Electrician interview questions and answers

"Explain how you safely isolate a circuit before working on it."

Describe every step in order, because the order is the point. Identify the correct circuit and its means of isolation. Switch off and isolate, then lock off with a personal lock and a warning tag so nobody can restore the supply. Prove your voltage indicator on a known live source, test that the circuit is dead between all conductors, then prove the indicator again. Only then begin work.

Add that you never rely on a switch being off or on someone else's word, and that you use a proving unit when no known live source is available.

"An RCD keeps tripping. How do you find the cause?"

Start by finding out when it trips: immediately, when a particular appliance is used, or seemingly at random, for example after rain. Unplug all appliances and see whether it holds. If it does, reconnect them one at a time to find the faulty one. If it still trips with nothing connected, isolate the circuits in turn to find the faulty circuit, then insulation-test that circuit to locate the leakage.

Mention the common causes: moisture in an outdoor socket or light fitting, a damaged cable, a failing appliance element, or neutral and earth connected together somewhere downstream.

"How do you choose the right cable size for a circuit?"

Start from the design current of the load and the rating of the protective device. Then consider the conditions that reduce a cable's current-carrying capacity: how it is installed, whether it is grouped with other cables, the ambient temperature and whether it passes through thermal insulation. Check the voltage drop over the length of the run. Finally, confirm that the cable and device together will disconnect a fault quickly enough.

A strong answer mentions using the tables in the applicable wiring standard rather than guessing from experience.

"What tests do you carry out on a new installation before it is energised?"

Explain the sequence and why it matters. With the supply off: continuity of protective conductors, insulation resistance between conductors and to earth, and polarity. After connection: earth fault loop impedance, to confirm protective devices will disconnect in time, RCD operation, and functional tests of switches and equipment. Record every result on the test certificate.

"Your supervisor wants a circuit energised before testing is finished, because the client is waiting. What do you do?"

Explain politely but firmly that you cannot energise an untested circuit, because it could injure someone or damage equipment, and that the tests protect the supervisor and the client as well. Offer to finish the tests as quickly as possible, or suggest energising only the circuits that are already fully tested.

Panels ask this to see whether you will hold a safety line under pressure. The right answer is always to hold it, respectfully.

"Tell us about a fault that was hard to trace."

Choose a real example. Describe the symptoms, the tests you used, how you narrowed down the location and what the cause turned out to be. Emphasise method: dividing the circuit, choosing the right instrument for each test, and recording what you found.

"How do you work with other trades on a building site?"

Talk about coordination: agreeing the sequence of work with plumbers, carpenters and plasterers, protecting cables already installed, attending site meetings, and telling people early when a change affects their work. Mention respect for shared spaces and for the site's safety rules.