NETA Circuit Breaker Maintenance 5 — Questions and Answers
Question 1: A sulfur hexafluoride (SF6) circuit breaker shows a low gas density alarm. According to NETA procedures, the FIRST action the technician should take is:
- Immediately operate the breaker to verify functionality
- Lock out the breaker and verify gas pressure with a calibrated gauge (Correct answer)
- Add SF6 gas to reach rated pressure, then return to service
- Perform a contact resistance test to determine if operation is safe
Correct answer: Lock out the breaker and verify gas pressure with a calibrated gauge
A low SF6 density alarm is a safety-critical condition; the breaker must be locked out and the gas pressure verified with a calibrated gauge before any further action.
Question 2: What does the 'close-open' (CO) operation rating on a circuit breaker signify?
- The breaker can close and immediately reopen on a fault without damage (Correct answer)
- The total number of mechanical operations before overhaul
- The rated current the breaker can interrupt at full voltage
- The maximum time the breaker can remain closed under overload
Correct answer: The breaker can close and immediately reopen on a fault without damage
A CO rating certifies that the breaker can close onto a fault and immediately reopen (trip), completing a full duty cycle without sustaining damage that impairs its ratings.
Question 3: When testing a molded-case circuit breaker's long-time delay element with a 300% rated current injection, approximately how long should the breaker trip for a properly functioning thermal-magnetic breaker?
- Instantaneously (under 1 cycle)
- 2 to 30 seconds (Correct answer)
- 1 to 5 minutes
- More than 10 minutes
Correct answer: 2 to 30 seconds
At 300% of rated current, a properly calibrated thermal-magnetic MCCB typically trips in 2 to 30 seconds through the thermal bimetal element, per NETA MTS acceptance criteria.
Question 4: Primary injection testing differs from secondary injection testing of a protective relay-circuit breaker system in that primary injection:
- Tests only the relay electronics without the CT circuit
- Injects current through the actual primary conductors and CTs (Correct answer)
- Applies voltage directly to the trip coil terminals
- Simulates faults using relay test set signal outputs only
Correct answer: Injects current through the actual primary conductors and CTs
Primary injection passes test current through the actual primary conductors and current transformers, verifying the entire protection scheme including CTs, wiring, relay, and breaker trip coil.
Question 5: According to NETA MTS, what minimum insulation resistance value (in megohms) is generally acceptable for a 480 V switchgear bus section at ambient temperature?
- 1 MΩ
- 10 MΩ
- 100 MΩ (Correct answer)
- 1000 MΩ
Correct answer: 100 MΩ
NETA MTS Table 100.1 specifies a minimum acceptable insulation resistance of 100 MΩ for 480 V class equipment tested with a 1000 V DC megohmmeter.
Question 6: The purpose of performing a 'minimum pickup' test on a circuit breaker's shunt trip coil is to:
- Verify the coil will operate at the lowest acceptable control voltage (Correct answer)
- Measure the coil's DC resistance for comparison to nameplate
- Confirm the coil does not operate above rated voltage
- Check the coil insulation resistance to ground
Correct answer: Verify the coil will operate at the lowest acceptable control voltage
Minimum pickup testing confirms the shunt trip coil will reliably operate the breaker at the lowest expected control voltage (typically 70% of rated), ensuring tripping under degraded battery conditions.
Question 7: During visual inspection of an oil circuit breaker, a technician observes the oil has turned dark brown with a burnt odor. This MOST likely indicates:
- Normal oil aging requiring a routine oil change on schedule
- The oil has experienced arc decomposition and requires immediate replacement and analysis (Correct answer)
- Contamination from external moisture ingress
- A chemical reaction with the tank gasket material
Correct answer: The oil has experienced arc decomposition and requires immediate replacement and analysis
Dark, burnt-smelling oil in an OCB indicates arc decomposition products are present, which reduces dielectric strength; the oil must be replaced immediately and analyzed before returning the breaker to service.
NETA Circuit Breaker Maintenance 5
A sulfur hexafluoride (SF6) circuit breaker shows a low gas density alarm.
According to NETA procedures, the FIRST action the technician should take is: