<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
boB,<br>
<br>
Most local codes, have granted a delay on implementing arc fault
protection. Also, the code only requires series faults to be
cleared.<br>
<pre class="moz-signature" cols="72">Kent Osterberg
Blue Mountain Solar, Inc.
<a class="moz-txt-link-abbreviated" href="http://www.bluemountainsolar.com">www.bluemountainsolar.com</a>
t: 541-568-4882</pre>
<br>
On 3/11/2012 8:28 PM, <a class="moz-txt-link-abbreviated" href="mailto:boB@midnitesolar.com">boB@midnitesolar.com</a> wrote:
<blockquote cite="mid:4F5D6D58.5060608@midnitesolar.com" type="cite">
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
On 3/11/2012 7:35 PM, Jeff Yago wrote:
<blockquote cite="mid:000e01ccfff8$c6bc4020$5434c060$@com"
type="cite">
<pre wrap="">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!).</pre>
</blockquote>
<br>
<br>
<span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Jeff,<br>
<br>
Arc fault was required as of NEC2011.</span><br>
<br>
boB<br>
<br>
<br>
<br>
<blockquote cite="mid:000e01ccfff8$c6bc4020$5434c060$@com"
type="cite">
<pre wrap="">
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
<a moz-do-not-send="true" class="moz-txt-link-abbreviated" href="mailto:jryago@dtisolar.com">jryago@dtisolar.com</a>
804-457-9566
</pre>
</blockquote>
</blockquote>
</body>
</html>