Electrical Engineer Interview Questions

"I got hooked in my second year when we rewound a small motor in the lab and I finally saw why the theory mattered. After that I chose power systems electives and did my final-year project on sizing a distribution board for a small workshop, including the load list, cables and breakers. My internship was with a contractor doing factory electrical work, and I liked being on site, seeing panels go from drawings to energised equipment. So the side I want to grow in is industrial power distribution and maintenance, where design and hands-on work meet. This role does both, which is exactly why I applied."

"From your site and the job post, you run several manufacturing lines with your own substation, so the work is mostly low-voltage distribution, motor control centres, drives and some medium-voltage switchgear, plus keeping downtime low. That matches what I've done: I've maintained motor circuits, set up drives and traced faults on production equipment. What appeals to me is the mix of planned projects and breakdown work, and the chance to get closer to the medium-voltage side and protection settings, which I've only touched so far. I'd rather join a team where the equipment is varied and the engineers own it end to end."

"On my own, I've wired and tested motor starters, done insulation resistance and continuity tests, set parameters on variable frequency drives, and helped commission a new distribution board, including checking the phase rotation. I've also written small PLC programs for conveyor sequences. What I've seen but not led is medium-voltage switching and relay testing; I've been present and followed the switching programme, but I wasn't the authorised person. That's my main gap. I'm working on it by reading the protection settings for our site and asking to shadow the senior engineer on every relay test, so I understand not just the steps but why each setting is what it is."

"At my last company we added a new packing line to an existing building. My part was the power side. I built the load list from the machine vendors' data, checked the spare capacity on the main board, and found we needed a new sub-distribution board. I drew the single-line diagram, sized the feeder and final circuits, including derating for a shared cable tray, and picked breakers that coordinated with the upstream device. During installation I checked terminations and labelling. At commissioning we did insulation tests, confirmed phase rotation before running motors, and checked every current against the design. One motor was connected for the wrong voltage, which we caught before start. The line went live on the planned date, and I handed over as-built drawings and a test sheet."

"I was checking a cable schedule for a new motor control centre, and several feeders ran together in one tray, but the sizes had been picked from the single-circuit rating with no grouping factor. When I applied the grouping and ambient derating, three cables came out too small for their breakers. I rechecked my numbers twice, then took the calculation to the engineer who'd prepared it and showed the table I'd used. He agreed straight away; it had been copied from an earlier project with a different installation method. We upsized those three before the cable was ordered. Afterwards we added a column to our schedule template for installation method and derating, so the assumption is visible to whoever checks it."

"I'd stop and not plan any isolation from that drawing, because if the drawing is wrong my lock might not isolate what I think it does. I'd trace what's actually there: which feeders come in, where they come from, and whether anything could back-feed, like a generator, a UPS or solar. I'd confirm it by testing, not by reading labels. Then I'd mark up the drawing by hand with what I found, date it and sign it, and send it to whoever controls the drawings so it becomes the new as-built. I'd also tell my supervisor, because if one panel is wrong, others on the same job might be too, and that changes how we plan future work."

"Power factor is real power in kilowatts divided by apparent power in kVA. Motors and transformers need reactive power to set up their magnetic fields, so the current lags the voltage and the plant draws more current than the useful work needs. That means bigger cables and transformers, more heat loss, more voltage drop, and many supply companies charge for it. To correct it, you add capacitors that supply reactive power locally. The size is the kilowatts times the difference in the tangents of the two angles. For 100 kilowatts at 0.7 raised to 0.95, the tangents are about 1.02 and 0.33, so it's 100 times 0.69, about 69 kvar, and I'd round up to around 70. I'd use an automatic bank with steps so it doesn't over-correct at light load, and with drives on site I'd check harmonics and use detuned reactors to avoid resonance."

"I start with the design current: for a motor, from the full-load current on the nameplate or the kilowatts, voltage and power factor. I pick a protective device rated at least that. Then the cable's current capacity, after derating, has to be at least the device rating. Derating depends on how it's installed, in air, tray, conduit or buried, the ambient temperature, how many circuits are grouped together and any thermal insulation. Next I check voltage drop at full load, and at start for a big motor, against the limit in the wiring rules we're working to; on long runs that often decides the size. Then short-circuit withstand, so the cable survives the fault energy until the device clears it. Last, fault loop impedance, so the protection disconnects an earth fault fast enough."

