Electrical Interview Questions Part I: Fundamentals, Motors & Protection
Calculate Smarter. Work Faster.
40 practical questions (Q1–Q40) on fundamentals, motors & protection — each with a definition, formula, key points and a model answer, plus linked calculators for every module.
How to Use This Question Bank
These questions cover topics that interviewers commonly explore for plant, utility, facility and process-industry maintenance roles: can you explain the theory, and — more importantly — can you apply it safely on a live plant? Each answer is split into a short definition, the formula where one applies, the points an interviewer listens for, and a model answer in first person that you can adapt with your own plant experience.
Answers are reviewed for technical consistency with common industry practice and written to be region-neutral. Where practice differs between countries — supply frequency (50 Hz vs 60 Hz), distribution voltage levels, or test acceptance limits — the answer says so. Treat specific values as typical; the equipment manufacturer's data and the standards that apply at your site always take priority.
The one framework that answers most troubleshooting questions. Panels are rarely testing whether you know a fault list by heart; they want to hear a safe, logical order. Walk through it out loud:
- Safety & LOTO
- Symptom / trip indication
- Measure
- Separate electrical vs mechanical
- Root cause
- Repair
- Test
- Controlled restart
- Document & RCA
At the end of each module you will find the calculators and guides on this site that match the topic, so you can work through the numbers behind an answer — motor current, voltage drop, breaker size — before the interview. This is Part I of a three-part, 120-question series; the other parts are linked above and listed in full at the bottom of this page.
No question matches that filter. Try a shorter term, such as “motor” or “relay”.
Electrical Fundamentals
Ohm's law, power triangle, three-phase relationships, voltage drop and demand — the basics every interview opens with.
Q1What are voltage, current, resistance and Ohm's law?
Definition
Voltage is the electrical potential difference that drives current through a circuit. Current is the flow of electrical charge, and resistance opposes that flow. Ohm's law links the three for a resistive element.
Formula
V = I × R
P = V × I (DC / resistive)
P = I² × RKey points
- For the same voltage, higher resistance means lower current.
- Loose or oxidised connections add contact resistance.
- Extra resistance at a joint turns into local heating (I²R) and voltage drop.
Model interview answer
“In maintenance I use the voltage–current–resistance relationship during troubleshooting. If a motor has normal supply voltage but is drawing excessive current, I check mechanical load, bearing condition, phase imbalance, cable termination and winding condition. A loose or high-resistance connection causes heating and voltage drop, so I also check termination tightness and temperature.”
Q2What is the difference between AC and DC?
Definition
AC (alternating current) periodically reverses direction and changes magnitude; DC (direct current) flows in one direction.
Key points
- AC: industrial power distribution, motors and transformers.
- DC: batteries, control circuits, PLC supplies and electronics.
- Supply frequency is 50 Hz in Europe, Africa, Australia and much of Asia (including India and China), and 60 Hz in North America, parts of South America and some Asian countries such as South Korea, Saudi Arabia and western Japan — always check the nameplate and local grid.
Model interview answer
“Most plant power systems use AC for motors and distribution, while DC is used for control, PLC, relay and battery-backed systems. During troubleshooting I first identify whether the circuit is AC or DC, because the measurement method, polarity and protection are different.”
Q3Explain kW, kVAR, kVA and power factor.
Definition
kW is active (real) power that does useful work, kVAR is reactive power that sustains magnetic fields, and kVA is apparent power — the vector sum of the two. Power factor is the ratio of active to apparent power.
Formula
kVA = √(kW² + kVAR²)
PF = kW / kVA = cos φModel interview answer
“In an industrial plant, motors consume both active and reactive power. A low power factor increases current for the same useful kW. I therefore monitor power factor and, where appropriate, use APFC/capacitor banks to reduce unnecessary reactive current.”
Q4What is power factor and why does a low power factor matter?
Definition
Power factor shows how effectively apparent power is converted into useful active power.
Formula
PF = kW / kVA
Three-phase: I = P / (√3 × V_L × PF)Key points
- Induction motors (especially lightly loaded ones) produce a lagging PF.
- Lower PF means higher current for the same kW.
- Higher current increases I²R losses in cables and transformers and uses up capacity.
