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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>All of this talk is reminding me to purchase black, occlusive,
array covering tarps for our job trailer. Does anyone have a recommendation for
a source?<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>David Palumbo <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Independent Power LLC<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Offices in Lamoille and the Champlain Valley<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>802.888.7194 <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>www.independentpowerllc.com <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Helvetica","sans-serif";
color:blue'>NABCEP Certified Solar PV Installer™</span><span
style='font-size:9.0pt;font-family:"Helvetica","sans-serif";color:black'><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Vermont RE Incentive Program Partner<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> re-wrenches-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@lists.re-wrenches.org] <b>On Behalf Of </b>Nick
Soleil<br>
<b>Sent:</b> Wednesday, September 08, 2010 1:34 PM<br>
<b>To:</b> gilligan06@gmail.com; RE-wrenches<br>
<b>Subject:</b> Re: [RE-wrenches] 1.56 ISC Minimum OCP is STUPID! (Was: ground
fault troubleshooting)<o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'>Hi Matt:<br>
How are things. We met a John Berdners house many
years ago (when SMA was based there.) Hope you are great.<br>
I see reason behind the 1.56 ISC fuse rating. Sure,
this can create a safety issue, but even if the fuse blows, a shorted array is
probably still faulted to earth, so the shock hazard is probably still
present. We generally need to proceed with caution if the GFCI fuse is
blown, so that should be our main warning that a shock hazard may exist.
The problem with reducing the the fuse size is that it will erroneously blow
due to high irradiance levels (edge of cloud effects or reflections), or
amperages that are pushed up by MPPT trackers. <br>
I do not want to always be accessing Soladeck
combiners under arrays to replace fuses that have erroneously tripped, just
because of a few minutes of artificially high irradiance.<o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'> <o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'>Nick Soleil<br>
Project Manager<br>
Advanced Alternative Energy Solutions, LLC<br>
PO Box 657<br>
Petaluma, CA 94953<br>
Cell: 707-321-2937<br>
Office: 707-789-9537<br>
Fax: 707-769-9037<o:p></o:p></span></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'><b><span style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'> Matt Lafferty <gilligan06@gmail.com><br>
<b>To:</b> RE-wrenches <re-wrenches@lists.re-wrenches.org><br>
<b>Sent:</b> Tue, September 7, 2010 7:25:52 PM<br>
<b>Subject:</b> Re: [RE-wrenches] 1.56 ISC Minimum OCP is STUPID! (Was: ground
fault troubleshooting)</span><o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Hi Kent,</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Thanks for the 98% vote. Now I'm gonna try to get the other 2% out
of you.... You're a smart guy, so it shouldn't be too difficult ;)</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>These aren't the days where we were lucky to have a customer with
barely enough money to afford a 300W system. We are commonly dealing with
>2500W strings of 200+ watt modules. It's a new paradigm and the risks
associated with faults continue to grow. You are 100% right... OCP in a
current-limited DC application ain't simple.</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>To be clear, I am completely in favor of 1.56 ISC as a minimum for
conductors. I want that inherent protection and any case I describe hereafter
assumes this condition. Also to be clear, I am not proposing that our
current-limited power sources should be able to trip the OCP from the source
based on amperage alone.</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>To your contention that "1.56 ISC isn't in any way responsible
for the danger"....</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Compared to a lower fuse rating.... Something like ISC for
instance.... 1.56 ISC as a MINIMUM OCP rating does indeed increase the
hazards. To both persons and property. By at least 40% in terms of raw
amperage. By more than that in terms of kCal/cm2. The biggest difference comes
in terms of time to blow and the amount of damage or injury caused during
that period. They call it Incident Energy.</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>For those who have not studied Time:Current curves of commonly used
fuses, you should. KLKD is a typical fuse I hope we are all familiar with. <a
href="http://www.littelfuse.com/data/en/Data_Sheets/KLKD.pdf" target="_blank">http://www.littelfuse.com/data/en/Data_Sheets/KLKD.pdf</a>
I'm using KLKD as an example because Littelfuse put a little table right on the
front of the datasheet to make it simple. Other commonly used fuses have
similar characteristics. </span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Note that this class of fuse will take 135% of it's rating for up
to 1 hour. 200% of its rating for up to 2 minutes. Be sure to check out the
fuse curves. These things will take ~125% of their ratings for pretty much
indefinitely. How much damage is caused waiting for it to blow? What about when
irradiance is <800 W/M2? How long then? Will your conductor hold up during
this time?</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>If you fault on the array side of a combiner fuse in a 3-string system,
you might NEVER BLOW that fuse. (Example: 8.4A ISC module with 15A fuse).
