Earthing and Circuit Protection Devices
What the exam tests
This domain tests the purpose and methods of earthing, earth continuity, earth resistance, earth electrodes and the relationship between earthing and circuit protection.
Core rules
- Earthing gives fault current a low-resistance path back to the supply so the protective device operates quickly and touch voltage stays low.
- Methods include plate earth, pipe earth and rod earth. The electrode must be in moist soil, below permanent moisture level where possible, and protected from corrosion.
- The earth continuity conductor must run to every socket and metal enclosure. It must not contain a fuse or switch.
- Earth resistance is measured with an earth tester. Lower resistance is better; multiple electrodes, longer rods or suitable backfill can reduce it.
- Protective multiple earthing and RCD protection are system features; they do not remove the need for correct earthing.
Worked example
A metal kettle gives a tingling shock. Insulation has failed, so the body is live. If the earth wire is continuous, fault current flows to earth and the MCB or RCCB trips. Check earth continuity from the plug earth pin to the kettle body. An open earth is a serious fault: repair it before returning the kettle to use.
Traps
Do not use the neutral as an earth. Do not rely on a water pipe as an earth electrode. Do not test earth resistance with the supply on. Do not assume an RCCB will work without a valid earth path.