Author Topic: Classic / MVPV6 grounding & ground fault protection questions  (Read 1376 times)

daklein

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I have a DC Solar trailer that I installed components from into a house off-grid (and charge from grid if needed in winter) battery backup system.  I initially set things up basically like on the trailer, but now I think I don't want the combiner box grounded the same way.  Maybe how it was wired on the trailer makes sense for a portable system, see picture of trailer combiner box wiring.   

I think on the trailer, if the PV+ shorts out, there would be no GFP warning, just the short circuit PV current potentially arcing or whatever.  On the trailer, there's not much to catch fire, the wiring can handle the panel short circuit current, and the PV- and Batt- are somewhat bonded to the trailer (through the ground bar in the combiner box, mounted on the metal plate, not sure there's an actual bond to the trailer frame).  Everything debatable in the right forum, but safe enough probably.  I think most people with the trailers wouldn't realize the Classic's GFP is defeated.

  There was a nice big #6 wire connecting the PV- and Gnd bus bars in the combiner box.  This does make sense, is that the traditional requirement, right?  To ground the PV-, there should be a single ground connection (and since the PV- and Batt- are both on the common neg in the Classic, this is where the Batt- 48v- is grounded in the trailers).   However, the Midnite CC has ground fault detection (Not same as GFCI in a house, just turn off CC so maybe no PV current and maybe prevent a fire), and that detection requires _not_ to have another connection between PV-(Bat-) and Gnd.  The Midnite CC becomes the single bond between Gnd & PV-(Batt-).       So that #6 wire going straight across defeats the Classic's GFP feature, right?   

Currently, with the system installed in garage, I have removed that bond in the combiner box, and the Classic GFP is turned on and the little jumper is installed.   So I should get a warning noise and Midnite should turn off, if there's a couple amp connection between PV+ and GND.   

Can I test this to confirm it works?  Make a connection between GND and PV+ on the CC side of breaker, turn one or more PV+ breakers back on and there should be beeping and Classic turns off and says GFP?   The connection I'd make would be say #10 wire capable to handle the PV current.    I have 3 parallel strings of 3 305w panels on the 250-CP Classic (no AFCI, but GFP should be active). 

Comments?   Thanks!!


daklein

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Re: Classic / MVPV6 grounding & ground fault protection questions
« Reply #1 on: April 04, 2021, 09:12:42 AM »
maybe I should have put this in the Classic section

Vic

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Re: Classic / MVPV6 grounding & ground fault protection questions
« Reply #2 on: April 05, 2021, 07:39:11 PM »
Hello daklein,

So,  it sounds like this a Backup system to an existing Grid-powered home.

Assume that that the main grid power system/panel has been inspected,  but this backup system will not be inspected, (assumptions).

Seems that there is a formal EGC (green wire) missing in/on that Combiner.   This would tie into the main panel.   All metal enclosures need to be connected together with a cable,   and meet NEC requirements (assume (yet again),  that you are in the US)).    I am not a Code expert.

Looks like there are two cables on CB4,  under one screw terminal.   This is probably fine for a test lash-up,   but  cannot be Code,  IMO.   You could use an insulated busbar to land the two cables,   and a single wire to that CB.   Guess that CB4 is the combined PV output,  leading to the CC.  Or,  you might be able to modify an MN Combiner output busbar  --  shortening it by cutting off one end,  and making it a "skip-tooth",  by removing one of the "teeth" that would normally connect to single-wide DIN breakers,  if you know what am trying to say.

Not to nit-pick too much,   one other little thing here or there might need a look  ...

Think that you are correct.   The safety grounding aspects of trailer-mounted DC/AC power systems,   are probably different than are most residential power systems.    The green wire is your friend.

Again,  am not  Code guru.   Good Luck,  Vic
Off Grid - Two systems -- 4 SW+ 5548s, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 3X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  Thanks MN for Great Products/Service/Support!

daklein

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Re: Classic / MVPV6 grounding & ground fault protection questions
« Reply #3 on: April 05, 2021, 11:41:10 PM »
Thanks Vic,   That combiner box picture is from one of the trailers (not hooked to a house), they are all set up about like that.  Yes they didn't have an EGC equipment ground back to the combiner box, that I've seen.   The main ground - neutral bond of the trailer was in the AC inverter output panel (for a trailer system, that's the power source), and that is on the same mounting plate as the combiner.  And the point about the multiple wires on the PV combined breaker is correct too, should have a busbar or something else.