Especially if there is an arcing DC fault. Temperatures of arcs are much higher
than 90C. Is your conductor up to that? I have been called to troubleshoot a
lot of low-performing and broke-down systems. One resi rooftop had a spot where
an arcing fault burned through the side of a NEMA 3R steel pull-can on the
roof and never blew a CODE COMPLIANT combiner fuse. This system had 3
strings and was down about 1/3 on power output after the original installer
replaced the GFP fuse following a "ground-fault". He had checked the
combiner fuses and they were good so he called me to troubleshoot it. He was
convinced that there must be a bad module and wanted a third party to verify it
for warranty claim. Hated to show him what he missed and that it was his fault.
(The fault had burned clear so the GFP didn't blow again.)</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Another situation that I am sadly familiar with has
been burning holes in a steel roof since 2007 <em><span style='font-family:
"Arial","sans-serif"'>(I told them not to do it over and over)</span></em>...
At least one spot is about the size of a softball. The others vary in size.
Their common characteristic is that you can see the sky thru them from inside
the building. The first time it happened was during an ice storm. This POS peel-n-stick
system typically burns thru the roof during low-irradiance periods. Most
of the time it will EVENTUALLY blow a combiner fuse, reducing the
current feeding the fault, and either weld to a short or burn clear (open).
This is ~500kW on central inverters with marginal GFP protection. I hear the
Tefcel front sheet is a nice insulator.... You want to walk out on that thing
in the daylight? I don't. This case happens to be a classic
installer-was-either-drunk-or-on-drugs-because-nobody-can-be-that-stupid situation.
Rolling UniSolar right over loose screws on the pan? WTF!!!!</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>I can list a lot of similar examples where damage has been caused
and fuses have either not blown or taken longer than they should have to blow.
The key here is that the wiring and OCP in ALL OF THESE CASES ARE CODE
COMPLIANT!</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>I bring these cases up because, if these things had fuses with
smaller current ratings, the fuses would blow before this much damage is done.
It takes a lot of heat to burn thru steel. You wouldn't think that a circuit
with a 10 or 15A fuse could do this much damage, but they do! Amperage = Heat.
The more Amps, the more Heat. When you use an Arc or Wire-feed welder, you
adjust Amps to get the heat you want. I want to minimize the amount of
potential Amps back-flowing into a fault to a lower level. A level that allows
safe and reliable NORMAL OPERATION, yet limits the catastrophic effects in an
ABNORMAL CONDITION.</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>My contention is that the NEC is flat out wrong requiring 1.56 ISC
as a MIMIMUM OCP rating. It creates undue hazard. It is in direct conflict with
the spirit, intent, and other long-standing precedents in the Code.... With the
exception of Emergency Fire Pumps and other mission-critical equipment that is
specifically intended to stay alive until it completely burns to the ground,
all other minimum OCP ratings are based on 125% CONTINUOUS OPERATING
CURRENT of the equipment. In our case, that equates to 125% Ipmax, as opposed
to ISC. An example of an AC equivalent to this asinine "minimum 1.56
ISC" OCP requirement would be requiring motor circuits to be OCP at 156%
of the Locked Rotor Amps. I'm sure you can imagine what this would do to the
size and cost of starters, etc. </span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Since the last time my favorite 6kW system put out 7.5kW was...
NEVER.... I'm gonna have to guess that a 1.25 Ipmax fuse would hold just fine.
This would be 100% consistent with the rest of the Code. It's
also not gonna blow with cloud-edge effect or other irradiance enhancing
events. Especially when you consider that it's gonna automatically withstand an
extra 20-25%% for an indefinite, possibly forever, period due to
the Time:Current curve relationship.</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>My NON-CODE-COMPLIANT-SELF prefers to size OCP at <em><span
style='font-family:"Arial","sans-serif"'>"ISC or next larger standard fuse
rating not larger than the LISTED Series Fuse Rating of the module"</span></em>.
You've got a 6-7% headstart between Ipmax and ISC plus the ~25%
indefinite Time:Current characteristic. Ain't no way in hell that a
NORMALLY OPERATING SYSTEM will blow fuses rated this way. And it's ~2/3 the amp
rating (or less) that is now required as a minimum. Which is exactly what we
want. Reliability during normal operation and safety.</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>The only time we want an OCP device to trip is during abnormal conditions
such as a short circuit. When we have this type of fault, we want that circuit
to open up as quickly as possible in order to minimize damage. At least I do.
