NOTE: Results and comments from this survey do not represent recommendations.

There is still quite a bit of discussion regarding arc flash hazards at 208 volts.

This week’s question is simple.

Do you believe 208 V systems pose an arc flash hazard?

Yes
No
Depends

Jim, I have had several persons indicate their belief that arc flash hazard doesn't exist at 208 VAC because it does not sustain. This seems wrong to me for two reasons: 1) anecdotal evidence (real injurious arc flash events) prove otherwise, and 2) IEEE 1584 does not indicate that we should not be calculating the IE at 208 VAC. Some of these people seem to be "quoting" persons like yourself, but I think they are misquoting. My questions to you:

1) Do you support the IEEE 1584 calculations at 208 VAC?

2) Can you post some proof of real events of 208 VAC arc flash?

Forgive me for not backing this up with a specific reference but I recall that 2019 IEEE 1584 discusses 2000 amps as being a point where arc flash is a hazard on 208V systems. I include labels for all 120/208V systems as if it is not a hazard then that is what the label valuably communicates.

In large cities, the utilities will use (3) Network transformers (with 208 V secondary) that are paralleled. The available fault current can be well over 100 kA. At these locations, there is definitely an Arc Flash (and blast) hazard. However, a low power source such as a 45 kVA 480 - 208 V distribution transformer most likely will not have enough energy to have a arc flash be self sustaining.

This is why I voted "Depends" because it really depends on the energy source (and equipment construction i.e. bus gap) as to whether the arc flash event will be self sustaining.

A bit of a TRICK QUESTION. (sorry)

This is just my view/opinion and not anything official from a particular standard.

- IEEE 1584 has the 2000A / 208 Exception (formerly 125 kVA transformer)
- NFPA 70E doesn't require arc rated PPE until the incident energy is ABOVE 1.2 cal/cm^2.

Because of the two statements above, some have come to believe "No Arc Flash Hazard Exists" for those cases. The results of the survey seem to indicate that might be the sentiment as well.

I look at work on any energized equipment - EVEN with lower incident energy (NFPA 70E) or low short circuit current (2000A IEEE 1584) as STILL being an arc flash hazard. The key is it may not be considered a major arc flash hazard but there can still be injury.

Case in point: I have run into situations where there are small burns on the finger(s), hand etc. from these low level arc flash events. Not life threatening but painful and an injury.

Also, don't forget the incident energy increases with reduced distance (getting closer to the equipment) so even if an arc flash label states the incident energy is less than 1.2 cal/cm^2, it is based on the working distance - typically 18, 24 or 36 inches. Any part of the body closer than that and the incident energy increases!

So, again my opinion but: don't ever use the language "No Arc Flash Hazard Exists" unless the system is in an electrically safe work condition.

Your thoughts? Fire Away!

Since there is such a small contribution to the calculations, does it make sense to model equipment under that 2000A threshold? Is a general label stating "potential arc flash hard exists" acceptable?

Great discussion!

I generally agree with Jim here on staying away from language that indicates "No Arc Flash Hazard Exists" for the same reasons.

I've come across these sorts of questions several times too on best way to approach calculations or labeling for single phase and/or lower voltage/lower energy - and in many cases settled on using NESC Table 410-1 values for these situations that may fall out of the recommended range of IEEE 1584 but still have some hazard. Even though the NESC may not be applicable in some cases, I found Table 410-1 useful and informative because it is based on actual test data. Generally, this has meant forcing the incident energy on the label to 4 calories in many cases, and promoting general use of 8 cal PPE.

This is definitely not the only answer, but kind of another tool in the bag that is applicable and helpful in certain situations. The combination of IEEE 1584 equations + NESC table allows you to calculate sometimes when you feel it is appropriate/accurate and fall back to table as well with good justification.

Hello,

From IEEE1584-2018 : "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."

That is typicaly possible to get 2000A of short-circuit current with a 45kVA transformer.

It may be argued that a fixed amount of energy could produce drastically different effects depending on how fast the energy was delivered. It explains why the 208V arc could make a damage to unprotected skin within a very limited time interval with the energy exposure less than 1.2 cal/cm2 (more at https://brainfiller.com/arcflashforum/viewtopic.php?f=34&t=2221).

I have never seen an actual arc flash event, other than in videos. Our plant uses SKM PTW for analysis. There are several 120/208 panels where the xfmr impedance limits current to a point where the upstream breaker will not trip until after 2 seconds, and the IE is >8 cal/cm^2. Ideally, we'd replace the feeder breakers to allow a lower current trip threshhold, but that requires time and manpower, of which both are in short supply. In the mean time, we have just labeled 120/208 panels with whatever PTW reports..in some cases this means guys wear a 40 cal suit to perform live work in these panels. I'm not saying this is right or wrong, it's just what we've done.

It is my understanding that testing has demonstrated that arcs cannot be struck or sustained at less than 120v ac. I was just asked how to welders strike and sustain arcs at less than 120v?

I was stumped. Any thoughts?

I'm not an expert on welding but the "bus gap" if you will for welding is a fraction of an inch as opposed to one to three inches for arc flash calcs. Might have something to do with it.