Has anyone read the paper by Chet Davis, P.E., titled "Calculating Arc Flash Energies and PPE for Systems <250V"? It seems to advocate utilizing a arc cutoff time of 0.5 sec for systems with voltages of less than 250V.

Opinions/discussion?

Not that Mr. Davis is President of ESA which makes EasyPower software. The paper shows how to change the time settings in EasyPower but I imagine the same could be done in other software packages.

As a disclaimer, I do use EasyPower but other than that am not associated with ESA, Inc.

Is the paper available on the internet? Do you have a link?

Would be interested in reading the paper and how the 0.5 second value is determined. I know from experience that calculations on 208 volt systems produce some very high IE levels that are questionable using the 2 second cutoff rule.

You can request it here. http://www.easypower.com/arc_flash/arc_flash_calculate_250.php

Apparantly the 0.5 cutoff was from recent testing by a utility.

Interesting... although the article states 0.5 sec cutoff for <250V, <125KVA xfmr is from a IEEE-1582 work group based on testing. It's not an adopted standard yet. Therefore, it can't be used to superceed the present IEEE 1584 2 sec cutoff. Anyone care to comment? Jim?

This is a great paper. I highly suggest people go to the Easypower website and download it. Chet Davis is highly regarded in the industry.

Since I am the head of the task group investigating this exact issue for the IEEE 1584 committee, I can state that we are looking into several options.

I would like to see the kVA lower limit changed to amps (I'm pretty sure this will happen). We are searching and debating where the lower limit will be but I believe it will likely be between 4kA and 10kA (many different opinions on this)

The 2 second cut off was based on reaction time, Chet's 0.5 second cut off was based on arc extinction from a series of tests. - 2 different issues.

Under worst case conditions with barriers and three phase initation, the duration does not seem to go beyond 1 second for low currents / 208V from what I have seen (< just FYI, nothing official yet).

Another issue we are just kicking around is if you do not have to study these circuits, how do you list the energy for selection of protection. We have a few ideas but I have to keep those confidential for now.

A few of us plan on conducting more testing to settle this issue but you are correct. For now, IEEE 1584 has not changed and the only "offical" cut off is 2 seconds. Anything other than that will not be based on the IEEE 1584 standard.

When I know more about the developments, I'll pass it along. We meet again in February 2011 and should have more then.

Thanks Jim for the response.

Great article, very informative.



I too would like the kVA lower limit to be addressed. My electric utility has hundreds of three phase padmount transformers that are <250V (208Y/120) and they are >125 kVA (kVA sizes: 150, 225, 300, 500, 750).

Unlike his recommendations for <250V, <125kVA, Mr. Davis is not explicit about recommending the 0.5 sec modification to IEEE 1584 calculations for <250V and >125kVA. Under the heading "Option 3: NFPA 70E Table 130.7", he makes the comment,
"For circuits served by transformers larger than 125 kVA, the 1584 equations should be used with or without modified arc duration time to insure an adequate PPE level." I added the underlined.

I do not use 70E task tables as an option, because as an electric utility, I focus on complying with the NESC. For now it seems that I should be using the IEEE 1584 calculations with a 2 second cutoff for my analysis, but I know that for some large 208V units the incident energy will be nearing 40 cal/cm2, which is quite different than the proposed 4 cal/cm2 in the 2012 NESC Table 410-1 for voltage range of 50-250V.