"I start with the sources, because that's where the danger comes from. Where's the incoming supply, at what voltage, and which transformers step it down, with their rating and impedance. Then any second source, like a standby generator, UPS or solar, and how it's kept from paralleling, usually interlocks or a changeover. Next I follow the power path: the busbars and whether they're split with a bus coupler, which breakers are normally open, and the rating of each incomer and feeder. Then the protection: where the current transformers sit, which relays or trip units protect each section, and where metering is. Finally the outgoing feeders and what they supply. From that I can tell how to isolate any part and what else goes off when I do."

"We had a pump feeder whose earth leakage protection tripped a few times a week, always overnight, and every time we reset it the pump ran fine. I started by logging when it tripped, and it lined up with rain and with the pump starting on level control. With the circuit isolated and locked off, I did insulation resistance tests. From the panel the whole run looked marginal, so I split it: disconnected at the motor and tested the cable and motor separately. The motor was fine. On the cable, one core was noticeably lower than the others. We found a joint in a cable pit that was taking in water. We replaced that section with a continuous run, re-tested, and it hasn't tripped since."

"First I'd find out what kind of trip it is. If it's a residual current device, I'm looking for leakage to earth; if it's the breaker's thermal part, it's overload; if it's instant, it's more like a short or a big inrush. Then the timing is my biggest clue. Mornings suggest something starts then, like heaters, a compressor or all the machines at once, or condensation in a cold building. I'd ask the users what they switch on and clamp the current while they do it. If the load is simply more than the circuit was designed for, that's a design fix, not a bigger breaker. If it's leakage, I'd isolate, lock off and insulation-test each circuit and appliance to find the one with low readings."

"First I'd measure the three phase currents. If that phase carries a lot more current, it's a load imbalance and I'd look at how circuits are spread across phases. If the currents are close and only that joint is hot, it's almost certainly a high-resistance connection: loose, corroded or badly made. Then I'd judge how urgent it is by how far above the other phases it is and whether it's rising. A joint like that gets worse on its own, because heat loosens it further. I'd report it and get an outage planned, sooner if it's severe. With it isolated and locked off, we'd open the joint, clean the surfaces, replace anything damaged and torque to the maker's value. After it's back on load, I'd scan it again to prove the fix."

"We were replacing a lighting distribution board. The circuit had been isolated and locked, and the team wanted to start. I did my own voltage test before touching anything, and one busbar still showed voltage to earth. I told everyone to step back and stopped the job. We traced it to a feed from a small backup generator that someone had tied into that board years earlier, and it wasn't on the drawing. So the isolation looked complete but wasn't. We isolated and locked the second source, re-tested, and then carried on. I also marked up the drawing and reported it so the next isolation on that board lists both sources. The job ran late by about an hour, and nobody argued once they'd seen the reading."

"I'd say no to working live, but I'd say it helpfully. The rule I work to is that equipment is made dead unless isolating it is truly impossible or would create a bigger danger, and losing output isn't one of those reasons. Arc flash and shock don't care about the production plan. Then I'd try to shrink the outage. I can do all the preparation while it's running, like getting parts, checking drawings and planning the isolation, so the actual dead time is short. I'd ask whether it can happen at a changeover, a break or the next planned stop. If they still push, I'd take it to my manager and the safety lead. I'd rather have that awkward conversation than an injury."

"First I prepare: I check the drawings and the equipment for every energy source, not just the main supply. That includes control supplies, back-feeds from generators or UPS, capacitors, the DC bus inside a drive, and mechanical energy like a spinning load. I tell the operators what's going off. Then I stop the equipment normally, open the isolator, and put my own personal lock and a signed tag on it; if several people work on it, each one adds a lock. I try to start it to confirm it won't run. Then I prove it's dead: I check my tester on a known live source, test every phase to each other and to earth, then check the tester again. Stored energy gets discharged or waited out. Only I remove my lock when the job's done."

