition sites, major building renovations and works of a similar nature can
greatly accelerate deterioration of wiring systems, through the arduous nature of
the works. The protective measures indicated below should therefore be supported
by regular and frequent formal inspection and testing and reports on the installation.
Of particular importance in such reporting is the physical and electrical condition
of enclosures, cable sheaths and terminations. A detailed examination should
be carried out to establish whether these have been subject to damage or excess
physical strain in service. The scope of BS 7671: 2001 does not include portable electrical
equipment or tools or their associated flexible cords, but there are other statutory
requirements covering their use and care.
The Regulations for Construction Sites apply to the site main switchgear and the
installation that is moveable or demountable, but not the fixed installation in the
site accommodation, which is covered by the general requirements of BS 7671: 2001.
The protective measures
The Regulations allow for the use of the following methods of electrical protection.
Supplies for general use
(1) Automatic disconnection and reduced low voltage, i.e. earthed centre-tapped
110V secondary or earthed star point 110V secondary transformers.
Note: BS 7671 requires a maximum disconnection time of 5 seconds and
0.2 seconds for fixed and moveable equipment respectively, at 230V. Moveable
equipment supplied at 400V must be disconnected in 0.05 seconds. Maximum
values of earth loop impedance to achieve these times, for 230V, are tabulated
in BS 7671 and should be ascertained by reference to the earth loop impedance
for the system and the characteristics of the particular protective device.
(2) The use of a 30mA residual current device – provided the inequality
Z sI D £ 25V can be satisfied, i.e. the touch voltage which could be experienced
in the event of an earth fault would be restricted to lower limits.
(3) SELV.
(4) Electrical separation – but on the basis of individual socket outlets protected
by individual separated sources.
(5) Automatic disconnection – applies only to 400V three-phase and 230V
single-phase supplies to the fixed installation and movable equipment such as
floodlights.
It is understood that it is common continental practice to use RCDs on construction
sites. However, the UK safety record with reduced low voltage systems is very
good and it should be borne in mind that the very nature of construction sites is