I keep running into low short circuit values causing devices to have long clearing times. I hear IEEE 1584 suggests cutting long clearing times off at 2 seconds. Is it better to be safe (and perhaps unreasonable) with long times or to cut it off at 2 seconds and perhaps be more realistic? Any ideas?

My understanding is the "2 second rule" is in the back of the standard. I heard that it is factoring in a persons response to an arc flash i.e. jumping back from it. If you are in a confined area where you can not jump back that would be a problem.

Long clearing times

We're seeing some of those instances too, where there seems to be arbitrary breaker settings or ultr-low short circuit currents and the clearing times are off the chart. When you calculate the incident energy on such devices, you always get this huge, seemingly out of whack number. As bad as it sounds, our current policy is to stick with the value we calculated at the true clearing time, just to be safe and lower the risk of liability on our part. Obviously we put in the report the reason thatthe number is so high, and give suggestions to mitigate it, but it just seems dangerous to cut numbers off. Just my opinion.

In agreement with what PDS Dave said, we were coming up with flash protection boundary calculations of a quarter mile in some cases. Using 2 second limit restored some credibility.

Gary B

2 second rule

I don't think it is dangerous to have a cut-off time. It is common sense really. The 2 second rule is based on the fact that in reality, someone will move away or be blasted away within 2 seconds. Really conservative folks will use 5 seconds, but most programs offer the 2 second cut-off times, and the standards support it (from a legal stand point). It does not make sense to have someone standing in front of a panel, etc. for 100 seconds while getting burned.

Note: If someone is in a confined space where it is impossible for them to move, you CANNOT use a 2 second rule.

The Standard have SOME legal standing but can easily be demolished in Court by an expert witness. One way they would be attacking it is citing selected comments from this forum.

The standard (IEEE 1584) is not a MANDATORY standard, it is a industry concensus standard, which is recommend as a guideline for performing arc flash hazard studies. There is quite a bit of material that is left to the judgement of the engineer.

Lot's of good opinions here. How many people actually use the 2 second cut off assuming there is good distance to jump back?

How many people actually use the 2 second cut off assuming there is good distance to

I work as a consultant for utilities and I am performing arc flash studies and most are using 2 seconds for padmounted equipment and 5 if they will be on a pole or in a bucket truck.

Can't find the 2 second reference

I can't locate the reference to the 2 second cutoff reference. Where exactly is it?

The 2 second limit is in Annex B.1.2 near the end. Page 76 in my copy

Got it. It was one of the things where my mind was scanning for "2 seconds" and the text is written "two seconds" and I just missed it.

Got it. It was one of the things were my mind was scanning for "2 seconds" and the text is written "two seconds" and I just missed it.

I wanted to point out though that the equations are based upon a working distance. The majority of the time what I have seen is 18 inches. For the 2 second rule not to be relevant a person would have to brace themselves like a foot ball player and stand there willingly and forcefully at 18 inches for the duration of the arc if it is longer than 2 seconds. This is not only crazy but not phisycally possible. I realize there are restraints such as walls and barriers that could change this but for the most part not gonna happen.

Remember that Calories are a measure of heat energy absorbed by the skin not temperature the surface might feel at a given time.

Two Seconds is Too Long

Beyond the personnel hazard, any flash that continues for more than two seconds is going to cause a lot of equipment damage. We had a squirrel, and defective 86 relay, cause an eight second flash on a Saturday afternoon when no one was around. The damage was impressive.

The point is, relays and CBs should extinguish an arc as soon as possible. More than two seconds is excessive. If you find these long burn times, perhaps your relaying should be changed to interrupt sooner.

Right it should, but sometimes it is not possible (Or at least really hard) to do due to the low arcing currents.

IEEE 1584 (2002), Annex B.1.2 near the end of the section.