In the case of 1.56 ISC, the NEC is GUARANTEEING GREATER DAMAGE AND INCREASED
HAZARDS compared to a lower-amp fuse. I have gone thru this logic with numerous
building inspectors over the years. Every single one agrees with it. Some, but
not all, have agreed to allow lower-amperage fuses. The only, and I mean ONLY
reason given by inspectors that have not allowed lower-amperage fuses is
because... <em><span style='font-family:"Arial","sans-serif"'>"The Code
requires 1.56 ISC so I have to require it."</span></em> </span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>String inverters only bother me so much in this regard. Central
inverter systems are where the real bad ju-ju starts to happen. For the sake of
definitions, I consider a string inverter to be one that has one or less
combiners and the modules are configured in series strings. A central inverter
is one that has string-level combiners and one or more re-combiners. </span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>I am seeing more and more faults in the DC feeders between string
combiners and re-combiners in these central systems. The power levels
you are dealing with here are pretty significant. 100% of the recent faults
I've been seeing are due to shi##y workmanship, particularly in conduit installation
and wire-pulling. Some of these faults would certainly have been avoided if
they had selected a tougher insulation such as XHHW-2, instead of Quik-Nick
THWN-2/THHN. (THWN stands for "This Heiffer Will Nick". THWN-2 stands
for "This Heiffer Will Nick 2wice")</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>I have zero tolerance for crappy workmanship and even less sympathy
for the people who do it. Just got off ANOTHER call this morning where the
installer has re-pulled one DC feeder four times and still can't pass megger
testing. They have re-pulled every feeder at least once. The spec only calls
for 250 Mohms even though wire and cable engineering formulas say the minimum
should be 688 Mohms for that size, length, and insulation type of wire. And
they can't even get it to 250. Every set of wires that has been pulled out
has obvious physical damage. The sub is crying, wanting more money and to
have the work accepted (NOT!). Come to find out, one of the field
guys working for the developer has witnessed these guys beating on the
1/0 AWG with a mallet to get it into the LB. </span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Says right here in my NECA 1-2006 Standard for
Good Workmanship in Electrical Construction... Section 9 Wire and
Cable.... "c) Wires and cables shall be installed so as not to damage the
insulation or cable sheath." Sounds like this electrician sub wannabe is
in violation of his contract.... You know that clause... "Workmanlike
manner". <em><span style='font-family:"Arial","sans-serif"'>(Sub, if you
are reading this... I am NOT your friend in this case. You WILL re-pull these
feeders correctly, at your own expense. I will repeat the advice already
provided: Use pulling condulets. I will add some advice: Fire your
electricians.)</span></em></span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>The scary thing is, this practice goes on every day. A LOT! The sad
fact is that many (most?) of these systems are not having thorough,
comprehensive Insulation Resistance Testing performed. And IRT will only catch
SOME of the future faults! I have been involved with post-mortem in two cases
where 500kW AC feeders have been properly IRT'd and blew up later. Not good!
(Side Note: Each of these cases involved big feeders in standard LB's. Make a
note of it.)</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>It's only a matter of time before these things go Pop Sizzle Smoke!
These failures WILL occur. A lot of them already have and the number is
growing. I hope and trust that most of us on this list practice Good
Workmanship on every project. That being said, none of us are perfect. What
about cases where we miss something or even cases like a tree falling across
one of our conduits?</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>My contention is that we should do whatever
we RESPONSIBLY can to minimize the damage when this happens and
the hazards when it's being troubleshot and repaired. It's a simple principle. </span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>One RESPONSIBLE way we can minimize the damage is to reduce the
fuse size by ~40%. <em><span style='font-family:"Arial","sans-serif"'>(i.e. ISC
or next higher standard fuse rating)</span></em> This method will provide
adequate operational reliability. It will also ensure that there is a
better chance of the fuse blowing sooner when there is a fault, thereby
minimizing the damage caused. It will minimize the hazard to personnel
performing troubleshooting and repair because the incident energy at the fault
will be reduced in all cases. By ~40%. Whether or not the fuse blew.</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Happy to discuss this issue with all who care and are not on NFPA
70 CMP. (Just kidding. You CMP guys are welcome to discuss it, too... Just be
ready to issue a memorandum/addendum to the 2011 NEC allowing OCP with lower
than 1.56 ISC...)</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Extra Credit for BOS Mfrs: Make a Combination Device that has DC
Arc-Fault Interruption and OCP that fits in a standard fuse configuration.
