Handbook of Electrical Installation Practice - BeKnowledge

258 Handbook of Electrical Installation Practice
potentially damaging to delicate devices such as RCDs. These should be regularly
checked for physical damage and to confirm their electrical characteristics.
If the required disconnection times are not achievable by this method, then supplementary
equipotential bonding can be applied and the inequality R £ 25/Ia may
be used to calculate the impedance of supplementary bonding to ensure that contact
with exposed conductive parts does not give rise to potentials that are considered
dangerous in the circumstances. R is the resistance of the supplementary bonding
conductor between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts
and Ia is the operating current of the protective device. This
is not considered to be a satisfactory means of achieving compliance with BS 7671,
due to the changing circumstances that prevail on construction sites and the danger
of disconnection of, or damage to, the bonding connection.
Supplies for portable hand lamps in confined or damp conditions
(1) SELV is suitable but the extra low voltage must be no greater than 50V.
However, lower voltages have been considered to be better (e.g. 25V).
(2) Reduced low voltage using single-phase and earthed centre tapped transformers.
Voltages up to 110V single-phase centre point earthed are allowed in
BS 7671 but lower voltages should be considered depending on the risk. A risk
analysis should be carried out and the special requirements of section 606 of
BS 7671: 2001 should be considered where the risks include restriction of
movement in locations.
Standards for equipment
•
•
•
Distribution assemblies should be to BS 4363 and BS EN 60439-4 (was BS 5486)
All other equipment should have an Index of Protection appropriate to the external
influence, but a minimum of at least IP44 should be utilised in open areas
All plugs, sockets and cable couplers must be to BS EN 60309-2 (was BS 4343).
Electricity supply arrangements
Installers should note that Distribution Network Operators (DNOs) may be unwilling
to make available a PME earthing terminal to construction sites. This means
that in the event of a conventional TN-S system, or a dedicated local transformer,
not being available, it will be necessary to establish a reliable connection with the
general mass of earth through the use of earth rods, or other independent means
of earthing.
AGRICULTURAL AND HORTICULTURAL PREMISES
The risks
The risks are from:
•
•
•
Wetness/wet skin
Proximity of earthed metal
Arduous conditions of service