Grounding Loop conflict with Magnum 4448PAE inverter install?

I am in the process of installing a new off-grid system at my home/home business.  While wiring the inverters (two Magnum 4448PAE's operating in a parallel stack) I noticed that the inverters have both a DC and AC ground that is common to the case.  The manual says to connect both.  Since I have two inverters and my DC ground is linked to my AC ground via a jumper between the AC/DC panels and my PV/DC ground is already linked in my DC panel via a 1amp trip ground fault protector (midnite DC-GFP 100) hooking up both would effectively create two ground loops between AC and DC ground.  In addition to this problem I also have two midnite solar 150 classic charge controllers, the install instructions for the DC-GFP 100 require that the PV ground be grounded to the case of the charge controllers (which makes sense) however, all of these components are in close proximity to each other and are connected via metal conduit and boxes.  So, now the result looks like 2 AC/DC ground loops plus 2 links directly from the PV array ground to AC/DC ground.

My goal was too keep the PV array ground isolated until it jumps via one jumper to the DC ground buss via the DC-GFP 100, the AC ground would also join to the DC ground buss at this point and from the DC ground buss go to the grounding rod - according to NEC as best as I can read it.

I have thought about not connecting the AC ground to the inverters and just connecting the DC ground - since it is connected to the AC ground buss via a jumper.  This would prevent the two loops AC/DC ground loops.  However it would still leave me with the problem of the PV ground being connected to the DC ground via the charge controller case and conduit (.3 Ohms, checked with a good meter). 

I have commercial experience with inverter installs and residential wiring but I have never installed a residential off-grid solar system before.

I am in a high lightning area and the arrays are mounted on the metal roof of my workshop, so I want to get the grounding system right - erroring on the side of lightning protection over NEC - which I know sometimes conflicts.  I do have 3 midnite solar MNSPD's (voltage clamps) installed (one on the main AC panel and one on each of my two solar combiners).

Thanks for reading, please let me know if you have any ideas?



Comments

  • cupcakecupcake Solar Expert Posts: 254 ✭✭✭
    Just ground all your DC stuff together (typcially in a dc shutoff box, etc) then take that final DC ground and stick it on your AC grounding bus bar... and from there it goes to earth (burried rods)

    :)

    ~1.5Kw PV in parallel
    Morningstar MPPT-60 controllers (2) in parallel
    3 Trojan tr-1275's in parallel 450ah total
    Samlex 2,000 watt 12-volt inverter hardwired


  • VicVic Solar Expert Posts: 2,976 ✭✭✭✭
    I am in the process of installing a new off-grid system at my home/home business.  While wiring the inverters (two Magnum 4448PAE's operating in a parallel stack) I noticed that the inverters have both a DC and AC ground that is common to the case.  The manual says to connect both.  Since I have two inverters and my DC ground is linked to my AC ground via a jumper between the AC/DC panels and my PV/DC ground is already linked in my DC panel via a 1amp trip ground fault protector (midnite DC-GFP 100) hooking up both would effectively create two ground loops between AC and DC ground.  In addition to this problem I also have two midnite solar 150 classic charge controllers, the install instructions for the DC-GFP 100 require that the PV ground be grounded to the case of the charge controllers (which makes sense) however, all of these components are in close proximity to each other and are connected via metal conduit and boxes.  So, now the result looks like 2 AC/DC ground loops plus 2 links directly from the PV array ground to AC/DC ground.

    My goal was too keep the PV array ground isolated until it jumps via one jumper to the DC ground buss via the DC-GFP 100, the AC ground would also join to the DC ground buss at this point and from the DC ground buss go to the grounding rod - according to NEC as best as I can read it.

    I have thought about not connecting the AC ground to the inverters and just connecting the DC ground - since it is connected to the AC ground buss via a jumper.  This would prevent the two loops AC/DC ground loops.  However it would still leave me with the problem of the PV ground being connected to the DC ground via the charge controller case and conduit (.3 Ohms, checked with a good meter). 

    I have commercial experience with inverter installs and residential wiring but I have never installed a residential off-grid solar system before.

    I am in a high lightning area and the arrays are mounted on the metal roof of my workshop, so I want to get the grounding system right - erroring on the side of lightning protection over NEC - which I know sometimes conflicts.  I do have 3 midnite solar MNSPD's (voltage clamps) installed (one on the main AC panel and one on each of my two solar combiners).

    Thanks for reading, please let me know if you have any ideas?




    Hi boxphoto..,

    When you said,  ...   "I have thought about not connecting the AC ground to the inverters and just connecting the DC ground - since it is connected to the AC ground buss via a jumper.  This would prevent the two loops AC/DC ground loops.  However it would still leave me with the problem of the PV ground being connected to the DC ground via the charge controller case and conduit (.3 Ohms, checked with a good meter... ",

    Are you saying that the Classic CCs have the have the PV negative connected to the CC case?  This is NOT the case with any Classics that I've seen  --  the battery and PV negatives float,  and are NOT connected to the Classic's case.

