Electrical Engineer interview questions and answers

Kirchhoff's current law: the sum of currents entering a node equals the sum leaving it, because charge doesn't build up at a point. Kirchhoff's voltage law: around any closed loop, the sum of the voltage rises equals the sum of the drops, because energy is conserved. Together they let you write the equations for any network, which is what nodal and mesh analysis do.

Ohm's law, V = IR, holds for linear resistors at constant temperature. It doesn't apply to non-linear devices such as diodes, transistors, arc lamps and thyristors, where current isn't proportional to voltage. A filament lamp's resistance also rises a lot as it heats up, so its cold and hot resistances differ.

Power factor is the ratio of real power to apparent power, P/S, which for sinusoidal signals is cos φ, the cosine of the angle between voltage and current. Inductive loads, such as motors and transformers, draw lagging reactive power. A low power factor means more current for the same real power: higher I²R losses, more voltage drop, larger cables and transformers, and penalties or a higher kVA demand charge from the utility.

It's corrected with capacitors, usually an automatic power factor correction (APFC) panel. The capacitor rating needed is Q = P(tan φ1 − tan φ2) kVAR. For 100 kW from 0.8 to 0.95, that's 100 × (0.75 − 0.33) ≈ 42 kVAR. Avoid overcorrecting to a leading power factor.

In star, line voltage = √3 × phase voltage, and line current = phase current. In delta, line voltage = phase voltage, and line current = √3 × phase current. Power is the same formula either way: P = √3 × VL × IL × cos φ, in watts, and apparent power S = √3 × VL × IL.

For India's low-voltage supply, 415 V between lines gives 415 / √3 ≈ 240 V from line to neutral, often quoted as 400/230 V. Example: a 415 V motor drawing 20 A at 0.85 power factor takes √3 × 415 × 20 × 0.85 ≈ 12.2 kW. Star gives a neutral point for single-phase loads and earthing, which is why distribution transformers' LV side is star-connected.

Two main causes. Unbalanced loading: the neutral carries the difference of the three phase currents, so heavy single-phase loads on one phase increase it. Harmonics: non-linear loads such as computers, LED drivers, UPS systems and VFDs draw third-harmonic currents, which are in phase in all three phases, so they add in the neutral instead of cancelling. With many such loads the neutral current can exceed the phase current, even when the phases look balanced.

I'd measure each phase and the neutral with a true-RMS clamp meter, and use a power quality analyser to check harmonics. Fixes: rebalance single-phase loads, use a larger neutral, add harmonic filters, and check for loose or overheated neutral connections.

For the same power, P = √3 VI cos φ, a higher voltage means a lower current. Line losses are I²R, so they fall with the square of the current: doubling the voltage cuts the current in half and the losses to a quarter. Lower current also means less voltage drop and thinner, cheaper conductors for the same power.

The limit is insulation: higher voltages need taller towers, longer insulator strings, larger clearances and costlier equipment, and bring corona loss. So voltage is stepped up at the power station, transmitted at levels such as 400 kV and 765 kV in India, then stepped down through 220 and 132 kV to 33 and 11 kV for distribution, and 415 V for consumers.

The Ferranti effect is when the voltage at the receiving end of a long transmission line is higher than at the sending end. It happens when the line is lightly loaded or open at the far end. The line's shunt capacitance draws a charging current, which leads the voltage, and that current flowing through the line's series inductance causes a voltage rise along the line instead of a drop.

It's significant in long extra-high-voltage lines and in underground cables, which have high capacitance. It's controlled with shunt reactors at the receiving end, which absorb the excess reactive power, and they're switched in during light load, for example at night.

Lightning arresters at the incoming lines and transformer divert lightning and switching surges to earth. Isolators provide visible isolation for maintenance and are operated only off-load. Circuit breakers, usually vacuum at this voltage, interrupt load and fault currents. Current and potential transformers step current and voltage down for meters and relays. The power transformer steps 33 kV down to 11 kV, usually with an on-load tap changer.

