Electrical Interview Questions Part II: Transformers, Cables, Testing & Instrumentation
Calculate Smarter. Work Faster.
40 practical questions (Q41–Q80) on transformers, cables, testing & instrumentation — each with a definition, formula, key points and a model answer, plus linked calculators for every module.
About Part II
Part II covers transformers, CT/PT, panels, cables and earthing, electrical testing and troubleshooting, and electronics and instrumentation — questions 41 to 80 of the 120-question series. Each answer gives a short definition, the formula where one applies, the key points an interviewer listens for, and a first-person model answer to adapt with your own plant experience.
Values such as test voltages, acceptance limits and protection settings are typical; the equipment manufacturer's data and the standards that apply at your site always take priority. Most testing and troubleshooting answers in this part follow the same safe order:
- Safety & LOTO
- Symptom / trip indication
- Measure
- Separate electrical vs mechanical
- Root cause
- Repair
- Test
- Controlled restart
- Document & RCA
Calculators and guides that match each module are listed at the end of it, and the full list of questions from the other parts is at the bottom of this page.
No question matches that filter. Try a shorter term, such as “motor” or “relay”.
Transformer & Power Distribution
Transformer principle, kVA rating, losses, oil testing, Buchholz relay, cooling and protection.
Q41Explain a transformer.
Definition
A transformer transfers AC energy between circuits by electromagnetic induction, normally changing voltage and current while keeping frequency the same.
Formula
V1 / V2 ≈ N1 / N2 ≈ I2 / I1Model interview answer
“I monitor transformer loading, oil and winding temperature, oil condition, bushings, cooling system and protection devices. Any abnormal temperature or oil condition is investigated before it develops into a major failure.”
Q42Why are transformers rated in kVA instead of kW?
Definition
Transformer losses — and therefore heating — depend on voltage (core loss) and current (copper loss), not on the power factor of the load. The manufacturer also doesn't know what PF the load will have.
Formula
Three-phase kVA = √3 × V_L × I_L / 1000Model interview answer
“A transformer supplies both active and reactive load, so its current and voltage determine thermal loading. That is why its capacity is expressed in kVA.”
Q43What are transformer losses?
Formula
Maximum efficiency occurs where copper loss = core lossTypes
- Core (iron) loss: hysteresis + eddy current — roughly constant whenever energised
- Copper (I²R) loss — varies with the square of load current
- Stray and dielectric losses — smaller
Model interview answer
“I monitor loading and temperature because copper loss rises with the square of current. Cooling performance becomes critical as loading increases.”
Q44Why is transformer oil used?
Definition
Transformer oil insulates the windings and transfers heat from the core and windings to the tank and radiators.
Model interview answer
“I check oil level, leaks, temperature, moisture, dielectric strength and, for larger units, dissolved gas analysis (DGA) trends. Deteriorated or contaminated oil reduces insulation reliability.”
Q45What is the BDV test?
Definition
The breakdown voltage test measures the dielectric strength of insulating oil by raising voltage across a standard electrode gap until it breaks down (commonly per IEC 60156 or ASTM D1816/D877).
Model interview answer
“Low BDV indicates moisture, particles or oil deterioration. I follow the applicable test standard and compare results with the acceptance values in the plant/OEM criteria or oil-maintenance standard (e.g. IEC 60422). Results from different methods (IEC 60156, ASTM D1816, ASTM D877) use different electrodes and gaps, so I don't compare them directly.”
Q46What is the transformer IR test?
Definition
Insulation resistance is measured between HV–LV, HV–earth and LV–earth, as applicable, with a DC insulation tester.
Model interview answer
“I isolate and discharge the transformer, then connect the tester in the approved test configuration — earthing or guarding the windings not under test as that procedure specifies. I compare temperature-corrected values with previous records, because the trend is more meaningful than one isolated number. PI and dielectric absorption ratio help assess moisture.”
Q47What is a Buchholz relay?
Definition
A Buchholz relay sits in the pipe between the main tank and conservator of conservator-type oil transformers, and detects gas accumulation and sudden oil surge caused by internal faults.