Start with midget-class so we can simply drop it into our string combiner
fuseholders.</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Pray for Sun!</span><o:p></o:p></p>
<p class=MsoNormal> <o:p></o:p></p>
<p class=MsoNormal><strong><i><span style='font-family:"Monotype Corsiva"'>Matt
Lafferty</span></i></strong><o:p></o:p></p>
<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:10.0pt;font-family:"Arial","sans-serif"'><a
href="mailto:gilligan06@gmail.com" target="_blank">gilligan06@gmail.com</a></span><o:p></o:p></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'><b><span style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'> re-wrenches-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@lists.re-wrenches.org] <b>On Behalf Of </b>Kent
Osterberg<br>
<b>Sent:</b> Tuesday, September 07, 2010 10:59 AM<br>
<b>To:</b> RE-wrenches@lists.re-wrenches.org<br>
<b>Subject:</b> [RE-wrenches] ground fault troubleshooting</span><o:p></o:p></p>
<p class=MsoNormal style='margin-bottom:12.0pt'>Matt,<br>
<br>
I agree with you on about 98% of this. You are 200% correct that a
faulted high-voltage or high-current PV array is a serious and dangerous
situation and that the person looking for the trouble in a faulted PV array
needs the proper tools and knowledge of how all the components work. But
the 156% rule for fuse sizing per NEC 690 is not in any way responsible for the
danger. The danger is a result of the nature of the PV module: a power
source with the current nearly proportional to the illumination and a short
circuit current that is only 10% greater than the normal operating
current. If one were to select a fuse that could blow when the array was
shorted, occasional edge of cloud irradiance enhancement would cause nuisance
trips and it still wouldn't clear a fault when the irradiance is 900 watts per
square meter. There will never be a simple fuse that can provide the
protection that is needed. The existing ground fault protection in the
inverters is inadequate and current plans for arc fault protection may not be a
satisfactory either. These have been slow incremental improvements; much more
is needed.<o:p></o:p></p>
<div>
<p class=MsoNormal style='margin-bottom:12.0pt'>-- <br>
Kent Osterberg<br>
Blue Mountain Solar<br>
541-568-4882<br>
<a href="http://www.bluemountainsolar.com/" target="_blank">www.bluemountainsolar.com</a><br>
<br>
<o:p></o:p></p>
<pre>Wrenches all,<br>
<br>
I 100% second Bill B's comment Correct that... I 200% second it. It should<br>
be the law.... "Don't begin to troubleshoot a faulted PV circuit without a<br>
reliable DC clamp meter."<br>
<br>
The MOST DANGEROUS PV system is a wounded PV system. This includes danger to<br>
persons and property. Safely and efficiently troubleshooting a faulted PV<br>
circuit requires a voltmeter AND an ammeter. And PPE. And adequate knowledge<br>
and understanding of operational and non-operational characteristics of PV<br>
systems.<br>
<br>
The simple reason for this is that, when one or more circuit conductors are<br>
faulted to a short condition, the voltage between the faulted elements is<br>
zero. Relying on just a voltage reading to determine whether or not to open<br>
a circuit under this condition will result<o:p></o:p></pre><pre> in an arc. The amount of energy<br>
in that arc will depend on the amount of available sunlight and the amount<br>
of PV that is feeding into it. The amount of potential hazard will<br>
correspond to these factors as well.<br>
<br>
Using a clamp style ammeter will allow you to understand where and how much<br>
current is flowing in a circuit before you decide to open it. It is one<br>
thing to know you have a 45 amp load in a circuit with a potential of ~450V<br>
because you clamp it before you break it. With this knowledge you can assess<br>
the situation. You can do something to mitigate or remove the potential<br>
hazards... Cover the array, open a disconnect somewhere, put your PPE on and<br>
go for it, select a different location to open the circuit, use insulated<br>
cable cutters, wait 'til dark.... You have choices.<br>
<br>
It is quite another to be surprised by the resulting arc in tight quarters<br>
because you measured the voltage and figured it was a<o:p></o:p></pre><pre> dead circuit! When you<br>
react to the startlement (word?) by dropping your screwdriver and yanking<br>
your hand back... Assuming you don't receive a shock, flash injury, or fall<br>
off the roof in the process, of course.... The result just may be that the<br>
now-dislodged conductor is arcing and zapping and spitting. Now you're gonna<br>
