[RE-wrenches] 1.56 ISC Minimum OCP is STUPID! (Was: ground fault troubleshooting)
Nick Soleil
nicksoleilsolar at yahoo.com
Wed Sep 8 10:33:32 PDT 2010
Hi Matt:
How are things. We met a John Berdners house many years ago (when SMA was
based there.) Hope you are great.
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.
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.
Nick Soleil
Project Manager
Advanced Alternative Energy Solutions, LLC
PO Box 657
Petaluma, CA 94953
Cell: 707-321-2937
Office: 707-789-9537
Fax: 707-769-9037
________________________________
From: Matt Lafferty <gilligan06 at gmail.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Sent: Tue, September 7, 2010 7:25:52 PM
Subject: Re: [RE-wrenches] 1.56 ISC Minimum OCP is STUPID! (Was: ground fault
troubleshooting)
Hi Kent,
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 ;)
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.
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.
To your contention that "1.56 ISC isn't in any way responsible for the
danger"....
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.
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.
http://www.littelfuse.com/data/en/Data_Sheets/KLKD.pdf 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.
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?
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.)
Another situation that I am sadly familiar with has been burning holes in a
steel roof since 2007 (I told them not to do it over and over)... 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!!!!
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!
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.
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.
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.
My NON-CODE-COMPLIANT-SELF prefers to size OCP at "ISC or next larger standard
fuse rating not larger than the LISTED Series Fuse Rating of the module".
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.
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... "The Code requires 1.56 ISC so I have to require it."
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.
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")
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.
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". (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.)
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.)
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?
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.
One RESPONSIBLE way we can minimize the damage is to reduce the fuse size by
~40%. (i.e. ISC or next higher standard fuse rating) 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.
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...)
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.
Pray for Sun!
Matt Lafferty
gilligan06 at gmail.com
________________________________
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent Osterberg
Sent: Tuesday, September 07, 2010 10:59 AM
To: RE-wrenches at lists.re-wrenches.org
Subject: [RE-wrenches] ground fault troubleshooting
Matt,
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.
--
Kent Osterberg
Blue Mountain Solar
541-568-4882
www.bluemountainsolar.com
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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