NETA System Commissioning and Analysis 2 — Questions and Answers
Question 1: During transformer commissioning, what is the primary purpose of performing a turns ratio test?
- Verify insulation resistance between windings
- Confirm the transformer ratio matches nameplate data within acceptable tolerance (Correct answer)
- Measure winding resistance to detect shorted turns
- Check for partial discharge activity under load
Correct answer: Confirm the transformer ratio matches nameplate data within acceptable tolerance
A turns ratio test confirms that the transformer's actual ratio matches the nameplate rating, typically within ±0.5%, ensuring proper voltage transformation.
Question 2: When commissioning a medium-voltage switchgear assembly, which test verifies that protective relay trip functions will operate the correct breaker?
- Insulation resistance test
- Contact resistance test
- Functional trip testing (Correct answer)
- Dielectric withstand test
Correct answer: Functional trip testing
Functional trip testing injects a test signal into the relay and verifies that the associated breaker trips, confirming correct wiring and relay-breaker coordination.
Question 3: A commissioning engineer measures 0.8 power factor on a newly energized motor. Which of the following is the most likely cause?
- Motor is operating under full load
- Motor is running at partial load or no load (Correct answer)
- Insulation is degraded
- Phase rotation is incorrect
Correct answer: Motor is running at partial load or no load
Induction motors have inherently low power factor at light or no load because magnetizing current dominates, causing high reactive current relative to real power.
Question 4: According to NETA standards, what is the recommended minimum insulation resistance value per kV of operating voltage for medium-voltage cables rated 5 kV and above?
- 1 MΩ per kV
- 100 MΩ per kV
- 1,000 MΩ per kV (Correct answer)
- 10 MΩ per kV
Correct answer: 1,000 MΩ per kV
NETA ATS recommends a minimum of 1,000 MΩ per kV of operating voltage for medium-voltage cable insulation resistance at 20°C.
Question 5: During commissioning of a generator paralleling system, what test confirms that generators will share load proportionally?
- Phase rotation test
- Load bank reactive power test
- Droop characteristic test (Correct answer)
- Insulation resistance test
Correct answer: Droop characteristic test
A droop characteristic test verifies each generator's governor response, ensuring proportional real power sharing when generators are operated in parallel.
Question 6: What does an unusually high contact resistance reading on a circuit breaker main contact indicate during commissioning?
- Contacts are properly lubricated
- Contacts may be pitted, eroded, or improperly aligned (Correct answer)
- The breaker has excessive spring tension
- Insulation is compromised between phases
Correct answer: Contacts may be pitted, eroded, or improperly aligned
High contact resistance indicates poor contact surface condition such as pitting, oxidation, or misalignment, which causes excessive heating under load.
Question 7: When performing an acceptance test on a grounding electrode system, what instrument is most commonly used to measure soil resistivity?
- Megohmmeter
- Earth ground tester using the four-point Wenner method (Correct answer)
- Clamp-on ammeter
- Oscilloscope
Correct answer: Earth ground tester using the four-point Wenner method
The four-point Wenner method using an earth ground tester measures soil resistivity by injecting current between outer probes and measuring voltage between inner probes.
NETA System Commissioning and Analysis 2
During transformer commissioning, what is the primary purpose of performing a turns ratio test?