Electrical Safety, Earthing and Protective Devices
This domain tests safe working around live and dead electrical equipment, earthing practice and protective device selection. It covers shock risk, earth electrodes and continuity, fuses, MCBs, MCCBs and RCCBs, and the difference between overload and earth-leakage protection.
Core rules
- Treat all conductors as live until tested and earthed. Use approved voltage detectors, gloves, mats and insulated tools. Maintain statutory clearances.
- Earthing provides a low-resistance path so fault current operates protection quickly and touch voltage stays low. System neutral earthing, equipment body earthing and lightning earthing serve different purposes.
- Earth resistance is measured with an earth tester. Lower is better; common LT installation targets are a few ohms, with local standards and soil conditions deciding the exact value. Parallel electrodes must be spaced well apart to avoid overlapping resistance zones.
- Fuses and MCBs protect against overload and short circuit. An RCCB/ELCB detects earth-leakage current and trips on residual current; a 30 mA RCCB is used for additional personal protection. An MCB does not replace an RCCB, and an RCCB does not replace proper earthing.
Worked example
An earth electrode measures 8 Ω, but the installation target is 5 Ω or less. A second identical rod is proposed. Two ideal independent rods in parallel would give 4 Ω, but in practice the resistance may be only about 5–6 Ω because the resistance zones overlap.
Drive the second rod at least its own length away from the first, connect it with a suitable conductor, and re-measure with the earth tester. If the result is still above target, extend the electrode, use a deeper rod, improve the soil around the electrode, or install a chemical/pipe electrode. Do not simply add rods close together and assume the resistance halves.
Traps that cost marks
- Using the neutral as a substitute for the protective earth.
- Relying on an RCCB when the earthing is defective.
- Testing an RCCB only with its test button and assuming the trip time is correct.
- Placing parallel earth rods too close together.
- Assuming a fuse protects against earth leakage; it may not operate at a low leakage current.