Electrical Interview Questions Part III: PLC, Reliability, UPS, HV, DG & ACB
Calculate Smarter. Work Faster.
40 practical questions (Q81–Q120) on PLC, reliability, UPS, HV, DG, boiler & ACB — each with a definition, formula, key points and a model answer, plus linked calculators for every module.
About Part III
Part III covers PLC and automation, maintenance management and reliability, UPS systems, 11/33/66 kV systems, DG sets, boiler electrical and control, and ACB protection and testing — questions 81 to 120 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. Breakdown and automation questions in this part are best answered in 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”.
PLC & Industrial Automation
PLC architecture, I/O troubleshooting, logic, communication, I/O mapping and backups.
Q81What is a PLC?
Definition
A programmable logic controller is an industrial computer that reads inputs, executes a program cyclically (the scan cycle) and controls outputs.
Main components
- CPU
- Memory
- Power supply
- Digital inputs/outputs
- Analog inputs/outputs
- Communication modules
Model interview answer
“A PLC continuously reads field inputs, executes the program and updates outputs. During troubleshooting I separate the problem into field device, I/O, logic, communication and output-device sections.”
Q82How do you troubleshoot PLC digital I/O?
Sequence
Model interview answer
“I start at the field device and follow the signal through wiring, terminal, PLC input LED and PLC status. For an output I trace from PLC logic to output module, relay/contactor and finally the field device.”
Q83How do you troubleshoot PLC analog I/O?
Model interview answer
“I verify the transmitter's 24 V supply and the actual 4–20 mA signal first. Then I check cable polarity, terminals, analog input configuration (range, 2-wire/4-wire), scaling, and grounding/shielding. Finally I compare the PLC value with the measured signal.”
Q84How do you troubleshoot PLC logic?
Key areas
- Inputs
- Permissives
- Interlocks
- Timers
- Counters
- Sequence steps
- Outputs
- Alarms
Model interview answer
“I identify the exact sequence step where the machine stopped, then find the first missing condition or permissive in the logic, using online monitoring. I never bypass or force a safety interlock just to make the machine run.”
Q85What causes PLC communication failure?
Possible causes
- Cable or connector fault
- Network switch failure
- Wrong IP/node address
- Duplicate IP
- Configuration mismatch
- Power failure
- Module failure
- Electrical noise, poor shielding or grounding
Model interview answer
“I first check power and the physical link, then network status, address and configuration and PLC/module diagnostics. If communication is intermittent, I also check shielding, grounding and noise sources.”
Q86What is HMI–PLC communication?
Definition
The HMI provides the operator interface — process values, alarms, commands and status — while the PLC performs the control logic.
Model interview answer
“If an HMI value is wrong, I determine whether the problem is in the field signal, the PLC value, the communication tag or HMI scaling. A wrong HMI display does not automatically mean the field instrument is faulty.”
Q87What is PLC I/O mapping?
Definition
I/O mapping assigns physical field devices to PLC addresses/tags and links them to the control logic.
Sequence
Model interview answer
“Incorrect I/O mapping can make a healthy field device look faulty. I verify the actual terminal, PLC address, tag and program logic before changing hardware.”
Q88An automatic machine sequence stops. How do you troubleshoot it?
Model interview answer
“I identify the exact sequence step and check the required sensor inputs, permissives and interlocks. If an input is missing, I troubleshoot the field device. If inputs are healthy, I check PLC logic and output, then follow the output through relay, contactor or solenoid to the machine.”
Q89What should be included in a PLC backup?
Key points
- PLC program (with comments/symbols)
- HMI/SCADA project
- Hardware configuration
- Network configuration
- VFD parameters
- Instrument/transmitter parameters
- Version and date
- Communication settings
Model interview answer
“I maintain a controlled backup of PLC and HMI programs together with VFD and network parameters, with version information, so the correct program can be restored quickly after a hardware failure.”
Q90Explain complete automation troubleshooting.
Sequence
Model interview answer
“At every stage I take an actual measurement or diagnostic indication. This quickly separates field, electrical, PLC and mechanical problems.”