Around them: busbars, the relay and control panel, a station battery with its charger, so protection works even when AC fails, an auxiliary transformer for the station's own supply, outgoing 11 kV feeders, capacitor banks, and the earth mat that keeps step and touch voltages safe.

Symmetrical faults are three-phase, with or without earth; unsymmetrical faults are line-to-ground, line-to-line and double line-to-ground. Single line-to-ground faults are by far the most common, around 70 to 80% of faults, while three-phase faults are rare but usually the most severe, so they set the breaker ratings. Unsymmetrical faults are analysed with symmetrical components.

Assuming an infinite source, fault MVA = rated MVA / per-unit impedance = 10 / 0.10 = 100 MVA. At 11 kV that's a fault current of 100 × 10⁶ / (√3 × 11 × 10³) ≈ 5.25 kA. The real value is lower once the source and cable impedances are included, so the breakers must be rated above it.

A load flow study calculates the voltage magnitude and angle at every bus, and the real and reactive power in every line, for a given load and generation. It's used for planning, checking voltage limits and line loading, finding losses, and as the starting point for fault and stability studies.

Buses are of three types. At a load or PQ bus, P and Q are known and V and the angle are found. At a generator or PV bus, P and V are known and Q and the angle are found. One slack bus has fixed V and angle, and supplies the losses. The equations are non-linear, so they're solved iteratively: Gauss-Seidel is simple but slow to converge, Newton-Raphson converges in few iterations and is the standard. Tools like ETAP and PSS/E are common.

A transformer's losses depend on voltage and current, not on the load's power factor. Core or iron losses, hysteresis and eddy-current losses, depend on voltage and frequency and are almost constant whenever it's energised. Copper losses are I²R in the windings and depend on current. Since heating, and so the rating, depends on volts and amps, and the manufacturer doesn't know the load's power factor, it's rated in kVA.

Efficiency is maximum when copper loss equals core loss. Core loss is found from the open-circuit test and full-load copper loss from the short-circuit test. Distribution transformers are designed for maximum efficiency below full load, since they're rarely fully loaded all day.

The stator's rotating field turns at synchronous speed, Ns = 120f / P. For a 4-pole motor on 50 Hz that's 120 × 50 / 4 = 1500 rpm. Slip is how far the rotor lags behind it: s = (Ns − N) / Ns. If the rotor runs at 1440 rpm, the slip is 60 / 1500 = 0.04, or 4%. Full-load slip is typically a few per cent.

The rotor can't reach synchronous speed because torque comes from currents induced in the rotor by the field cutting its conductors. At synchronous speed there would be no relative motion, no induced voltage, no rotor current and no torque, so the rotor would slow down. That's why it's also called an asynchronous motor.

In a series motor the field winding carries the armature current. At no load the current is very small, so the field flux is very weak. Speed is roughly proportional to back EMF divided by flux, so as the flux falls, the speed rises to dangerous levels, and the motor can run away until the armature is damaged by centrifugal force.

That's why series motors are always connected directly or through gears to their load, never by a belt that might slip off. They're used where very high starting torque is needed, such as traction, cranes and hoists, because torque rises roughly with the square of current at low loads. A shunt motor runs at nearly constant speed and is safe at no load.

At start, an induction motor draws roughly six to eight times its full-load current, which causes voltage dips for other loads and stresses the supply. A direct-on-line starter is simple and gives full starting torque, so it suits small motors. A star-delta starter starts the motor in star, so each winding gets 1/√3 of the line voltage, and starting current and torque fall to one-third of DOL; it suits light starting loads like fans and some pumps, and the motor must be designed for delta running.

A soft starter ramps the voltage up smoothly with thyristors. A VFD controls both voltage and frequency, giving low starting current with good torque and speed control, and saves energy in pumps and fans, at a higher cost and with harmonics.

