Substations Factsheet

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Background

Low power substations are found about 150-200 metres apart in a typical urban area. They are often grey metal boxes in a fenced enclosure. Sometimes they are inside brick or plastic structures. They have a 'Danger of Death' yellow sign attached to the fence. This is to warn the public of the danger of electric shocks. They change a high voltage, often 11,000 volts, into 415/230 volts to be fed to the nearby houses. Rural areas may have small grey box transformers attached part way up a wooden pole. Sometimes on the outskirts of a town there may be a large 'nest' of transformers, and overhead cables leading to them. There may also be underground cables leading to or away from the transformers.

In cities, flats, workplaces and sometimes houses, can have substations next to, or under the property (in a basement), as part of the building structure. These can produce magnetic field levels higher than those associated with health effects in rooms on the same floor as the substation or in the floor above.

The size of substations is variable, depending on whether they serve mainly residential properties, or also commercial and industrial units. Schools and institutions such as hospitals often have their own substation. A study in 2005 reported that visible substations and cables reduce the value of a property; the percentage reduction depended on the type of property^{[Sims 2005]}.

There are two types of field produced by overhead and underground cables and the substation equipment itself; electric fields and magnetic fields. The strength of the electric field depends on the voltage. Electric fields from substation equipment are unlikely to extend beyond the equipment housing, as they are screened by practically all building materials. Magnetic fields are caused by electric current flowing when people use electrical power. For all practical purposes magnetic fields cannot be stopped and will travel through walls as if they were not there. Only careful design of wiring and of electrical equipment can significantly reduce magnetic fields. Magnetic fields also come from underground cables. The electric fields will be zero as they are screened by earth, concrete, sand etc. The magnetic fields are very high near to the cable, higher than from overhead cables because they are closer to you. They fall off more rapidly than the fields from overhead wires, because the cables are closer together and cancel out each other's effects more quickly.

Houses, or ground floor or basement flats, with very small or no front garden, may have high magnetic fields in their front rooms from distribution cables running underneath the pavement. In many built up areas the electricity companies often connect neutrals from different substations together, this can produce "net currents" which flow round the system the wrong way and can give rise to high magnetic fields over wide areas (e.g. round 4 or 5 streets).

References