"We had an eight-hour weekend shutdown to replace an incoming breaker. The swap went fine, but when we tried to close the new breaker it wouldn't, because the interlock with the generator changeover wasn't satisfied. With about two hours left, I kept the team from guessing. I took the drawings and checked the interlock circuit point by point with a meter, and found two wires swapped on the new breaker's auxiliary contacts, because its terminal numbering differed from the old one. We corrected it, tested the interlock both ways, then closed. Power was back with about forty minutes to spare. Since then I insist on a bench check of new switchgear auxiliaries against our drawings before the shutdown starts, not during it."

"A few habits. When the wiring rules or a standard we use gets revised, I read the summary of changes, because that's where the things that affect our designs show up. For equipment, I read the manufacturer's manual before I touch something new; drives and protection relays in particular have settings that bite if you guess. I do the short courses equipment makers run when I can. And I learn a lot from the older electricians, who've seen failures I haven't. When I pick up something useful, like a better way to test a drive or a change to a rule, I write a short note for the team or bring it to the toolbox talk, so it doesn't stay in my head."

"I go in assuming they know things I don't, because they usually do. Before I finalise anything on an existing site, I walk it with the electrician who maintains it and ask what keeps failing and what's awkward to reach. That's saved me from some bad designs. When I hand over work, I explain why, not just what, so if something doesn't fit on site they can make a sensible call or ring me. Where I don't bend is safety and design limits. If someone wants to skip a test or change a cable size, I explain the reason calmly, show the numbers if I can, and if we still disagree I bring in the supervisor. I've found that when they see I listen, they listen back."

"The rotation is easy: I'd isolate, lock off and swap any two of the three phases, then bump-test to confirm. On a pump or fan, running backwards can itself change the load, so I'd re-check the trip after fixing rotation, but I wouldn't assume that explains it. Next I'd look at the overload setting against the nameplate full-load current, because it's often set wrong on a new install. Then I'd check the motor is connected star or delta as the nameplate says for our supply voltage, since the wrong connection makes it draw too much current under load. I'd clamp all three phase currents to look for a missing phase or big imbalance, and finally check the driven equipment isn't jammed or overloaded."

"A transformer has two windings on a shared iron core. AC in the primary makes an alternating magnetic flux in the core, and that changing flux induces a voltage in the secondary. The voltage ratio follows the turns ratio, and current changes the opposite way, so power in and out stay nearly equal. Losses come in two kinds. Core losses, from hysteresis and eddy currents, are roughly constant whenever it's energised. Copper losses in the windings rise with the square of the load current. If you put DC on it, the flux doesn't change, so nothing is induced and there's no back-voltage to oppose the supply. Only the small winding resistance limits the current, so it draws a very large current and overheats or trips."

"The three-phase currents in the stator windings create a magnetic field that rotates at synchronous speed, which is 120 times the frequency divided by the number of poles. A four-pole motor on 50 hertz gives 1500 rpm, or 1800 on 60 hertz. That rotating field cuts through the rotor bars, induces a current in them, and the interaction between that current and the field produces torque. The rotor can never quite catch up. If it turned at synchronous speed, the field wouldn't cut the bars, no current would flow and there'd be no torque. The difference is slip: synchronous speed minus actual speed, over synchronous speed. A four-pole motor running at 1450 rpm on 50 hertz has a slip of about 0.033. More load means more slip and more rotor current."

"An induction motor always runs a little below synchronous speed because it needs slip to induce rotor current. A synchronous motor has its own rotor field, from DC excitation or permanent magnets, which locks onto the rotating stator field, so it runs at exactly synchronous speed whatever the load, up to its pull-out torque. The catch is starting: it can't pull into step from rest on its own. Large ones usually start as induction motors using damper windings, then excitation is applied near full speed, or they're started with a drive. The big advantage is that by changing the excitation you control power factor, and an over-excited machine runs at leading power factor, helping correct the rest of the plant. So I'd pick one for large constant-speed loads like big compressors or mills."

"At standstill an induction motor draws a big inrush, often six to eight times its full-load current, which can dip the voltage for everything else and jolt the machinery. Direct-on-line is simplest and fine for small motors on a strong supply. Star-delta starts the motor in star, so each winding sees the line voltage divided by root three, which cuts the starting current and torque to about a third, then switches to delta. It's cheap, but there's a current spike at changeover and it only suits loads that start light. A soft starter ramps the voltage up smoothly with thyristors, good for pumps and conveyors, but torque still drops with voltage. A variable frequency drive is the most flexible: low starting current, good torque from low speed, and speed control that saves energy on fans and pumps."