Four conditions: the incoming machine's voltage must equal the bus voltage, its frequency must be equal or very slightly higher, the phase sequence must be the same, and the phase angle between the two voltages must be zero at the moment the breaker closes. Closing out of phase causes very large currents and torque that can damage the machine and its coupling.

Voltage is matched with the excitation and frequency with the prime mover's governor. Phase sequence is checked once, at commissioning, with a phase sequence meter. The phase angle is watched on a synchroscope, closing when it's turning slowly in the fast direction and at the 12 o'clock position. Plants use a check-synchronising relay so the breaker can't close outside the limits.

A fuse melts when current is too high and must be replaced. An MCB is a resettable breaker with two trips: a thermal bimetal for overloads, and a magnetic coil for short circuits. An RCCB compares the current going out on the phase with the current returning on the neutral, and trips if the difference, the leakage to earth, exceeds its rating: 30 mA for protecting people, and 100 or 300 mA for fire protection. An RCCB alone gives no overload or short-circuit protection; an RCBO combines both.

MCB curves set the magnetic trip: B trips at 3 to 5 times rated current, for lighting and resistive loads; C at 5 to 10 times, for general and small motor loads; D at 10 to 20 times, for high inrush such as transformers.

Earthing connects the metal bodies of equipment to the general mass of earth. If a live wire touches the body, the fault current flows to earth through a low-resistance path, the protection trips quickly, and the body doesn't stay at a dangerous voltage. Earthing the system neutral also keeps voltages stable. India's code of practice for earthing is IS 3043.

Neutral is a current-carrying conductor, the return path for load current; earth carries current only during a fault. The neutral-to-earth voltage should be close to zero; a high value suggests a loose neutral or a poor earth. Earth resistance is measured with an earth tester using the three-point fall-of-potential method, ideally in dry season, when it's highest.

For internal faults: differential protection (87T), which compares currents on both sides, with second-harmonic restraint so the magnetising inrush current when the transformer is switched on doesn't trip it; restricted earth fault protection (64R) on the star winding for earth faults near the neutral; and the Buchholz relay, which detects gas from slow internal faults or a sudden oil surge.

As backup and for external faults: overcurrent and earth fault relays (50/51 and 50N/51N) on both sides. Mechanical and thermal protections: oil and winding temperature indicators with alarm and trip, a pressure relief device, a surge relay for the on-load tap changer, and a low oil level alarm. Lightning arresters protect it from surges.

Air circuit breakers are used at low voltage, typically as the incomer and main feeders of a 415 V panel, where currents run into thousands of amps; MCCBs handle smaller LV feeders. Vacuum circuit breakers dominate medium voltage, 3.3 to 33 kV: the arc burns in metal vapour inside a sealed vacuum bottle and goes out at the first current zero, with very fast recovery of dielectric strength and little maintenance. SF6 breakers are used at 66 kV and above, and in gas-insulated substations, because SF6 has excellent arc-quenching and insulating properties.

Mention that SF6 is a very potent greenhouse gas, so leaks are monitored and gas is recovered, not vented. Breakers are chosen by voltage, normal current and breaking capacity in kA.

Lockout-tagout makes sure equipment can't be switched on while someone works on it. The steps: inform the people affected; stop the equipment normally; isolate it at the source, the breaker or isolator, not just a control switch; put your own lock on the isolator and a tag with your name, date and reason, and keep the key yourself; release stored energy, such as discharging capacitors and earthing conductors; and then verify it's dead with a tested voltage detector, checking the tester on a known live source before and after.

Only the person who applied a lock removes it. With several workers, each puts their own lock on a multi-lock hasp, so the supply can't be restored until everyone has finished.

Don't touch the person while they're in contact, or you'll be shocked too. Switch off the supply at the switch, plug or breaker. If you can't do that quickly on a low-voltage circuit, stand on something dry and push the person away with a dry non-conducting object, such as a wooden stick. For high voltage, don't approach at all; keep everyone back and get the line switched off by the utility or control room.

