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Consumer Unit Selection

In a world of ever-changing legislation and best practice it is a constant challenge to remain current, ensuring that electrical suppliers are providing the best possible service to their customers. In most residential installations the biggest challenges are with the selection of the correct consumer unit. Take, for instance, the introduction of the 17th edition wiring regulations and the implications they had with Electrical Contractors, Designers and Consultants.

What do the building regulations say?

Electrical equipment is referenced in two separate parts of the Building Regulations: Part P of the Building Regulations relates to electrical safety in dwellings. The approved document prescribes that switches, sockets and consumer units in new dwellings should be easy to reach, in accordance with Part M of the building regulations.

Part M which deals with access to and use of buildings, and recommends that switches sockets and other equipment should be located between 450mm and 1200mm from finished floor level and does not specifically mention the consumer unit. However, Part P suggests that one way to comply is by mounting the consumer unit so that the switches are between 1350mm and 1450mm from the finished floor level. If, however, you are installing a multi-row consumer unit, it will not be possible to have all of the switches between 1350mm and 1450mm. In this instance the advice from BEAMA (British Electrotechnical and Allied Manufacturers Association), is that the bottom row of switches should be at the designated 1350mm to 1450mm height to avoid easy access to young children whilst remaining accessible to other people when standing or sitting.

The consumer unit also needs to be accessible; therefore the location and the height need to be taken into consideration. For instance a small under-stair cupboard may be classed as difficult to access and hence be an unsuitable place to install a consumer unit. Care should also be taken that the consumer unit is placed in a position where it not likely to be damaged by impact.

What do the 17th edition wiring regulations say?

In residential applications we have to assume that the building is used by ’ordinary persons’ e.g. home owners.

Socket Outlets
Under the 17th edition it is likely that every socket outlet in a domestic installation requires residual-current device (RCD) protection not exceeding 30mA. Regulation 411.3.3 requires protection from an RCD not exceeding 30mA for socket outlets up to 20A and mobile equipment up to 32A that is for outdoor use. Exceptions to 411.3.3 are permitted where the socket outlets are specifically labelled or suitably identified for a particular item of equipment.
Cables buried in walls
The normal practice in residential applications is to bury cables in the walls, but this brings its own challenges in the form of how these cables are positioned or protected. In section 522 we are told that for cables which are not buried at least 50mm in the wall, the cables need to be mechanically protected against damage by the use of SWA cable, earthed metal conduit, earthed metal trunking, another means of protection against penetration by nails or screws, or be installed in a safe zone. If, however the cables are installed in the safe zone they need to be protected by a RCD not exceeding 30mA, this would apply to most domestic installations where thermoplastic (PVC) wiring systems are used.
Bath and shower rooms
Regulation 701.411.3.3 requires that all circuits within a bath or shower room are shall be additionally protected by an RCD not exceeding 30mA. This means that any 230V lighting, the 230V to the source for SELV, a shower circuit or bathroom heater for exampleall require RCD protection.
Division of installation
Section 314 calls for the circuits in the installation to be divided to avoid hazards and minimise inconvenience in the event of a fault and reduce the possibility of unwanted tripping of the RCD due to excessive protective conductor currents. Regulation 314.1.(iv).
To comply with these requirements the circuits of an installation should not be connected to a single RCD. This could lead to loss of supply to the entire installation in the event of a fault on one circuit, clearly inconvenient for the user of the building.
Smoke detectors
Chapter 56 states that all new dwellings are required to be fitted with fire detection and an alarm system. Consideration needs to be given to the supply for this circuit. In a standard house a grade D, category LD3 system is required, a mains powered alarm which has an integral standby supply. Such a system is required to be either permanently wired from an independent circuit at the distribution board or from a local, regularly used lighting circuit. Where RCD protection is required for smoke detector circuit one option is to supply that circuit individually with a residual current circuit breaker with overcurrent protection (RCBO).

Consumer Unit Arrangements

Option 1
Option 2
Option 3
Option 4
Option 5