When electrical installations meet flora and fauna

Where exposed to conditions of fauna or flora and/or mould growth that may constitute a hazard, wiring systems and other equipment of an installation are required to be appropriately selected, or special measures adopted. (Regulation Groups 512.2, 522.9 and 522.10 of BS 7671 refer).

The term ‘fauna’ (animal life) includes insects, small animals and birds. Examples of flora (plant life) include plants, trees and other forms of vegetation; mould also has to be considered.

Many electrical installations in external locations, as well as in some internal locations (such as greenhouses), may be exposed to fauna or flora and/or mould growth that could impair safety or functionality.

As with all external influences, it is best to avoid installing electrical equipment where it will be exposed to adverse effects. For example; distribution equipment (such as switchgear and consumer units) supplying outdoor installations should, where practicable, be installed in an indoor location.

If not suitably selected or adequately protected, equipment enclosures installed in contact with the ground (or buried in it) might become infested by insects.

Luminaires and other equipment designed to be installed outdoors may not necessarily prevent the ingress of insects and should be used only in accordance with the manufacturer's instructions. Cable joints for buried cables are usually compound filled and so ingress by fauna is not possible.

Equipment enclosures provided with small holes to allow collected or condensed water to drain away should not be installed where they are liable to become infested by insects.

Damage to cables from the gnawing of rats, mice and squirrels etc. is a relatively common occurrence. There is no evidence to show that these rodents are uniquely attracted to one type of cable. All types of cable, for example, thermoplastic (pvc), rubber and lead sheathed and even steel wire armoured cables, seem susceptible to attack, resulting in exposure of the conductors (Fig 1).

Where conductors exposed by the gnawing action are surrounded by rodents’ nesting materials, which are generally combustible, there is a risk of fire. This is in addition to the obvious danger of electric shock to persons from direct contact with an exposed live conductor (Fig 2).

Regulation 522.10.1 requires wiring systems to be suitably resistant to damage where the presence or actions of fauna constitutes a hazard and where not suitably resistant to such effects of itself, additional protective measures should be provided to counteract adverse effects. Such measures could include:

Therefore, where cables are installed in areas that are, or are liable to be, inhabited by rodents, such as might be found in the roof voids and cellars of certain older properties, the wiring system must be capable of resisting damage caused by gnawing. It is advisable to employ cables or wiring systems such as the following for use in such areas:

Cables are less likely to be attacked by rodents where they are attached as far up a wall as possible, or, where installed in a roof void, fixed to the underside of the roof structure.

The enclosures of electrical switchgear, accessories and wiring systems (such as trunking) should be free of any openings through which rodents could gain entry to build nests and cause damage to cables or other equipment. Cable entries to equipment, such as luminaires, must be effected through correctly selected cable entries using, if necessary, cable glands (Fig 3).

For further guidance on the properties and limitations of cables refer to; BS 7540-1:2005 Electric cables – Guide to use for cables with a rated voltage not exceeding 450/750 V.

For other guidance and publications please see the ELECSA website. Information about the ELECSA Domestic Installers schemes, visit www.elecsa.co.uk

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