Then call for help on 112 or 108. Check breathing; if they aren't breathing normally, start CPR, 30 chest compressions at 100 to 120 a minute and 2 rescue breaths, until help arrives. Cool burns with clean water. Anyone who had a shock needs a medical check, because heart rhythm problems can appear later.

A permit to work is a written authorisation from the person in charge of the system to the person in charge of the work. Before issuing it, the issuer isolates the feeder at every possible source, locks the switches, posts caution boards, proves it dead and applies earths on both sides of the work point. The permit lists exactly what is isolated and earthed and the work allowed. The receiver signs, briefs the team and works only within those limits.

When the work is done, everyone and every tool is withdrawn, the receiver signs it back, and only after the permit is cancelled are the earths removed and the feeder re-energised. Never restore supply by a pre-agreed time; that practice has caused fatal accidents.

That an open breaker isn't proof of a dead circuit. The feeder can be energised from elsewhere: back-feed from a DG set, a rooftop solar inverter, a UPS or a ring main fed from the other end. Labels can be wrong, and a breaker can be racked in or closed by someone else. Cables and capacitors hold a charge, and lines running beside live ones can carry induced voltage.

So the rule is isolate, lock, test and earth: isolate all sources with visible break, apply locks and tags, test for absence of voltage with a proven tester, and earth it before touching. Under the CEA safety regulations, which replaced the Indian Electricity Rules, 1956, only designated, authorised persons may carry out such work, and HV work needs a permit.

Keep it to about a minute and focus on your electrical exposure. For example: "I'm an EEE graduate from [college]. I'm most interested in [power systems / machines / protection], and I did my industrial training at [substation, plant or company], where I saw [what you saw, for example transformer maintenance and relay testing]. My final-year project was [project], where I [your part and result]. I'm comfortable with [AutoCAD, ETAP or MATLAB]. I'm looking for a role where I can work on real installations and grow into [an O&M, design or project engineer]."

Avoid listing every subject you studied; give one or two practical things they can ask about.

Interviewers ask this to see whether you observed and understood, not just attended. Describe the place, two or three things you learned in detail, and one insight. For example: "I trained for [weeks] at a [132/33 kV substation]. I learned the single-line diagram of the station, how the operators carry out switching under a permit, and how relay settings are tested. What struck me was how strictly they follow isolation and earthing before any work."

Be ready for follow-ups on anything you name, such as the transformer's rating, the breaker type or the protection used, so revise your training notes before the interview.

Answer honestly, and if you're willing, show you see the value. For example: "Yes. Maintenance and breakdown work is where I'll learn the most about how equipment really behaves. I'm comfortable staying near the site, and I understand that a breakdown at night has to be attended because production depends on it. I'd like to learn the plant's single-line diagram and equipment quickly, so I can be useful on calls."

If you have a real constraint, such as family responsibilities, mention it now and politely. Interviewers prefer that to someone who agrees and leaves after three months.

Use STAR, and show both speed and safety. "A [motor / feeder / transformer] tripped during the peak shift and stopped [the line] (situation). I was the shift engineer (task). I checked the relay flags, which showed earth fault, isolated and locked the feeder, and tested the cable and motor insulation with a megger, which found the cable's insulation had failed at a joint. We moved the load to a standby feeder under a permit, restored production in [time], and replaced the joint the next day (action). Afterwards I added insulation testing of that cable to the preventive maintenance plan (result)."

Mention the safety steps; interviewers listen for them.

Be firm about safety and helpful about the deadline. Explain that you won't skip isolation, testing or the permit, because a shortcut can kill someone and is against the safety rules; then do everything you can to be fast within them. For example: "I told the manager I understood the cost of downtime and gave a realistic time. I used a second team to prepare the permit while we diagnosed, and updated him every 15 minutes. Supply was back within the time I promised, and nobody was put at risk."

Interviewers want to see that you hold safety lines calmly, escalate if pushed, and still respect the production pressure.