- Many utilities apply PF penalties or kVA-based demand charges.
Model interview answer
“If plant current rises while useful kW stays the same, poor power factor is a likely reason. I check APFC operation, capacitor steps, CT sensing and polarity, harmonics and capacitor health before taking corrective action.”
Q5Single-phase vs three-phase supply — what is the difference?
Definition
A three-phase system has three AC voltages displaced by 120 electrical degrees; single-phase has one.
Formula
Three-phase: P = √3 × V_L × I_L × PF
Single-phase: P = V × I × PFModel interview answer
“Three-phase supply is preferred for industrial motors because it produces a rotating magnetic field and transfers power more efficiently. During maintenance I check phase-to-phase voltages, phase sequence and voltage/current imbalance.”
Q6Explain star and delta connections.
Definition
In star (wye), one end of each winding joins at a common neutral point. In delta, the windings are connected end-to-end in a closed loop.
Formula
Star: V_L = √3 × V_ph, I_L = I_ph
Delta: V_L = V_ph, I_L = √3 × I_phModel interview answer
“In motor maintenance I verify the nameplate connection before changing the terminal links. An incorrect star/delta connection can cause abnormal current, low torque or overheating — for example, a winding designed for delta at the supply voltage will be under-fluxed if left in star.”
Q7What are frequency and phase sequence?
Definition
Frequency is the number of AC cycles per second (Hz). Phase sequence is the order in which the three phase voltages reach their positive peaks (e.g. R-Y-B / L1-L2-L3); it sets the rotation direction of a three-phase motor.
Formula
Ns = 120 × f / P (P = number of poles)Key points
- 4-pole motor: 1500 rpm synchronous at 50 Hz, 1800 rpm at 60 Hz.
- Reversing phase sequence reverses a three-phase motor.
Model interview answer
“Frequency directly sets motor speed and phase sequence sets rotation direction. Before commissioning a motor or pump I verify phase sequence with a phase-sequence meter to prevent reverse rotation.”
Q8How do you calculate electrical load current?
Formula
Three-phase: I = P / (√3 × V_L × PF)
Single-phase: I = P / (V × PF)
Motor (P = rated shaft output): I = P / (√3 × V_L × PF × η)Use P as the motor's rated shaft (output) power when dividing by η. If P is already the electrical input power, do not divide by efficiency again. V_L is the line-to-line RMS voltage.
Model interview answer
“I calculate load current using voltage, power factor and — for motors — efficiency, then compare the result with the actual measured current. This supports correct selection of cable, breaker, transformer and generator capacity.”
Q9What is voltage drop?
Definition
Voltage drop is the reduction in voltage between source and load caused by the impedance of the circuit.
Formula
Basic (DC / resistive): V_d = I × R
Three-phase AC: V_d ≈ √3 × I × L × (R cos φ + X sin φ)R and X are conductor resistance and reactance per unit length (Ω/km), L is the one-way route length, and V_d is the line-to-line drop. The X sin φ term is for a lagging PF and changes sign for a leading PF. Many cable manufacturers publish mV/A/m values that already include this — check which convention a table uses before mixing methods.
Model interview answer
“If a motor sees low voltage at its terminals while the source voltage is normal, I check cable length and size, termination resistance, loading and phase imbalance. Excessive voltage drop causes overheating and poor starting, especially during motor acceleration.”
Q10Explain kW, kWh, maximum demand and load factor.
Definition
kW is instantaneous power, kWh is energy over time, maximum demand is the highest average demand recorded over the utility's integration period (commonly 15 or 30 minutes, depending on the tariff), and load factor compares the average demand with that peak over the same period.
Formula
Energy (kWh) = kW × hours
Load factor = Average demand in the period / Maximum demand in the same periodModel interview answer
“I use maximum demand and load factor to understand plant loading and energy utilisation. A sudden increase in maximum demand can indicate a process change, additional equipment, simultaneous motor starts or poor load management.”
Calculators & guides for this module
Motors & Motor Maintenance
Induction motor theory, nameplate data, overheating, vibration, starting methods and VFDs.
Q11Explain the working principle of a three-phase induction motor.
Definition
Three-phase current in the stator creates a rotating magnetic field. This field induces EMF and current in the rotor, and the interaction between the stator field and rotor current produces torque.
