[RE-wrenches] 1.56 ISC Minimum OCP is STUPID! (Was: ground fault troubleshooting)

[Matt Lafferty]

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 <http://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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