Model interview answer
“Slow gas accumulation gives an alarm, while a severe oil surge initiates a trip. After operation I collect and analyse the gas where possible and investigate the transformer before re-energisation.”
Q48Explain conservator, breather, ONAN and ONAF.
Definition
The conservator accommodates oil expansion. The silica-gel breather dries air entering the conservator. ONAN = oil natural, air natural cooling; ONAF = oil natural, air forced (fans).
Model interview answer
“I inspect oil level, breather silica gel colour and cooling fans. For a transformer with separate ONAN and ONAF nameplate ratings, loss of the fans means loading must be reduced towards the ONAN rating, following the manufacturer's instructions.”
Q49Why does a transformer overheat or hum abnormally?
Possible causes
- Overload
- Poor cooling (blocked radiators, failed fans)
- High ambient temperature
- Harmonics
- Overvoltage / over-fluxing
- Loose core clamping or connections
- Internal fault
Model interview answer
“I compare load, temperature, voltage and cooling status with normal conditions. Abnormal humming together with temperature rise needs further investigation rather than assuming it is normal magnetostriction noise.”
Q50What are common transformer protection systems?
Key points
- Differential protection
- Overcurrent and earth fault
- Restricted earth fault (REF)
- Buchholz
- Oil and winding temperature
- Pressure relief device
- Surge arresters
- Proper earthing
Model interview answer
“I treat transformer protection as a complete system. After a trip I identify which protection operated, collect relay records and investigate the actual fault before restoration.”
Calculators & guides for this module
Panels, Cables & Earthing
CTs and PTs, MCC/PCC panels, thermography, cable selection, terminations, earthing and safe isolation.
Q51What are a CT and a PT/VT?
Definition
A current transformer steps a high primary current down to a proportional small current (typically 1 A or 5 A) for meters and relays. A potential/voltage transformer steps high voltage down to a safe measurable level (commonly 110 V or 100 V class, 120 V in North America).
Key points
- A CT primary is in series with the circuit.
- A PT/VT is connected across (in parallel with) the voltage.
- Metering CTs and protection CTs have different accuracy classes.
Model interview answer
“In a plant, CTs and PTs feed meters and protection relays. During maintenance I check ratio, polarity, insulation, burden, connections and secondary circuit condition.”
Q52Why must a CT secondary never be left open?
Definition
With primary current flowing, an open CT secondary has no opposing ampere-turns, so the core saturates and a dangerously high voltage appears across the secondary terminals.
Risks
- Shock hazard
- Insulation breakdown
- CT overheating/damage
- Meter or relay damage
Model interview answer
“I never open a CT secondary while primary current is flowing. Before disconnecting a CT-connected meter or relay, I use the approved CT shorting/test-block arrangement and follow the site safety procedure. If a CT secondary is found open on a live circuit, I don't try to handle or bridge it by hand — the circuit is made safe under the approved procedure first.”
Q53What are MCC and PCC panels?
Definition
An MCC (motor control centre) controls and protects motors; a PCC (power control centre) distributes power to downstream boards and large loads.
Key components
- Incomer
- Busbars
- MCCB/ACB
- Contactors
- Overload relays
- CTs and meters
- Control circuits
- Earthing
Model interview answer
“In an MCC I focus on motor feeders, contactors, overloads, protection and control circuits. In a PCC I focus on power distribution, incomers, busbars, feeder protection and load management.”
Q54How do you perform electrical panel maintenance?
Definition
Panel maintenance covers inspection, cleaning, tightening, testing and condition assessment of the electrical components.
Checkpoints
- Loose terminals
- Busbar condition
- Dust and moisture
- Hotspots
- Breakers and contactors
- Earthing
- Insulation
- Cable terminations
Model interview answer
“I isolate the panel under LOTO and verify the dead condition. Then I inspect busbars, breakers, contactors, terminals, cables, insulation and earthing. I carry out torque checks to the manufacturer's values and thermography where applicable, and record abnormal findings for corrective action.”