have to stick something back into that box to deal with it. In the meantime,<br>
a number of possible things can happen, most of which are not favorable....<br>
Melting insulation and conductor material are the most common. The degree<br>
(not just a pun) of damage and remaining hazard will be determined by the<br>
amount of sunshine and amount of PV feeding into the arc.<br>
<br>
The MOST DANGEROUS single point on the DC side of a PV system is ANYPLACE on<br>
the Inverter side of a fuse(s). This is a simple function of the assinine<br>
"1.56 ISC minimum fuse" rule in the NEC. The source cannot create enough<br>
current to blow<o:p></o:p></pre><pre> the fuse(s). If you have a fault between a combiner and the<br>
inverter, you WILL have current flowing into the fault as long as the sun is<br>
up! If you are relying on just a voltmeter in a central-inverter plant, you<br>
could very well be in for a 15-20kW surprise, or greater!<br>
<br>
The combination of shi##y wire, sloppy conduit installation, and crappy<br>
wire-pulling methods have resulted in too many DC feeder faults to count. It<br>
boggles my mind every time I hear of yet another guy nearly joining the dead<br>
because he touched or opened up a connection somewhere in a faulted circuit<br>
without de-energizing it. Time and time again I hear that they tested it for<br>
voltage and it was "dead". Sometimes they even opened up the service<br>
disconnect at the string combiner, "just to make sure". Time and again it's<br>
a "journeyman electrician". I like it best when it's the same card-carrying<br>
jackass who "built" the thing.<br>
<br>
I consider<o:p></o:p></pre><pre> THWN-2 to be on the list of shi##y wire types for DC, by the way.<br>
I'm an XHHW-2 guy, personally. Why would anybody select an insulation that<br>
is easy to nick/slice/tear when you can have a super-tough insulation for a<br>
couple pennies more? Why would anybody select an insulation that only has<br>
about 5% of the dielectric resistance of one that is a couple pennies more?<br>
Why? Oh, I know... It's that race to the bottom on BOS costs... <br>
<br>
Which leads to the next step in stupidity... Designing and building LARGE PV<br>
plants without sufficient DC SERVICE disconnects... This is what's going on<br>
out there.... PV plants with 500kW Central-inverters being installed without<br>
string-combiner disconnects. Without any DC service disconnects. <br>
<br>
The NEC considers the fuseholder in the combiner &/or the connector on the<br>
module to be a "disconnect" and does not require a "service disconnect" in<br>
the circuit. So these smart-ass<o:p></o:p></pre><pre> engineers and project developers are out<br>
there building this shi#. Some of these projects are being built by PV<br>
module manufacturers masquerading as developers. "Vertically integrated..."<br>
Others are being designed & built by formerly respected integrators who have<br>
either sold out or lost their conscience altogether. The trend is to build<br>
them to sell to PPA companies who ostensibly own and "operate" them. These<br>
solar timebombs are being built on both sides of the fence. Frosty ain't the<br>
only one with a solar flamethrower!<br>
<br>
All in the race to the bottom of the $/Watt pile that they are now calling<br>
LCOE. Har Dee Har Har! <br>
<br>
I hate to say this, but I hope somebody gets really hurt out there, and<br>
soon. I hope it's the same smart-ass engineer (or his boss) who thought it<br>
was alright to design this way after some field technician walks away from<br>
it because it's dangerous. And then I hope his family sues the<o:p></o:p></pre><pre> crap out of<br>
the company and companies involved with designing, supplying, building, and<br>
owning it so they STOP DOING THIS SHI#! And then I hope he takes his cooked<br>
carcass on the road doing safety awareness training so others don't repeat<br>
these stupid, avoidable catastrophes! And then I hope these cheap-ass<br>
developers go out to every site that doesn't have sufficient disconnects and<br>
re-fits the systems with them to avoid further injuries and $$$$<br>
settlements. What is the levelized cost of energy for that system now, Mr.<br>
CFO?<br>
<br>
Unfortunately it isn't likely to be that smart-ass engineer. Or his boss. It<br>
is far more likely to be a Wrench. A Wrench without a DC clamp and the<br>
knowledge that he needs one. A Wrench without the proper PPE because he<br>
"tested it and it was dead" so, even if he had his gear on to "test it", he<br>
took his gloves and face-shield off to work on it. A Wrench who doesn't<br>
