[RE-wrenches] Finding ground faults

Tue May 22 20:57:57 PDT 2012

It is enviable that you are in a situation to narrow the ground fault down to 4 panels by opening fuses. Good situation. Below is the best string on the topic.

Nick Vida

---------------------------- Original Message ----------------------------
Subject: [RE-wrenches] Getting "GFDI Open" Message When It's Not
From:    "Jeff Clearwater" <jeffc at villagepower.com>
Date:    Sun, September 21, 2008 10:42 am
To:      "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
--------------------------------------------------------------------------

Good Day All,

I'm getting a "GFDI Fuse Open" message on a SB3000 and the fuse is good.

What I've checked:

1) I've checked POS to GND and NEG to GND both floating and when
connected to the integral DC Disco both for continuity and voltage .
I show expected Voc on both strings and no continuity to GND nor
voltage to GND when the fuse is pulled.  There appears to be no
ground faults or miswiring.

2)  I've visually inspected all array wires - no pinching can be
seen.  I've checked for Voltage and Continuity to GND from both the
array side and the inverter side to isolate the wires in the conduit
- no problems there either.

3)  I've replaced the fuse even though it shows good with a good fuse
with no change.

4)  I've cycled the SB several times - no change.

5)  I've run through John B's method below twice - no change.

6)  I've confirmed that the GFDI fuse is in the top position of the
two possible GFDI position - the one for negative ground.

About the only thing I forgot to do was check if the Jumper for
Negative or Positive GND was in the correct position - it dawned on
me after coming home that if somehow that got set wrong at the
factory that it would perhaps show a fault but not blow a fuse?
Could this be it?  (I hope!)

Anyone experience this condition before?

Next step is to just swap out the inverter but as it's Sunday SMA
tech support is probably not open so I hesitate to do that without
their go ahead.

Thoughts?

Thanks!

Jeff C.

At 12:18 PM -0800 11/21/07, John Berdner wrote:
Subject: RE: SMA Ground Fault troubleshooting advice. [RE-wrenches]

>
>Dave/Wrenches:
>
>Locating ground faults in the array is fairly straight forward albeit a
>little dangerous.
>A DVM and some spare fuses (just in case) is all you really need.
>Be very careful when you are doing all of this because you will be
>working with live wires hanging in space and PV negative is no longer
>necessarily at ground potential.  If the equipment grounding on the
>array is not up to snuff it is also possible to see hazardous voltages
>on the module frames or structure so measure everything and be really
>careful.  Treat everything as hot and potentially lethal until your
>meter says otherwise. Remember each time you disconnect something the
>whole situation can change so measure again to be sure.  Also, depending
>where the ground fault is and the order in which you pull the wires out
>of the inverter (or disconnect) it is possible to see double the array
>Voc.  This can be a very bad thing since the spacing on all components
>was designed for 600 Vdc max and you could be at double that.  A 960 Vdc
>arc at  5 to 8 Amps is pretty ugly.
>
>Start by opening the DC disconnect and removing the GFDI fuse in the
>inverter.
>Measure the array open circuit voltage form the PV plus in the dc
>disconnect to the PV negative in the inverter.
>Lets say it is 480 Vdc.
>Now measure PV positive to ground and then the PV negative to ground.
>An array without a ground fault should give you roughly equal values of
>around 240 Vdc but the measurement should decay down towards zero.  What
>you are seeing is a discharge of the capacitive coupling between the
>array and ground.
>
>In an array with a ground fault you will measure a fixed voltage to
>ground on both the PV positive and PV negative.
>The sum of the PV positive to ground and PV negative to ground
>measurements should equal the open circuit voltage.
>The difference in this case is that the voltage will be fixed and will
>not decay towards zero.
>The voltages you measure actually tell you where to start looking for
>the ground fault.
>
>For example: with 12 modules in series and an open circuit voltage of
>480, each module is equal to 40 Vdc.
>If you measure PV positive to ground and get 0 volts then you should
>see a PV negative to ground voltage of - 480 Vdc.  This tell you the
>ground fault is in the PV positive output conductor.
>Alternatively, if you were to see PV positive to ground at 400 Vdc and
>PV negative to ground at -80 Vdc this tells you the ground fault is
>between the 2nd and 3rd modules "up" from the PV negative.
>
>To figure out which string the ground fault is in, isolate each string
>(remove the wires from the disconnect/inverter) and repeat the PV
>positive to ground and PV negative to ground measurements.  On one
>string the measurement will float around and the other will give you a
>fixed measurement to ground.  The fault is in the string that has the
>fixed measurement to ground.  My suggestion is to pull both positive and
>negative wires for string 1 and measure.  If you are lucky you picked
>the right string the first time out and you don't need to pull the
>second string at all.
>It is unlikely but also possible to see a ground fault in the "middle"
>of a module.  In this case you might see 380 Vdc from PV positive to
>ground and -100 Vdc from negative to ground.  This would indicate a
>fault in the middle of the third module "up" from the PV negative.
>Normally this means the module itself has a ground fault due to a nick
>in the backskin, fault in the edge seal, etc. You can verify this by
>isolating (unplugging) the suspect module then measuring PV positive to
>ground and PV negative to ground for that isolated module.
>
>
>Lastly, In some very rare case the ground fault will actually be in the
>string combiner, dc disconnect, or (heaven forbid) in the inverter
>itself. You can verify this if the array voltage measurements show a
>ground fault when connected to the combiner/disconnect/inverter and then
>both strings float when measured in hanging in free space.  While
>unlikely, if you find this situation reconnect the PV back up then
>disconnect one thing at a time and measuring relative to PV positive and
>PV negative to ground until you find where the fault is.
>
>Best Regards,
>
>John Berdner
>
>
>
>>>>  ipl at sover.net 11/21/2007 07:40:51 AM >>>
>
>Wrenches,
>
>I have a grid tied system with a SB3800U inverter being fed by 2
>strings of
>(12) Evergreen 180's (4.32kW). UniRack rail mounted on garage roof.
>Good
>installation all wiring neat and well supported. Only DC bond to ground
>is
>in the inverter etc. And no external monitoring equipment.
>
>The system has been operating very well (over 5,000 kWh over 2 months
>in
>Northern Vermont), this is the first problem.
>
>We are in the middle of a snowy spell and the array is covered in snow,
>but
>I should be able to check out the array within a few days. According to
>the
>error message on the Sunny Boy the GFDI 1 amp fuse is open. The array
>has
>made it through lots of rain, ice and snow over the past year.
>
>My suspicion is in the MC connector(s) themselves. We had a bad batch
>of MC
>cables purchased from a large distributor 12 to 18 months ago. The MC
>connector would pull off the wire when tested. Of course not all of
>them
>where tested before they got into the field, so one could be on this
>roof.
>
>Obviously I will check connections in the combiner, the dc disconnect
>etc,
>could be a moisture issue. Of course it could be a squirel I guess.
>
>Any advice on troubleshooting this ground fault would be appreciated.
>Safety
>wise do we need to cover the array with a dark tarp?
>
>Dave
>
>
>
>
>--
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--
~~~~~~~~~~~~~
Jeff Clearwater
Senior Design Engineer
NABCEP (tm) Certified Solar PV Installer
http://www.nabcep.org/
Village Power Design
Turnkey Solar Design & Installation for the Commercial Sector
http://www.villagepower.com
gosolar at villagepower.com

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