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 Post subject: Back to Basics
PostPosted: Wed Jul 22, 2020 6:53 am 

Joined: Mon Mar 09, 2020 9:27 am
Posts: 5
What reasoning is there that we do not include less than 3 phase 208 circuits in an arc flash study? I understand the IEEE model does not cover under 208V but does that exempt the assessment requirement from NFPA 70E? Should every circuit, down to 120 be evaluated, even if it is a cursory, " that's a 120v device so there is no chance of injury from arc flash"? Is there a guideline that says 120 V circuits will not cause greater than 1.2 cal/cm^ and therefore they are exempt? I know the NFPA 70E is a little open ended with things like table 130.5 c and its "likelihood of occurrence", so is it ok to use any study we find that states an arc flash is unlikely in X scenario as justification for not doing a study?

Hope you can make sense of that. I'm sure i'm over thinking this, but I'm new so any guidance is appreciated.


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 Post subject: Re: Back to Basics
PostPosted: Wed Jul 22, 2020 7:20 am 

Joined: Thu Apr 18, 2019 11:42 am
Posts: 30
120V, 1 ph circuits by themselves, don't have enough energy to sustain arcs. So with regards to risk assessments and 70E, you would only be concerned with shock protection. Jim would be able to elaborate more on this topic.

This article might be useful: https://etap.com/docs/default-source/te ... 0f5b57f_18.

Mike


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 Post subject: Re: Back to Basics
PostPosted: Wed Jul 22, 2020 12:58 pm 

Joined: Wed Jul 25, 2012 11:41 am
Posts: 12
Location: Halifax, NS
A 120V, single phase to ground fault is very unlikely to have a sustained arc, and in most cases that it would, the circuit protection is likely to limit the duration drastically.

aguywithfeet wrote:
that's a 120v device so there is no chance of injury from arc flash"


There is very little chance that, but the risk is still not 0.

Like mpparent says, shock is still a a concern.
mpparent wrote:
So with regards to risk assessments and 70E, you would only be concerned with shock protection.


What I recommend in most cases is that the label on all single phase switches, equipment, etc warn that the equipment should be de-energized to remove the shock harzard, and either with judgement or a study, determined the arc flash potential. Typically it will be found that the vast majority of these cases will have little potential incident energy, and in those cases, if energized work is necessary, I recommend using at least PPE that is suitable for 2 cal/cm2 for energized work.

Of course, if the 120V that the worker is exposed to is control voltage, with a small control transformer you will find that there is not enough energy for an arc to be sustained, eliminating the arc flash incident energy hazard.

The tipping point that I start to look at the equipment is if it is serviced with a transformer larger than 100kVA, but that is still very conservative in most cases, I hope.

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Jeff M
My Electrical Safety Page


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 Post subject: Re: Back to Basics
PostPosted: Mon Aug 03, 2020 8:19 am 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1528
Location: Scottsdale, Arizona
aguywithfeet wrote:
What reasoning is there that we do not include less than 3 phase 208 circuits in an arc flash study? I understand the IEEE model does not cover under 208V but does that exempt the assessment requirement from NFPA 70E? Should every circuit, down to 120 be evaluated, even if it is a cursory, " that's a 120v device so there is no chance of injury from arc flash"? Is there a guideline that says 120 V circuits will not cause greater than 1.2 cal/cm^ and therefore they are exempt? I know the NFPA 70E is a little open ended with things like table 130.5 c and its "likelihood of occurrence", so is it ok to use any study we find that states an arc flash is unlikely in X scenario as justification for not doing a study?

Hope you can make sense of that. I'm sure i'm over thinking this, but I'm new so any guidance is appreciated.

A little bit of history.

The 2002 Edition of IEEE 1584 was valid from 208 V to 15 kV so it did/and still does include 208 V systems. There was the issue of whether an arc flash on a 208 V panel on the secondary side of a small transformer would sustain or be more of a small bang (still a hazard - just considered a small arc flash hazard). A few tests lead to language that stated:

Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.

At issue, was this type of event was considered to not sustain. It would be a very limited duration also limited by a lower short circuit current from the smaller transformer. 120 V was never specifically called out.

Single phase was not addressed since it was considered that some larger single phase arc flash events could escalate to three phase from the conducting plasma striking the second and then third phases. However, I have not seen a 120 V arc flash of lower fault current become a large arc flash. If anyone has heard of this happening, please jump in here with your story. The problem is there is no way to state it would NEVER be a problem (liability issue) Keep in mind there are lot of "moving parts" to this. The gap, enclosure size, short circuit current all can affect the sustainability.

Fast forward to the 2018 Edition of IEEE 1584. We found we were a bit optimistic in 2002. Newer language states:

Sustainable arcs are possible but less likely in three-phase systems operating at 240 V nominal or less with an available short-circuit current less than 2000 A.

We had a couple of tests under very specific conditions sustain longer than anticipated for currents down to 2500 Amps. With lawyers watching what we do, we had to get creative with the language since we could NOT guarantee there would never be a significant arc flash hazard and also took 80% of what sustained, hence the 2000 A reference.

So, is a major arc flash likely under these conditions in the quote above? No (personal opinion)
Could one guarantee it would NEVER happen? No - can't guarantee.

Regarding single phase - easy answer. We simply have not conducted testing..... YET. It's on the radar screen for sometime in the future.

Hope this helps a bit. Probably more than you wanted to know :)


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 Post subject: Re: Back to Basics
PostPosted: Mon Aug 10, 2020 9:33 am 
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Joined: Mon May 25, 2015 1:17 pm
Posts: 15
Location: Northern California
My approach is to label everything 2000A and above regardless of whether it is 120V or higher.
But this brings a question how do we know that something has low AF unless it is labelled accordingly? Hence I also label all panelboards.


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