Arc Flash & GE AK-50 Circuit Breaker

[whycliffrussell]

I'm doing an arc flash study, and according to ETAP if I delay the LSIG settings and increase the pickup time on a GE AK-50 (1600A frame) breaker the arc flash will increase - the converse is true if I turn down both of the settings...

Somehow this does not seem correct to me? ie: The CPT of the breaker is still the same regardless of it's setting therefore wouldn't the arc flash still be the same? ie: The charging spring that opens the breaker still exerts the same force on the contacts and therefore it still takes the same amount of time to open the breaker...energy is proportional to contact parting time!

Can someone point me in a direction to clarify this? Does IEEE 1584-2000/2002 say anything about this?

 

[davidbeach]

The breaker is interrupting an arcing fault downstream of the breaker. The arc flash energy is proportional to the total clearing time, so increasing the time for the breaker to start to open will certainly increase the amount of arc flash energy.

 

[dpc]

If you are thinking about a fault within this breaker, you have to look upstream for the protective device.

As David said, the program is using the clearing time for the breaker, in conjunction with the arcing current to determine arc-flash energy for faults downstream of the breaker, such as at the MCC it might be feeding. If you slow down the LSIG, the arc lasts longer and energy is higher. In 480V trip units, the breaker clearing time is built into the time current curves.