Tricky Electrical interview questions

75 questions with answers (EEE-426 to EEE-500). Try each one aloud before reading the answer.

Circuit Theory

EEE-426 · "Can the neutral wire carry current?"

Trap: Assuming neutral current is always zero. It is zero only for a perfectly balanced linear load. Unbalanced loads produce neutral current, and triplen harmonics (3rd, 9th...) add in the neutral, so it can even exceed phase current.

EEE-427 · A 100 W and a 40 W bulb (both 230 V) are connected in series. Which glows brighter?

Trap: Assuming the higher-wattage bulb is brighter. The 40 W bulb has higher resistance (R = V²/P), and with the same series current it dissipates more power (I²R), so it glows brighter.

EEE-428 · Can the voltage across a capacitor in a series RLC circuit exceed the supply voltage?

Trap: Believing no element can exceed the source voltage. At resonance, V_C = V_L = Q × V_supply, so with Q = 10 the capacitor sees ten times the supply voltage.

EEE-429 · Does maximum power transfer give maximum efficiency?

Trap: Equating the two. At matched load, half the power is lost in the source resistance, so efficiency is only 50%. Power systems keep source impedance far below load impedance for high efficiency.

EEE-430 · Does capacitor current peak when its voltage peaks?

Trap: Assuming they peak together. i = C dv/dt, so current peaks when voltage changes fastest, at its zero crossing, and is zero at the voltage peak (current leads by 90°).

EEE-431 · Three identical resistors take the same power in star or delta, right?

Trap: Thinking connection does not matter. In delta each resistor sees √3 times the voltage, so total power is three times that in star on the same supply.

EEE-432 · Does unity power factor mean the circuit has no inductance or capacitance?

Trap: Equating unity PF with a purely resistive circuit. A series RLC circuit at resonance, or an inductive load with exact capacitive compensation, has unity PF while containing large L and C.

Electromagnetics

EEE-433 · Does a stronger magnet always induce more EMF in a nearby coil?

Trap: Thinking EMF depends on field strength alone. EMF depends on the rate of change of flux linkage, e = −N dΦ/dt. A strong stationary magnet induces nothing.

EEE-434 · A magnet dropped through a copper pipe falls freely because copper is non-magnetic. True?

Trap: Ignoring induced currents. The moving magnet induces eddy currents in the conductive pipe, and their field opposes the motion (Lenz's law), so the magnet falls slowly.

EEE-435 · Does adding more iron to a core keep raising inductance indefinitely?

Trap: Treating permeability as constant. Once the core saturates, incremental permeability collapses toward μ₀ and inductance drops sharply, which is why gapped cores are used under DC bias.

EEE-436 · Are eddy currents always undesirable?

Trap: Seeing them only as losses. They are used deliberately in induction heating and cooking, eddy current brakes, induction disc meters, metal detectors and non-destructive testing.

EEE-437 · Doubling the turns on an inductor doubles its inductance, right?

Trap: Assuming linear scaling. L = N²/S, so doubling turns on the same core quadruples inductance.

EEE-438 · Does a magnetic field do work on a moving free charge?

Trap: Assuming magnetic force always does work. The force qv × B is perpendicular to velocity, so it changes direction but not speed or kinetic energy. In machines, energy conversion happens through induced EMF and the electrical source.

Transformers

EEE-439 · "What happens if you connect a transformer to DC?"

Trap: Expecting it to step DC up or down. Steady DC produces no changing flux, so there is no induced EMF or back-EMF. The primary draws a huge current limited only by winding resistance, saturates the core and overheats.

EEE-440 · "Why are transformers rated in kVA and not kW?"

Trap: Thinking rating relates to useful power. Losses depend on voltage (iron loss) and current (copper loss), not on load power factor, which the manufacturer cannot know. kVA defines the thermal limit regardless of PF.

EEE-441 · Does transformer core flux increase as load increases?

Trap: Assuming more load means more flux. Flux is set by V/(4.44 fN) and stays nearly constant. Extra primary current cancels the secondary MMF; only leakage flux grows with load.

EEE-442 · Can a 60 Hz transformer be used safely on 50 Hz at the same voltage?

Trap: Assuming frequency does not matter. Flux ∝ V/f rises by 20% at 50 Hz, which can saturate the core, increase magnetizing current and overheat it. A 50 Hz unit on 60 Hz is generally fine magnetically.

EEE-443 · Is a transformer most efficient at full load?

