What is Circuit Protective Conductor for Low Voltage Wiring System

CPC or Circuit Protective Conductor 

The circuit protective conductor (increasingly called the CPC) is a system of conductors joining together all exposed conductive parts and connecting them to the main earthing terminal. Strictly speaking, the term includes the earthing conductor as well as the equipotential bonding conductors.

Common types of allowable CPC include:

1. a single core cable
2. a conductor in a multicore cable
3. an insulated or bare conductor in a common enclosure with insulated live conductors
4. a fixed or bare insulated conductor
5. the sheath or armouring of a cable
6. a metal conduit, cable management system, or electrical continuous support system
7. an extraneous-conductive-part (subject to regulations)

When considering CPC, CableEngineering allows the following:

1. cable armour by itself
2. a separate conductor by itself
3. armour combined with a separate conductor

For every socket outlet, a separate circuit protective conductor ofadequate size should be provided connecting the earthing terminalof the socket outlet and the earthing terminal inside the enclosureaccommodating the socket outlet if the CPC is formed by theenclosure.

For every length of flexible conduit, a separate circuit protectiveconductor of adequate size should be provided to ensure the earthcontinuity of the installation between the two ends of the flexibleconduit.

Adjacent sections of a busbar trunking used as a CPC should beconnected by a protective conductor of adequate size e.g. coppertape or link to ensure the earth continuity between the two sections.This requirement may be waived if it is certified by the manufacturerof the busbar trunking that the earth continuity between theadjacent sections of the busbar trunking is ensured by using theconnection accessories provided by the manufacturer.

The circuit protective conductor of every ring final circuit (other thanthat formed by the metal covering or enclosure of a cable) should berun in the form of a ring having both ends connected to the earthingterminal at the origin of the circuit.

CPC actually parts of earthing exposed conductive parts. Types of exposed conductive parts:

1. Exposed conductive parts include;
2. Exposed conductive parts do not include; &
3. Circuit protective conductor (CPC).

Exposed conductive parts include: 

(i) metallic enclosure of current using equipment, other thandouble insulated equipment; 

(ii) metallic conduit, trunking and ducting for enclosure of cable(s); 

(iii) metallic enclosures of current distribution equipment such asswitchgear and controlgear assemblies.

Exposed conductive parts do not include: 

(i) wall brackets and metal parts connected to overhead lineinsulators if such parts are not readily accessible; 

(ii) inaccessible steel reinforcement in steel reinforced concretepoles;

(iii) small isolated metal parts such as bolts, rivets, nameplates andcable clips which owing to their small dimensions or theirdisposition cannot be gripped or contacted by a major surfaceof the human body in excess of 50 mm×50 mm; 

(iv) fixing screws for non-metallic accessories provided that there isno appreciable risk of the screws coming into contact with liveparts; 

(v) short lengths of metal conduit for mechanical protection ofcables having a non-metallic sheath.

Unless other effective precautions are taken to prevent danger, such asthe use of double insulated equipment or the use of isolating transformerto BSEN 61558 or equivalent, all exposed conductive parts of equipment(other than live parts) should be connected by means of circuit protectiveconductors (CPC) to the main earthing terminal of the installation andthe terminal should be connected to earth electrode(s) via earthingconductor(s).