Calculators & guides for this module
Maintenance Management & Reliability
PM, PdM, RCA, job planning, spares, MTBF/MTTR and reliability improvement.
Q91What are preventive, predictive and breakdown maintenance?
Definitions
- Preventive: planned maintenance based on time or usage.
- Predictive: condition-based maintenance driven by measured data.
- Breakdown (corrective): repair after failure.
Model interview answer
“My objective is to reduce unplanned breakdowns with the right mix of PM and PdM. For critical equipment I prefer condition monitoring and trend-based intervention wherever practical.”
Q92How do you prepare an electrical PM schedule?
Key points
- Equipment criticality
- OEM recommendations
- Operating hours and duty
- Failure history
- Safety and statutory requirements
- Inspection/testing frequency
Model interview answer
“I classify equipment by criticality, then define inspection, testing and servicing frequencies. Motors, MCCs, transformers, VFDs, batteries, cables, earthing and protection systems are included according to their risk and operating conditions.”
Q93What is predictive maintenance?
Definition
Predictive maintenance uses equipment condition data to decide when intervention is needed.
Methods
- Thermography
- Vibration analysis
- IR/PI trending
- Oil analysis and DGA
- Temperature trending
- Motor current signature analysis
- Ultrasound
Model interview answer
“I use trends rather than only fixed intervals. For example, rising motor bearing temperature or vibration gives an early warning before a breakdown.”
Q94What is RCA?
Definition
Root cause analysis identifies the fundamental reason for a failure rather than only correcting the immediate symptom.
Methods
- 5 Why
- Fishbone (Ishikawa)
- Fault tree analysis
- Trend analysis
- Failure history review
Model interview answer
“If a motor bearing repeatedly fails, replacing the bearing is corrective action, not root-cause correction. I investigate lubrication, alignment, vibration, shaft condition, coupling and load. If misalignment is the root cause, I correct alignment and then monitor vibration and temperature.”
Q95How do you plan a maintenance job?
Key points
- Scope
- Manpower and skills
- Tools and test instruments
- Spares
- Safety, PTW and LOTO
- Shutdown window
- OEM requirements
- Testing and restoration plan
Model interview answer
“Before a critical job I prepare manpower, tools, spares, drawings, safety requirements and test instruments. Good planning reduces downtime and prevents equipment sitting open because a critical spare or tool is missing.”
Q96How do you manage electrical spare parts?
Definition
Spares management keeps critical parts available when needed without excess inventory.
Key points
- Critical spares
- Fast-moving items
- Long-lead items
- Min/max stock levels
- Lead time
- Obsolescence
- Consumption history
Model interview answer
“I classify spares by equipment criticality and lead time. Critical breakers, contactors, overloads, VFDs, PLC modules, fuses and sensors may need strategic stocking depending on plant risk.”
Q97Explain MTBF and MTTR.
Formula
MTBF = Total operating time / Number of failures
MTTR = Total repair time / Number of repairs
Inherent availability = MTBF / (MTBF + MTTR)Model interview answer
“MTBF shows equipment reliability, while MTTR shows how quickly we restore equipment after a failure. If MTBF is falling, I look for recurring failure modes and RCA. If MTTR is high, I look at manpower, access, troubleshooting time, procedures and spares availability.”
Q98How can you improve electrical equipment reliability?
Key points
- Criticality analysis
- PM and PdM
- RCA
- Thermography
- Vibration monitoring
- IR testing
- Correct protection settings
- Correct cable sizing
- Critical spares
- PLC/VFD backups
Model interview answer
“My focus is preventing failures rather than only responding to them. I use condition monitoring, RCA, proper protection, critical spare planning and historical data, and measure improvement through reduced breakdown frequency, downtime and repeat failures.”
Q99How do you manage a major plant electrical breakdown?
Sequence
Model interview answer
“During a major breakdown my first priority is personnel and equipment safety. After isolation I establish the fault, mobilise manpower and spares, execute the repair and test properly before a controlled restoration. After stabilisation I document the event and perform RCA if the failure is critical or repetitive.”
Q100What is your approach as a senior electrical maintenance engineer to improve plant reliability?
