Electrical Engineer interview questions and answers

In a star connection, line current equals phase current, and line voltage is root three times phase voltage. That is why a 415 V three-phase supply in India gives about 240 V between any phase and neutral. In a delta connection it is the other way round: line voltage equals phase voltage, and line current is root three times phase current. Star gives us a neutral point, which we need for single-phase loads and for earthing the system. Three-phase power is the same formula in both cases: root three times line voltage times line current times power factor.

Power factor is real power in kW divided by apparent power in kVA. A low power factor, usually from induction motors and lightly loaded transformers, means more current for the same useful work. That raises I²R losses, loads cables and transformers more, and can attract penalties or higher bills from the utility. To correct it, required kVAr equals kW multiplied by the difference between tan of the existing angle and tan of the target angle. For a 100 kW load going from 0.8 to 0.95, that is 100 × (0.75 − 0.33), roughly 42 kVAr. In plants I prefer an automatic power factor correction panel with steps, so we never overcorrect at light load.

I start with full load current. For 30 kW at 415 V, with power factor 0.85 and efficiency 0.9, that is 30,000 divided by (1.732 × 415 × 0.85 × 0.9), about 55 A. Then I apply derating factors for ambient temperature, grouping with other cables in the tray and the laying method, buried or in air, using the cable manufacturer’s tables. I pick a size whose derated rating is above the load current. Next I check voltage drop for the actual run length, both running and during starting, against the limit in the project specification. Finally I check that the cable can withstand the prospective short-circuit current for the breaker clearing time.

A transformer’s heating comes from two losses. Copper loss depends on current, and iron or core loss depends on voltage. Neither depends on the power factor of the load. The manufacturer does not know what power factor the customer will run at, so the safe limit is expressed as volts times amps, which is kVA. A 500 kVA transformer can carry 500 kVA whether the load power factor is 0.7 or 0.95, but the useful kW will be different. The same logic applies to UPS and generator ratings, where you also check the kW rating separately.

MCBs protect final circuits like lighting and socket outlets, with fixed thermal-magnetic trips and lower current ratings, typically up to about 125 A. MCCBs cover feeders and larger loads, with higher breaking capacity and, in many models, adjustable trip settings. ACBs sit on main LT panel incomers and large outgoing feeders, carrying thousands of amps, with electronic releases for long-time, short-time, instantaneous and earth fault protection. An RCCB does a different job: it trips on earth leakage, such as 30 mA for protecting people, but gives no overcurrent protection, so it always needs an MCB upstream or is replaced by an RCBO.

Direct-on-line starting draws roughly six to eight times full load current. On a large motor that causes voltage dips that disturb other loads, and it stresses the motor and coupling. A star-delta starter cuts starting current and torque to about one third, which suits pumps and fans that start lightly loaded, but there is a current spike at changeover. A soft starter ramps voltage smoothly and suits conveyors and compressors where we just want a gentle start. A VFD gives the smoothest start and also speed control. On centrifugal pumps and fans that saves real energy, because power varies roughly with the cube of speed.

System earthing connects the neutral of the transformer or generator to earth, which fixes the system voltage relative to ground and gives earth fault current a return path so protection operates. Equipment earthing connects metal enclosures, panels and motor bodies to earth, so a person touching a faulty body is not exposed to a dangerous voltage. In India we follow IS 3043 for earthing practice. Electrodes can be pipe, plate or chemical earthing. To measure resistance I use a three-point fall-of-potential test with an earth tester, placing the current and potential spikes at proper distances, and I compare the reading with the design value.

I start with a single line diagram, fault levels at each bus, CT ratios and load currents. For each IDMT relay I set pickup above maximum load current, including motor starting, and below the minimum fault current it must detect. Then I grade from the farthest feeder towards the source. The downstream relay gets the fastest time multiplier, and each upstream relay is set to operate later by a grading margin, typically around 0.3 seconds, to cover breaker opening time and relay errors. I plot all curves together, often in ETAP, and check the instantaneous elements do not overlap. Finally, settings are verified by secondary injection testing.

I would suspect harmonics. Six-pulse VFDs draw non-sinusoidal current, mainly 5th and 7th harmonics, and plain capacitors have low impedance at those frequencies. The bank can also form a parallel resonance with the transformer inductance near a harmonic order, which amplifies currents and blows fuses or bulges capacitors. First I would log the main incomer with a power quality analyser for a full production cycle, checking current and voltage THD and individual harmonics. Common fixes are replacing the plain bank with a detuned bank using series reactors, adding line reactors or DC chokes on drives, and fitting an active harmonic filter if distortion is still high against IEEE 519 limits.

