arc flash and siemens breaker

[steve66]

I'm doing an arc flash study using SKM software. I have several switchboards with 3000 amp or 4000 amp Siemens WL circuit breakers which have ETU745 trip units.

Do I need to manually add the .05 second total clearing time of the breaker, or is that already included in the devices trip curve?

 

[Jraef]

Published trip curves are supposed to be INCLUSIVE of clearing time. So Trip Curve = Sense + Unlatch + Mechanical Separation + Arc Extinguish.

Qualifier: that is from UL489 for molded case breakers. I can't say for sure if it's the same for UL1066 for Power breakers, because I don't have a copy any longer. But I can't imagine they would do it differently.

 

[steve66]

The specs for the breaker list a "Break Time (with active ETU)" of 34 ms. That seems to match the trip curve in the spec sheet, and I think it also matches the trip curve in the SKM library.

But the specs also list a "Total Clearing Time (with active ETU)" of 50 ms.

What is the difference between break time and total clearing time? Do I need to add the 16 ms difference in SKM to make sure my arc flash calculations are right?