What Does (CPC) Mean?

BS7671 defines a circuit protective conductor (CPC) as a protective conductor connecting exposed-conductive-parts of equipment to the main earthing terminal. The term is actively used in the United Kingdom.

Circuit Protective Conductor Requirements

  • A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point.
  • Except where Regulation 412.1.3 [1] applies, a circuit supplying one or more items of Class II equipment shall have a circuit protective conductor run to and terminated at each point in wiring and at each accessory.
  • In every installation a main earthing terminal shall be provided to connect to the earthing conductor the circuit protective conductors.
  • Except where the circuit protective conductor is formed by a metal covering or enclosure containing all of the conductors of the ring, the circuit protective conductor of every ring final circuit shall also be run in the form of a ring having both ends connected to the earthing terminal at the origin of the circuit.
  • Where two or more similar radial circuits supply socket-outlets in adjacent areas and are fed from the same distribution board, have identical means of short-circuit and overcurrent protection and circuit protective conductors of the same cross-sectional area, then a second protective conductor may be provided at the final socket-outlet on one circuit by connection to the protective conductor of the adjacent circuit.
  • Where supplementary bonding is to be applied to a fixed appliance which is supplied via a short length of flexible cable from an adjacent connection unit or other accessory, incorporating a flex outlet, the circuit protective conductor within the flexible cable shall be deemed to provide the supplementary bonding connection to the exposed-conductive-parts of the appliance, from the earthing terminal in the connection unit or other accessory.
  • All metal parts of the heater or boiler which are in contact with the water (other than current-carrying parts) shall be solidly and metallically connected to a metal water pipe through which the water supply to the heater or boiler is provided, and that water pipe shall be connected to the main earthing terminal by means independent of the circuit protective conductor.
  • In medical locations of Group 2, where PELV is used, exposed-conductive-parts of equipment, e.g. operating theatre luminaires, shall be connected to the circuit protective conductor.
  • In TN and TT systems, a value of 25 V a.c. or 60 V d.c. may be obtained by the provision of protective equipotential bonding in conjunction with circuit protective conductors for the particular circuit. In the case of TT systems a satisfactory value of RA will also be required.
  • All circuit protective conductors shall be incorporated in a multicore cable or in a conduit together with the live conductors.

Circuit Protective Conductor Example

Illustration of earthing and protective conductor terms
Figure 1. Illustration of earthing and protective conductor terms

Types of CPC

Common types of allowable CPC include:

  • A single-core cable
  • A conductor in a cable
  • An insulated or bare conductor in a common enclosure with insulated live conductors
  • A fixed bare or insulated conductor
  • A metal covering, for example, the sheath, screen or armouring of a cable
  • A metal conduit, metallic cable management system or other enclosure or electrically continuous support system for conductors
  • An extraneous-conductive-part complying with Regulation 543.2.6 [1].

Steel conduit is permitted for use as a circuit protective conductor (CPC).

References

  1. BS7671