While a properly designed temporary electrical distribution system should be installed to supply site lighting, power tools and equipment, and welfare facilities on a construction site, there will be instances where such a supply system is not considered necessary.
Where work of limited scope and duration is carried out in predominantly complete premises, power tools and lighting can be supplied from the general socket-outlets, so long as their current rating does not exceed that of the socket-outlets and of the circuits from which the supply is taken.
This would be the case, for example:
At the very early stages of some construction projects where a distributed temporary electrical supply may not be available on-site, and only minor work involving the need for power tools and/or lighting is being carried out.
When certain alterations or additions are being made to an existing, otherwise complete, premises.
During commissioning or ‘snagging’ activities.
Where the nature of the work being carried out does not warrant the need for power tools requiring an external supply source.
A number of options are available for use where the above or similar scenarios exist. This article considers the safe use of portable tool transformers, use of an RCD-protected low voltage circuit of the premises and battery-powered tools.
Use of an RCD-protected low voltage circuit
If the environmental conditions are sufficiently benign, 230 V equipment may be used and connected directly to the socket outlets of the installation. Where such an approach is taken, additional protection should be provided by an RCD having a rated residual current not exceeding 30 mA. It may not be possible to determine whether the circuit of the fixed installation being used as the source of supply has such protection for reasons including:
Lack of knowledge – not all persons working on construction sites are electrically competent.
Inability to identify and/or gain access to the relevant distribution boards from which the circuits are supplied.
As a result, to ensure that such protection is provided, the portable equipment employed should be supplied via an RCD safety adaptor, RCD safety plug or in-line RCD conforming to BS 70711. Where such an adaptor is used, it should be determined that it is suitable for the rated voltage (for example 110V/230 V).
Wherever RCDs not forming part of the fixed installation are relied upon for safety, their continued functionality should be verified prior to each use by pressing the integral test button.
Portable equipment, as a result of its being moved around, connected and reconnected and stored with other equipment, is more vulnerable to damage than permanently installed equipment.
Portable tool transformers
Portable tool transformers are readily available having a 110 V or 230 V primary and a 230 V, 110 V or SELV secondary as appropriate for the portable equipment that is to be supplied. The transformer will provide electrical separation, and the heavy-duty flexible cable to H07RN-F (BS EN 50525-2-21) or equivalent, used on both the input and output sides, will provide sufficient protection against mechanical damage. Further details are shown below.