The standard cable used in domestic and commercial wiring in the UK since the 1960s is a flat PVC twin-and-earth type, alternatively known as 6242Y cable. This comprises a flat formation of PVC-insulated live and neutral cores separated by a bare earth conductor, the whole assembly being PVC-sheathed to produce a flat cable rated at 300/500 V.
The cable is also available with three insulated cores and bare earth, for use on double-switched lighting circuits. These forms of cable are ideal for installation under cladding in standard-depth plaster. They are defined in BS 6004, which covers a large size range, only the smaller sizes of which are used in domestic and commercial circuits.
There are other cable types included in BS 6004 which have more relevance to nondomestic installations. These include cables in both flat and circular form, similar to the 6242Y type but with an insulated earth conductor.
Circular cables designated 6183Y are widely used in commercial or light industrial areas, especially where many circuits are mounted together on cable trays.
Related: Industrial Cables
Also included in BS 6004 are insulated conductors designated 6491X which are pulled into conduit or trunking in circuits where mechanical protection or the facility to re-wire are the key factors.
Of recent years, LSF versions of the twin-and-earth cables and the conduit wires have become available and are being used in installations where a particular emphasis on fire performance is required. Such cables are included in BS 7211.
These are commonly known as 6242B (for twin flat) or 6491B (for single-core conduit wire). A significant change occurred in 2004 for all fixed wiring in electrical installations in the UK. An amendment was published to BS 7671 (the IEE Wiring Regulations) which specified new cable core colors to bring the UK more closely into line with practice in mainland Europe; the term harmonized core colors are often used.
For single-phase, fixed installations, the red phase, and black neutral are replaced by brown phase and blue neutral, as used for many years in flexible cables for appliances. For three phase cables, the new phase colors are brown, black and grey instead of red, yellow and blue, with the neutral now blue instead of black.
In both cases, the protective conductor is still identified by a green-yellow combination. An alternative for three-phase cables is for all phase cores to be brown and marking of L1, L2, and L3 to be carried out at terminations. The neutral will be blue again in this case.
Electrical installations commenced after 31 March 2004 may use either the new harmonized core colors or the pre-existing colors, but not both. New installations after 31 March 2006 must only use the harmonized core colors.
An alternative type of cable with outstanding impact and crush strength is mineral-insulated cable (MICC) manufactured to BS EN 60702-1. This is often known by its trade name, Pyrotenax. In a MICC cable, the copper line and neutral conductors are positioned inside a copper sheath, the spaces between the copper components being filled with heavily compacted mineral powder of insulating grade.
Related: Low Voltage (LV) Polymeric Cables
Pressure or impact applied to the cable merely compresses the powder in such a way that the insulation integrity is maintained. The copper sheath often acts as the circuit earth conductor. An oversheath is not necessary but is often provided for reasons of appearance or for external marking. An MICC cable has a relatively small cross-section and is easy to install.
In shopping and office complexes or in blocks of flats there may be a need for a distribution sub-main to feed individual supply points or meters. If this sub-main is to be installed and operated by the owner of the premises, then a 0.6/1 kV split-concentric service cable to BS 4553 may be used.
This comprises a phase conductor insulated in PVC or XLPE, around which is a layer of copper wires and an oversheath. Some of the copper wires are bare and these are used as the earth conductor. The remainder is polymer-covered and they make up the neutral conductor. For larger installations, 3-core versions of this cable are available to manufacturers’ specification.
In circuits supplying equipment for fire detection and alarm, emergency lighting and emergency supplies, regulations dictate that the cables will continue to operate during a fire. This continued operation could be ensured by measures, such as embedding the cable in masonry but may be achieved by cables which are fire-resistant in themselves. BS 5839-1:2002, the code of practice for fire detection and fire alarm systems for buildings, recommends the use of fire resisting cables for mains power supply circuits and all critical signal paths in such systems.
Fire-resistance tests for cables are set down in BS 6387, BS 8434-1, BS 8434-2 and EN 50200. The latter three tests are called up in BS 5839-1 although two levels of survival time are specified, 30 minutes for ‘standard’ and 120 minutes for ‘enhanced’. The three types of cable are recognized in the code of practice are to BS EN 60702-1 (as described earlier), BS 7846 and BS 7629 (both as described below).
The MICC cables to BS EN 60702-1 should comply with the ‘enhanced’ performance since the mineral insulation is unaffected by the fire. An MICC cable will only fail when the copper conductor or sheath melts and where such severe fires might occur the cable can be sheathed in LSF material to assist in delaying the onset of melting. MICC cable is also categorized CWZ in BS 6387.
A number of alternatives to MICC cables for fire resistance have been developed and standardized. Some rely on a filled silicone rubber insulation which degrades to an insulating char, which continues to provide separation between the conductors so that circuit integrity is maintained during a fire. Other types supplement standard insulation with layers of mica tape so that even if the primary insulation burns completely the mica tape provides essential insulation to maintain supplies during the fire.
Both cable types are standardized in BS 7629, and in addition to complying with performance levels up to CWZ of BS 6387, designs also may comply with either the ‘standard’ or ‘enhanced’ performance required by BS 5839-1.
Some circuits requiring an equivalent level of fire resistance need to be designed with larger cables that are found in BS 7629. Such circuits might be for the main emergency supply, fire-fighting lifts, sprinkler systems, and water pumps, smoke extraction fans, fire shutters or smoke dampers.
These larger cables are standardized in BS 7846, which includes the size range and LSF performance of BS 6724, but through the use of layers of mica tape to supplement the insulation these cables can be supplied to the CWZ performance level in BS 6387, and additionally to the ‘standard’ or ‘enhanced’ performance levels specified in BS 5839-1. An additional fire test category in BS 7846, called F3, may be considered to be more appropriate for applications where the cable might be subject to fire, impact and water spray in combination during the fire.
Source: Newnes – Electrical Power Engineers