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<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>Hi Kent,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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 ;)</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial><SPAN class=093142120-07092010><FONT color=#0000ff size=2
face=Arial>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.</FONT></SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial><SPAN
class=093142120-07092010></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial><SPAN class=093142120-07092010>To your contention that "1.56
ISC isn't in any way responsible for the danger"....</SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial><SPAN
class=093142120-07092010></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>For those who have not studied Time:Current curves of commonly
used fuses, you should. </FONT></SPAN><SPAN class=093142120-07092010><FONT
color=#0000ff size=2 face=Arial>KLKD is a typical fuse I hope we are all
familiar with. <A
href="http://www.littelfuse.com/data/en/Data_Sheets/KLKD.pdf">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. </FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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? </FONT></SPAN><SPAN
class=093142120-07092010><FONT color=#0000ff size=2 face=Arial>Will your
conductor hold up during this time?</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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. </FONT></SPAN><SPAN class=093142120-07092010><FONT
color=#0000ff size=2 face=Arial>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.)</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>Another situation that I am sadly familiar
with has been burning holes in a steel roof since 2007 <EM>(I told
them not to do it over and over)</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. </FONT></SPAN><SPAN
class=093142120-07092010><FONT color=#0000ff size=2 face=Arial>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). </FONT></SPAN><SPAN
class=093142120-07092010><FONT color=#0000ff size=2 face=Arial>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!!!!</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT size=2
face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010></SPAN><SPAN
class=093142120-07092010><FONT color=#0000ff size=2 face=Arial>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. </FONT></SPAN><SPAN
class=093142120-07092010><FONT color=#0000ff size=2 face=Arial>The key here is
that the wiring and OCP in ALL OF THESE CASES ARE CODE
COMPLIANT!</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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</FONT></SPAN><SPAN class=093142120-07092010><FONT color=#0000ff size=2
face=Arial>, 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
</FONT></SPAN><SPAN class=093142120-07092010><FONT color=#0000ff size=2
face=Arial>Locked Rotor Amps. I'm sure you can imagine what this would do to the
size and cost of starters, etc. </FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT size=2
face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010></SPAN><SPAN
class=093142120-07092010><FONT color=#0000ff size=2 face=Arial>My
NON-CODE-COMPLIANT-SELF prefers to size OCP at <EM>"ISC or next larger standard
fuse rating not larger than the LISTED Series Fuse Rating of the module"</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. </FONT></SPAN><SPAN
class=093142120-07092010><FONT color=#0000ff size=2 face=Arial>Which is exactly
what we want. Reliability during normal operation and
safety.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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>"The Code requires 1.56 ISC so I have to require it."</EM>
</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial><EM></EM></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>String inverters only bother me so much in this regard.
Central inverter systems are where the real bad ju-ju starts to happen. <SPAN
class=093142120-07092010><FONT color=#0000ff size=2 face=Arial>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.
</FONT></SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010></SPAN><SPAN
class=093142120-07092010><FONT color=#0000ff size=2
face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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")</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>I have zero tolerance for crappy workmanship and even less
sympathy for the people who do it. </FONT></SPAN><SPAN
class=093142120-07092010><FONT color=#0000ff size=2 face=Arial>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.
</FONT></SPAN><SPAN class=093142120-07092010><FONT color=#0000ff size=2
face=Arial>Every set of wires that has been pulled out has obvious physical
damage. </FONT></SPAN><SPAN class=093142120-07092010><FONT color=#0000ff size=2
face=Arial>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. </FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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>(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.)</EM></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>The scary thing is, this practice goes on every day. A LOT!
</FONT></SPAN><SPAN class=093142120-07092010><FONT color=#0000ff size=2
face=Arial>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.)</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT size=2
face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010></SPAN><SPAN
class=093142120-07092010><FONT color=#0000ff size=2 face=Arial>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?</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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.
</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>One RESPONSIBLE way we can minimize the damage is to reduce
the fuse size by ~40%. <EM>(i.e. ISC or next higher standard fuse rating)</EM>
This method will provide adequate operational reliability. It will
also </FONT></SPAN><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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...)</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>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.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial>Pray for Sun!</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=093142120-07092010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><STRONG><I><SPAN
style="FONT-FAMILY: 'Monotype Corsiva'; mso-bidi-font-family: Tahoma">Ma<SPAN
class=093142120-07092010>t</SPAN>t Lafferty</SPAN></I></STRONG></DIV>
<DIV class=Section1>
<P class=MsoNormal><SPAN style="FONT-FAMILY: Arial; FONT-SIZE: 10pt"><A
href="mailto:gilligan06@gmail.com">gilligan06@gmail.com</A></SPAN><BR></P></DIV>
<DIV dir=ltr lang=en-us class=OutlookMessageHeader align=left>
<HR tabIndex=-1>
<FONT size=2 face=Tahoma><B>From:</B> 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<BR></FONT><BR></DIV>
<DIV></DIV>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.<BR><BR>
<DIV class=moz-signature>-- <BR>Kent Osterberg<BR>Blue Mountain
Solar<BR>541-568-4882<BR><A
href="http://www.bluemountainsolar.com/">www.bluemountainsolar.com</A><BR><BR><BR><PRE>Wrenches all,
I 100% second Bill B's comment Correct that... I 200% second it. It should
be the law.... "Don't begin to troubleshoot a faulted PV circuit without a
reliable DC clamp meter."
