[RE-wrenches] Starting fires for fun with string wiring

Mon Mar 12 09:09:28 PDT 2012

I've been wondering about this since I read over the 2011 code:

Are there any listed DC arc-flash products on the market yet, or scheduled to come to market within the next year?

Also: will arc-flash protection come in the form of breakers that need to be installed externally, or will it be something internal to the inverters or charge controllers to which source circuits are wired? Will they need to be installed on each string, or can they function on a combined circuit?

Do systems with DC optimizers or with micro-inverters have arc-fault protection at the module level?

Jeffrey Quackenbush
NABCEP Certified PV Installer
Peripatetic Solar Technician

________________________________
 From: "boB at midnitesolar.com" <boB at midnitesolar.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org> 
Sent: Monday, March 12, 2012 12:37 AM
Subject: Re: [RE-wrenches] Starting fires for fun with string wiring

On 3/11/2012 9:26 PM, Kent Osterberg wrote: 
boB,
>
>Most local codes, have granted a delay on implementing arc fault
      protection. Also, the code only requires series faults to be
      cleared.
>

I mentioned this because arc fault is already in the 2011 NEC
    whether it is adopted or not.

I have a feeling that Jeff was referring to some of the changes that
    are in the works for 2014.

As far as I understand, the code cannot be required unless there are
    products that fill the requirement.

Also, the UL spec was not quite complete, but that  didn't stop many
    of us from starting to work on it
before even 2011 code was out, even releasing "something", even if
    it did not have a spec
to test to yet.

This code is going to get interesting for sure.   It's also going to
    make installations more expensive so
it's good that PV prices are falling some to help make up for it !

boB

Kent Osterberg
Blue Mountain Solar, Inc. www.bluemountainsolar.com t: 541-568-4882
>On 3/11/2012 8:28 PM, boB at midnitesolar.com wrote: 
>On 3/11/2012 7:35 PM, Jeff Yago wrote: 
>>I just sent a post about people not understanding the dangers of fire in the
array strings between the modules and before the combiner because its high
voltage DC and there are no fuses or circuit breakers to shut down the power
source (at least not until the next code requires arc-fault sensing!).
>>
>>Jeff,
>>
>>Arc fault was required as of NEC2011.
>>
>>boB
>>
>>
>>
>>
>>Anyway, we have a 3 string, 4-module per string off-grid test system behind
the shop wired for 88 volts peak into a charge controller and 24 volt
battery bank. Last week as a test I temporarily disconnected the charge
controller and re-wired the 3 strings together into a single string of 12
modules with a 264 volt open circuit voltage DC output at about 7 amps at
dead short.  When compared with most grid-tie systems, this is not that high
a voltage or current, so what happened next would be far worse for a larger
voltage array. I took a 2 x 4 and attached USE-2 single conductor wires using "wire
staples" with the (+) and (-) cables routed side by side and down the 2 x 4.
At the lower wire staple, I deliberately nailed through the wire insulation
which nicked the insulation but did not cut the wires.  After routing the
array power through these test wires nothing happened since the wire staple
was not making a very good contact with the bare wire at the insulation
nick.  I then moved the wires slowly from side to side by hand, which could
have been caused in a real installation from wind, temperature
expansion/contraction, somebody pulling on them, or just the action of age
on the nicked wire insulation.  Anyway, after only a few seconds of slowly
moving these wires, suddenly the wiring where it passed under the wire
staple burst into flame and started to arc.  In about 5 seconds the 2 x 4
was burning and the string wires were burning like a powder fuse running up
the 2 x 4.  As the wire insulation was burning, it exposed the energized
wires and allowed the arcing to continue up the wood 2 x 4 which was now
also in flames .  At times, the arc would pass between wires that were over
1" apart once the arc had started.  In other words, the arc would not
normally jump 1" through the air, but once an arc started it would continue
to arc even when the wires were far apart since the plasma formed from the
vaporized copper wire was conducting the electricity across the gap. I know the code requires any string wires to be in metal conduit from the
point it enters a building to the first disconnect,  but I am not sure this
is enough protection.   DC voltage is a very strange animal and even trained
electricians do not always realize the strange behavior it has if they have
only worked with AC wiring.  Arc faults in older AC electrical panels are
becoming a real danger to service technicians since sometimes just opening
the panel door can cause something loose to fall across the interior buss
bars and start the arc.   Arc faults are really dangerous because the plasma formed from vaporized
copper can be thousands of degrees in temperature in under a second, and
since it is not technically a "short", an arc-fault condition will not
usually trip the circuit breaker since there is enough resistance in the
higher voltage arc to limit the amp flow to below the rating of the circuit
breaker, which makes it very hard to stop.  I think the solar industry will
start seeing more of this type hazard as these array voltages continue to
increase.  I strongly suggest everyone should wear arc fault rated face
shield and hand protection when working around energized high-voltage DC
string circuits, as electricians gloves alone are not enough.    I will try and down-load this video from my camera and convert to a file I
can send as an email attachment.  If interested, send me your email address. Jeff Yago jryago at dtisolar.com 804-457-9566 
>
>
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