Q55What is thermography and why is it used?
Definition
Thermography is a non-contact condition-monitoring technique that uses infrared imaging to find abnormal temperature patterns.
Maintenance focus
Loose connections, overloaded feeders, contact resistance, busbar hotspots and phase imbalance. Scans should be done under reasonable load, as a lightly loaded joint can hide a defect.
Model interview answer
“If one MCC feeder termination is significantly hotter than similar feeders at comparable load, I suspect a loose termination, high contact resistance, overload or imbalance. I verify the load, isolate safely, correct the defect and perform a follow-up scan.”
Q56How do you select a power cable?
Definition
Cable selection must satisfy current-carrying capacity (after derating), voltage drop, short-circuit withstand and installation conditions.
Formula
I_b ≤ I_n ≤ I_z
I_2 ≤ 1.45 × I_z
Adiabatic short-circuit check: S ≥ √(I² × t) / kI_b = design current, I_n = protective device rating, I_z = derated cable capacity, I_2 = current ensuring effective device operation (IEC 60364 / BS 7671 approach). In the adiabatic check, S = conductor area (mm²), I = prospective fault current (A), t = device disconnection time (s, valid up to about 5 s) and k depends on conductor material, insulation and its initial/final temperatures.
Key points
- Design load current
- Copper or aluminium conductor
- Installation method
- Ambient temperature
- Grouping
- Derating factors
- Voltage drop
- Short-circuit withstand
- Insulation type
- Future load
Model interview answer
“I calculate design current, then select the cable considering derating, installation method, voltage drop and short-circuit withstand. I also verify that breaker and cable are properly coordinated.”
Q57What causes cable overheating?
Causes
- Overload
- Undersized cable
- Loose termination
- Poor crimping
- High ambient temperature
- Cable grouping without derating
- Harmonics (especially neutral current)
- Damaged insulation
Model interview answer
“I compare actual current with the derated cable capacity and inspect the termination. If the cable body is normal but the lug is much hotter, I suspect contact resistance or poor crimping. Thermography is useful here.”
Q58What is cable termination?
Definition
Cable termination is the process of safely connecting a cable to equipment using the correct lugs, glands, crimping and insulation.
Model interview answer
“I check correct lug size, conductor stripping, crimp tool and die, gland, heat-shrink or cold-shrink insulation, mechanical support and terminal torque. A poor termination creates high contact resistance and can eventually cause fire or equipment failure.”
Q59What is electrical earthing?
Definition
Earthing provides a controlled path for fault current so protection operates quickly, and keeps exposed conductive parts at a safer potential.
Key points
- Equipment (protective) earthing
- System earthing (TN, TT, IT)
- Earth continuity
- Earth electrodes and grid
- Equipotential bonding
Model interview answer
“I verify that exposed metallic parts are properly bonded to the earthing system. During maintenance I check earth continuity and measure earth resistance per the applicable site procedure and standard (e.g. fall-of-potential method).”
Q60What are the basic electrical safety requirements during maintenance?
Key points
- Permit to work (PTW)
- Isolation from all sources
- Lockout/tagout (LOTO)
- Discharge stored energy (capacitors, cables)
- Test for dead with a proven tester
- Apply temporary earths where required
- Appropriate PPE (including arc-flash rated)
- Barricading
- Authorised personnel only
Model interview answer
“My first priority is safety, not troubleshooting speed. I isolate the source, apply LOTO, discharge stored energy, and verify absence of voltage with a tester proven before and after use before starting work. I never bypass a safety interlock to restore production.”
Calculators & guides for this module
Electrical Testing & Troubleshooting
Test instruments, commissioning tests and structured troubleshooting sequences.
Q61What is a multimeter?
Definition
A multimeter measures electrical quantities such as AC/DC voltage, resistance and continuity, and many models also measure current, frequency and capacitance.
Model interview answer
“I select the correct function and range before testing. For live measurements I use an instrument with an appropriate CAT rating (e.g. CAT III/IV for distribution work) and proper PPE.”
Q62What is a clamp meter?
