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# NETA 2 Exam Questions - Circuit Breakers and Fault Currents

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May 2017
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## NETA 2 Exam Questions - Circuit Breakers and Fault Currents

I came across a few questions today that I haven't seen in the past and honestly had me completely stumped. Any input would be great.

A power factor test was performed on a medium-voltage oil circuit breaker. The watt loss sum of the two open-breaker tests for the A-phase was 20 milliwatts. The closed-breaker test watts loss was 22 milliwatts. What is the tank loss index (TLI) for this phase?

What is the expected contact-resistance measurement for a 4000A 480V air circuit breaker in good condition?
- This one kind of threw me, in my experience every breaker is different. I can honestly say no-one has taught me that there is a designated value.

A three-phase bolted fault would primarily produce which type of current?
- Not even sure what the question is asking

2. Originally Posted by adamstraub
What is the expected contact-resistance measurement for a 4000A 480V air circuit breaker in good condition?
- This one kind of threw me, in my experience every breaker is different. I can honestly say no-one has taught me that there is a designated value.
You can usually find recommended values in circuit breaker instruction books, and if you compare them, you will find that they are all pretty similar. Check out this thread for more info: https://testguy.net/threads/1369-Exp...ker-frame-size

3. Originally Posted by adamstraub
A power factor test was performed on a medium-voltage oil circuit breaker. The watt loss sum of the two open-breaker tests for the A-phase was 20 milliwatts. The closed-breaker test watts loss was 22 milliwatts. What is the tank loss index (TLI) for this phase?
TLI = (closed breaker test in Watts) - (sum of open breaker losses in Watts)

See this document for more info on circuit breaker power factor:

https://www.omicronenergy.com/downlo...E-0A78861B762F

4. Originally Posted by adamstraub
A three-phase bolted fault would primarily produce which type of current?
- Not even sure what the question is asking
Read up on symmetrical components:

https://en.m.wikipedia.org/wiki/Symmetrical_components

https://www.gegridsolutions.com/smar...ymmetrical.pdf