Jeff, excellent advice. Even seasoned electricians are negligent in this area.
We've done lots of large commercial electrical installations over the years and
a rule that was strictly enforced over the years in our shop was to stand to
the
side and look to the side when operating breakers. On the larger breakers we
stand to the side and use a 2x4. On one occasion a few years ago we had a 2000
amp breaker in a stand up panel blow when energized. Had the electrician been
standing in front of the breaker he would have no doubt died.........
Max Balchowsky
Design Engineer
SEE Systems
1048 Irvine Ave Suite 217
Newport Beach, Ca. 92660
760-403-6810
________________________________
From: Jeff Yago <jry...@netscape.com>
To: RE-wrenches <re-wrenches@lists.re-wrenches.org>
Sent: Tue, August 31, 2010 7:24:57 PM
Subject: Re: [RE-wrenches] Arc flash concerns
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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