Trap: Assuming full load is best. Maximum efficiency occurs where copper loss equals iron loss, often designed at partial load, especially for distribution transformers.

EEE-444 · Does an autotransformer isolate the load from the supply?

Trap: Treating it like a two-winding transformer. Primary and secondary share a winding, so there is a direct electrical connection; a break in the common section can put full input voltage on the load.

EEE-445 · Is the open-circuit test done on the HV side?

Trap: Thinking either side is equivalent. It is done on the LV side at rated LV voltage with HV open, because the required voltage and instruments are easier to handle. The open HV terminals still carry rated HV and must be guarded.

DC Machines

EEE-446 · If the field of a running DC shunt motor opens, the motor just stops. True?

Trap: Assuming loss of field stops the motor. Flux falls to residual, so speed can rise dangerously (N ∝ 1/Φ) and armature current surges. Field-failure protection is needed.

EEE-447 · Reversing the supply polarity reverses a DC shunt motor. True?

Trap: Thinking polarity reversal reverses direction. Both field and armature currents reverse, so torque direction is unchanged. Reverse either the armature or the field connections, not both.

EEE-448 · Can a DC series motor run on AC?

Trap: Assuming DC motors never run on AC. In a series motor, field and armature current reverse together, so torque stays unidirectional. With a laminated field it becomes a universal motor, used in drills and mixers.

EEE-449 · Is back EMF a loss in a DC motor?

Trap: Treating it as wasted voltage. E_b I_a is exactly the electrical power converted to mechanical power. Back EMF also self-regulates current to match load.

EEE-450 · Does higher armature resistance improve a shunt motor's speed regulation?

Trap: Thinking more resistance stabilizes speed. Higher R_a increases the I_a R_a drop with load, so speed falls more and regulation worsens. It is only used deliberately for below-base speed control.

EEE-451 · Should brushes always sit at the geometric neutral axis?

Trap: Ignoring armature reaction. Without interpoles, the magnetic neutral shifts with load, so brushes are shifted forward in generators and backward in motors. With interpoles, brushes stay at the geometric neutral.

Induction Machines

EEE-452 · Does an induction motor become efficient at light load because current is low?

Trap: Equating low current with efficiency. Magnetizing current stays nearly constant while useful output is small, so power factor and efficiency are poor at light load. Oversizing motors wastes energy.

EEE-453 · Does adding rotor resistance increase the maximum torque?

Trap: Confusing starting torque with maximum torque. Maximum torque is independent of rotor resistance; added resistance only shifts it to higher slip, which can raise starting torque.

EEE-454 · Will a running three-phase motor stop if one phase fails?

Trap: Assuming single-phasing always stops it. A running motor may keep turning on two lines but draws much higher current and overheats. It will not start from rest. Phase-loss protection is required.

EEE-455 · Does lower supply voltage make a loaded induction motor run cooler?

Trap: Assuming less voltage means less current. With constant load torque, torque ∝ V² forces slip and rotor current up, so the motor draws more current and overheats.

EEE-456 · Do squirrel-cage rotor bars need insulation from the core?

Trap: Assuming every winding needs insulation. Induced rotor voltages are very small, and bar resistance is far lower than the core path, so current stays in the bars. Cast aluminium cages are uninsulated.

EEE-457 · Running a 50 Hz motor at 60 Hz on a VFD gives more torque because it runs faster. True?

Trap: Assuming faster means stronger. Above base speed the drive cannot raise voltage, so flux falls (field weakening). Power stays roughly constant and available torque falls with speed.

Synchronous Machines

EEE-458 · Increasing field current on a grid-connected generator increases its MW output. True?

Trap: Confusing excitation with active power. On an infinite bus, real power is set by prime mover input. Excitation changes only reactive power and internal EMF.

EEE-459 · Does a synchronous motor slow down as load increases?

Trap: Expecting slip like an induction motor. Speed stays exactly synchronous; only the load angle δ increases until pull-out torque is exceeded and synchronism is lost.

EEE-460 · Can a synchronous motor only run at lagging power factor?

Trap: Assuming all motors lag. Over-excited synchronous motors run at leading PF and supply reactive power, which is why they are used for PF correction.

EEE-461 · Is the frequency of an isolated alternator controlled by its excitation?

Trap: Mixing up control loops. Frequency is set by rotor speed, f = PN/120, controlled by the governor. Excitation controls voltage.

EEE-462 · Can a generator accidentally become a motor?

