colburnpratt wrote:
Using the 2-second rule on adjustable instantaneous breakers; do you use the instantaneous amperage in the arc flash calculation; or do you use the frame rating amperage (i.e. 150 amp) when calculating the arc flash. If the high point is at rated at 1,640 amp, would 1,640 be used in the IEEE 1584 calculation, or would you use the frame rating amperage of 150 amp?
None of the above. It is much more complex than you describe. Clause 6.9 in the 2018 Edition of IEEE 1584 contains the 2 second language. It is not really a rule as it is guidance that requires judgement from the person performing the study.
More on the 2 second language:
https://brainfiller.com/videos/2-second-cut-off-ieee-1584-arc-flash-studies/What is missed here is this is a function of the calculated arcing short circuit current and it's relationship to the upstream protective device's fault clearing time derived from a time current curve. Not sure what you mean by "high point" If it is the current to trip the device, that and the frame size from a time current curve has nothing to do with this. It is a matter of where the arcing current falls on the time current curve to define the clearing time. 2 seconds may possibly be used as a cut off for long clearing times but it subject to many conditions and judgment.
