withstand ratings of equipment/circuit breakers at 600V and less 1

[mpparent]

Gents:

When equipment is manufactured, eg circuit breakers or switchgear, how long is the short time withstand test?

 

[dpc]

It depends on the equipment type and who is testing it. Some devices, such as molded-case circuit breakers essentially have no short-time rating.

Do you have a specific device in mind?

 

[REDDOG]

UL requires 3 cycle of fault current.
3 cycles is the clearing time for circuit breakers(.05 seconds).
Current limiting fuses must have a clearing time of.5 cycle
(.0083 seconds).

 

[dpc]

REDDOG: I believe the 3-cycle requirement applies only to molded-case circuit breakers? Low-voltage power circuit breakers generally have a much longer short-time rating - 30 cycles I think.

This is why it is possible to apply LVPCB without instantaneous trip, while UL-listed MCCB must have instantaneous trip to protect itself.

dpc

 

[REDDOG]

The question was what is the length of the withstand test for LV electrical equipment.
The Bus structure in the LV switchgear withstandind rating is based upon 3 cycles of fault current at the rating of the bus structure withstand rating. Therefore under NEC 110-3B the equipment must be installed in accordance with the Listing requirements of the testing lab.If you misapply the equipment...then you are in violation of the Code.
If equipment based upon 3 cycle clearing time is connected to protection devices of greater than 3 cycle clearing time the bus system within the equipment may be destroyed.

 

[dpc]

Reddog:

I think maybe we are talking about two different things.

The standard you are referring to is for LV switchboards - basically NEMA PB-2, I think. This has a 3 cycle limitation.

However, Low Voltage switchgear is built to ANSI C37.20, and NEMA SG-5. This would be GE AK type gear for one example. It is also UL labeled.

The bus is thermally-rated as opposed to the switchboard spec which is based on current density.

The low voltage power circuit breakers in LV switchgear have a through-fault rating up to 30 cycles and do not require instantaneous trip. The bus rating and testing requirements are based on this fact.

This is why LVPCBs provide much better coordination than molded-case circuit breakers. And I don't believe this violates any aspect of the NEC as they are in common use throughout the U.S.

Regards,

dpc