Strong closing line
“I don't want to be only a breakdown responder; I want to make the equipment more reliable so that the same failure doesn't happen again.”
Model interview answer
“My approach isn't limited to restoring equipment after a breakdown. I focus on equipment criticality, preventive and predictive maintenance, RCA of repeated failures, condition monitoring, critical spare planning, protection coordination, PLC/VFD backups and MTBF/MTTR improvement. I coordinate closely with production and safety teams. My objective is higher availability, lower unplanned downtime, safe operation and continuous improvement.”
Calculators & guides for this module
UPS Systems
UPS topologies, UPS vs inverter, battery backup estimation and bypass systems.
Q101What is a UPS? Explain its working principle and major components.
Definition
A UPS (uninterruptible power supply) provides continuous power to critical loads even when the normal AC supply fails or becomes abnormal.
Sequence
Main components
- Rectifier/charger
- DC bus
- Battery bank
- Inverter
- Static bypass
- Maintenance bypass
- Control and protection system
Maintenance focus
Battery voltage and health, DC bus, rectifier, inverter, cooling fans, bypass operation, alarm history and output voltage/frequency.
Model interview answer
“An online (double-conversion) UPS continuously supplies the critical load through its inverter. Normally the rectifier converts AC to DC and charges the battery while the inverter feeds the load. On mains failure the battery immediately supports the DC bus and the inverter continues without interruption. If the inverter faults or is overloaded, the static bypass transfers the load to the bypass source.”
Q102Explain online, offline and line-interactive UPS. How does a UPS differ from an inverter?
Definition
A UPS is designed for uninterrupted, conditioned power to critical loads. An inverter (in the home/backup sense) mainly converts battery DC to AC during an outage.
| Feature | Online (double-conversion) | Line-interactive | Offline (standby) | Home/backup inverter |
|---|---|---|---|---|
| Normal load path | Through the inverter continuously | Mains, with AVR tap-changing | Mains | Mains via changeover relay (typical) |
| Transfer on mains failure | No transfer — zero break | Short transfer (a few ms) | Short transfer (typically a few ms) | Longer changeover, design-dependent |
| Voltage conditioning | Excellent | Moderate | Limited | Depends on model |
| Typical use | PLC, servers, instrumentation | Small IT and office loads | Non-critical PCs | Homes, general backup loads |
Model interview answer
“The main difference is architecture and application. An online UPS continuously powers the load through its inverter, so it suits sensitive PLC, automation and instrumentation loads. A conventional inverter mainly provides backup conversion from battery DC to AC and may not give the same power conditioning or seamless transfer.”
Q103How do you calculate UPS battery backup time? What affects it?
Formula
Backup (h) ≈ (N × V_battery × Ah × η_inverter × DoD) / Load (W)N = number of batteries in the string, DoD = usable depth of discharge. This is a first estimate; high discharge rates reduce effective Ah (Peukert effect).
Factors
- Load in watts
- Battery voltage and Ah
- Battery age and condition
- Temperature
- UPS/inverter efficiency
- Discharge rate
- Number and configuration of batteries
- End-of-discharge voltage setting
Model interview answer
“I calculate theoretical backup from battery energy and load, but actual backup is lower because of inverter efficiency, ageing, temperature and discharge characteristics. For accurate figures I use the battery manufacturer's constant-power discharge tables. During maintenance I check individual battery voltage, terminal condition, temperature and overall bank health.”
Q104What are UPS bypass systems? Explain static and maintenance bypass.
Definition
A bypass provides an alternative path to supply the critical load without passing through the UPS inverter.
Types
- Static bypass — automatic electronic transfer
- Maintenance (manual) bypass — isolates the UPS for servicing
Important
Before operating any bypass, verify synchronisation, source availability, load condition and the approved switching procedure — a wrong switching sequence can drop the critical load.
Model interview answer
“Static bypass is an automatic electronic transfer path used during UPS overload, inverter failure or certain abnormal conditions. Maintenance bypass provides a safe alternative supply so the UPS can be isolated for maintenance while the load stays energised, where the system architecture allows.”