"I'd explain it respectfully but clearly: the breaker is sized to protect that cable. If we fit a bigger one, the cable can carry more current than it's rated for without anything tripping, and it overheats inside the wall or tray. That's a fire risk, not a fix. So I'd ask to find out why it's tripping first. If the load has genuinely grown, the fix is to split the circuit or run a bigger cable with a matching breaker, then check it still coordinates with the device upstream. If it's inrush from a motor or transformer, there may be a breaker with a different trip curve that still protects the cable. And if it's a fault, we need to find the fault. I'd offer to take the measurements myself so it's not just my word against his."

"A fuse is a deliberately weak link: the element melts when current is too high for too long. It's simple, very fast on big faults and can break very high fault currents, but you have to replace it and it only senses current. An MCB does the job itself and can be reset. Its thermal element handles slow overloads and its magnetic coil trips instantly on a short circuit. The trip curve decides how much inrush it tolerates. I'd use them on final circuits and small feeders. A protection relay doesn't break current at all. It measures through current and voltage transformers, decides whether there's a fault, and signals a circuit breaker to open. That lets you use overcurrent, earth fault or differential protection with adjustable settings, so it's used on transformers, big feeders and medium-voltage switchgear."

"Earthing is about making a fault safe and cleared fast. Equipment earthing means bonding every exposed metal part, like a motor frame or panel door, back to earth with a protective conductor. If a live wire touches the frame, a large fault current flows through that path, the breaker or fuse sees it and trips quickly, so the metal isn't left at a dangerous voltage long enough to hurt someone touching it. System earthing is connecting the supply's neutral or star point to earth, which holds the phase voltages steady relative to earth and gives earth faults a return path. The earthing arrangement differs between countries and supplies. Either way, I'd test the earth electrode and the fault loop impedance, because a poor earth means the device may not trip in time."

"Ohm's law says the voltage across a resistance equals current times resistance, V equals I R. Power is voltage times current, and substituting Ohm's law gives two other forms: I squared R, which is handy for cable losses, and V squared over R. For the heater, it's a resistive load, so power factor is one and I is just P over V: 2000 watts divided by 230 volts is about 8.7 amps. Its resistance when hot is V over I, roughly 26 ohms. On a 120-volt supply the same heater would draw about 16.7 amps, which is why the supply voltage matters when you size the circuit."

"DC flows in one direction at a steady voltage, like a battery. AC swaps direction many times a second, at 50 or 60 hertz depending on the grid. AC became the standard because a transformer can step its voltage up or down very simply. That matters because for the same power, higher voltage means lower current, and losses in a line go with current squared, so you transmit at high voltage and step down near the user. DC still wins in some places. Over very long distances and in undersea cables, high-voltage DC has lower losses and avoids the charging current that long AC cables suffer from. It can also link two grids that aren't synchronised. And solar panels, batteries and most electronics are DC at heart."

"Three-phase is used because the three waveforms are spaced a third of a cycle apart, so a balanced load draws steady power rather than pulsing, which suits big machines. You carry more power per amount of copper than single-phase, and it naturally creates a rotating magnetic field, so induction motors start and run without extra tricks. For a balanced load, real power is root three times the line voltage times the line current times the power factor. Say 400 volts line to line, 20 amps and a power factor of 0.85: that's about 11.8 kilowatts. The line and phase relationship depends on the connection. In star, line voltage is root three times phase voltage and the currents are equal; in delta, the voltages are equal and line current is root three times phase current."

"A PLC is a rugged industrial computer for control. It runs in a loop called the scan: it reads all its inputs into memory, runs the program from top to bottom using that snapshot, writes the results to the outputs, does its communication and checks, then starts again, often every few milliseconds. For start-stop, I'd wire the stop button normally closed, so the input is on when everything's healthy. If the wire breaks, the input drops and the motor stops, which is the safe way to fail. In the rung, the start button in parallel with a contact from the motor output forms a latch, then in series the stop input and the overload contact, driving the motor output. An emergency stop I'd hardwire through a safety relay too, not rely on the PLC alone."