This is not an arc flash question, but here it goes.

A LVPCB can be set or purchased without INST setting and according to IEEE C37 test, it can hold the short circuit for 30 cycles. What happens after the 30 cycles if the short circuit current is still present? Will the LVPCB open instantaneously, protecting itself like an override? Or will still remain closed after the 30 cyles?

I believe that Zog answered this question that was also posted on the eng-tips forum as:

Yes, he answered the question, but I no not think it is the right answer and I wanted to have additional opinions.

His answer was that eventually it will open, but will depend on the fault level and the trip unit installed on the LVPCB.

I was not asking about the tripping time, but if the LVPCB would have an override after the 30 cycles, and if that override is instantaneous.

I am assuming that the fault current is just below the IR of the LVPC and that the LVPC does not have INST trip, independently of the trip unit. No mater which trip unit is installed, the breaker has an override or noes does not have an override after the 30 cycles.

Thanks,

I believe that will depend somewhat on what the trip unit is on the breaker. Do you have a specific breaker and trip unit you are inquiring about?

Some breakers do have a built in overide. For example: A SQD Masterpact NW 800/1600A frame with an interrupting code of H1 has a short time withstand rating of 65kA but no built in instantaneous overide. A breaker with an interrupting code of L1 has a short time withstand rating of 30kA with a built in instantaneous override of 80kA.

Thanks for the answer.

I am not concerned about ICCB's or LVCB's but LVPCB (low voltage power circuit breaker). I believe the MasterPact is an ICCB.

The answer I look for does not depend on the type of triping unit or manufactuer, or frame of the LVPCB It is more a general behaviour of LVPCB's accroding to standard.

The IEEE C37 standard that applies to LVPCB's allow the LVPCB to NOT open in a very high short circuit current during a maximum of 30 cycles.

ICCB's and LVCB's are not tested for 30 cycles because they use another standard (UL 489).

The question is what happens if the high short circuit current (lets say just below the IR of the LVPCB) is not interrupted by any upstream or downstream device. The LVPCB will withstand this high short circuit current for 30 cycles (as tested), but what happens after the 30 cycles. Does the LVPCB has an automatic override and open instantly at the moment that the 30 cycles are exceeded? If it does not open in override instantly, the LVPCB will be exposed to a large current beyond the 30 cycle test.

Thanks.

I would think if it didn’t open and the high current was still there it would weld itself together or blow up.

Agreed. The point of any test is that the device in question will continue to function afterward. If test parameters are exceeded, all bets are off. Which standard are you referring to?

I believe this 30 cycle rating only applies to the duty which the CB can withstand before physical damage occurs. In reality it will should never get there, even though there is no instantaneous trip, the overload will trip once it gets hot enough. Although not as fast as an instantaneous element, it should occur within the 30 cycle duty. If not the CB will be damaged.

Hey,
A LVPCB can be set or purchased without INST setting and according to IEEE C37 test, it can hold the short circuit for 30 cycles. What happens after the 30 cycles if the short circuit current is still present? Will the LVPCB open instantaneously, like an override? Or will still remain closed?

The Masterpact L1 and H1 styles are both ANSI C37 Certified and UL1066 Listed. The Instantaneous override, when provided, is a function of the breaker mechanism construction, it has nothing to do with the trip unit.

I would expect any breaker without an override function to be suspect if its ratings have been exceeded. I am sure there are tolerances and safety factors built into the breaker such that the 30 cycles is not an absolute, but they should not be relied on. However, I have never seen this type of data published.

If a breaker is not built with a published override point, I would expect its behavior to be undefined once its fault ratings are exceeded. In my opinion, its future operation may be suspect and it should be replaced.