Safety of 480 vs 240.

[ActionDave]

I admit that I have not read the stated publication, so I do not know what the exact wording and context is, however in practice I am quite skeptical of this "initiate" or "sustain" an arc idea. I can envision certain scenarios where this would be valid, however as has been stated, I believe most hot work incidents involve a substantial metal object such as a wrench or bare conductor hitting a live part. How would a few hundred volts make any difference?

480V can sustain an arc better than anything below it or above it.

My boss was involved in a incident where a 480V safety switch blew up from an underground phase to phase fault. It threw fire like a Roman Candle, tripped the re-closer at the substation three times, and pretty much ruined his plans for the weekend. It was at night so he could see the plasma trails, (or whatever you call them) moving through the air, and taste the copper in his teeth for the next day.

 

[electrofelon]

480V can sustain an arc better than anything below it or above it.

I would like to understand the physics of how 480 happens to be the voltage most likely to sustain an arc.

 

[ActionDave]

I would like to understand the physics of how 480 happens to be the voltage most likely to sustain an arc.

I'm just a wire pulling grunt, not a physicist. What I have gathered is anything below it doesn't blow up as bad and anything above it goes POOF before anything else can happen.

 

[kwired]

I would like to understand the physics of how 480 happens to be the voltage most likely to sustain an arc.

I think in particular it is 480/277 wye systems.

277 to ground is high enough that ground faults will have enough push behind them to sustain a high resistance arc, go below 200-250 volts they self extinguish much easier. Go much higher with voltage or even a line to line 480 volt fault on the wye system and the current is usually driven high enough that overcurrent protection will have much better chance of a quick response.

Might be main reason ground fault protection is only required on solid grounded 480/277 wye systems and not other systems.

 

[iwire]

I would like to understand the physics of how 480 happens to be the voltage most likely to sustain an arc.

I can't answer why but I can tell you it is one of the reason for the GFP requirement for over 150 volts to ground.

 

[electrofelon]

I can't answer why but I can tell you it is one of the reason for the GFP requirement for over 150 volts to ground.

I do recall the commentary in the handbook saying something like, "...because of the large number of meldowns on these system.." I question if this is just because of its commonality and position at the higher end of LV. Clearly NEC systems of higher voltage are relatively rare. and would pretty much all be in MV territory. For arguments sake, say 800 volt systems were a thing and utilized the same methods and equipment as 480, and were equally as common. Would we have the same risks and GFP requirements on those?

 

[JFletcher]

That would make sense, 210.6(A). No 277V lighting in the rooms.

true but ptacs can be 277V. Ive only seen one hotel with that setup, an old 168 room Ramada outside of Richmond. I remember because it was the first time I saw 480/277 gear outside of an industrial facility.

 

[banion23]

Not exactly sure what you are asking for but if you are interested in more on the incident energy calculations and how they were arrived at, I would suggest you obtain a copy of IEEE 1584-2002 and read through it.

I agree.

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[banion23]

I just ran your figures thru the DC arc flash calculation in NFPA 70E-2015 Annex D and get a value of 0.06 cal/cm2 for an arc time of 2 secs, 18 in working distance. Using an arc time of 0.5 secs, the incident energy is 0.02 cal/cm2.

Awesome. Lol

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[Sahib]

480V can sustain an arc better than anything below it or above it..

Sorry, 600V system also has high probability of arc flash hazard as 480V per same IEEE Std.

 

[wbdvt]

Sorry, 600V system also has high probability of arc flash hazard as 480V per same IEEE Std.

Sustaining an arc versus the probability of initiating an arc flash are two different things.

 

[Sahib]

Sustaining an arc versus the probability of initiating an arc flash are two different things.

600V and 480V do both.