After a visual check of the nameplate, oil level, breather silica gel and earthing of body and neutral, I do insulation resistance with a megger between HV, LV and earth, and work out the polarisation index. Then winding resistance on all taps, turns ratio on every tap position, magnetising current, and vector group verification. I send an oil sample for breakdown voltage testing. I check that the Buchholz relay, oil and winding temperature indicators and their trip wiring work. Protection relays on the HT breaker are tested with secondary injection. Only after reviewing results against factory test reports do I charge at no load and watch for abnormal sound or tripping.

In one rung, I put the start push button as a normally open contact in series with the stop button, the overload relay contact and the interlock, for example a guard door switch. The motor output coil sits at the end. Parallel to the start button I add a contact of the motor output itself as a seal-in, so the motor stays on after the button is released. For fail-safe design, the stop button, overload and interlock are wired normally closed in the field, so their inputs are on when healthy and the rung checks for that. A broken wire then stops the motor. I would also add a run feedback from the contactor auxiliary, and raise an alarm if feedback does not come within a few seconds.

At a packaging plant near Nashik, a main conveyor motor tripped on overload during a night shift and kept tripping on restart. Production stood still. I isolated the feeder, applied a lock and tag, and checked the motor insulation and winding resistance, which were fine. Current readings showed one phase much higher, so I traced back and found a loose, burnt lug in the MCC starter. I replaced the lug and contactor, retightened the terminations and restarted within about two hours. Next morning I proposed thermography for all MCC panels every quarter. The first scan found two more hot joints before they failed.

During a panel extension at a commercial building, I saw a contractor’s technician about to work on a busbar chamber with only the outgoing breaker off. The incomer was still live, and there was no permit, lock or voltage test. I stopped the job, explained calmly why the busbar was still energised, and arranged a proper shutdown with the facility team that evening. We issued a permit, locked out the incomer, tested for dead and applied temporary earths. The job took four hours longer, and the contractor supervisor was unhappy at first. Later he asked me to run a short safety briefing for his whole crew.

We had to replace an old LT incomer breaker with an ACB at a pharma plant, and the client could give us only a twelve-hour Sunday shutdown. Two weeks before, I made an hour-by-hour plan, pre-fabricated the busbar links, pre-tested the new ACB and its release, and kept a spare set of lugs and bolts. We walked through the plan with the utility, the client’s maintenance head and our crew on Friday. On the day, one cable end was shorter than drawings showed, so we used a pre-arranged link. We finished testing and energised the panel about ninety minutes early.

In my first project as a graduate engineer, I prepared a load schedule for a small office floor and forgot to apply the diversity factor correctly on the lighting and small power circuits, so the distribution board incomer came out one size too large. My senior caught it during review because the number looked high against similar floors. It was not unsafe, but it would have added cost. I corrected it, and since then I keep a checklist for every load calculation: connected load, demand factor, diversity, spare capacity and a sanity check against a past project. I also ask for a peer review before issuing anything.

A hotel owner kept facing nuisance tripping on the kitchen feeder and wanted us to simply fit a bigger breaker. That would have removed the protection. I explained it using his own bills and equipment: the kitchen had added three new fryers and a dishwasher, so the cable was now running close to its limit and getting hot. A bigger breaker would let the cable overheat before tripping. I showed him a thermal camera photo of the hot cable end. He approved a new, larger cable and separate circuits for the fryers, and the tripping stopped completely.

On a residential tower project, I found the contractor had run smaller cables than the approved schedule for two floor distribution boards, saying the larger size was not available. I did not argue on the spot. I photographed the cables, noted the tag numbers and raised a non-conformance report the same day. Then I sat with their site engineer, checked the actual load and voltage drop, and confirmed the smaller size failed the voltage drop check on the longer run. We agreed they would replace those runs at their cost, and I adjusted the inspection sequence so the delay did not hold up other trades.

Yes. I understand that electrical work happens where the equipment is, and shutdowns usually fall on nights or weekends. In my current maintenance role I already work a rotating shift every third week, and I have spent six weeks at a solar plant site during commissioning. I would only ask for clarity on how long typical postings last and how site allowances and travel are handled, so I can plan for my family. If the role starts with a long site posting, I am fine with that, because commissioning experience is what I want to build at this stage.

Many of my friends took IT offers, and I respect that. But the part of engineering I enjoyed most was the hands-on side: my internship at a substation, my final-year project on a solar charge controller, and working with real switchgear in the lab. I want to understand how power actually reaches a factory floor and how to keep it safe and reliable. Your company does panel manufacturing and site commissioning, so I would get both design and field exposure. I plan to stay in core electrical work and later take the electrical supervisor licence for my state.

My current CTC is ₹6.2 lakh a year as a maintenance engineer with four years in a process plant. Since this role adds HT substation responsibility and commissioning travel, I am looking for around ₹7.5 to 8 lakh fixed, plus the usual site allowances. I am flexible on the split if the total package reflects the added responsibility. My notice period is 60 days. I have an annual shutdown coming up that I am leading, and I would like to finish it properly, but if your project needs me earlier, I can discuss an early release with my manager.