Electrical engineering

What interviewers look for

• Safety as a habit: you isolate, lock, tag and prove a circuit dead before you touch it, every time, and you can explain each step.
• Solid basics under pressure: Ohm's law, power, single and three phase, kVA and kW, worked out on paper with units.
• You know what each protective device does (MCB, RCD, fuse, surge protection, earthing) and can choose the right one for a circuit.
• A method for fault finding: you check the supply, measure, divide the circuit and prove the cause, instead of changing parts until it works.
• For solar and telecom posts: you can size panels, batteries and an inverter from a load list, and you know how batteries and generators fail in the field.

Questions they ask

1. 1“How would you size a solar system for a small health post?”

   Why they ask: Whether you work from the load, step by step, instead of guessing a panel size. The panel checks daily energy, battery capacity, depth of discharge and inverter size.

   How to answer

   • Start with a load list: each appliance, its watts, hours of use per day and quantity, giving daily energy in Wh. Mark critical loads such as the vaccine fridge.
   • Panels: daily energy divided by the local peak sun hours, then increased for losses such as dust, heat and cables.
   • Batteries: daily energy × days of autonomy, divided by the system voltage and the usable depth of discharge.
   • Inverter: rated above the largest load running at the same time, with room for motor starting surges; then check the charge controller and cable sizes.

   I start with a load list made with the staff: lights, the vaccine fridge, a laptop, a small lab centrifuge, and how many hours each one runs. That gives me the daily energy in watt-hours. I divide it by the peak sun hours for the area and add a margin for dust, heat and cable losses to get the panel size. For the batteries I multiply daily energy by the days of autonomy the clinic needs, then divide by the system voltage and the depth of discharge, which is lower for lead-acid than for lithium. The inverter must cover everything running together, plus starting surges.

2. 2“What is the difference between an MCB and an RCD, and why would a circuit need both?”

   Why they ask: Whether you understand protection: one protects the cable from too much current, the other protects people from electric shock.

   How to answer

   • MCB: trips on overload and short circuit; its rating is chosen to protect the cable.
   • RCD: compares the current in line and neutral; if some leaks to earth, for example through a person, it trips, usually at 30 mA when protecting people.
   • An RCD does not protect against overload, and an MCB will not trip on a small leakage that can still kill; so socket circuits and wet areas usually have both, or an RCBO that combines them.
   • Add that both depend on good earthing and correct wiring to work as designed.

3. 3“A three-phase motor trips its breaker every time it starts. How would you find the cause?”

   Why they ask: Whether you fault-find with a method and safely, and whether you know the common causes of motor trips.

   How to answer

   • Ask and observe first: is the fault new, does it trip at once or after a few seconds, which device trips (MCB, overload relay, RCD), any smell or noise.
   • Isolate and lock off, then test: insulation resistance of the cable and windings to earth, resistance of the three windings (they should be about equal), and the supply voltage on all three phases.
   • Check the mechanical side: a seized pump or bearing makes the motor draw a very high current.
   • Check the starter and protection: a direct-on-line start draws a high current for a moment, so the breaker type or overload setting may be wrong for a motor.

   First I would ask the operator what changed, and watch which device trips and when. Then I isolate, lock and tag the supply, and prove it dead. I test the insulation resistance of the cable and the motor windings to earth, and measure the resistance of the three windings; they should be almost equal. If the electrical tests are fine, I turn the shaft by hand, because a seized pump or bearing makes the motor pull a very high current. Finally I check the breaker type and the overload setting against the motor nameplate.

4. 4“Explain lock-out tag-out. When do you use it?”

   Why they ask: Whether safety is a routine for you. The panel listens for “prove it dead”, and for what stops someone else switching the supply back on.

   How to answer

   • Use it every time you work on equipment that could be energised or start up: panels, motors, generators, pumps.
   • Steps: inform people, switch off, isolate at the correct point, fit your own lock and tag, then test that it is dead.
   • Prove your tester: check it on a known live source, test the circuit, then check the tester again.
   • Only the person who fitted the lock removes it, after the work is finished and everyone is clear.

   I use it on any job where the supply could come back while I am working, for example a distribution board in a hospital or a generator changeover. I tell the people affected, switch off, isolate at the breaker or isolator, and fit my own padlock and a tag with my name and the date. Then I prove it dead with a voltage tester that I first check on a known live source, and check again afterwards. My lock stays on until my work is finished and I have told everyone the supply is coming back.

5. 5“How do you choose the size of a generator for a building?”

   Why they ask: Whether you understand kVA versus kW and motor starting, and that a generator should not run overloaded or almost empty.

   How to answer

   • Add up the running load in kW from a load list, and note the largest motors: pumps, air conditioners, compressors.
   • Convert to kVA with the power factor; generators are rated in kVA, usually at a power factor of 0.8.
   • Add a margin for motor starting and future loads, but do not oversize a lot: diesel engines run badly at very light load for long periods.
   • Include a changeover switch (manual or ATS) so the generator can never feed back into the grid.

6. 6“Why is earthing important, and how do you check it?”

   Why they ask: Whether you know that protection depends on a good earth path, and that earthing is measured, not assumed.