Formula
Ns = 120 × f / PModel interview answer
“I always relate motor theory to maintenance. The rotor must run below synchronous speed, because slip is needed to induce rotor EMF and produce torque. During maintenance I monitor current, temperature, vibration, bearing condition, insulation and phase balance.”
Q12What are synchronous speed and slip?
Formula
Slip % = (Ns − Nr) / Ns × 100Key points
- Typical full-load slip for standard industrial motors is only a few percent (larger motors usually have lower slip).
Model interview answer
“If slip increases significantly in operation, I investigate overload, low voltage, rotor problems such as broken bars, or mechanical resistance. Higher slip usually comes with higher current and heating.”
Q13What information is on a motor nameplate?
Nameplate data
- Rated output (kW/HP)
- Voltage and connection (star/delta)
- Rated current (FLA)
- Frequency
- Rated speed (rpm)
- Power factor and efficiency (or efficiency class, e.g. IE2/IE3)
- Duty type (S1, S3…)
- Insulation class and temperature rise
- IP rating and frame size
- Service factor (on NEMA motors)
Model interview answer
“Before setting overload protection or VFD parameters, I always refer to the nameplate. I don't depend only on calculated current when the manufacturer's rated current is available.”
Q14What is motor full-load current (FLC/FLA)?
Definition
Full-load current is the current the motor draws at rated voltage, frequency and rated output.
Formula
I = P / (√3 × V × PF × η)Model interview answer
“I use nameplate FLA as the primary reference for overload protection and compare it with actual running current. Sustained current above the rated value needs investigation.”
Q15How do you select a motor?
Selection criteria
- Power and torque (starting and running)
- Speed
- Duty cycle
- Voltage and frequency
- Starting method
- Enclosure (IP) and insulation class
- Ambient temperature and altitude
- Efficiency class
- VFD compatibility (inverter-duty insulation, bearing currents)
Model interview answer
“I first understand the driven equipment and its torque–speed requirement. Then I select power, speed, enclosure, insulation, duty and starting method — not just kW.”
Q16Why does a motor overheat?
Common causes
- Overload
- Voltage imbalance or single phasing
- Low or high supply voltage
- Bearing failure
- Blocked ventilation or damaged cooling fan
- Too many starts per hour
- High ambient temperature
- Insulation deterioration
- Harmonics / VFD operation at low speed without forced cooling
Model interview answer
“My first step is to compare running current with nameplate current. Then I check all three phase voltages and currents, the cooling fan and air path, bearings, mechanical load and starting frequency. I find the root cause instead of simply replacing the motor.”
Q17What causes motor vibration and noise?
Common causes
- Unbalance
- Misalignment
- Bearing defects
- Coupling problems
- Soft foot / loose foundation
- Bent shaft
- Electrical asymmetry (e.g. rotor bar faults, imbalance)
- Resonance
Model interview answer
“I compare vibration trends and check whether the vibration is mechanical or electrical in origin — for example, electrically caused vibration typically disappears the instant power is cut. I inspect alignment, coupling, bearings, foundation and phase/current balance before deciding on repair.”
Q18Explain motor bearings and common bearing failures.
Definition
Bearings support the rotating shaft and reduce friction.
Typical failure causes
- Wrong lubricant, over- or under-greasing
- Contamination (dust, water)
- Misalignment and excessive belt tension
- Improper fitting
- Shaft/bearing currents on VFD-fed motors (fluting)
Model interview answer
“Bearing failure can result from incorrect lubrication, contamination, misalignment, excessive load or improper installation. I check temperature, vibration, noise, grease condition and shaft play and compare them with historical trends.”
Q19What motor winding tests do you perform?
Formula
PI = IR at 10 min / IR at 1 minTypical tests
- Insulation resistance (phase-to-earth, phase-to-phase where accessible)
- Polarization index (PI) where applicable
- Winding resistance (compare phase-to-phase balance)
- Continuity
- Surge comparison test in a workshop, where available
Model interview answer
“I isolate the motor first and then perform insulation and winding resistance tests per the equipment and manufacturer requirements (IEEE 43 is a common reference for IR/PI). I compare phase values and historical readings rather than judging one reading in isolation.”
