Ed Wood, Qualifying Supervisor at GA, explains all about Site Temps Wiring Standard
The electrical installation on a construction site is there to provide lighting and power to enable work to proceed. Those working on the site may be ankle deep in mud and using power tools, making them very susceptible to receiving a shock to earth. The supply on a building site must not use a TN-C-S (PME) earthing system. This is due to the difficulty in bonding all exposed metal work on a construction site. The possibility of creating a potential to earth in the event of a fault is extremely high. A TN-S supply can be used only if a TT supply is not feasible (for example a construction site in a penthouse ).
In all other cases the supply will be converted (if an earth has been provided by the supplier) into a TT system using an earth stake and a 100mA time-delay RCD immediately after the meter. A high external earth loop impedance is therefore likely to be present and for this reason the RCD discrimination must be addressed in order to meet the maximum disconnection times of 0.2 seconds for final circuits up to 32A, and 1 second for all distribution circuits and final circuits over 32A.
All portable equipment on site should be supplied via a 110V transformer, with the secondary winding centre tapped to earth ensuring the maximum voltage to earth will not exceed 55V. The primary winding of the transformer must be protected by at least a 100mA RCD. The internal wiring of site cabins is not classed under these regulations as they will not change as construction progresses, and so are covered by the general requirements of BS7671. Plugs and sockets used to supply site equipment must be non-interchangeable to prevent misconnection.
The maximum period between inspections of electrical installations on construction sites is 3 months as shown in Guidance Note 3, table 3.2.
As shown here, any circuit supplying a socket outlet with a rated current up and including 32A and any circuit supplying hand-help equipment up to and including 32A must be either reduced low voltage, supplied by a 30mA RCD or be SELV or PELV.
Any socket outlet with a rated current over 32A should be protected by an RCD with an operating current not exceeding 500mA.
Cables should not be installed across a site road or walkway unless adequate protection against mechanical protection is provided. Appropriate flexible cable should be used for all portable equipment for use on reduced low voltage systems.
The device for isolating the incoming supply must be suitable for locking in the OFF position by padlock, or by its location inside a lockable enclosure. Simply isolating a protective device or removing a fuse is not adequate protection for safe isolation.
Reference BS 7671: 2008 Section 704 and Guidance Note 3