fully understand the<o:p></o:p></pre><pre> operation of GFP circuits. A Wrench who doesn't<br>
understand that not all faults are ground faults and the characteristics of<br>
a fault change in terms of potential and magnitude with varying<br>
environmental conditions. A Wrench that doesn't fully understand that power<br>
can be coming from both directions. A Wrench who figures he doesn't have the<br>
time to completely isolate a section of a circuit because there AIN'T NO<br>
REAL DISCONNECTS. I hope it's not your Wrench.<br>
<br>
As the size of the inverter grows, so does the hazard. To a point. The<br>
idiotic 1.56 ISC rule only increases the potential hazards. Central-inverter<br>
plants should not be serviced by anybody who doesn't have an extremely<br>
comprehensive understanding of these systems, and the tools and PPE to<br>
safely work on it. For systems with inverter-integral re-combiners, the most<br>
dangerous spot in these systems are the feeders between string combiners and<br>
re-combiners.<o:p></o:p></pre><pre> Anything between the output of a string combiner and the input<br>
of a re-combiner. For systems with standalone re-combiners, a fault between<br>
the re-combiner output and the line side of the next disconnect is the most<br>
dangerous point, but certainly not the only dangerous point. If either of<br>
these systems are built without load-break disconnects at the<br>
string-combiner level, the cost to service goes thru the roof. It either<br>
goes thru the roof to do it safely or it goes thru the roof in terms of risk<br>
to do it not safely. Pick one.<br>
<br>
There is an interesting dynamic between the potential hazard on a faulted DC<br>
homerun feeder and the kW of the inverter. The less re-combiner inputs you<br>
use, the greater the potential hazard on faulted input feeders. Again, this<br>
is because of the UNSAFE AND STUPID 1.56 ISC rule. In systems with a<br>
relatively low number of re-combiner inputs, there are large portions of<br>
time when there isn't<o:p></o:p></pre><pre> enough combined amperage in the non-faulted feeders to<br>
blow the re-combiner fuse of the faulted feeder. If your system only has 4<br>
or 5 re-combiner inputs and it's winter-time, it is quite likely that a<br>
faulted feeder is being fed from both ends. (Commonly 100A fuses in the<br>
re-combiner with ~60A ISC feeding a string-combiner) That feeder can be fed<br>
from the re-combiner end, by anything up to about 105% of the fuse rating,<br>
for pretty much ever without blowing the fuse. The more parrallel inputs<br>
there are, the more likely there will be sufficient current generated by the<br>
other feeders to blow the fuse. Since the vast majority of systems out there<br>
don't have load-break disconnects at the re-combiner inputs, the technician<br>
needs to be able to open disconnects at each string combiner in order to<br>
isolate this feeder. But what about systems without DC service disconnects?<br>
Repair at night?<br>
<br>
My hope is that anybody on<o:p></o:p></pre><pre> this list will refuse... Say it with me now...<br>
R-E-F-U-S-E to install PV systems without adequate disconnect provisions to<br>
isolate faulted feeders. And only allow technicians with proper knowledge<br>
and equipment to work on a busted PV system. "Journeyman electrician" does<br>
NOT automatically mean that person has the proper knowledge to do it safely.<br>
Safely working on a faulted PV DC circuit requires ALWAYS clamping the thing<br>
for starters. It might also mean "not working" on it until the sun goes<br>
down. A technician with the proper knowledge and equipment should be able to<br>
determine the proper course of repair.<br>
<br>
In the case of the faulted lightning arrestor, it was "only" a small<br>
circuit, but it got the guy's attention and apparently nobody got hurt. The<br>
bigger these systems get, the bigger the potential hazard. <br>
<br>
To answer Tom's question about jumping around a fault: Maybe, maybe not,<br>
depending on the nature of<o:p></o:p></pre><pre> the fault (+/-, +/G, -/G) and the location of the<br>
jumper relative the fault and the power source. Even if jumping to ground<br>
eliminates the arcing when you are working with the terminal, you will still<br>
have arcing when you land/un-land the jumper &/or remove the fault. If the<br>
sun is shining and you have a DC fault, you will have arcing at some point<br>
when you make/break the circuit. Hopefully it's safely contained and<br>
localized to the contacts of a service disconnect!<br>
<br>
Pray for Sun!<br>
<br>
Matt Lafferty<o:p></o:p></pre>
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