HK Grade B Electrical Worker Exam: Electrical Safety

Electrical Safety is the domain where the paper checks whether you can work on and near live equipment without injuring yourself or anyone else. Questions are practical: the isolation sequence, hazard recognition, your legal duties, and the correct response to an accident.

The isolation sequence

1. Identify the circuit and every source of supply, including backfeeds from PV, UPS, generators, control circuits and shared neutrals.
2. Notify affected users and agree the point of isolation.
3. Isolate by opening all live poles at a device suitable for isolation.
4. Lock off and tag, and keep the key with you; a tag alone is not a lock.
5. Prove dead: prove the voltage indicator on a proving unit, test L–L, L–N, L–E and N–E at the point of work, then prove the indicator again.
6. Work, replace covers, remove locks and re-energise deliberately.

Working live is permitted only where isolation is unreasonable in the circumstances and the work is justified, risk-assessed and authorised. A 30 mA RCD, insulated tools or rubber gloves are not substitutes for isolation.

Legal and emergency duties

The Electricity Ordinance places a general duty on persons working on electrical installations to ensure safety, and the Code of Practice expects live work to be avoided. After an accident, do not touch the casualty: switch off, or separate the person using insulated material, call 999, and start CPR or use an AED. Electrical accidents and dangerous occurrences must be reported, and the employer also carries duties under Labour Department safety legislation. Records of the incident, the isolation and the test results belong in the maintenance log.

Worked example

A 16 A Type B lighting circuit gives a measured earth fault loop impedance of 3.2 Ω. Disconnection requires Ia = 5 × 16 = 80 A, so Zs must not exceed 220 / 80 = 2.75 Ω. The circuit fails. The correct response is not to ignore it: check the CPC termination at the last point, verify the measured Ze at the origin, recalculate the expected Zs, then shorten the run, increase conductor size or, where the Code permits, provide 30 mA RCD protection as the means of fault protection. Record the outcome.

Traps that cost marks

Using a multimeter instead of an approved voltage indicator; testing only L–N and declaring the circuit dead; trusting a breaker marked OFF when a second supply or a borrowed neutral is present; failing to re-prove the tester after the test; and answering "wear PPE" when the question is about the sequence of isolation.