Wednesday, December 23, 2009

Temporary electric supply

What is a temporary electric supply?

A temporary electric supply is normally associated with the temporary electrical installation of a construction site. The term ‘temporary’ brings up a vision of a length of a length of twin and earth cable, or a four-core twisted cable and an undersized green earth wire, that is connected into a 30A single or 4-phase and neutral switch-fuse, trailing across the rough ground of the construction site to terminate into a seasoned self-fabricated distributed (with or without metal-clad enclosure).

On the so-called ‘distribution board’, a length of three flexible extension cord is connected to a clumsily assembled socket outlet with or without the use of a three-pin plug. How would you connect a three-core extension cord to a three-pin 13 A socket outlet? Somewhere on this blog, you can see clearly how it is done. It even has had various ways of doing it.

The extension cord run at high level near the soffit of floor slab, or some just run an the scattered floor to a temporary metalclad 13A switched socket outlet some 30 meters away.

The construction contract cost hundreds of millions, but the temporary electric supply system has been ‘engineered’ to fulfill all the site electrical requirements for the minimum price possible.

The main contractor has the responsibility to ensure the temporary electricity supply system installed is not only functional and meets all his electrical needs, but also safe for all involved in the construction work. The supply system need to be good enough to provide reliable power distribution, whether that period of the construction contract is three months or three years. Or whether the site supply requirement is 4 kVA or 3-megawatt supply.

What specifications to use for the temporary electric supply equipment?
Generally, what applied to low voltage installation is in the IEE Wiring regulations also apply to the temporary supply system. However two more British standards should be used to cover the gaps not covered there: BS 4363 (Specification for distribution units and electric supplies for construction sites and building sites) and BS 7375 (Code of practice for distribution of electricity in construction and building sites).

Source of the temporary supply

The temporary electric supply can be obtained from either the distribution network of the local electric supply authority or an independent electric generator installed at the site. Which one to use is usually just a matter of judgment on the cost involved.

However a few other factors may also need to be carefully considered which include practical problems that are usually associated with the distribution of the electric power safely and effectively throughout the site.

If the supply is taken from the local electric supply authority, a lead-time is usually required, as the authority would need time to arrange for the connection. The main contractor also need to submit sufficient details on the peak demand that will be required during the course of the contract, the positions of the point of supply intake and also the estimated contract period.

The authority usually requires enough details on the types and size of electrical load, e.g. lighting, heating, motors, etc. Motor loads usually need more details such as types of motors and the method of starting (direct-online, auto-transformer starting, etc).

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