When a time current curve indicates the arc flash duration is considered long, do you use the 2 second cut off as discussed in IEEE 1584 Annex B.1.2? (provided there is sufficient room for escape?)

Upon initial calculations we had some 480V transformers with unbelievable approach distances; using 2 second cutoff made the results more credible.

Application Specific

I believe this is applicaiton specific. We recently evaluated a short cirucit / coordination / arc flash study that was completed for a client by a third party, and one thing caught my eye. They utilized the two second rule. For the application I thought this may be a bad Idea would like to get your thoughts.

Design intent:
EE locally laied out a system of a bus duct mounted 45' above the floor adjacent to a catwalk. The bus plug was caculated at a level 3 utilizing the two second rule. Without the two second rule the breaker clearing time was upwards of 8-10 seconds (seems like an eternity to me).

Some argued that after two seconds the "arc" would not be able to re-strike. I am still struggling with this one, however.

My concern where do you go on a catwalk? Bigger concern the blast pressure was estimated somewhere at 16K per sq. ft. meaning that if there was an incident it may be the fall that causes a fatal injury from 45' up. As it turns out by changing some settings on the main breaker trip unit (taking out the I^2 and setting the short time lower) it picked up the fault exponentially faster at the 85% or worst case level, and still provided good coordination with downstream devices. It was amazing what a couple of setting changes can make.

Is there a voltage limitation on the 2 second rule? Presently our company standard it to us 5 seconds for above 1000 volts.

Thanks.

2 seconds in the electrical world is HUGE......