Definition
A clamp meter measures current by sensing the magnetic field around a conductor, without breaking the circuit. True-RMS models are needed for non-sinusoidal currents such as VFD inputs.
Model interview answer
“It is ideal for measuring motor and feeder current in operation. I compare all three phase currents to identify imbalance without opening the circuit. Clamping only one conductor is essential — clamping around a whole cable reads roughly zero.”
Q63What is a megger (insulation resistance tester)?
Definition
An insulation resistance tester applies a DC test voltage (commonly 500 V, 1 kV, 2.5 kV or 5 kV depending on equipment rating) and measures insulation resistance.
Applications
- Motors
- Cables
- Transformers
- Panels and switchgear
Model interview answer
“I isolate the equipment, disconnect sensitive electronics such as VFDs and PLC cards, and verify the circuit is safe before testing. I discharge the equipment after the test, record the IR value and compare it with history and the applicable equipment or manufacturer criteria.”
Q64Explain continuity, polarity and phase-sequence testing.
Definition
Continuity confirms a complete electrical path, polarity confirms correct connection of line/neutral or DC +/−, and phase-sequence testing confirms the order of the three-phase supply.
Model interview answer
“Before commissioning I verify continuity and correct wiring, then confirm phase sequence. This is especially important for pumps, fans and compressors, where reverse rotation can cause damage.”
Q65What electrical tests are performed before commissioning?
Key tests
- Visual and mechanical inspection
- Continuity
- Insulation resistance
- Earth continuity
- Phase sequence
- Protection testing (injection)
- Control circuit testing
- Functional/interlock testing
- Motor rotation (uncoupled where possible)
Model interview answer
“I follow a structured pre-commissioning checklist and verify wiring, insulation, earthing, protection, control logic and motor rotation before allowing equipment to run under load.”
Q66How do you troubleshoot a motor that suddenly trips?
Sequence
Model interview answer
“First I check the trip indication and whether it was overload, short circuit, earth fault or another protection. Then I check incoming voltage and phase balance, isolate the motor, test cable insulation and winding condition, and inspect bearings and mechanical load. I verify protection settings before a controlled restart.”
Q67An MCC motor feeder is repeatedly tripping. How do you troubleshoot it?
Possible causes
- Motor overload
- Short circuit
- Earth fault
- Cable insulation failure
- Mechanical jam
- Bearing failure
- Incorrect overload setting
- Winding fault
Model interview answer
“I separate electrical and mechanical possibilities. I check the trip indication, measure operating current, inspect the driven equipment, test cable and motor insulation and verify overload settings. I do not keep resetting the feeder without identifying the cause.”
Q68Why does a contactor chatter or burn?
Chatter causes
Low or unstable coil voltage, loose control wiring, wrong coil voltage, damaged coil or shading ring, auxiliary contact problems, mechanical obstruction or dirty pole faces.
Contact burning causes
Excessive current, frequent switching (inching/jogging), arcing, low contact pressure, or the wrong utilisation category (e.g. an AC-3 rating used for AC-4 duty).
Model interview answer
“If a contactor chatters, I first measure the coil voltage during operation, because unstable control voltage is a common cause. For burned contacts I check load current, switching frequency, contact condition and whether the contactor is correctly rated for the duty.”
Q69How do you troubleshoot an electrical panel?
Sequence
Model interview answer
“I start with the symptom and the electrical drawing. Then I check incoming supply, control voltage, protection devices, contactors/relays, terminals and outgoing feeders in sequence. I use measurements to find the failed point rather than replacing components at random.”
Q70Explain your complete breakdown troubleshooting method.
Sequence
Model interview answer
“For repeated or critical failures I also perform a root cause analysis, so the same failure doesn't come back.”
Calculators & guides for this module
Electronics & Instrumentation
Diodes, rectifiers, SMPS, relays, sensors, 4–20 mA loops and signal tracing.
Q71What is a diode?
Definition
A diode is a semiconductor device that allows current to flow primarily in one direction.
