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

Kent Osterberg kent at coveoregon.com
Sun Mar 11 21:26:52 PDT 2012


boB,

Most local codes, have granted a delay on implementing arc fault 
protection. Also, the code only requires series faults to be cleared.

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
>>
>>
>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20120311/d1a248cd/attachment-0003.html>


More information about the RE-wrenches mailing list