Trap: Thinking a generator cannot motor. If the prime mover fails while connected to the grid, the machine draws power and drives the turbine as a synchronous motor, which can damage turbines. Reverse power relays trip it.

EEE-463 · Does every synchronous machine pull out at δ = 90°?

Trap: Applying the round-rotor result universally. In salient-pole machines the reluctance term shifts maximum power below 90°. Transient stability limits under faults are different again.

Power Generation

EEE-464 · Do solar panels produce maximum power on the hottest days?

Trap: Linking heat with sunlight output. PV voltage and efficiency fall as cell temperature rises, so cool, clear, sunny days give the best output.

EEE-465 · Does wind turbine output rise in proportion to wind speed?

Trap: Assuming linear behaviour. Available power ∝ v³, so doubling speed gives eight times the power, up to rated output. Above cut-out speed the turbine shuts down.

EEE-466 · Are capacity factor and efficiency the same thing?

Trap: Confusing utilization with conversion. Efficiency is output energy ÷ input energy. Capacity factor is actual output ÷ maximum possible output over time; a very efficient plant can still have a low capacity factor.

EEE-467 · Does a nuclear plant produce electricity directly from fission?

Trap: Imagining direct conversion. Fission produces heat, which raises steam that drives a turbine-generator, like a thermal plant.

EEE-468 · Rising grid frequency means demand is too high. True?

Trap: Reversing the relationship. Frequency rises when generation exceeds load, as rotors accelerate. Excess load causes frequency to fall.

EEE-469 · Does pumped storage produce net energy?

Trap: Seeing it as a generator of energy. It is a net consumer, with round-trip losses of roughly 20–30%. Its value lies in shifting energy from low-demand to peak periods and providing fast reserve.

Transmission & Distribution

EEE-470 · Doubling transmission voltage halves line losses. True?

Trap: Assuming linear scaling. For the same power and PF, current halves and losses ∝ I², so losses fall to one-quarter.

EEE-471 · Should the highest possible voltage always be used to minimize losses?

Trap: Optimizing losses alone. Higher voltage raises insulation, tower, clearance, corona and substation costs, and line charging. Voltage is chosen by overall economics for the distance and power.

EEE-472 · Does HVDC transmission have no losses?

Trap: Thinking DC removes all losses. Conductor I²R losses remain, plus converter station losses. HVDC only avoids reactive current, skin effect and charging current.

EEE-473 · Does the Ferranti effect appear at heavy load?

Trap: Associating overvoltage with heavy load. It appears at light or no load when line charging current dominates. At heavy load, receiving voltage falls below sending voltage.

EEE-474 · Are overhead transmission conductors insulated?

Trap: Assuming all live conductors are insulated. HV overhead conductors are bare; air and insulators provide insulation. Touching or approaching them is lethal.

EEE-475 · Should conductors be strung as tight as possible to remove sag?

Trap: Seeing sag as a defect. Conductors contract in cold weather and carry ice and wind loads; too little sag raises tension and can break conductors or towers. Sag is designed for both clearance and tension.

Protection & Switchgear

EEE-476 · "Is a higher-rated fuse safer?"

Trap: Thinking a bigger fuse gives more protection. It may not blow before the cable overheats. The fuse must be rated to protect the cable it feeds.

EEE-477 · Does an RCD protect against overload and short circuit?

Trap: Assuming RCDs do everything. A plain RCD responds only to residual (earth leakage) current. Overcurrent protection needs an MCB or fuse, or an RCBO combining both.

EEE-478 · Does an MCB protect people from electric shock?

Trap: Thinking any breaker protects people. MCBs protect cables from overcurrent and trip at amperes, far above lethal levels. Shock protection relies on earthing with fast disconnection and 30 mA RCDs.

EEE-479 · Does current stop the instant breaker contacts separate?

Trap: Ignoring the arc. An arc keeps current flowing until it is extinguished, normally at a current zero in AC, which is why DC breakers are harder to design.

EEE-480 · Can an isolator be opened on load if done quickly?

Trap: Thinking speed makes it safe. Isolators have no arc-quenching ability; opening on load draws a sustained arc that can destroy the equipment and injure the operator. Open the breaker first, using interlocks.

EEE-481 · Is the fastest possible protection always best?

Trap: Ignoring selectivity. Upstream devices must wait for downstream ones to clear so only the faulted section is lost, and settings must ride through inrush and motor starting.

EEE-482 · Does differential protection trip for faults outside its zone?

Trap: Thinking it sees all faults. It is designed to stay stable for external faults, because through-fault current enters and leaves the zone equally. Backup protection covers external faults.

