[RE-wrenches] ground fault troubleshooting

Mon Sep 6 13:04:35 PDT 2010

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

  _____  

From:  Bill Brooks
Sent: Monday, April 05, 2010 8:12 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] ground fault troubleshooting

Tom,

Your dc clamp-on meter would have saved you from removing a conductor that
had current on it. It is likely a lightning event caused the LA602 to do its
job-thus the cracked case. The dc clamp-on meter would have shown the
current flowing into the SOV and on to ground.

Bill.

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Tom DeBates
Sent: Monday, April 05, 2010 7:04 PM
To: re-wrenches at lists.re-wrenches.org
Subject: [RE-wrenches] ground fault troubleshooting

hello Wrenches,
  We recently had an interesting (amusing?) experience that I thought might
be worth sharing and to get some feedback.
  I received a call from a fellow contractor that had a ground fault in his
PV system (Wattsun DA tracker, 24 Siemens (Shell?)110, 1 string, Sunny Boy
2500). The GF fuse in the Sunny Boy had blown. Another installer and I went
to his site to try to determine what the cause was. fortunately the Ground
Fault article by Paul Mync had just been in Solar Pro and we used this as a
template for troubleshooting the problem. To make a long story short, we
spent the better part of a day troubleshooting and still could not solve the
problem. What seemed to be happening was that we were getting a different
ground fault current based upon how many modules we had eliminated for the
string and it seemed to be approximately proportional. 
  A few days later our friend was going to run through the procdedure again
and take better notes. Since I was not there, the details may be a bit
sketchy, but basically are as follows. As he reconnected the
positive(ungrounded) home-run at the module j-box, the lightning arrestor
(Delta LA602DC), proceeded to "go off like a Roman candle". As he pulled the
conductor off the terminal he got a nice arc between them. The obvious thing
to do was to eliminate the arrestor.... that solved the problem. Upon
further inspection, the arrestor had a crack in the case. Had the arrestor
had a slight fault (due to moisture) initially that lead to leakage current
based upon voltage applied and then finally "fully-faulted"?
  At least two lessons here:
1. When troubleshooting a GF, always eliminate any accessories from the
system.
2. When dealing with a faulted array never assume that opening the grounded
conductor will open the circuit. Just wondering...would a jumper form
grounded conductor to ground had prevented the acring he experienced?
thanks,
tom

Tom DeBates
Habi-Tek
524 Summit St.
Geneva,IL. 60134
630-262-8193
fax 630-262-1343

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