CPC
Circuit Protective Conductor
The earth wire of a circuit. It links the metal parts of the equipment the circuit feeds back to the main terminal. It is not the normal load- return path, although equipment can produce protective-conductor current in normal operation. During an earth fault it provides a path for fault current, working with the earthing arrangement and protective device to achieve disconnection. In UK wiring it is green and yellow, or bare inside twin-and-earth cable, with green-and-yellow sleeving wherever it is exposed.
How the CPC fits into fault protection
Community draft — not yet reviewedWhat this means
This question has not yet been reviewed by a qualified electrician. It is a community draft: check anything you plan to act on against BS 7671 and your own supervisor before you rely on it. Completing it does not make you competent, certified or registered.
The CPC provides a protective path. ADS describes the protective measure that disconnects the supply when a fault occurs. A connected CPC alone does not guarantee timely disconnection: the earthing arrangement and protective device must work together and be verified.
Is the CPC the normal return path for load current?
No. In a simple single-phase circuit, line and neutral carry the normal load current. The CPC has a protective role. Real equipment can also produce protective-conductor current during normal operation, so “there can never be current in the earth wire” is an unsafe assumption.
Are earthing and protective bonding the same thing?
Earthing connects exposed conductive parts of electrical equipment to the main earthing terminal. Protective bonding connects relevant parts that can introduce a potential from outside the installation. Their roles are related, but deciding whether a service needs bonding requires more information than whether it looks metallic.
Concept revision only. This does not establish competence to inspect, test or alter an installation.
Further reading: IET earthing and bonding FAQs · IET protective-conductor currents.
How the ideas connect
Keep three ideas separate: CPC names the conductor, continuity describes an unbroken path, and R1+R2 combines the resistance of the line conductor and CPC. That is why this term connects earthing with dead testing.
Where CPC fits in the learning path
The Installation section covers these units:
- Wiring Systems & Enclosures
- Cable Selection & Circuit Design
- Earthing, Bonding & Consumer Units
Curriculum mapping: C&G 2365 Units 203/204/305.
See this section in the full curriculum
This section is on the Plus path. Lesson availability depends on content review and your learning path.
Read, practise, return
1. Read it in context
- Earthing & BondingInstallation
- UK Cable ColoursSparks Basics
2. Try recalling, then explore practice
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Follow a topic above to explore how the term is used, then use the learning entry below to find your path.
3. Return to the meaning
After reading or practising, explain the idea in your own words. Revisit the CPC definition to check what you remembered.
Recall a related term
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Connecting an installation's exposed metalwork to earth and back to the supply source. That gives fault current a deliberate low-impedance path home, turning a live casing into a large, brief fault current that the protective device can detect and disconnect — without it, the casing simply stays live.
Earthing
Read the Earthing entryCompare related terms
Open a meaning to compare its role with CPC, or follow its full entry.
Earthing
Connecting an installation's exposed metalwork to earth and back to the supply source. That gives fault current a deliberate low-impedance path home, turning a live casing into a large, brief fault current that the protective device can detect and disconnect — without it, the casing simply stays live.
Read the Earthing entryR1+R2
The combined resistance of a circuit's line conductor (R1) and its circuit protective conductor (R2), measured end-to-end during dead testing. It proves the protective conductor is continuous, and added to Ze it predicts the circuit's earth fault loop impedance before the supply is ever switched on.
Read the R1+R2 entryContinuity testing
Low-resistance measurement proving that conductors which must be complete actually are. That means protective conductors back to the earthing terminal, bonding conductors to services, and the legs of ring final circuits. Performed dead with a low-ohm meter, it is the first step of the test sequence because everything after it assumes the earth path exists.
Read the Continuity testing entryA definition is a start. Using it in a real question is where it sticks.