Power to the cabins

Electrical installations on building sites emerged from the dark ages in 1968 with the publication of BS 4363. This standard and its sister code of practice, CP 1017, defined a series of distribution assemblies and installation practices for fixed (415 V) and portable (110 V) supplies on building sites.

More recently, safety levels have moved forward again, with particular emphasis on increased protection for 415 V circuits and improved segregation within switchboards to facilitate live working.

In 1998 the standards affecting construction site equipment were updated: BS 4363 now covers 110 V equipment only, with mains voltage site equipment covered by BS EN 60439: part 4. Although the philosophy behind the standards differs, the harmonised European Standard sits comfortably alongside the British Standard.

Improved protection of mains voltage circuits

Some 15 years ago electricity boards began to adopt a policy of only providing a temporary builder's supply when the consumer's installation was protected at source by a medium sensitivity rcd. There were various reasons for this, most commonly it was because the electricity board could not guarantee the earth on protective multiple earth (pme) supplies. Therefore automatic disconnection protection for the installation (subsequently Regulation 413 of BS 7671) could only be achieved through the use of an rcd.

The effect of deploying an rcd at the source of a temporary installation where previously there had been an hrc fuse was startling. Many sites found themselves without power for days on end while electricians hunted down neutral earth faults and faulty kettles.

Contractors realised that it would be necessary to install rcds on all significant sub-circuits to isolate the fault close to its occurrence and prevent the main rcd from tripping. This has resulted in most high current site mains distribution assemblies incorporating rcd protection on every outgoing circuit, each with time delay to achieve discrimination with devices upstream and downstream. The consequence is that the average site installation incorporates many more rcds than an equivalent fixed installation.

The 16th Edition of the Wiring Regulations (BS 7671) has a section dedicated to construction site installations – Section 604. In this section a maximum disconnection time of 0·2 seconds is specified for the mains voltage parts of a construction site installation (section 60404-01) in comparison to a 5 second disconnection time for a conventional installation. Given the constantly changing nature of a temporary installation there is really no alternative but to adopt rcds in order to achieve disconnection time compliance.

The effect of the Wiring Regulations can also be seen in the wide use of 30 mA rcds to protect mains voltage socket-outlets on sites. While BS 4363 did not envisage the widespread use of mains voltage sockets, the benefits associated with feeding pumps, welding sets, etc, via plugs and sockets has resulted in the installation of 415 V sockets. As a typical site cannot be classified as an equipotential zone, the protection of socket-outlets by 30 mA rcds is required to comply with Regulation 417-16-01.

Safe live working

A construction site installation expands and then contracts over the course of its existence. Supplies to portacabins and plant have to be connected and disconnected often and at short notice. In addition, the need to change mains cables due to physical damage is much more likely on a site than in a permanent installation.

These factors, when coupled to the tight time deadlines, mean that there is pressure on electricians to work live. To accommodate this, site mains distribution assemblies have been developed that are constructed to BS EN 60439 Form 4 Type 3, with full segregation between individual functional units (mccbs) and busbars. This enables any circuit to be connected or disconnected safely while the assembly is live.