Power Electronics

EEE-483 · Can an SCR be turned off by removing the gate signal?

Trap: Treating an SCR like a transistor. Once latched, the gate loses control. Anode current must fall below holding current, usually with reverse bias, for turn-off. GTOs and IGBTs can be turned off by gate.

EEE-484 · Does a buck converter's output depend on load?

Trap: Assuming V_o = D V_in always. That holds only in continuous conduction. At light load in discontinuous conduction, output rises above D V_in and depends on load, so closed-loop control is needed.

EEE-485 · Is a PWM inverter's output voltage a sine wave?

Trap: Confusing current with voltage. The output voltage is a train of rectangular pulses; motor inductance filters it, so the current is nearly sinusoidal. A sine filter is needed for a true sine voltage.

EEE-486 · Does increasing switching frequency always improve efficiency?

Trap: Seeing only smaller filters. Switching loss rises in proportion to frequency, so efficiency falls beyond an optimum. The choice balances filter size, losses and EMI.

EEE-487 · Can MOSFETs not be paralleled because they hog current like BJTs?

Trap: Applying BJT behaviour to MOSFETs. R_DS(on) rises with temperature, so a hotter device carries less current and sharing is naturally stable in conduction. Layout and gate drive still need matching.

EEE-488 · A capacitor-input diode rectifier draws current in phase with voltage, so PF is unity. True?

Trap: Considering only displacement PF. Current flows in short pulses near voltage peaks, so displacement PF is near unity but harmonic distortion makes true PF poor. PFC circuits fix this.

Electrical Measurements

EEE-489 · Can a multimeter's resistance range check insulation?

Trap: Thinking any ohmmeter will do. A multimeter applies only a few volts and cannot stress insulation, so weak insulation can read healthy. Use an insulation tester at the appropriate test voltage.

EEE-490 · Is a low-resistance voltmeter fine because it reads more current?

Trap: Ignoring loading. A voltmeter must have high resistance; a low-resistance meter draws current, changes the circuit and reads low, especially across high-resistance branches.

EEE-491 · Does a PMMC meter read the RMS value of AC?

Trap: Assuming it responds to AC. PMMC torque follows instantaneous current, so it reads the average, which is zero for symmetrical AC. A rectifier is needed for AC measurement.

EEE-492 · Is a class 0.5 analog meter accurate to 0.5% of every reading?

Trap: Reading the class as percent of reading. For analog instruments it is percent of full scale, so a reading at 10% of scale may be 5% wrong. Choose the range so readings fall in the upper part of the scale.

EEE-493 · Clamping around a whole two-core cable shows the load current. True?

Trap: Ignoring cancellation. Line and neutral carry equal and opposite currents, so their fields cancel and the meter reads near zero, apart from any leakage. Clamp a single conductor.

EEE-494 · One wattmeter reads negative in the two-wattmeter method. Is it faulty?

Trap: Assuming a negative reading means a fault. When PF is below 0.5, one wattmeter reads negative. Reverse its coil connection and subtract its reading from the other.

Electrical Safety, Wiring & Drives

EEE-495 · "Why doesn't a bird on a high-voltage line get shocked?"

Trap: Thinking high voltage alone causes shock. Both feet are at the same potential, so there is no potential difference across the bird and no current. Touching another phase or earthed structure would complete the circuit.

EEE-496 · Is mains voltage too low to be lethal?

Trap: Thinking only high voltage kills. Harm comes from current through the body, and 230 V can drive tens of milliamperes through the chest, enough for ventricular fibrillation.

EEE-497 · Do rubber-soled shoes make it safe to touch live conductors?

Trap: Trusting incidental insulation. Shoes may be wet, worn or conductive, and contact with earthed metal or another conductor bypasses them. Only isolation and proving dead make work safe.

EEE-498 · Is the neutral safe to touch because it is at earth potential?

Trap: Treating neutral as dead. Unbalance raises neutral voltage, and a broken neutral can put it near line voltage. Neutral conductors must be treated as live.

EEE-499 · Does a low earth electrode resistance alone guarantee safety?

Trap: Focusing on one number. Safety also depends on equipotential bonding, step and touch voltages, fault-clearing time and correct protective device operation.

EEE-500 · Switching off a circuit makes it safe to work on. True?

Trap: Trusting the switch. It may switch only the neutral, other sources or backfeeds may exist, and capacitors can hold charge. Isolate all sources, lock off, prove dead and earth where required.

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