Calculators & guides for this module
High-Voltage Systems (11 kV / 33 kV / 66 kV)
Why distribution uses higher voltages, current at different voltage levels, 11 kV protection and commissioning.
Q105Why are 11 kV, 33 kV and 66 kV used in distribution? Why '11' kV and not 10 kV?
Definition
11 kV, 22 kV, 33 kV, 66 kV, 132 kV and 220 kV are standardised nominal voltages used in India, the UK and many Commonwealth-influenced networks. Other regions use different levels — for example 10/20 kV in much of continental Europe and 12.47/13.8/34.5 kV in North America.
Formula
P = √3 × V × I × PF
For the same power: I ∝ 1 / V
Conductor loss: P_loss = 3 × I² × R
Classic explanation: sending-end V ≈ 1.1 × receiving-end V (10 kV × 1.1 = 11 kV)Raising voltage reduces current for the same power, which reduces I²R losses (by the square of the current ratio) and voltage drop.
| Receiving end (nominal) | 10 kV | 20 kV | 30 kV | 60 kV | 120 kV | 200 kV |
|---|---|---|---|---|---|---|
| Sending end (+10%) | 11 kV | 22 kV | 33 kV | 66 kV | 132 kV | 220 kV |
Points to cover
- The widely taught explanation: lines were designed so that roughly 10% extra voltage at the sending end covers the voltage drop along the line, so a line delivering about 10 kV is sent at 11 kV. The same pattern gives 22, 33, 66, 132 and 220 kV.
- Why about 10%: voltage drop varies with load current, so a round allowance was chosen for typical line lengths and loading — not an exact calculated figure.
- It is not the form factor (1.11): 10 kV × 1.11 = 11.1 kV and 120 kV × 1.11 = 133.2 kV, which do not match the real 11 kV and 132 kV levels. Form factor (RMS ÷ average of a sine wave) is unrelated.
- Not every level is a multiple of 11: 400 kV, 765 kV and 800 kV are not. These EHV levels came from later standard voltage series, and such networks hold voltage with reactive compensation and voltage control rather than a fixed 10% margin.
- The same idea shows up at LV: distribution transformer secondaries are often rated slightly above the nominal system voltage at no load (for example 433 V on a 415 V system) to allow for regulation under load.
- Why the levels stayed: once breakers, transformers, insulators and cables were standardised around these ratings, adding new levels (44, 55, 77 kV…) would add cost and complexity without much benefit.
Interview tip
Present the 10% voltage-drop reason as the classic explanation, then show depth by rejecting the form-factor myth and noting that 400/765 kV and non-Commonwealth levels such as 10/20 kV or 13.8 kV don't follow the pattern. Actual standard voltages are listed in IEC 60038 and national standards.
Model interview answer
“11 kV is a standardised distribution voltage. The commonly accepted reason for the '11' is that about 10% extra voltage is sent to cover line voltage drop, so a nominal 10 kV at the receiving end is sent at 11 kV — the same logic gives 22, 33, 66, 132 and 220 kV. It is not related to the 1.11 form factor, and it doesn't apply to EHV levels like 400 or 765 kV, where voltage is controlled with compensation. Higher voltage is used for the same power because current falls, which reduces I²R loss and voltage drop.”
Q106Calculate current at 11 kV, 33 kV and 66 kV for the same power.
Formula
I = P / (√3 × V × PF)
10 MW, PF = 1 at 11 kV: I ≈ 525 A
at 33 kV: I ≈ 175 A
at 66 kV: I ≈ 87.5 ACheck: 10,000,000 ÷ (1.732 × 11,000) = 524.9 A. At PF 0.9 each current rises by a factor of 1/0.9.
Maintenance focus
Transformer ratio, HV insulation, protection, cable terminations, VCB, CT/PT and earthing.
Model interview answer
“The same power needs about one-third of the current when voltage rises from 11 kV to 33 kV, and about one-sixth at 66 kV, at the same power factor. Because loss is proportional to I², going from 11 kV to 33 kV cuts conductor loss to about one-ninth for the same conductor.”
Q107Explain an 11 kV system, its equipment and protection arrangement.
