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The consumer unit has been, and remains, the standard piece of equipment for distributing electrical energy within an installation. There has been much discussion about what a consumer unit is and how it should be installed.
SHORT-CIRCUIT RATING
Regulation 536.4.201 is a UK requirement dealing with fault current (short-circuit) ratings. This regulation requires that a consumer unit being supplied from a 230 V single-phase supply having a rating not exceeding 100 A under the control of ordinary persons must comply with BS EN 61439-3 having a suitable fault current (short-circuit). However, where a consumer unit incorporates components and protective devices specified by the manufacturer complying with BS EN 61439-3, it is recognised that the 16 kA conditional short-circuit rating will be met where the supply cable is at least 2 m in length.
The practical application of this is that it is permissible to use circuit-breakers rated at 6 kA or 10 kA even where the prospective short circuit current is 16 kA. However, this is only the case where the consumer
unit and its associated equipment has been suitably tested by a reputable manufacturer. There should be no intermixing of components from different manufacturers unless permission has been given by the manufacturer. Under all circumstances manufacturer’s instructions should be met (536.4.203 and its associated notes).
METALLIC
Regulation 421.1.201 requires consumer units in domestic premises to have their enclosure manufactured from non-combustible material or be enclosed in a cabinet or enclosure constructed of non-combustible material. In practice this means that all consumer units are now made of steel. There is no specific requirement to ‘fire proof’ a consumer unit. However, care should be taken to advis the client not to store combustible material near consumer units irrespective of the material they are constructed from (421.1.2).
BASIC PROTECTION BY BARRIERS/ENCLOSURES
A consumer unit provides basic protection (protection against direct contact) by barriers and enclosures as detailed in Regulation Group 416.2. Regulation 416.2.1 requires that live parts shall be inside enclosures or behind barriers providing a degree of protection of at least IP2X or IPXXB; ideally any openings should be smaller than this.
Where the top surface of the consumer unit is accessible, Regulation 416.2.2 requires the degree of protection not to exceed IP4X or IPXXD. Furthermore, Regulations 416.2.3 and 416.2.4 require that the barrier/enclosure is firmly secured in place and, amongst other things, that the removal of the barrier/enclosure is only possible by the use of a key or tool.