Performance of MCB instantaneous trip when tested single-phase and 3-p
Performance of MCB instantaneous trip when tested single-phase and 3-p
(OP)
Let's say I test a molded case circuit breaker instantaneous trip two ways:
1 - apply current to phase A and check current at which trip occurs. Repeat for B and C. Record the lowest of the three as the "trip point".
2 - Wire thee phases in series and apply test current and record trip point.
Is there any reason we would expect method 2 to give lower trip point then method 1?
1 - apply current to phase A and check current at which trip occurs. Repeat for B and C. Record the lowest of the three as the "trip point".
2 - Wire thee phases in series and apply test current and record trip point.
Is there any reason we would expect method 2 to give lower trip point then method 1?
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RE: Performance of MCB instantaneous trip when tested single-phase and 3-p
Incidentally, in IEC parlance a Moulded Case Circuit Breaker is an MCCB, not an MCB. An MCB is an entirely different device, the minature circuit breaker. Is the terminology different in the US?
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RE: Performance of MCB instantaneous trip when tested single-phase and 3-p
However, depending on how you do the testing, it may be difficult to be sure it tripped on the magnetic element rather than the thermal. If you increase the current too slowly, it may trip on thermal rather than instantaneous. If that is the case, the three-phase fault may trip sooner due to the increased heat within the breaker.
For electronic trip, I'm not sure, but in theory the results should be the same.
Also, at very high levels of fault current, the three-phase fault may cause the contacts to open more quickly than the single-phase fault because of the contact design in modern breakers. The contacts have a "reverse current" configuration that generates magnetic forces that tend to force the contacts apart. This force "assists" the spring in opening the contacts. For very high current faults, the contacts can part before the breaker actually trips. So maybe a three-phase fault would tend to open a little faster if current levels were very high. But at the pickup setting, this effect would be minimal, I would guess.
Scotty: "MCCB" is used for molded case circuit breaker in the US, but probably not by everyone. "MCB" is not used much here, at least in my experience.
RE: Performance of MCB instantaneous trip when tested single-phase and 3-p
Just re-read your post and saw 'test a molded case circuit breaker instantaneous trip'. Must have replied while my fingers were in gear and my brain in freewheel.
Listen to dpc instead.
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Sometimes I only open my mouth to swap feet...
RE: Performance of MCB instantaneous trip when tested single-phase and 3-p
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RE: Performance of MCB instantaneous trip when tested single-phase and 3-p
I get the right solution answering a question I mis-read!! I was thinking more of the thermal trip behaviour when I wrote it but, yes, it would apply to the instantaneous trip test too. Thanks for the LPS - I'm almost embarrassed to accept it though!
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Sometimes I only open my mouth to swap feet...