Electrical Maintenance Supervisor Interview Q/A

Q1.   Why use UPS when there is a generator?
ANSWER: UPS provides immediate backup during generator startup delays.
ANSWER: The faulty cell should be isolated and replaced while maintaining
the correct voltage.
Q3.  Why use 192 cells in UPS?
ANSWER: DC power is stable and efficient for battery storage and
UPS systems.
Q5.  How many battery cells are connected in a rectifier and why?
ANSWER: Depends on output voltage. Rectifiers convert AC to DC for
battery charging.
Q6.  Difference between an Inverter & Rectifier?
ANSWER: Rectifier: Converts AC to DC.
Inverter: Converts DC to AC.
Q7.   What is AVR and its purpose?
ANSWER: AVR stabilizes voltage fluctuations and prevents electrical
damage.
Q8.  How to calculate kilowatt-hour consumption?
ANSWER: 500kw*24*30 = 300 kw\month
Q9.  What is UPS and why use a bridge circuit in UPS?
ANSWER: UPS provides backup power; bridge circuits help in
rectification and inversion.
Q10.  What is the difference between UPS & Inverter?
ANSWER: UPS provides instant backup with no delay; inverters take a
few seconds to switch.
Q11.  How to calculate 15KW load backup in rectifier (1500Ah, 2
banks)?
ANSWER:  Check indicators for IR, ISD, IG, and II faults.
Q12.  Why is 48V used in DC?
ANSWER: 48V is a standard telecom and UPS voltage for efficiency
and safety.
Q13.  How to check battery impedance?
ANSWER: Use an impedance tester to measure resistance across
terminals.
Q14.  What is the function of ATS?
ANSWER: Automatic Transfer Switch shifts load from the main supply
to the generator.
Q15.  What is the difference between AMF and ATS?
ANSWER: AMF (Auto Mains Failure) starts the generator; ATS switches
load automatically.
Q16.  What is the working principle of a generator?
ANSWER: Converts mechanical energy into electrical energy via
electromagnetic induction.
Q17.  How to maintain a generator?
ANSWER: Regular oil changes, battery checks, and load testing.
Q18.  What is free cooling in data centers?
ANSWER: Uses outside air for cooling instead of air conditioning.
Q19.  What is the role of a capacitor panel?
ANSWER: Improves power factor and reduces reactive power losses.
Q20.  Difference between MCB and MCCB?
ANSWER: MCB is for low-current protection, MCCB for higher loads
and adjustable trips.
Q21.  How does HVAC function in fire incidents?
ANSWER: Fire suppression shuts HVAC off, and emergency
ventilation activates.
Q22.  What is the function of a transformer?
ANSWER: Steps up/down voltage for transmission and distribution.
Q23.  What is the difference between a dry-type and oil-type
transformer?
ANSWER: Dry-type uses air cooling, oil-type uses oil for insulation and
cooling.
Q24.  What is the function of a capacitor bank?
ANSWER: Stores electrical energy and improves voltage regulation.
Q25.  What is a fire addressable system?
ANSWER: A fire alarm system that can identify the exact location of an
activated detector or device.
Q26.  What are transformer protection systems?
ANSWER: Include Buchholz relay, differential protection, overcurrent,
and temperature monitoring systems.
Q27.  What is AVR?
ANSWER: Automatic Voltage Regulator (AVR) stabilizes voltage
fluctuations to ensure safe and consistent power supply.
Q28.  What is the capacity of your data center?
Data center capacity is typically measured in power (kW),
cooling capacity, and available rack space.
Q29.  What is VFI?
ANSWER: Voltage and Frequency Independent (VFI) is a mode in UPS
systems where output remains stable regardless of input
variations.
Q30.  What is digital input and analog input?
ANSWER: Digital input: Signals that exist in two states (on/off, 0/1).
Analog input: Continuous signals that vary over a range, such
 as temperature or voltage levels.
Q31.  What is the VESDA system?
ANSWER: Very Early Smoke Detection Apparatus (VESDA) is an advanced
air sampling system that detects smoke particles before a fire
develops.
Q32.  What is the difference between CRAC and CRAH?
ANSWER: CRAC (Computer Room Air Conditioner) uses refrigerants,
while CRAH (Computer Room Air Handler) uses chilled water
