EGSA System Troubleshooting and Maintenance 4 — Questions and Answers
Question 1: A three-phase generator produces balanced voltage at no load, but voltage becomes severely unbalanced under load. What is the most probable cause?
- Incorrect fuel injection timing
- Single-phasing due to an open stator winding or blown phase fuse (Correct answer)
- Incorrect oil viscosity
- Faulty air filter restriction indicator
Correct answer: Single-phasing due to an open stator winding or blown phase fuse
An open stator winding or blown phase fuse causes one phase to lose voltage under load, creating severe three-phase imbalance.
Question 2: What does the acronym MTBF represent, and why is it significant in generator maintenance planning?
- Maximum Time Between Faults — limits warranty claims
- Mean Time Between Failures — used to predict maintenance intervals and reliability (Correct answer)
- Minimum Transfer Bus Frequency — sets paralleling thresholds
- Master Terminal Block Function — defines wiring hierarchy
Correct answer: Mean Time Between Failures — used to predict maintenance intervals and reliability
MTBF (Mean Time Between Failures) is a reliability metric used to schedule preventive maintenance and predict when components are likely to fail.
Question 3: Which procedure should be performed before removing a high-voltage generator set from service for internal inspection?
- Run the engine at full load for 30 minutes
- Perform a LOTO (Lockout/Tagout) procedure on all energy sources (Correct answer)
- Increase coolant temperature to purge system
- Verify oil pressure is above minimum
Correct answer: Perform a LOTO (Lockout/Tagout) procedure on all energy sources
Lockout/Tagout (LOTO) must be applied to all energy sources before internal inspection to prevent accidental energization.
Question 4: A generator's starting battery consistently shows low voltage after a week of standby. The battery charger output is verified correct. What is the most likely cause?
- Low engine oil level draining the battery
- A parasitic load or leakage current discharging the battery between tests (Correct answer)
- Incorrect coolant mixture
- Worn fan belt causing charging failure
Correct answer: A parasitic load or leakage current discharging the battery between tests
A parasitic drain or leakage current not related to the charger circuit will slowly discharge the battery between charge events.
Question 5: What is the function of a diesel engine's prelube system in a generator application?
- Filters fuel before injection
- Supplies oil pressure to engine bearings before cranking to reduce startup wear (Correct answer)
- Regulates coolant flow during warmup
- Controls exhaust backpressure
Correct answer: Supplies oil pressure to engine bearings before cranking to reduce startup wear
A prelube system circulates oil through bearings before engine cranking, preventing the dry-start wear that accounts for most mechanical failures.
Question 6: When testing a generator's voltage regulator response to a sudden load application, the acceptable transient voltage dip and recovery time are typically specified in which document?
- The transfer switch installation manual
- The generator manufacturer's technical specification or NEMA MG-1 standards (Correct answer)
- The building electrical permit
- The engine coolant specification sheet
Correct answer: The generator manufacturer's technical specification or NEMA MG-1 standards
Generator transient response limits for voltage dip and recovery are defined in the manufacturer's technical specification and NEMA MG-1 standards.
Question 7: A diesel generator experiences a sudden loss of load due to a breaker trip. The resulting overspeed causes a high-RPM shutdown. What component is primarily responsible for preventing overspeed?
- The automatic voltage regulator
- The engine governor and overspeed protection relay (Correct answer)
- The coolant thermostat
- The oil pressure switch
Correct answer: The engine governor and overspeed protection relay
The engine governor controls fuel to limit speed, and an overspeed relay provides backup shutdown protection if governor response is insufficient.
EGSA System Troubleshooting and Maintenance 4
A three-phase generator produces balanced voltage at no load, but voltage becomes severely unbalanced under load.
What is the most probable cause?