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

boB at midnitesolar.com boB at midnitesolar.com
Sun Mar 11 21:37:11 PDT 2012


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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