Two-way switching using cpc

A potentially dangerous snag is the use of the circuit protective conductor of a flat twin and earth cable as a live conductor in a two-way switching circuit. Danger may arise because an electrician working on the installation at a future date may, reasonably, make the assumption that the centre conductor is at Earth potential.

Basic protection must be provided by one or more of the basic protective measures such as insulation of live parts or protective by a barrier or an enclosure.

Regulations 416.1 and 416.2 refer. In the case of a cable, a live conductor must be completely covered by insulation. The centre conductor of a twin and earth flat sheathed cable is not covered in insulation, it is merely sheathed.

Furthermore, if the centre conductor of a flat twin and earth cable is used as a live conductor, then, probably, no circuit protective conductor will have been run to and terminated at each point in wiring and at each accessory. (Regulation 411.3.1.1 refers)

Solution

Cable incorporation three cores, colour coded brown, black and grey, plus a circuit protective conductor, represents one available option for such circuits. It must not be forgotten that, in this particular configuration, the black and grey cores are being sued as line conductors and should be suitably marked by means such as brown sleeving.

Regulation 416.1

Live parts shall be completely covered with insulation which can only be removed by destruction. For equipment, the insulation shall comply with the relevant standard for such electrical equipment

NOTE: the insulation is intended to prevent contact with live parts.

Paint, varnish, lacquer, or similar products are generally not considered to provide adequate insulation for basic protection in normal service.

Regulation 416.2 (part of)

NOTE: Barriers or enclosures are intended to prevent contact with live parts

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