Q20What are single phasing and voltage imbalance?
Definition
Single phasing is the loss of one supply phase. Voltage imbalance means the three phase voltages are unequal.
Formula
% Voltage unbalance ≈ Max deviation from average / Average × 100Key points
- A small voltage unbalance can cause a current unbalance several times larger.
- Motor derating is commonly applied above about 1% voltage unbalance (NEMA MG 1).
Model interview answer
“I check the incoming supply, MCC breaker, fuses, contactor, cable and motor terminals in sequence. Voltage imbalance produces much larger current imbalance and heating, so I don't ignore a small voltage difference.”
Q21Why is motor starting current high?
Definition
At standstill the slip is 1, so the rotor's effective resistance (R2/s) is at its lowest and the motor behaves like a transformer with a short-circuited secondary. It therefore draws several times its rated current — typically around 5–8 × FLC on direct-on-line starting, depending on design.
Model interview answer
“I check whether starting current and acceleration time are within expected limits. If acceleration is slow, I investigate load torque, supply voltage dip, mechanical resistance, starter settings and motor condition.”
Q22What is DOL starting?
Definition
Direct-on-line (DOL) starting connects the motor directly to full supply voltage.
Key points
- Simple and low cost
- Highest starting current
- Full starting torque
- Mechanical shock on the drive train
Model interview answer
“For smaller motors, or where the supply can accept the starting current, DOL is simple and reliable. I ensure correct breaker, contactor, overload relay and cable selection.”
Q23Explain star-delta starting.
Definition
The motor starts connected in star, then switches to delta after it accelerates. It requires a motor with six terminals that is rated for delta connection at the supply voltage.
Key points
- Starting line current ≈ 1/3 of DOL
- Starting torque ≈ 1/3 of DOL — only suitable for low-torque starts
- An open-transition changeover can cause a current and torque transient
Model interview answer
“I check the timer, main/star/delta contactors, electrical and mechanical interlocks and the motor terminal configuration. If the changeover happens at the wrong time, or star and delta contactors close together, it creates a serious fault.”
Q24What is an autotransformer starter?
Definition
It starts the motor at reduced voltage using autotransformer taps (commonly 50/65/80%) and then applies full voltage.
Formula
At tap ratio k: motor current ≈ k × I_DOL, line current ≈ k² × I_DOL, torque ≈ k² × T_DOLModel interview answer
“It is useful for larger motors where reduced-voltage starting is required with a selectable torque. I inspect the transformer connections, tap selection, switching mechanism and transition sequence.”
Q25What is a soft starter?
Definition
A soft starter uses thyristors (SCRs) to ramp up motor voltage gradually during starting, and often to ramp down during stopping.
Model interview answer
“I use soft starters where smooth acceleration and reduced mechanical shock matter, such as pumps, fans and conveyors. I check current limit, ramp time, bypass contactor, heatsink and thermal condition. A soft starter controls voltage, not frequency, so it cannot control running speed.”
Q26What is a VFD?
Definition
A variable frequency drive controls motor speed and torque by rectifying AC to DC and then synthesising variable-frequency, variable-voltage AC with an inverter (typically IGBT PWM).
Sequence
Model interview answer
“A typical VFD has a rectifier, DC bus and inverter section. By controlling frequency and voltage together it controls motor speed and torque. I monitor parameters, cooling fans and heatsinks, DC bus voltage, motor current and fault history.”
Q27Which VFD parameters must be entered?
Key parameters
- Motor rated voltage, current, frequency, speed and power
- Acceleration and deceleration time
- Minimum and maximum frequency
- Control mode (V/f, sensorless vector, closed loop)
- Motor thermal protection settings
- Auto-tune, where supported
Model interview answer
“I enter the actual motor nameplate data carefully and run auto-tune when the drive supports it. Incorrect motor parameters cause overcurrent trips, poor torque control, overheating or unstable operation.”
Q28Explain common VFD faults.
Common fault types
- Overcurrent (OC)
- Overvoltage (OV) — often during deceleration/regeneration
- Undervoltage (UV)
- Overload
- Overtemperature
- Earth fault
- Communication failure
Model interview answer
“I record the exact fault code and operating condition instead of resetting immediately. For overcurrent I check motor cable and insulation, mechanical load, acceleration time and motor parameters. For overvoltage I check deceleration time, braking resistor/chopper and regeneration. After correction I restart under observation.”
