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

[boB at midnitesolar.com]

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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