CPC

A circuit protective conductor is the wire that carries fault current safely to earth when insulation fails or a live conductor touches metal. In electrical installations, the CPC is distinct from the neutral conductor, which carries normal load current back to the source. The CPC sits idle during normal operation but becomes critical during a fault, routing dangerous current away from people and equipment before protective devices open the circuit.

The CPC is typically colour coded green and yellow striped in modern installations, though older work may use plain green, black, or other colours depending on local standards. Copper or aluminium are the standard conductors; copper is preferred in smaller cross-sections and damp environments. The conductor must be sized relative to the phase conductors it protects: typically 1 mm² where phase conductors are 1.5 mm² or smaller, scaling up in proportion for larger circuits.

Routing and connection

The CPC must be continuous from the final outlet or equipment back to the main earthing point, typically the buried rod or metal water pipe at the building entrance. It cannot be switched off or interrupted, and it cannot pass through any protective device that would break the circuit. Every metal enclosure, conduit, or case that a person might touch must be bonded back to the CPC network through this conductor.

A CPC that is undersized, broken, or poorly connected defeats the entire protective scheme. Corrosion at crimped joints, missing earth sleeves on crimp terminals, or unauthorised modifications can leave a fault path that is too high resistance to trigger the breaker, leaving hazardous voltage on accessible metalwork. Testing the continuity and resistance of the CPC is standard practice during installation and periodic inspection, typically checking that resistance from any point does not exceed 0.1 ohms.

The CPC sits at the foundation of electrical safety codes worldwide. Standards such as IEC 60364 and national regulations (NEC in North America, BS 7909 for temporary installations, for example) all mandate its presence and correct sizing. Many incidents of electric shock occur in installations where the CPC is missing or damaged, making it one of the most important but least understood parts of the electrical infrastructure.