Sequence
Major equipment
- Power transformer
- VCB
- CT/PT
- Protection relay
- Busbar
- 11 kV cable
- Surge arrester
- Earthing system
Protection
Overcurrent and earth fault (including sensitive/restricted earth fault), transformer differential where applicable, Buchholz and temperature for transformers, and under/over-voltage where required.
Model interview answer
“On an 11 kV feeder fault I expect the VCB to trip via the protection relay. I read relay indications and event records first, then check the cable, termination, transformer or connected load depending on the fault location. Before re-energisation, insulation and other required tests must be completed.”
Q108How do you test and commission an 11 kV cable or transformer before energisation?
Cable checks
Visual inspection, continuity and phase identification, insulation resistance, sheath test where applicable, termination inspection, earth continuity and the specified HV withstand test (e.g. VLF), per the applicable standard and cable manufacturer.
Transformer checks
IR, winding resistance, turns ratio, vector group, magnetising current, oil tests where applicable, protection and alarm checks, earthing, tap position and functional checks.
Model interview answer
“I follow the approved commissioning checklist and the applicable standard/OEM requirements. I verify mechanical condition, insulation, connections, earthing, protection and functional operation. Only after satisfactory results and formal authorisation do I proceed with controlled energisation.”
Calculators & guides for this module
DG Sets (Diesel Generators)
DG working principle, AVR vs governor, synchronisation, load sharing, reverse power and troubleshooting.
Q109Explain the working principle of a DG set.
Definition
A DG set converts chemical energy in diesel fuel into mechanical energy in the engine, and the alternator converts that into electrical energy.
Sequence
Major components
- Diesel engine
- Alternator
- AVR
- Governor
- Fuel system
- Cooling system
- Lubrication system
- Starting battery
- Control panel
- Protection system
Model interview answer
“The engine provides torque to rotate the alternator. The AVR controls excitation and therefore voltage, and the governor controls engine speed and therefore frequency. I monitor voltage, frequency, oil pressure, coolant temperature, battery condition and load.”
Q110What are an AVR and a governor? How do they differ?
Formula
f = P × N / 120 (from Ns = 120f / P)Example: a 4-pole alternator runs at 1500 rpm for 50 Hz or 1800 rpm for 60 Hz.
AVR (automatic voltage regulator)
Controls alternator excitation to hold output voltage.
Governor
Controls fuel to the engine to hold speed, which sets frequency.
Model interview answer
“If DG voltage is unstable I investigate the AVR and excitation system. If frequency is unstable I investigate engine speed, governor and load response. I don't mix up the two because they control different parameters.”
Q111What is DG synchronisation? What must match before paralleling?
Definition
Synchronisation connects a generator to a live bus after matching its electrical conditions to the bus.
Conditions
- Voltage magnitude
- Frequency (incoming set typically slightly faster so it picks up load)
- Phase sequence
- Phase angle
- Synchronising permissive/interlock (check-sync)
Model interview answer
“Before closing the DG breaker onto a live bus, voltage, frequency, phase sequence and phase angle must be within limits. After synchronisation, active power (kW) is controlled mainly through the governor/load-sharing system and reactive power (kVAR) through excitation/AVR.”
Q112What are DG load sharing and reverse power protection?
Load sharing
When generators run in parallel, total kW and kVAR are shared between them — kW via governors (droop or isochronous load sharing) and kVAR via AVRs (droop or VAR sharing).
Reverse power
Power flowing from the bus into a generator, so it runs as a motor driving its own engine.
Possible causes of reverse power
Engine fuel starvation, governor problems, loss of prime-mover power or incorrect load-sharing operation.
Model interview answer
“During parallel operation the governor/load-sharing system controls real power sharing and excitation controls reactive power sharing. Reverse power protection trips the generator breaker to stop a generator being motored if its prime mover loses power.”
Q113A DG has low voltage, unstable frequency, high temperature or low oil pressure. How will you troubleshoot it?
Low voltage
AVR, excitation, sensing connections, alternator condition, load and engine underspeed.
Frequency problem
Governor, engine speed, fuel system and sudden load changes.