The MOST DANGEROUS PV system is a wounded PV system. This includes danger to
persons and property. Safely and efficiently troubleshooting a faulted PV
circuit requires a voltmeter AND an ammeter. And PPE. And adequate knowledge
and understanding of operational and non-operational characteristics of PV
systems.
The simple reason for this is that, when one or more circuit conductors are
faulted to a short condition, the voltage between the faulted elements is
zero. Relying on just a voltage reading to determine whether or not to open
a circuit under this condition will result in an arc. The amount of energy
in that arc will depend on the amount of available sunlight and the amount
of PV that is feeding into it. The amount of potential hazard will
correspond to these factors as well.
Using a clamp style ammeter will allow you to understand where and how much
current is flowing in a circuit before you decide to open it. It is one
thing to know you have a 45 amp load in a circuit with a potential of ~450V
because you clamp it before you break it. With this knowledge you can assess
the situation. You can do something to mitigate or remove the potential
hazards... Cover the array, open a disconnect somewhere, put your PPE on and
go for it, select a different location to open the circuit, use insulated
cable cutters, wait 'til dark.... You have choices.
It is quite another to be surprised by the resulting arc in tight quarters
because you measured the voltage and figured it was a dead circuit! When you
react to the startlement (word?) by dropping your screwdriver and yanking
your hand back... Assuming you don't receive a shock, flash injury, or fall
off the roof in the process, of course.... The result just may be that the
now-dislodged conductor is arcing and zapping and spitting. Now you're gonna
have to stick something back into that box to deal with it. In the meantime,
a number of possible things can happen, most of which are not favorable....
Melting insulation and conductor material are the most common. The degree
(not just a pun) of damage and remaining hazard will be determined by the
amount of sunshine and amount of PV feeding into the arc.
The MOST DANGEROUS single point on the DC side of a PV system is ANYPLACE on
the Inverter side of a fuse(s). This is a simple function of the assinine
"1.56 ISC minimum fuse" rule in the NEC. The source cannot create enough
current to blow the fuse(s). If you have a fault between a combiner and the
inverter, you WILL have current flowing into the fault as long as the sun is
up! If you are relying on just a voltmeter in a central-inverter plant, you
could very well be in for a 15-20kW surprise, or greater!
The combination of shi##y wire, sloppy conduit installation, and crappy
wire-pulling methods have resulted in too many DC feeder faults to count. It
boggles my mind every time I hear of yet another guy nearly joining the dead
because he touched or opened up a connection somewhere in a faulted circuit
without de-energizing it. Time and time again I hear that they tested it for
voltage and it was "dead". Sometimes they even opened up the service
disconnect at the string combiner, "just to make sure". Time and again it's
a "journeyman electrician". I like it best when it's the same card-carrying
jackass who "built" the thing.
I consider THWN-2 to be on the list of shi##y wire types for DC, by the way.
I'm an XHHW-2 guy, personally. Why would anybody select an insulation that
is easy to nick/slice/tear when you can have a super-tough insulation for a
couple pennies more? Why would anybody select an insulation that only has
about 5% of the dielectric resistance of one that is a couple pennies more?
Why? Oh, I know... It's that race to the bottom on BOS costs...
Which leads to the next step in stupidity... Designing and building LARGE PV
plants without sufficient DC SERVICE disconnects... This is what's going on
out there.... PV plants with 500kW Central-inverters being installed without
string-combiner disconnects. Without any DC service disconnects.
The NEC considers the fuseholder in the combiner &/or the connector on the
module to be a "disconnect" and does not require a "service disconnect" in
the circuit. So these smart-ass engineers and project developers are out
there building this shi#. Some of these projects are being built by PV
module manufacturers masquerading as developers. "Vertically integrated..."
Others are being designed & built by formerly respected integrators who have
either sold out or lost their conscience altogether. The trend is to build
them to sell to PPA companies who ostensibly own and "operate" them. These
solar timebombs are being built on both sides of the fence. Frosty ain't the
only one with a solar flamethrower!
