CPC stands for Circuit Protective Conductor (sometimes referred to as the EARTH) which is the wire in electrical systems that connects any exposed conductive parts of equipment to the main earthing terminal of the electrical system.
When we think about electrical wiring, there is the Line Conductor ("the live"), the Neutral conductor and the earth. The earth is actually the circuit protective conductor and is responsible for removing electrical fault currents from the electrical circuit, making a circuit breaker or RCD/GFCI device trip and goes some way to preventing electric shock. The CPC helps to achieve automatic disconnection of supply by tripping or blowing protective devices in the event of an electrical fault.
There should be a CPC in almost all modern electrical circuits that were wired recently. In accordance with UK wiring regulations BS7671, a CPC needs to be run to each point whether needed or not by the equipment on the circuit.
The earth wire is responsible for connecting the Main Earthing Terminal to the means of earthing. The CPCs of each circuit connect into the main earthing terminal and the earthing conductor then links these to EARTH.
The cover of the CPC and Earth cables in modern wiring systems should be a green/yellow mixture. In older wiring systems the CPC was often light or dark green only.