   How to answer

   • Earthing gives fault current a safe path so the breaker or RCD trips quickly, and stops metal parts staying live.
   • Know the parts: earth electrode (rod), earthing conductor, main earthing terminal, and bonding of metal pipes and frames.
   • Test the electrode with an earth resistance tester, check the continuity of the protective conductors, and record the values.
   • In dry, sandy ground the reading can be high; the usual fix is more or deeper rods, as the designer or standard requires.

7. 7“Tell us about a fault you found and fixed.”

   Why they ask: Whether your experience is real. The panel follows up on details: what you measured, with which instrument, and what the reading showed.

   How to answer

   • Pick a real fault and set it up in one or two sentences: where, what equipment, what the users saw.
   • Explain your steps in order and the instruments you used: multimeter, clamp meter, insulation tester.
   • Give the cause, the fix, and how you tested before switching back on.
   • End with what you changed so it would not happen again, such as a regular check, a label or training.

   At a school with a solar system, the lights went off every night by nine. The staff thought the panels were broken. I measured the panel voltage and the charging current at midday with a clamp meter, and both were normal. Then I checked the batteries one by one after charging: one of the four had a much lower voltage than the others and was pulling the whole bank down. We replaced it, checked the charge controller settings for that battery type, and I showed the school guard how to read the battery display each evening.

Example answers are in English, the language most panels use. Say it in your own words.

Topics to revise

• Ohm's law and power (V = I × R, P = V × I)Voltage equals current times resistance, and power equals voltage times current. For AC loads with motors, also multiply by the power factor. Expect questions like: what current does a 1,150 W kettle draw at 230 V? (5 A).
• Single-phase vs three-phaseSingle phase has one line and a neutral, around 220–230 V, for homes and offices. Three phase has three lines with about 380–400 V between them, for motors and larger buildings; balance the loads across the phases. Three-phase power = √3 × V × I × power factor.
• MCB, MCCB, RCD and RCBOMCBs and MCCBs protect cables from overload and short circuit; an RCD trips on current leaking to earth (30 mA to protect people); an RCBO does both in one device. Know which device trips for which fault.
• Earthing and bondingConnecting the system and exposed metal parts to earth, so faults trip the protection quickly. Know the earth rod, the main earthing terminal, bonding of pipes and metal frames, and testing with an earth resistance tester.
• Cable sizing and voltage dropA cable is chosen for the current it carries, how it is installed and its length, because long runs lose voltage. They may ask why a long cable to a borehole pump needs a bigger size than the current alone suggests.
• Load calculation and diversityList each load with its watts and hours to get daily energy (Wh) and maximum demand. Diversity accepts that not everything runs at once, but critical loads such as a vaccine fridge are never reduced.
• Solar PV sizing (panels, batteries, inverter)Panels from daily energy and peak sun hours plus losses; batteries from daily energy, days of autonomy and depth of discharge; the inverter from the largest simultaneous load and surges. Panels in series add voltage, in parallel add current. An MPPT charge controller usually gets more energy from the panels than a PWM one.
• kVA, kW and power factorkW is the real power that does work; kVA is the apparent power used to size generators, transformers and cables. kW = kVA × power factor. A 50 kVA generator at a power factor of 0.8 gives about 40 kW.
• Generators and changeover (ATS)Know the daily checks (fuel, oil, coolant, battery), run hours, why long periods at light load harm a diesel engine, and why a changeover switch or ATS is needed so the generator and the grid are never connected together.
• 48 V DC telecom powerTelecom sites usually run their equipment on 48 V DC from rectifiers, with a battery bank as backup, often with solar and a generator as a hybrid site. Know the rectifier, the batteries, and the alarms for low voltage and generator failure.
• Wiring standards and colour codesEmployers usually follow IEC 60364 or the British BS 7671 rules. In the IEC colour code, brown is line, blue is neutral and green-yellow is earth; older installations may use red and black. Say which standard you have worked to, and never trust colour alone: test.
• Test instrumentsMultimeter for voltage, resistance and continuity; clamp meter for current without cutting the cable; insulation resistance tester (megger) for insulation; earth tester for earth electrodes. Use meters with the right CAT rating for the work.

Practical tasks you may get

1. 1Written test (60–90 minutes): current from power, three-phase power, kVA to kW, voltage drop, and a solar sizing problem. Write the formula, the numbers with units, and the answer; practise ten of these by hand the week before.
2. 2Wiring task on a board: wire a circuit with an MCB, an RCD, a switch and a socket, or a two-way lighting circuit. They mark safety, neat terminations, correct colours, and whether you test before switching on.
3. 3Fault-finding task: a trainer board or a real panel with a planted fault. Talk through your steps aloud, isolate before touching, and use the meter on the correct range.
4. 4Reading a single-line diagram or a solar schematic: explain where the protection is, what each device does, and what you would check first if the system failed.

Mistakes to avoid

• Saying “I switch off and start working” without isolating, locking and proving the circuit dead.
• Confusing kVA and kW, or sizing a generator exactly to the load with no margin for motor starting.
• Sizing solar from panel watts only, forgetting batteries, depth of discharge and cloudy days.
• Claiming high-voltage or substation experience you do not have; panels check it with one follow-up question or a reference call.
• Identifying wires by colour alone in an old installation instead of testing them.
• Answers without units, or mixing up volts, amps and watts.

Quick check

5 questions. Answer each one to see the explanation.