All in the race to the bottom of the $/Watt pile that they are now calling
LCOE. Har Dee Har Har!
I hate to say this, but I hope somebody gets really hurt out there, and
soon. I hope it's the same smart-ass engineer (or his boss) who thought it
was alright to design this way after some field technician walks away from
it because it's dangerous. And then I hope his family sues the crap out of
the company and companies involved with designing, supplying, building, and
owning it so they STOP DOING THIS SHI#! And then I hope he takes his cooked
carcass on the road doing safety awareness training so others don't repeat
these stupid, avoidable catastrophes! And then I hope these cheap-ass
developers go out to every site that doesn't have sufficient disconnects and
re-fits the systems with them to avoid further injuries and $$$$
settlements. What is the levelized cost of energy for that system now, Mr.
CFO?
Unfortunately it isn't likely to be that smart-ass engineer. Or his boss. It
is far more likely to be a Wrench. A Wrench without a DC clamp and the
knowledge that he needs one. A Wrench without the proper PPE because he
"tested it and it was dead" so, even if he had his gear on to "test it", he
took his gloves and face-shield off to work on it. A Wrench who doesn't
fully understand the operation of GFP circuits. A Wrench who doesn't
understand that not all faults are ground faults and the characteristics of
a fault change in terms of potential and magnitude with varying
environmental conditions. A Wrench that doesn't fully understand that power
can be coming from both directions. A Wrench who figures he doesn't have the
time to completely isolate a section of a circuit because there AIN'T NO
REAL DISCONNECTS. I hope it's not your Wrench.
As the size of the inverter grows, so does the hazard. To a point. The
idiotic 1.56 ISC rule only increases the potential hazards. Central-inverter
plants should not be serviced by anybody who doesn't have an extremely
comprehensive understanding of these systems, and the tools and PPE to
safely work on it. For systems with inverter-integral re-combiners, the most
dangerous spot in these systems are the feeders between string combiners and
re-combiners. Anything between the output of a string combiner and the input
of a re-combiner. For systems with standalone re-combiners, a fault between
the re-combiner output and the line side of the next disconnect is the most
dangerous point, but certainly not the only dangerous point. If either of
these systems are built without load-break disconnects at the
string-combiner level, the cost to service goes thru the roof. It either
goes thru the roof to do it safely or it goes thru the roof in terms of risk
to do it not safely. Pick one.
There is an interesting dynamic between the potential hazard on a faulted DC
homerun feeder and the kW of the inverter. The less re-combiner inputs you
use, the greater the potential hazard on faulted input feeders. Again, this
is because of the UNSAFE AND STUPID 1.56 ISC rule. In systems with a
relatively low number of re-combiner inputs, there are large portions of
time when there isn't enough combined amperage in the non-faulted feeders to
blow the re-combiner fuse of the faulted feeder. If your system only has 4
or 5 re-combiner inputs and it's winter-time, it is quite likely that a
faulted feeder is being fed from both ends. (Commonly 100A fuses in the
re-combiner with ~60A ISC feeding a string-combiner) That feeder can be fed
from the re-combiner end, by anything up to about 105% of the fuse rating,
for pretty much ever without blowing the fuse. The more parrallel inputs
there are, the more likely there will be sufficient current generated by the
other feeders to blow the fuse. Since the vast majority of systems out there
don't have load-break disconnects at the re-combiner inputs, the technician
needs to be able to open disconnects at each string combiner in order to
isolate this feeder. But what about systems without DC service disconnects?
Repair at night?
My hope is that anybody on this list will refuse... Say it with me now...
R-E-F-U-S-E to install PV systems without adequate disconnect provisions to
isolate faulted feeders. And only allow technicians with proper knowledge
and equipment to work on a busted PV system. "Journeyman electrician" does
NOT automatically mean that person has the proper knowledge to do it safely.
Safely working on a faulted PV DC circuit requires ALWAYS clamping the thing
for starters. It might also mean "not working" on it until the sun goes
down. A technician with the proper knowledge and equipment should be able to
determine the proper course of repair.
In the case of the faulted lightning arrestor, it was "only" a small
circuit, but it got the guy's attention and apparently nobody got hurt. The
bigger these systems get, the bigger the potential hazard.
To answer Tom's question about jumping around a fault: Maybe, maybe not,
depending on the nature of the fault (+/-, +/G, -/G) and the location of the
jumper relative the fault and the power source. Even if jumping to ground
eliminates the arcing when you are working with the terminal, you will still
have arcing when you land/un-land the jumper &/or remove the fault. If the
sun is shining and you have a DC fault, you will have arcing at some point
when you make/break the circuit. Hopefully it's safely contained and
localized to the contacts of a service disconnect!
Pray for Sun!
Matt Lafferty
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