A 2 second maximum arc flash duration is a reasonable cut off based on IEEE 1584-2002. This is provided there is room for a person to escape during an arc flash. Jim Phillips explains how to use the 2 second cut off time.
The 2002 edition of IEEE 1584 contains language that suggests that you could cut your arc duration or your protective device clearing time off at a value of 2 seconds. And you might think two seconds, that’s a really long time. Well, what could happen, and I’ve shown this in other videos, is that if you have a low enough arcing short circuit current and a high enough protective device setting, your protective device, according to the time-current curve, could take seconds– 10, 20, 30, 40 seconds before it actually operates and clears the arc flash.
Is that realistic, 30, 40, seconds? Well, according to the time-current curve, that’s the value we have, but is it realistic? Well, there’s a couple things to consider.
Number one, if an arc flash were to be able to sustain that long, which I don’t see how it could, because the bus would be consumed, but if were able to sustain that long, would somebody actually stand there for that whole amount of time? No. Your human reaction is you’re going to be jumping backwards trying to get out of the way. That’s just the human response to a threat.
The other part of this is, as I mentioned, could the arc flash actually last 20 or 30 seconds? Well, never say never, but there’s a pretty good chance that the bus is going to end up being consumed. So within the 2002 edition of IEEE 1584 there’s this two second language. A lot of people refer to it as the two second rule. It’s not really a rule, but that’s how it gets referenced. And all that means is that if you have one of these long clearing time situations, that it’s reasonable to cut the current time off at two seconds.
And where the two seconds comes from, it’s based on reaction time. It’s a pretty well-established reaction time. Can people actually react faster than two seconds? Yeah, I would think so. But as far as being able to put your finger on something that’s in a standard that you can actually reference, two seconds is what we have.