Applications
- Rectification
- Reverse-polarity protection
- Flyback (freewheeling) protection across relay coils
- Switching
Model interview answer
“I use the multimeter's diode-test mode to check forward conduction (roughly 0.5–0.7 V for a silicon diode) and reverse blocking. A shorted or open diode can cause power-supply and control-circuit failures.”
Q72What is a rectifier?
Definition
A rectifier converts AC into unidirectional (pulsating) DC, which is then smoothed by filtering.
Types
- Half-wave
- Full-wave (centre-tapped)
- Bridge
Model interview answer
“Bridge rectifiers are common in industrial power supplies and drives, before DC filtering or switching stages. If a DC supply fails I check the input, rectifier, DC bus capacitor and downstream circuit.”
Q73What is an SMPS?
Definition
A switched-mode power supply uses high-frequency switching to convert and regulate power efficiently.
Sequence
Model interview answer
“When an SMPS fails I check the input fuse, rectifier, switching device, DC bus, output section and feedback circuit. I also check whether an external short or overload is making the supply shut down or hiccup.”
Q74Relay vs SSR — what is the difference?
Definition
A conventional relay switches with electromechanical contacts; a solid-state relay switches with semiconductors.
Key points
- Relay: physical contacts, galvanic isolation, contact wear, small leakage.
- SSR: fast, silent, no contact wear, but has leakage current and voltage drop.
- SSRs need heat dissipation (heatsink) and usually fail short-circuited.
Model interview answer
“I select the device by switching frequency, load type, voltage/current and environment. For high-frequency switching such as heater control an SSR may suit, but I check its thermal design.”
Q75What is a proximity sensor?
Definition
A proximity sensor detects an object without physical contact.
Types
- Inductive (metal targets)
- Capacitive (most materials, including liquids)
- Photoelectric
- Ultrasonic
Model interview answer
“In plant automation, proximity sensors are used for position, counting and speed detection. If a sensor 'fails', I check supply voltage, sensing distance and target, LED indication, wiring and the PLC input before replacing it.”
Q76Explain PNP and NPN sensors.
Definition
A PNP sensor sources positive voltage to the load/PLC input when active; an NPN sensor sinks current to 0 V when active.
Key points
- PNP sensors pair with PLC sinking inputs (common to 0 V).
- NPN sensors pair with PLC sourcing inputs (common to +24 V).
- Naming conventions for 'sink' and 'source' inputs vary by manufacturer — follow the input wiring diagram.
Model interview answer
“Before replacing a sensor I verify whether the PLC input is configured for PNP or NPN. A mismatch means a perfectly healthy sensor never produces a PLC input signal.”
Q77What is a 4–20 mA signal?
Definition
4–20 mA is a standard analog current signal for transmitting process variables in industry.
Formula
PV = LRV + (mA − 4) / 16 × (URV − LRV)Example, 0–100 bar: 4 mA = 0 bar, 12 mA = 50 bar, 20 mA = 100 bar.
Model interview answer
“4–20 mA is robust against voltage drop and electrical noise and gives a 'live zero' — a reading near 0 mA indicates a broken loop rather than a zero process value. During troubleshooting I measure the actual loop current and compare it with the PLC/HMI value.”
Q78What is an industrial transmitter?
Definition
A transmitter converts a physical process variable such as pressure, temperature, flow or level into a standardised signal (e.g. 4–20 mA, often with HART or a fieldbus).
Model interview answer
“For a transmitter fault I check the process condition, 24 V loop supply, wiring, loop current, calibration and PLC scaling. I don't immediately assume the transmitter itself is faulty.”
Q79How do you troubleshoot a solenoid valve or sensor?
Model interview answer
“I follow the signal path from supply to field device to PLC I/O and then to the actuator. For a solenoid I check coil voltage, coil resistance, PLC output, interposing relay/DO module and mechanical valve operation (including air supply). For a sensor I check supply, output signal and PLC input.”
Q80Explain instrumentation fault tracing.
Sequence
Model interview answer
“I trace the signal step by step instead of replacing instruments at random. If the HMI shows an incorrect pressure, I first verify the actual process and transmitter output, then measure the 4–20 mA signal at the JB and PLC input, and finally check PLC scaling.”
