[RE-wrenches] Arc flash concerns

[R Ray Walters]

Oh boy, just when we thought all we had to worry about was getting electrocuted or falling off a roof.
The cooked dummy at the end of the last video is a very serious reminder.
Besides the arc flash gear, are there other things we should be watching for to prevent this from happening? (we definitely get lizards inside our gear)
What happens on DC? at higher voltages, I would think it could be even worse than AC.
What happened to the poor guy in the first video?

R. Walters
ray at solarray.com
Solar Engineer




On Aug 31, 2010, at 8:24 PM, Jeff Yago wrote:

> First, I am not talking about NEC requirements for "arc fault" circuit breakers in bedrooms!
>  
> I hope those on the list who are licensed electricians are very familiar with the safety issues of an "arc flash" but I have run into many pv installers who do not have a clue, and believe as long as the breaker they are working on is disconnected there is no danger when working with the panel energized.  This is a VERY dangerous problem and it can happen on brand new or old electrical switchgear.  An arc flash starts with something across the buss bars of a breaker panel or from a buss to ground.  What is hard to understand is this is not a true short circuit and will NOT trip any circuit breaker. 
>  
> This makes it very dangerous when working on any energized panel.  It starts with any small or poor conductor across the phases or buss bars.  This could be anything from a fine wire, a dead lizard, rust falling off the interior when you slam the door closed, filings from drilling a hole in the panel side, or even removing or installing a new or old circuit breaker even if it is not connected to any load.  This is just enough contact to allow an arc to form, and as soon as the arc forms it goes into melt down which produces 35,000 degree temperatures and blows plasma of vaporized copper out the front of the enclosure.   Having been near one I can tell you the sound is like holding a 12 ga. shot gun next to your ear.
>  
> Starting in 2004 the NEC started requiring arc flash studies and labeling on all electrical panels in all non-residential facilities.  Most likely if you run into one of these panels there will be a label on the door saying "the arch flash safe distance rating of this panel is 6 feet"  and indicate that arc flash protective clothing must used before touching this panel.   Any commercial building owner who does not comply with these new regulations can face criminal prosecution if somebody gets fried.   The safe way to approach any circuit breaker panel is to stand to the hinge side of the door as you open it, and not stand directly in front as most people do.  This will deflect the high temperature plasma away from your face if it happens.
>  
> Here are several short YouTube videos, one of a security camera view of two licensed electricians doing normal maintenance on new switchgear.  One guy is standing in front of a new large circuit breaker that he is "cranking" into position.  Later analysis indicated one of the rear pins was bent, and as he cranked he was deforming it sideways towards the ground buss.  This new breaker was not connected to any load. Let me say that again, there was no load and no current passing through this circuit breaker. 
>  
> Since an arc has resistance, it allows enough amps to maintain the arc, but not enough amps to trip the breakers, which is why this will continue to burn and not trip anything. Also, this may cascade back through multiple sub-panels as their can be enough impedance in a long cable between a main and sub panel to hold down the amps below the trip rating of the next breaker up the line. 
>  
> Regardless of what you hear, this is not just a high voltage issue, and definitely is a concern down to 208 volts AC which puts it into the normal range of a residential 120/240 VAC main panel.   If you are adding or removing a breaker in an existing panel to connect a new inverter, please make sure the panel is de-energized and wear approved arc flash gear!
>  
> Jeff Yago, DTI Solar
>  
>   http://www.youtube.com/watch?v=m3_ear4k-Jg
>  
>  http://www.youtube.com/watch?v=-iClXrd50Z8&feature=related    
>  
>  http://www.youtube.com/watch?v=AFlBLQjOAJI&feature=related
>             
> 
>  
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