High temperature
Overload, coolant level, radiator, fan and belt, airflow and ambient temperature.
Low oil pressure
Oil level, filter, pump, leaks, engine condition and the pressure sensor itself.
Model interview answer
“I first identify the exact alarm or trip and record operating parameters. I separate electrical faults from engine faults. I never bypass low-oil-pressure or high-temperature protection to keep the DG running, because those protections prevent major engine damage.”
Q114What DG protections are important? How do you troubleshoot a DG starting failure?
Sequence
Important protections
- Over/under voltage
- Over/under frequency
- Overcurrent and short circuit
- Earth fault
- Reverse power
- Low oil pressure
- High coolant temperature
- Overspeed
- Emergency stop
Model interview answer
“If the DG doesn't start, I check battery voltage and terminals, starter operation, emergency-stop status, fuel availability, control fuses and starting permissives. Then I check whether the engine cranks but doesn't fire, or doesn't crank at all — that immediately separates the fault category.”
Calculators & guides for this module
Boiler — Electrical & Control
Boiler electrical equipment, draft fans, feed pumps, interlocks and three-element drum-level control.
Q115Explain the working principle of an industrial boiler and its major electrical equipment.
Definition
A boiler converts water into steam using heat from fuel combustion or another heat source.
Major electrical/automation equipment
- FD fan
- ID fan
- PA fan where applicable
- Feed-water pump
- Burner and ignition system
- Motors with VFDs/starters
- PLC/BMS and HMI
- Level, pressure and temperature transmitters
- Solenoid valves
- Safety interlocks and flame scanner
Model interview answer
“From an electrical perspective I focus on reliable operation of fans, feed pumps, burner systems, VFDs, PLC I/O and instrumentation. Boiler operation is heavily interlocked, so electrical and instrumentation reliability directly affects plant safety.”
Q116Explain ID fan, FD fan, PA fan and feed-water pump. What electrical problems occur?
Common electrical problems
- Motor overload
- Bearing failure
- VFD fault
- Single phasing
- Cable fault
- Contactor/ACB/MCCB trip
- Voltage imbalance
FD fan
Forced draft — supplies combustion air.
ID fan
Induced draft — extracts flue gas and maintains furnace draft.
PA fan
Primary air — carries/dries fuel where the firing system needs it (e.g. pulverised coal, some fluidised-bed boilers).
Feed-water pump
Supplies water to the boiler drum against drum pressure.
Model interview answer
“If an ID fan motor trips, I check current, voltage, VFD/breaker status, bearings, damper position, fan vibration and mechanical load. For a feed-water pump I also check process conditions such as suction and discharge pressure, and whether the pump is mechanically loaded correctly.”
Q117What are boiler interlocks and trips? Why must they never be bypassed?
Definition
Interlocks allow equipment to operate only when safe conditions are satisfied. Trips shut down equipment or the process when a dangerous condition occurs.
Examples
- Low drum water level
- High steam pressure
- Flame failure
- Fan failure
- Feed-water failure
- High temperature
- Emergency stop
- Purge not complete
Model interview answer
“Boiler interlocks are safety-critical. If a permissive is missing, I find the actual cause through the sensor, wiring, PLC logic and actuator. I do not bypass a safety interlock to restart the boiler, because it may be preventing a dangerous condition such as a furnace explosion or low-water damage.”
Q118What is three-element drum-level control?
Definition
Three-element control uses drum level, steam flow and feed-water flow together to control drum level during changing steam demand.
Maintenance focus
Level transmitter and its density compensation, steam-flow and feed-water-flow measurement, PLC/DCS logic, control valve and signal scaling.
Model interview answer
“Single-element control responds only to drum level. Three-element control adds steam flow as a feed-forward and feed-water flow as a cascade loop, improving response to load changes and compensating for shrink and swell — where level temporarily moves the 'wrong' way during a load change.”
Calculators & guides for this module
ACB Protection & Testing
LSIG protection, breaking capacity ratings, spring charging, interlocks and injection testing.
Q119Explain ACB construction and LSIG protection.