for cooling data centers.
Q33.  What is an air-cooled chiller and water-cooled chiller?
ANSWER: Air-cooled chiller dissipates heat into the air, while water-
cooled chiller uses water for heat removal, requiring a cooling
tower.
Q34.  What is the function of surge arrestor?
ANSWER: Protects electrical equipment from voltage spikes and transients.
Q35.  What are the components of VFD?
ANSWER:  Rectifier, DC bus, inverter, and control circuit.
Q36.  What are the main generator faults?
ANSWER: Overload, overheating, oil leakage, and voltage fluctuation.
Q37.  How to check generator battery voltage and rating?
ANSWER: Use a multimeter to measure the voltage; rating is specified by
manufacturer.
Q38. How to design UPS capacity?
ANSWER: Determine load requirement, runtime, efficiency, and safety
factors.
Q39.  How does an ATS function?
ANSWER: Automatically switches power between primary and backup
sources.
Q40.  What is the working principle of a generator?
ANSWER: Converts mechanical energy into electrical energy using
electromagnetic induction.
Q41.  What is a double knock pre-action system?
ANSWER: Requires two detection signals before water release to prevent
false alarms.
Q42.  What is a fire addressable system?
ANSWER: A fire alarm system that identifies the exact location of an
activated device.
Q43.  What is the role of a capacitor panel?
ANSWER: Improves power factor and reduces reactive power losses.
Q44.  How do you check the thermal imaging in LV panels?
ANSWER: Use an infrared thermal camera to detect hotspots.
Q45.  What is the function of an ELCB?
ANSWER: Detects leakage current and prevents electric shock.
Q46.  What is the difference between AMF and ATS?
ANSWER: AMF starts the generator automatically, ATS transfers load
between sources.
Q47.  What is VESDA system?
ANSWER: Very Early Smoke Detection Apparatus detects smoke particles
before visible fire.
Q48.  What is double conversion in UPS?
ANSWER: Converts AC to DC, then DC back to AC for a stable output.
Q49.  What is a fire suppression system?
ANSWER: FM200, CO2, water sprinklers, and dry chemical systems to
extinguish fires.
Q50.  What is a transformer protection system?
ANSWER: Includes Buchholz relay, differential protection, overcurrent, and
temperature monitoring.
Q51.  How to automatically start a generator when main supply fails?
ANSWER: Use an Automatic Mains Failure (AMF) panel to detect failure and
start the generator.
Q52.  What is the function of a capacitor bank?
ANSWER: Improves power factor and reduces load on electrical systems.
Q53.  What is the difference between dry-type and oil-type transformers?
ANSWER: Dry-type uses air cooling, oil-type uses oil for insulation and cooling.
Q54.  What is an ATS panel?
ANSWER: Automatic Transfer Switch panel shifts power supply between mains
and backup generators.
Q55.  What is the role of AVR in a generator?
ANSWER: Automatic Voltage Regulator maintains voltage stability and prevents
fluctuations.
Q56.  What is a circuit breaker and its types?
ANSWER: A circuit breaker interrupts excessive current flow to protect
equipment.
Types: MCB, MCCB, ACB, VCB, SF6, RCCB.
Q57.  What is an MCCB and how is it different from MCB?
ANSWER: MCCB (Molded Case Circuit Breaker) handles higher currents and is
adjustable.
MCB (Miniature Circuit Breaker) is used for lower currents and is non-
adjustable.
Q58.  What is the function of a busbar in an electrical panel?
ANSWER: Conducts and distributes electrical power within a switchboard or
substation
Q59.  What is the difference between a fuse and a circuit breaker?
ANSWER: A fuse melts under excessive current; a circuit breaker trips and can
be reset.
Q60.  What is an SMPS?
ANSWER: Switched Mode Power Supply converts AC to regulated DC efficiently.
Q61.  How does a generator synchronize with the main supply?
ANSWER: Through phase matching, voltage regulation, and frequency
adjustment.
Q62.  What are the components of a diesel generator?
ANSWER: Engine, alternator, fuel system, cooling system, control panel.
Q63.  What is the function of an isolation transformer?
