IEC British Standard Electrical Wiring Installations

IEC – British Standard

Typical residential wiring diagram issued from BS 7671 requirements for electrical installations.

INSTALLATION GUIDELINES

BS 7671, “Requirements for Electrical Installations” is published by the Institution of Engineering and Technology and BSI, it governs all domestic electrical installations (and many other types).

SUPPLY

Since 1 January 1995 nominal voltage in UK is 230 V +10% / - 6% AC at 50 Hz. Earth is normally supplied but may be local.

HOUSE SERVICE CUT-OUT

The Electricity Boards protective device, usually a 80 A or a 100 A HRC Fuse. It is sealed in a special housing to prevent tampering.

METER

Usually a dual meter for normal and off-peak energy consumption, sealed to prevent tampering.

CONSUMER UNIT

It houses the main switch which isolates the total installation and the individual circuit protection devices. The consumer unit should comply with BS EN 60439-3. Circuit protection is normally provided by miniature circuit breakers to BS EN 60898 (older installation may have fuse protection). The consumer unit may also contain one or more residual current devices providing additional protection in all or part of the installation (older installations may not have RCD protection provided).

POWER CIRCUITS

Appliances having heavy current consumption (cookers, waterheaters, etc.) should each be supplied on a specific circuit of the appropriate rating. In general, a double pole switch controls and isolates the appliance when necessary and the connection of the appliance can be made either directly to the switch or via a flexible cable outlet. Switches should conform to BS EN 60669 and Cooker Control unit to BS 4177.

SOCKET OUTLET CIRCUITS

Socket outlets should comply with BS 1363-2 and are usually of the switched type. They are usually supplied via a ring final circuit which is a circuit running from the protection device to each outlet and then returning to the protection device. Permanent connections to a ring final circuit and branches off the ring are made via fused connection units to BS 1363-4. Socket outlets to BS 546 are no longer used in domestic installations in the UK but are still widely used in some other countries.

PLUGS

Plugs of domestic appliances should conform to BS 1363-1. They contain a fuse link to BS 1362 of a rating appropriate to the flexible cord of the appliance (max =13 A). Domestic appliances sold in UK must be fitted with a BS 1363-1 plug, further the UK plug and socket safety regulations require mandatory third party certification of plugs.

LIGHTING CIRCUITS

Usually a circuit supplies several lighting points in turn. A lighting point usually consists of a ceiling rose in which incoming, outgoing and switch connections are made and a pendant flexible cord supplying an attached lamp holder or may be a complete luminaire. Light switches should meet BS EN 60669-1. Dimmers should meet BS EN 60669-2-1.

BATHROOMS

The wiring regulations are very strict. Every switch or other means of electrical control or adjustment shall be so situated as to be normally inaccessible in the bathroom. The required Ingress protection is managed through a series of defined zones progressively from the bath or shower. Pull cord switches are allowed, shaver sockets with isolating transformers are also allowed but should conform to BS EN 61558-2-5.

OUTDOORS

Any socket outlet provided outdoors or intended to supply outdoor equipment (e.g. electric lawn-mowers should have a 30 mA RCD protection).

EARTHING

Socket outlets to BS1363-2 have provision for earthing. A protective conductor (which could also be steel conduit) is generally required for all low voltage circuits (1000 V a.c. between conductors) and its continuity must be proved. All main incoming services, for example, water and gas pipes and metallic parts of the building structure, etc. must be bonded and connected to the main earthing terminal of the installation. In addition it may be necessary to supplementary bond water and waste pipes, sinks and other metallic items such as central heating radiators. However in rooms with a fixed bath or shower, supplementary bonding must be applied to simultaneously accessible metal parts unless RCD protection of all circuits in the bathroom is provided.

POLARITY

The polarity is conserved and marked throughout the installation:

Live:

Terminals marked L are for brown coloured insulated conductors, both solid conductors and flexible conductors are coloured brown

Neutral:

Terminals marked N are for blue coloured insulated conductors, both solid conductors and flexible conductors are coloured blue.

Earth:

Terminals marked E or are for green/yellow coloured insulated earth conductors, both solid conductors and flexible conductors are either coloured green or yellow or sleeved as such.

CABLING

Most domestic wiring is done in flat p.v.c. insulated and sheathed 3 core cable (flat twin and earth). Maximum use is made of floor and internal wall voids to run cables. Surface installations are run in conduit or trunking, these can be plastic or metal. Various British standards govern the design and construction of conduits, trunking and cable. Wiring accessories may be installed in plastic or (more often) metal flush boxes to BS 4662. Surface installations are made in appropriate proprietary boxes.