Definition
An ACB is a high-current LV circuit breaker that uses air as the arc-interrupting medium, with main and arcing contacts, arc chutes, a stored-energy mechanism and an electronic trip unit.
L — Long time
Protects against sustained overload (inverse-time).
S — Short time
Delayed short-circuit protection that allows downstream devices to clear first (selectivity).
I — Instantaneous
Trips without intentional delay for very high fault current.
G — Ground/earth fault
Detects earth-fault current — residual or neutral-return sensing, depending on the arrangement.
Model interview answer
“Electronic trip units let protection be set according to the feeder and the coordination study. I check long-time pickup, short-time pickup and delay, instantaneous setting and earth-fault settings, keeping them within the equipment and coordination requirements.”
Q120What are ACB Icu, Ics, Icw, trip unit, spring charging, interlocking, and primary/secondary injection tests?
Maintenance checks
- Contact condition and contact resistance
- Insulation
- Trip unit
- Closing and tripping coils
- Spring mechanism
- Auxiliary contacts
- Interlocks
- Terminal torque
- Protection settings
Icu
Rated ultimate short-circuit breaking capacity (IEC 60947-2).
Ics
Rated service short-circuit breaking capacity, expressed as a percentage of Icu (25/50/75/100%); after an Ics interruption the breaker must remain fit for service.
Icw
Rated short-time withstand current for a stated duration (e.g. 1 s) — key for selectivity using short-time delay.
Trip unit
Electronic unit that senses current through CTs/sensors and commands the breaker to trip.
Spring charging
Stores mechanical energy for closing (and enables a fast open–close–open sequence).
Interlocking
Prevents unsafe operations — e.g. racking while closed, or closing two incomers in parallel.
Primary injection
Injects high current through the main primary circuit to test the whole chain: sensors, trip unit and mechanism.
Secondary injection
Injects test signals into the trip unit/relay sensing circuit to test settings and logic without high primary current.
Model interview answer
“For ACB maintenance I inspect the mechanism, contacts, arc chutes, terminals, spring charging system, trip and closing coils and the electronic trip unit. I perform contact resistance, insulation and protection tests as applicable. Primary injection verifies the complete protection path, while secondary injection is useful for testing trip-unit settings and logic.”
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
What is the difference between an AVR and a governor on a DG set? +
The AVR controls alternator excitation and therefore output voltage and reactive power sharing. The governor controls engine fuel and therefore speed, which sets frequency and real power sharing. Voltage problems point to the AVR/excitation system, while frequency problems point to the governor, fuel system or load changes.
Why is 11 kV used instead of 10 kV? +
The classic explanation is that about 10% extra voltage is sent to cover the voltage drop along the line, so a nominal 10 kV at the receiving end is sent at 11 kV; the same pattern gives 22, 33, 66, 132 and 220 kV. It is not linked to the 1.11 form factor of a sine wave, and it does not apply to EHV levels such as 400 kV or 765 kV, which use reactive compensation instead. These levels are standard in India, the UK and many other networks, while other regions use values such as 10 kV or 20 kV in parts of Europe and 13.8 kV in North America.
What is the difference between MTBF and MTTR? +
MTBF (mean time between failures) is total operating time divided by the number of failures and reflects reliability. MTTR (mean time to repair) is total repair time divided by the number of repairs and reflects maintainability. Inherent availability can be estimated as MTBF divided by the sum of MTBF and MTTR.
What is the difference between an online and an offline UPS? +
An online (double-conversion) UPS feeds the load through its inverter all the time, so there is no transfer when the mains fails and the output is continuously conditioned. An offline (standby) UPS feeds the load from the mains and switches to its inverter only when the mains fails, with a short transfer time. Online UPS systems are preferred for PLCs, servers and instrumentation.
Why must boiler interlocks never be bypassed? +
Boiler interlocks and trips stop the burner or the boiler when a dangerous condition exists, such as low drum level, flame failure, incomplete purge or loss of a draft fan. Bypassing one can allow a furnace explosion or low-water damage. The correct approach is to trace the missing permissive through the sensor, wiring, PLC logic and actuator and fix the actual cause.
More Electrical Interview Questions
This is Part III 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 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.