Q29Explain motor braking methods.
Types
- Mechanical (friction) braking
- DC injection braking
- Dynamic (resistor) braking
- Regenerative braking
- Plugging (reverse-phase braking) — high stress, used rarely
Model interview answer
“I select the braking method according to load inertia, required stopping time and drive capability. For high-inertia loads I pay particular attention to braking energy, resistor rating and drive thermal limits.”
Q30How do you reverse a three-phase motor?
Definition
For a direct three-phase connection, swap any two supply phases. On a VFD, direction is normally changed by command or parameter, not by rewiring.
Model interview answer
“Before a rotation test I confirm the driven machine can safely run in either direction. I never reverse a motor without checking the mechanical requirement — some pumps and compressors are damaged by reverse rotation.”
Calculators & guides for this module
Protection & Switchgear
Breakers, fuses, residual-current devices, overloads and protection coordination.
Q31Explain MCB, MCCB, ACB, VCB and fuse.
Definition
These devices provide switching and/or overcurrent and fault protection at different voltage and current levels.
Key points
- MCB — low-current final circuits
- MCCB — higher-current LV feeders, often adjustable
- ACB — high-current LV incomers and bus couplers
- VCB — medium-voltage switching (vacuum interrupter)
- Fuse — simple, fast, current-limiting protection
Model interview answer
“I select the device based on operating voltage, load current, prospective fault level, breaking capacity and protection requirements. I never select a breaker only from normal load current.”
Q32MCB vs MCCB — what is the difference?
Key points
- MCCBs cover much higher current ratings.
- MCCBs commonly offer adjustable trip settings; MCBs usually have fixed thermal-magnetic curves (B, C, D).
- MCCBs generally have higher breaking capacity and more accessories (shunt trip, aux contacts, motor operators).
Model interview answer
“For a high-current industrial feeder I evaluate MCCB parameters including trip settings and breaking capacity, while MCBs are used for smaller final circuits.”
Q33What is an ACB?
Definition
An air circuit breaker is a high-current low-voltage breaker that uses air as the arc-interrupting medium, usually with an electronic trip unit.
Model interview answer
“I inspect ACB contacts, arc chutes, terminals, mechanism, spring charging system, trip unit and interlocks. Protection settings must be coordinated with downstream devices.”
Q34What is a VCB?
Definition
A vacuum circuit breaker interrupts the arc inside a sealed vacuum interrupter and is widely used in medium-voltage systems (roughly 3.3 kV to around 36 kV).
Model interview answer
“For an 11 kV system I inspect the vacuum interrupter condition (including contact erosion indication), operating mechanism, trip/close coils, spring charging and the protection relay interface.”
Q35What is an HRC fuse?
Definition
A high rupturing capacity fuse can safely interrupt very high fault current within its rated breaking capacity, and is strongly current-limiting.
Model interview answer
“I select the fuse based on voltage, current, breaking capacity and time–current characteristic (e.g. gG for general cable protection, aM for motor circuits). After a fuse operates I investigate the cause before replacing it with an identical type and rating.”
Q36Overload vs short circuit vs earth fault
Definitions
- Overload — current above rating in an otherwise healthy circuit, usually due to load.
- Short circuit — a very low-impedance path between conductors, producing very high current.
- Earth fault — unintended connection between a live conductor and earth or an exposed conductive part.
Model interview answer
“I identify the protection indication and isolate the circuit before inspection. Then I check whether the fault originated in the cable, motor, load or equipment insulation.”
Q37RCCB, RCBO and ELCB — what is the difference?
Key points
- RCCB: residual-current protection only.
- RCBO: residual-current plus overcurrent protection in one device.
- ELCB: older term; voltage-operated ELCBs are obsolete, and the term is often used loosely for current-operated devices.
Model interview answer
“An RCCB alone does not provide overload or short-circuit protection, so appropriate overcurrent protection must also be present upstream or in series.”
Q38What is a motor overload relay?