    Have not looked at Magnum inverters,   but those that I have looked at  DO have isolation between the battery negative and the AC neutral ...

    FWIW,   Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • boxphotographerboxphotographer Registered Users Posts: 5
    Vic, Thanks for your reply.  Nope, I am not saying PV negative.  I am saying PV ground (frame and mount grounds for the arrays).  I currently have it wired as per Midnite instructions here: http://www.midnitesolar.com/pdfs/MNDC-GFP100-Dual_manual_.pdf  (3rd diagram, 2 controllers, self tripping 1 amp GF).  I have been doing some research and I am thinking of using the built it GFP in the classics instead and switching the the MNDC to shunt trip control from one of the classics.  Then I could just tie all of the grounds together as cupcake suggested.  I was trying to keep PV ground isolated from equipment grounds except for one jumper at the bussbar which leads to the single grounding rod - as per what many lightning protection articles and experts seem to agree on.  Most of my experience is in mobile applications - where grounding is a whole different beast.

    The Magnum does have isolation between DC/AC grounds to neutral just not between the AC/DC ground (again, not batt -).  The magnum install instructions require a separate cable for each DC (case) and AC ground despite them both being common.  Of interesting note, when I looked at the midnite install diagrams for magnum, they only show one case ground...  So, I am thinking I will follow midnite's lead and only connect one ground to each inverter.

    Does this make sense?  I am not at my best today, so I would understand if not...
  • boxphotographerboxphotographer Registered Users Posts: 5
    cupcake - it is tempting.
  • cupcakecupcake Solar Expert Posts: 254 ✭✭✭
    It's ALL case-grounding... you can have every piece of metal in your system with 50 ground wires going out it... in fact the more the better... AC or DC doesnt matter... you are grounding (connecting/bonding/tieing together) METAL to EARTH... [email protected] voltages like metal and like to go to the earth.. that's all you're doing...

    ac/dc doesnt matter, you are simply making an escape route or 'drain' for sh*thouse voltages that wind up on metal surfaces (ie 'eqipment' or 'cases')... since current cannot move without voltage the logic is that if you remove voltage you remove current as well... thus you are simply making all your metal stuff be at 'zero volts' by connecting them to the earth...

    Most all inspectors are programmed with the 'ground everything' logic due to this principle..

    so mash all your CASE grounds together... ac or dc... obviously its easy to keep the ac grounds and dc grounds 'segregated' because all your DC stuff winds up in a Dc shutoff box, and all your AC stuff winds up in a typical AC service panel box...  the inverter comes with a small lug for the AC case ground and a fatter lug for the DC case ground, however BOTH SERVE THE SAME PURPOSE, to connect metal to earth to keep voltage at zero....

    :)




    ~1.5Kw PV in parallel
    Morningstar MPPT-60 controllers (2) in parallel
    3 Trojan tr-1275's in parallel 450ah total
    Samlex 2,000 watt 12-volt inverter hardwired


  • VicVic Solar Expert Posts: 2,976 ✭✭✭✭
    Vic, Thanks for your reply.  Nope, I am not saying PV negative.  I am saying PV ground (frame and mount grounds for the arrays).  I currently have it wired as per Midnite instructions here: http://www.midnitesolar.com/pdfs/MNDC-GFP100-Dual_manual_.pdf  (3rd diagram, 2 controllers, self tripping 1 amp GF).  I have been doing some research and I am thinking of using the built it GFP in the classics instead and switching the the MNDC to shunt trip control from one of the classics.  Then I could just tie all of the grounds together as cupcake suggested.  I was trying to keep PV ground isolated from equipment grounds except for one jumper at the bussbar which leads to the single grounding rod - as per what many lightning protection articles and experts seem to agree on.  Most of my experience is in mobile applications - where grounding is a whole different beast.

    The Magnum does have isolation between DC/AC grounds to neutral just not between the AC/DC ground (again, not batt -).  The magnum install instructions require a separate cable for each DC (case) and AC ground despite them both being common.  Of interesting note, when I looked at the midnite install diagrams for magnum, they only show one case ground...  So, I am thinking I will follow midnite's lead and only connect one ground to each inverter.

    Does this make sense?  I am not at my best today, so I would understand if not...


    OK, box..,

    Thanks for the clarification ...   Do not even know where you are located.   There is a bit of semantics here ... and,   do not know what Codes you are required to meet,  etc.

    Seldom ever comment on just how one should handle Bonding and Grounding,  as "it all depends",  and we almost never really know many of the fine details of any one system,  based on someone's description.

    However,  I would not ever bring the earthing of PV frames into a structure,   rather,  choosing to generally this to earth,  outside of a structure,   this also all depends upon many variables.

    AHJs (the Inspecting Authority)  usually dictates just how they want to see these things handled.  There is Bonding of equipment,  and there is Grounding,   and the best way to connect them,   all depends.

    Good Luck,   Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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