ANSWER: Separates primary and secondary circuits to improve safety and
reduce noise.
Q64.  What is a relay and its types?
ANSWER: A relay is an electrically operated switch.
Types: Electromagnetic, solid-state, thermal, differential
Q65.  What are the common UPS faults?
ANSWER: Battery failure, overload, overheating, rectifier failure.
Q66.  What is the purpose of earthing in electrical systems?
ANSWER: Prevents electrical shock and protects equipment from surges.
Q67.  What is an IP rating in electrical enclosures?
ANSWER: Ingress Protection rating defines resistance to dust and water.
Q68.  What is the difference between primary and secondary distribution?
ANSWER: Primary: High-voltage transmission to substations.
Secondary: Distribution from substations to consumers.
Q69.  How do you size a generator for a building?
ANSWER: Based on total load demand, peak power usage, and efficiency.
Q70.  What is a harmonic filter in electrical systems?
ANSWER: Reduces electrical harmonics that can affect performance.
Q71.  How does a UPS protect against power surges?
ANSWER: Uses surge suppressors and voltage regulation to absorb spikes.
Q72.  What is the efficiency of a UPS system?
ANSWER: Typically, 90-98% depending on type and operation mode.
Q73.  What are the types of UPS topologies?
ANSWER: Offline, Line-interactive, Online Double Conversion.
Q74.  What is meant by power factor correction?
ANSWER: Improving power factor to reduce reactive power and improve
efficiency.
Q75.  What is the role of a synchronizing panel?
ANSWER: Ensures multiple generators operate in sync with equal load sharing.
Q76.  What is transient voltage and how is it mitigated?
ANSWER: Temporary voltage spike managed by surge protectors and filters.
Q77.  What is a residual current device (RCD)?
ANSWER: Detects leakage current and trips to prevent electric shocks.
Q78.  How to prevent UPS overheating?
ANSWER: Proper ventilation, cooling fans, load management.
Q79.  What is meant by black start capability?
ANSWER: Ability of a generator to start without external power.
Q80.  How does an automatic fire suppression system work?
ANSWER: Detects fire and activates extinguishing agents automatically.
Q81.  What is an energy meter and its function?
ANSWER: Measures electrical energy consumption in kWh.
Q82.  How to maintain a generator?
ANSWER: Regular oil changes, battery checks, and load testing.
Q83.  What is free cooling in a data center?
ANSWER: Uses outside air or water to cool IT equipment and reduce energy
consumption.
Q84.  What is double conversion in UPS?
ANSWER: Converts AC to DC and then back to AC to ensure a stable output.
Q85. How to isolate three UPS if all are working in parallel?
ANSWER: Fuse: Protects circuits by melting under excessive current.
Q86.  What is an air-cooled chiller and a water-cooled chiller?
ANSWER: Air-cooled: Uses ambient air for heat dissipation.
Water-cooled: Uses water and requires a cooling tower.
Q87.  What is an integrated BMS system with electrical components?
ANSWER: A Building Management System that controls HVAC, lighting,
security, and electrical systems.
Q88.  Difference between MCB and MCCB?
ANSWER: MCB: Used for low current protection.
MCCB: Handles higher currents and is adjustable.
Q89.  What is SLD (Single Line Diagram)?
ANSWER: A simplified diagram showing an electrical system’s components
and connections.
Q90.  What is transformer protection?
ANSWER: Includes Buchholz relay, differential protection, overcurrent, and
temperature monitoring.
Q91.  What is AVR?
ANSWER: Automatic Voltage Regulator stabilizes voltage output to ensure a
steady power supply.
Q92.  What type of fire system is used at a site?
ANSWER: Common types include FM200, CO2, water sprinklers, and dry
chemical systems.
Q93.  What is a dry-type transformer and an oil-type transformer?
ANSWER: Dry-type: Uses air for cooling.
Oil-type: Uses insulating oil for cooling and insulation.

Q2.  How to bypass a faulty battery cell in UPS?

ANSWER: The number of battery cells ensures the required DC voltage for

stable operation. 

Q4.  Why use DC?