Definition
An overload relay protects a motor against sustained overcurrent and, depending on type, phase loss or imbalance. Trip classes (e.g. Class 10, 20, 30) define the maximum tripping time in seconds at 7.2 × the set current from cold (IEC 60947-4-1) — so a higher class tolerates a longer motor start.
Model interview answer
“I set the overload according to the motor's rated current and application — choosing a trip class that suits the motor's acceleration time and the manufacturer's coordination guidance. Repeated trips mean checking load, bearings, phase balance, voltage and motor condition.”
Q39Why does a circuit breaker trip repeatedly?
Possible causes
- Overload
- Short circuit
- Earth fault
- Incorrect setting or wrong curve
- Cable fault
- Motor fault
- Breaker mechanism or trip unit fault
- Loose/overheated terminals causing thermal trips
Model interview answer
“I first identify the trip indication. Then I isolate the downstream circuit and inspect the cable, motor and load. I measure current and insulation resistance and verify protection settings. I never repeatedly reset a breaker without finding the cause.”
Q40What is selectivity (protection coordination)?
Definition
Protection coordination ensures that the protective device closest to a fault operates first, while upstream devices stay closed so healthy circuits remain energised.
Model interview answer
“I review prospective fault levels and time–current characteristics and coordinate breaker and relay settings (and manufacturer selectivity tables). This avoids unnecessary plant-wide shutdowns during feeder faults.”
Calculators & guides for this module
These answers summarise common industrial practice for interview preparation. Test voltages, acceptance limits, protection settings and statutory requirements vary by equipment, manufacturer and country — always follow the OEM documentation, applicable standards and your site's safety procedures.
Frequently Asked Questions
How should I prepare for an electrical maintenance engineer interview? +
Revise the fundamentals (Ohm's law, power triangle, three-phase formulas), then practise explaining real troubleshooting cases in a clear sequence: safety and isolation first, then symptom, measurement, root cause, repair, testing and controlled restoration. Interviewers for maintenance roles usually value a structured, safety-first method and practical examples more than memorised definitions.
How should I answer a troubleshooting question in an interview? +
Start with safety: isolate, apply lockout/tagout and test for dead. Then describe how you read the trip indication or symptom, which measurements you take and in what order, how you separate electrical from mechanical causes, how you confirm the root cause, and how you test before a controlled restart. Ending with documentation or RCA shows a reliability mindset rather than a reset-and-hope approach.
What topics are most commonly asked in an electrical maintenance interview? +
Common topics include induction motors and starters, VFDs, breakers and protection coordination, transformers and oil testing, cable sizing and terminations, earthing, insulation resistance testing, PLC and instrumentation troubleshooting, DG sets, UPS systems, LOTO and permit-to-work, and maintenance management terms such as PM, PdM, RCA, MTBF and MTTR.
Which motor starting method gives the lowest starting current? +
A VFD can usually start a motor at or near its rated current because it raises frequency and voltage together, while still providing good torque. Soft starters and autotransformer starters reduce current by lowering voltage, which also reduces starting torque, and a star-delta starter cuts line current and torque to about one-third of DOL. The right choice depends on the load's torque requirement, not only on current.
Are the answers on this page suitable to memorise word for word? +
They are best used as a structure rather than a script. Interviewers quickly notice memorised text, so adapt each model answer with a real example from your own plant, equipment and failures. Always confirm specific values such as test voltages, acceptance limits and protection settings against the equipment manufacturer's data and the standards that apply in your country.
More Electrical Interview Questions
This is Part I of a three-part, 120-question series. Every question below opens directly on its own page.
Part II: Transformers, Cables, Testing & Instrumentation
Questions 41–80
Transformer & Power Distribution
- Q41Explain a transformer.
- Q42Why are transformers rated in kVA instead of kW?
- Q43What are transformer losses?
- Q44Why is transformer oil used?
- Q45What is the BDV test?
- Q46What is the transformer IR test?
- Q47What is a Buchholz relay?
- Q48Explain conservator, breather, ONAN and ONAF.
- Q49Why does a transformer overheat or hum abnormally?
- Q50What are common transformer protection systems?
Panels, Cables & Earthing
- Q51What are a CT and a PT/VT?
- Q52Why must a CT secondary never be left open?
