Performance of MCB instantaneous trip when tested single-phase and 3-p 1

[electricpete]

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?

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[ScottyUK]

Are these tests for traditional thermal-magnetic breakers or for those with electronic tripping units? In the former case the sensing elements are likely to be of the bimetal type using a heater which carries the load current. I can understand why for an old-fashioned design the trip point would be lower for all three phases in series, purely because the actuating force for the trip mechanism is available from all three of the sensing elements. For the single pole test the actuating force is only from one element. The variation in trip setting between phases is likely to be due to the physical design of the tripping mechanism giving some mechanical advantage to one phase relative to the others.

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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[dpc]

For a conventional thermal-mag breaker, I would not expect the results to be substantially different. (But I don't actually know for sure.) The instantaneous trip elements are separate for each phase and are mechanically linked to a common tripping mechanism that releases the spring.

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.

 

[ScottyUK]

epete,

Just re-read your post and saw [blue]'test a molded case circuit breaker instantaneous trip'[/blue]. Must have replied while my fingers were in gear and my brain in freewheel.

Listen to dpc instead.

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[electricpete]

Scotty, I think you had the right answer. All three instantaneous elements can push against the plunger. With three pushing, the force required of each may be less. Over 10 or so tests, we saw around 10% less on the three in series method than the minimum of the three individual phase tests.

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[ScottyUK]

Damn,

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...