NETA Circuit Breaker Maintenance 4 — Questions and Answers
Question 1: According to NETA MTS, what is the recommended maximum contact resistance for a 15 kV vacuum circuit breaker measured across each pole?
- 10 microhms
- 50 microhms
- 100 microhms (Correct answer)
- 200 microhms
Correct answer: 100 microhms
NETA MTS specifies that contact resistance for medium-voltage vacuum circuit breakers should not exceed 100 microhms per pole unless the manufacturer specifies otherwise.
Question 2: Which dielectric test method is used to verify the vacuum integrity of a vacuum interrupter bottle?
- DC hi-pot test across open contacts (Correct answer)
- Megohmmeter test to ground
- Power factor (dissipation factor) test
- Partial discharge measurement at rated voltage
Correct answer: DC hi-pot test across open contacts
A DC hi-pot test applied across the open contacts of a vacuum interrupter checks the vacuum integrity; a degraded vacuum will allow current to flow at lower voltages than expected.
Question 3: When performing a trip timing test on a low-voltage power circuit breaker, what parameter is being verified?
- Insulation resistance of the trip coil
- Time from trip signal initiation to contact separation (Correct answer)
- Contact bounce duration after closing
- Charging motor current draw
Correct answer: Time from trip signal initiation to contact separation
Trip timing tests measure the elapsed time from when the trip signal is applied to when the main contacts physically separate, verifying the breaker responds within manufacturer specifications.
Question 4: A power circuit breaker's charging spring fails to charge after the close-open operation. The MOST likely cause is:
- Worn contact arcing tips
- Defective anti-pump relay
- Failed spring charging motor or limit switch (Correct answer)
- Excessive contact erosion
Correct answer: Failed spring charging motor or limit switch
The spring charging motor or its associated limit switch is the most common cause of failure to recharge the closing spring after an operation.
Question 5: NETA recommends that circuit breaker insulation resistance tests be performed at what minimum test voltage for 600 V class breakers?
- 250 V DC
- 500 V DC (Correct answer)
- 1000 V DC
- 2500 V DC
Correct answer: 500 V DC
For 600 V class equipment, NETA MTS recommends a 500 V DC megohmmeter test voltage to measure insulation resistance.
Question 6: Which condition does the 'wipe' measurement on a circuit breaker describe?
- The travel distance of the contacts after initial touch during closing (Correct answer)
- The arc gap distance when contacts are fully open
- The total stroke from open to fully closed position
- The contact spring compression pre-load force
Correct answer: The travel distance of the contacts after initial touch during closing
Contact wipe (also called overtravel) is the distance the moving contact continues to travel after it first makes contact with the stationary contact, ensuring proper contact pressure.
Question 7: During a circuit breaker maintenance inspection, you find carbon tracking on the arc chute. The CORRECT action is to:
- Clean with wire brush and return to service
- Replace the arc chute assembly (Correct answer)
- Apply silicone spray to inhibit further tracking
- Measure contact resistance and accept if within tolerance
Correct answer: Replace the arc chute assembly
Carbon tracking on arc chutes creates a conductive path that can cause flashover; arc chutes with carbon tracking should be replaced, not cleaned and returned to service.
NETA Circuit Breaker Maintenance 4
According to NETA MTS, what is the recommended maximum contact resistance for a 15 kV vacuum circuit breaker measured across each pole?