- Q53What are MCC and PCC panels?
- Q54How do you perform electrical panel maintenance?
- Q55What is thermography and why is it used?
- Q56How do you select a power cable?
- Q57What causes cable overheating?
- Q58What is cable termination?
- Q59What is electrical earthing?
- Q60What are the basic electrical safety requirements during maintenance?
Electrical Testing & Troubleshooting
- Q61What is a multimeter?
- Q62What is a clamp meter?
- Q63What is a megger (insulation resistance tester)?
- Q64Explain continuity, polarity and phase-sequence testing.
- Q65What electrical tests are performed before commissioning?
- Q66How do you troubleshoot a motor that suddenly trips?
- Q67An MCC motor feeder is repeatedly tripping. How do you troubleshoot it?
- Q68Why does a contactor chatter or burn?
- Q69How do you troubleshoot an electrical panel?
- Q70Explain your complete breakdown troubleshooting method.
Electronics & Instrumentation
- Q71What is a diode?
- Q72What is a rectifier?
- Q73What is an SMPS?
- Q74Relay vs SSR — what is the difference?
- Q75What is a proximity sensor?
- Q76Explain PNP and NPN sensors.
- Q77What is a 4–20 mA signal?
- Q78What is an industrial transmitter?
- Q79How do you troubleshoot a solenoid valve or sensor?
- Q80Explain instrumentation fault tracing.
Part III: PLC, Reliability, UPS, HV, DG, Boiler & ACB
Questions 81–120
PLC & Industrial Automation
- Q81What is a PLC?
- Q82How do you troubleshoot PLC digital I/O?
- Q83How do you troubleshoot PLC analog I/O?
- Q84How do you troubleshoot PLC logic?
- Q85What causes PLC communication failure?
- Q86What is HMI–PLC communication?
- Q87What is PLC I/O mapping?
- Q88An automatic machine sequence stops. How do you troubleshoot it?
- Q89What should be included in a PLC backup?
- Q90Explain complete automation troubleshooting.
Maintenance Management & Reliability
- Q91What are preventive, predictive and breakdown maintenance?
- Q92How do you prepare an electrical PM schedule?
- Q93What is predictive maintenance?
- Q94What is RCA?
- Q95How do you plan a maintenance job?
- Q96How do you manage electrical spare parts?
- Q97Explain MTBF and MTTR.
- Q98How can you improve electrical equipment reliability?
- Q99How do you manage a major plant electrical breakdown?
- Q100What is your approach as a senior electrical maintenance engineer to improve plant reliability?
UPS Systems
- Q101What is a UPS? Explain its working principle and major components.
- Q102Explain online, offline and line-interactive UPS. How does a UPS differ from an inverter?
- Q103How do you calculate UPS battery backup time? What affects it?
- Q104What are UPS bypass systems? Explain static and maintenance bypass.
High-Voltage Systems (11 kV / 33 kV / 66 kV)
- Q105Why are 11 kV, 33 kV and 66 kV used in distribution? Why '11' kV and not 10 kV?
- Q106Calculate current at 11 kV, 33 kV and 66 kV for the same power.
- Q107Explain an 11 kV system, its equipment and protection arrangement.
- Q108How do you test and commission an 11 kV cable or transformer before energisation?
DG Sets (Diesel Generators)
- Q109Explain the working principle of a DG set.
- Q110What are an AVR and a governor? How do they differ?
- Q111What is DG synchronisation? What must match before paralleling?
- Q112What are DG load sharing and reverse power protection?
- Q113A DG has low voltage, unstable frequency, high temperature or low oil pressure. How will you troubleshoot it?
- Q114What DG protections are important? How do you troubleshoot a DG starting failure?
Boiler — Electrical & Control
- Q115Explain the working principle of an industrial boiler and its major electrical equipment.
- Q116Explain ID fan, FD fan, PA fan and feed-water pump. What electrical problems occur?
- Q117What are boiler interlocks and trips? Why must they never be bypassed?
- Q118What is three-element drum-level control?
ACB Protection & Testing
- Q119Explain ACB construction and LSIG protection.
- Q120What are ACB Icu, Ics, Icw, trip unit, spring charging, interlocking, and primary/secondary injection tests?