Thanks for the nitpicks, May I have another?   Here's a picture of the combiner box now mounted in the house.   

I probably could do with a PV+ combiner busbar on the breakers outputs.  For now, I butt crimp spliced together a PV+ combined harness of the MNSPD pos, and #10 from each breaker to #6 input to the CC.   (I just ordered a MNPV6-250 BUSBAR, that will be better).

 :-[   Nevermind the garden hose and pex are the last two feet into the combiner box, from EMT where another box is to be installed with AFCI items in it.   The 3 PV strings pos & neg/gnd are in separate EMT from the rooftop Soladeck box.

There's an EGC #6 stranded bare to the ground bar, back through other items on the wall, inverters, battery + breaker box, to a ground rod outside and rest of house ground from AC panels and out to the N-G bond at main service.  In the combiner box, #6 to the CC case ground, and #10 ground to the PV array and the MDSPD.  The PV array & racking is all weeb'ed / bonded and has ground down to another rod.

There's no direct connection between  PV-/Batt-  and GND in the combiner or elsewhere, just the Classic with GFP turned on and little jumper installed to right of screw terminals. So I think now the Classic GFP would work.  (That's still my main question:  if GFP should work now and how to test it.)
« Last Edit: April 06, 2021, 12:18:19 AM by daklein »

Vic

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Re: Classic / MVPV6 grounding & ground fault protection questions
« Reply #4 on: April 06, 2021, 01:03:13 PM »
Hi daklein,

Yes,  knew that the main question that you were asking,   has not yet been answered  ...

Am not a fan of Ground Fault interruption,  and only use it to pass inspections,   when needed.

Distracted,  here,  right now.

Here is a long discussion on this topic,   from ten years ago,  on this Forum:
http://midniteftp.com/forum/index.php?topic=142.0

A very smart Engineer has written a White Paper of GF.  This is Linked in the body of that discussion,  but here that is:
http://www.midnitesolar.com/pdfs/DC-GFP-Draft3-5.pdf

EDIT:  OK,  that Link IS broken.   Tried hacking "midniteftp.com/forum/"    in place of the front part of that Link,  but that was not quite it.  There have been a number of Server changes,  over the years.  Perhaps that paper did not migrate,  or more likely,  it is here,  but my guess of the preamble is wrong   ... did Search on the Wind-Sun Forum,  but could not find it there,   either   ...

More later,  hope that the old GF White Paper Link above,  still works  (EDIT),   alas,  that Link does NOT work.    Vic
« Last Edit: April 06, 2021, 04:55:45 PM by Vic »
Off Grid - Two systems -- 4 SW+ 5548s, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 3X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  Thanks MN for Great Products/Service/Support!

Vic

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Re: Classic / MVPV6 grounding & ground fault protection questions
« Reply #5 on: April 06, 2021, 04:58:13 PM »
Will add this as a separate reply:

BB Bill,  SuperModerator on the Wind-Sun Forum, was kind enough to provide a Link to that GF White Paper:

https://us.v-cdn.net/6024911/uploads/attachments/512/1965.pdf

For those who are curious.   Later,   Vic
Off Grid - Two systems -- 4 SW+ 5548s, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 3X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  Thanks MN for Great Products/Service/Support!

daklein

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Re: Classic / MVPV6 grounding & ground fault protection questions
« Reply #6 on: April 07, 2021, 11:31:43 AM »
Thank you very much Vic for explaining and finding the white paper.   (and thanks to BB Bill,  Ryan, Robin & Bob for writing that up)     

I had read the 10yr old GF discussion thread, but I didn't quite get the shortcoming of NEC DC GFP until reading that white paper.  The diagrams are very helpful.   

So I will not test the GFP in the Classic as I have it set up now.   I have a RPi hooked up to the Classic and the inverters.  It might be isolated, but still that may not like a 'test', or I might touch Batt- thinking it's safe which it's not at that time. 

