Lighting Circuits

Lighting circuits installed before 1966 often did not include a circuit protective conductor (cpc).

If Class I equipment, such as a metal luminair (lighting fitting) switchplate or the like is subsequently installed, the addition of a circuit protective conductor is essential

Without a cpc, there will be a risk of electric shock under fault conditions

Solution

There is no legal requirement, and no regulation in BS 7671, requiring and existing lighting circuit to be rewired or otherwise upgraded to current standards.

Furthermore, it is permissible to extend or alter and existing lighting circuit having no cpc. However, the new work must be carried out in accordance with the current edition of BS 7671, and the safety of the existing installation must not be impaired.

The following are some of the main requirements of BS 7671 that must be taken into account where an existing lighting circuit without cpc is to be extended or altered.

1. No alteration or addition may be made to an existing installation unless it has been ascertained that the rating and condition of any existing equipment, including that of the distributor, which will have to carry the additional load is adequate for the altered circumstances and the earthing and bonding on which the alteration or addition will rely for safety is adequate (Regulation 131.8 refers). Amongst other things, the adequacy of the cross-sectional-area (csa) of the existing circuit conductors and the type and rating of the protective device must be established.
   
   If, as is likely, protection against electric shock is provided by Automatic Disconnection of Supply, a cpc must be run to and terminated at each new point in wiring and at each new accessory (Regulation 411.3.1.1 refers).
2. A cpc must also be run to and terminated at any existing point in wiring or accessory that is changed from the all-insulated type to the type having metallic parts required to be earthed
3. Furthermore, it is advisable that a cpc should be connected to any existing Class I equipment connected to the circuit if this equipment is not already satisfactorily earthed. Advice on dealing with existing Class I equipment that may not be earthed is given in a best practice guide entitled Replacing a consumer Unit in Domestic Premises Where Lighting Circuits Have No Protective Conductor, published by the Electrical Safety Council. This may be downloaded from NICEIC Group Ltd at www.niceicgroup.com
4. Like a cpc used for any other purpose, a cpc installed to an existing point or accessory may consist of a separate green-yellow covered copper conductor. However, where a cpc is not an integral part of a cable (such as a twin and earth cable) and is not contained in an enclosure formed by a wiring system (such as trunking), it must have a cross-sectional area not less than:
   • 2.5mm² if protection against mechanical damage is provided, or
   • 4mm² if protection against mechanical damage is not provided (Regulations 543.1.1 and 543.3.1 refer)
5. Where a cpc consists of a separate green-yellow covered copper conductor, it must still be incorporated in the same wiring system as the live conductors or in their immediate proximity (Regulation 543.6.1 refers). This would require the cpc to be run along the same cable route(s) as the existing cable. The requirement does not apply where a residual current device is used for protection against electric shock

In practice, rather than making changes to an existing lighting circuit having no cpc, the designer or contractor may persuade the customer that it would be safer and more practicable to rewire the circuit

Regulation 131.8

No addition or alteration, temporary or permanent, shall be made to an existing installation, unless it has been ascertained that the rating and the condition of any existing equipment, including that of the distributor, will be adequate for the altered circumstances. Furthermore, the earthing and bonding arrangements, if necessary for the protective measure applied for the safety of the addition or alteration, shall be adequate.

Regulation 411.3.1.1 (part of)

A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lamp holder having no exposed-conductive-parts and suspended from such a point

Regulation 543.1.1 (part of)

... The cross-sectional area shall be not less than 2.5 mm² copper equivalent if protection against mechanical damage is provided, and 4 mm² copper equivalent if mechanical protection is not provided

Regulation 543.3.1

A protective conductor shall be suitably protected against mechanical and chemical deterioration and electrodynamic effects

Regulation 543.6.1

Where over-current protective devices are used for fault protection against electric shock, the protective conductor shall be incorporated in the same wiring system as the live conductors or in their immediate proximity

This article is extracted from Snags and Solutions Part 1 or Part 2 published by NICEIC