7.5.1 - Introduction

The electrical installation on a construction site is there to provide lighting and power to enable the work to proceed. By the very nature of the situation, the installation will be subjected to the kind of ill treatment which is unlikely to be applied to most fixed installations. Those working on the site may be ankle deep in mud and thus particularly susceptible to a shock to earth, and they may be using portable tools such as drills and grinders in situations where danger is more likely than in most factory situations. The difficulty of ensuring that bonding requirements are met on construction sites means that PME supplies must not be used to supply them.

Installations will also, by definition, he temporary. As the construction proceeds they will be moved and altered. It is usual for such installations to be subjected to thorough inspection and testing at intervals which will never exceed three months. A formal visual inspection of 110 V equipment should take place monthly; the effectiveness of each RCD should be tested before every use by pressing its test button.

As well as the erection of new buildings, the requirements for construction sites will also apply to:

1. - sites where repairs, alterations or additions are carried out

2. - demolition of buildings

3. - public engineering works

4. -civil engineering operations, such as road building, coastal protection, etc.

The special requirements for construction sites do not apply to temporary buildings erected for the use of the construction workers, such as offices, toilets, cloakrooms, dormitories, canteens, meeting rooms, etc. These situations will not change as construction progresses, and are thus subject to the general requirements of the Regulations.

The equipment used must be suitable for the particular supply to which it is connected, and for the duty it will meet on site. Where more than one voltage is in use, plugs and sockets must be non interchangeable to prevent misconnection. Six levels of voltage are recognised for a construction site installation. They are:

1. - 25 V single-phase SELV for portable hand-lamps in damp and confined situations,

2. - 50 V single-phase, centre-point earthed for hand lamps in damp and confined situations,

3. - 400 V three phase, for use with fixed or transportable equipment with a load of more
----- than 3750 Watts,

4. - 230 V single phase, for site buildings and fixed lighting,

5. - 110 V three phase, for transportable equipment with a load up to 3750 Watts, and

6. - 110 V single phase, fed from a transformer, often with an earthed centre-tapped secondary winding, to feed transportable tools and equipment, such as floodlighting, with a load of up to 2 kW. This supply ensures that the voltage to earth should never exceed 55 V (see {Fig 7.5}). The primary winding of the transformer must be RCD-protected unless the equipment fed is to be used indoors.

Fig 7.5 Arrangement of transformer for safety 110 V supply for a construction site

Supplies will normally be obtained from the Electricity Supply Company. Where a site is remote, so that a generator must be used (IT supply system) special protective requirements apply which are beyond the scope of this Guide, and the advice of a qualified electrical engineer must be sought. Attention is drawn to BS 7375: 1966 Code of Practice for the distribution of electricity on construction and building sites.