I'll put back a hard ground between Batt- and EGC.   Effectively like it was in the trailer, with PV-/Batt- hard wired to EGC, but directly at the battery.

I'll remove the GFP jumper in the Classic, so there are not two neg paths back to battery   (batt-,  and Pv-/Batt- to EGC to batt-)

I'll get AFCI detection set up as planned to shut off the PV, to have some protection & notification of any chewed or loose PV cables.  A full content Classic would have anyway.

Vic

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Re: Classic / MVPV6 grounding & ground fault protection questions
« Reply #7 on: April 07, 2021, 03:12:22 PM »
Hi daklein,

Yes,  BB Bill is a very sharp engineer,  and gives much of his time,  tirelessly (31,600+ Posts, there),  on the Wind-Sun Forum,  as you probably know.

That White Paper is very well done,  appreciate all of his efforts,  and those of Robin,  boB and Ryan,  of MidNite.

Your bottom line on wiring your PV-based power system sound pretty good to me.

Guess that the R Pi for the Classic/s is mostly because the Classics are 250 CPs?

Good luck with the projects.  Missed the auction auction activities on these DC Solar trailers/hardware.  Interesting building-blocks.    Thanks,  Vic



« Last Edit: April 07, 2021, 03:14:22 PM by Vic »
Off Grid - Two systems -- 4 SW+ 5548s, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 3X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  Thanks MN for Great Products/Service/Support!

daklein

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Re: Classic / MVPV6 grounding & ground fault protection questions
« Reply #8 on: April 08, 2021, 09:14:44 AM »
Thanks Vic,
      The RPi is running the ClassicMQTT code to get and log from the CC.   There is a CAN adapter that is listening to the SMA CAN messages which some are obvious and figured them out.   I have a rs485 piggyback board for the SMA and an 485-usb adapter that I'll hook up to the RPi in the future, that should let me change parameters in the SMAs.     For now, I marginally have enough info to do what I want:  logging and graphing, and turn on/off some other dump loads in the house when the SMA's main batteries are around target voltage, to avoid them kicking off the AC coupled solar microinverters.

       I switched the grounding last night, seems to work this morning still.

       There are still some trailers being sold, maybe for a while.  They still go for a little more now than before, but still a good deal for what's on them.  The auction company bought them all and is reselling them in smaller quantities each week or two.    bidindustrial.com/dc-solar-mobile-solar-generator-trailers/

Thanks,   Dale

ClassicCrazy

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Re: Classic / MVPV6 grounding & ground fault protection questions
« Reply #9 on: April 08, 2021, 11:01:30 AM »
Thanks Vic,
      The RPi is running the ClassicMQTT code to get and log from the CC.   There is a CAN adapter that is listening to the SMA CAN messages which some are obvious and figured them out.   I have a rs485 piggyback board for the SMA and an 485-usb adapter that I'll hook up to the RPi in the future, that should let me change parameters in the SMAs.     For now, I marginally have enough info to do what I want:  logging and graphing, and turn on/off some other dump loads in the house when the SMA's main batteries are around target voltage, to avoid them kicking off the AC coupled solar microinverters.

       I switched the grounding last night, seems to work this morning still.

       There are still some trailers being sold, maybe for a while.  They still go for a little more now than before, but still a good deal for what's on them.  The auction company bought them all and is reselling them in smaller quantities each week or two.    bidindustrial.com/dc-solar-mobile-solar-generator-trailers/

Thanks,   Dale

I looked at some of those video's on the link . Sounds like a deal if  you got a good bid.
And also I had never heard about that company that swindled lots of people.

Larry
system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
Midnite KID MPPT 24 DC in to  12 volt AGM batterie

Vic

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Re: Classic / MVPV6 grounding & ground fault protection questions
« Reply #10 on: April 10, 2021, 11:39:03 AM »
Hi Dale,

Thanks for the info on your Monitoring approach.   Sounds like you have a good handle on that.

Thanks,  too for the Link to the DC Solar Auction trailer site.

Good Luck with the system.    Vic
Off Grid - Two systems -- 4 SW+ 5548s, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 3X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  Thanks MN for Great Products/Service/Support!