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
Why should a CT secondary never be left open while the primary is carrying current? +
With primary current flowing, an open secondary removes the opposing ampere-turns, so the core saturates and a dangerously high voltage can appear across the secondary terminals. This creates a shock hazard and can damage the CT insulation and connected meters or relays. CT secondaries are shorted using an approved test block or shorting link before any meter or relay is disconnected.
Why are transformers rated in kVA and not kW? +
Transformer losses depend on voltage (core loss) and current (copper loss), not on the power factor of the connected load, and the manufacturer cannot know the load power factor in advance. Rating in kVA therefore describes the thermal capacity of the transformer regardless of whether the load is resistive or inductive.
What insulation resistance value is acceptable for a motor winding? +
There is no single universal number. IEEE 43 recommends minimum one-minute values corrected to 40 °C, for example about 100 MΩ for most form-wound AC windings built after about 1970 and about 5 MΩ for random-wound machines and form-wound coils rated below 1 kV. The manufacturer's criteria, the test voltage and the trend against earlier readings matter as much as the absolute value.
How do you convert a 4–20 mA signal into engineering units? +
Use PV = LRV + (mA − 4) ÷ 16 × (URV − LRV), where LRV and URV are the lower and upper range values of the transmitter. For a 0–100 bar transmitter, 12 mA corresponds to 50 bar. A reading close to 0 mA usually means a broken loop rather than a zero process value, which is why the live zero at 4 mA is useful.
What is the difference between an MCC and a PCC panel? +
An MCC (motor control centre) contains motor feeders with contactors, overload relays and control circuits for starting and protecting motors. A PCC (power control centre) distributes power to downstream boards and large loads through incomers, busbars and feeder breakers. Maintenance on an MCC focuses on starters and control wiring, while PCC maintenance focuses on breakers, busbars and load distribution.
More Electrical Interview Questions
This is Part II of a three-part, 120-question series. Every question below opens directly on its own page.
Part I: Fundamentals, Motors & Protection
Questions 1–40
Electrical Fundamentals
- Q1What are voltage, current, resistance and Ohm's law?
- Q2What is the difference between AC and DC?
- Q3Explain kW, kVAR, kVA and power factor.
- Q4What is power factor and why does a low power factor matter?
- Q5Single-phase vs three-phase supply — what is the difference?
- Q6Explain star and delta connections.
- Q7What are frequency and phase sequence?
- Q8How do you calculate electrical load current?
- Q9What is voltage drop?
- Q10Explain kW, kWh, maximum demand and load factor.
Motors & Motor Maintenance
- Q11Explain the working principle of a three-phase induction motor.
- Q12What are synchronous speed and slip?
- Q13What information is on a motor nameplate?
- Q14What is motor full-load current (FLC/FLA)?
- Q15How do you select a motor?
- Q16Why does a motor overheat?
- Q17What causes motor vibration and noise?
- Q18Explain motor bearings and common bearing failures.
- Q19What motor winding tests do you perform?
- Q20What are single phasing and voltage imbalance?
- Q21Why is motor starting current high?
- Q22What is DOL starting?
- Q23Explain star-delta starting.
- Q24What is an autotransformer starter?
- Q25What is a soft starter?
- Q26What is a VFD?
- Q27Which VFD parameters must be entered?
- Q28Explain common VFD faults.
- Q29Explain motor braking methods.
- Q30How do you reverse a three-phase motor?
Protection & Switchgear
- Q31Explain MCB, MCCB, ACB, VCB and fuse.
- Q32MCB vs MCCB — what is the difference?
- Q33What is an ACB?
- Q34What is a VCB?
- Q35What is an HRC fuse?
- Q36Overload vs short circuit vs earth fault
- Q37RCCB, RCBO and ELCB — what is the difference?
- Q38What is a motor overload relay?
- Q39Why does a circuit breaker trip repeatedly?
- Q40What is selectivity (protection coordination)?
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?