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  • Grounding a transformerless inverter

    A little background - I do not have an electrical background, but I am in the process of installing a ground mount system with (25) 305 watt panels, in two strings, connected to an SMA Sunny Boy 7.0 transformerless string inverter. I did re-wire all the outlets and lights when we renovated our house, so I do have some experience in straightforward electrical work.

    I live in an area where coal is king and electric rates are very low, so there is little-to-no solar being installed, and no installers nearby, and electricians are not familiar with solar. Hence, I am doing it myself. I did re-wire all the outlets and lights when we renovated our house, so I do have some experience is straightforward electrical work. When I pulled the permit for the solar, the local inspector did not require a drawing, and told me straight out that he has no experience with solar, so I am on my own to ensure that the panels and inverter are wired correctly.

    I feel pretty confident in everything except for the grounding. I have found so much information and so many drawings online that seem to conflict and I am a little paranoid about frying my equipment.

    My panels are getting mounted on Ironridge XR rails, and bonded with UFO connectors. Each of the rails will have a grounding lug with a 6 gauge wire connecting them. Here is where I get a little fuzzy. Should that 6ga wire then be connected to a grounding rod in the earth, or should it go back to the inverter and get connected to the equipment grounding terminal? I know that I am misunderstanding, but in the inverter manual it says, "Do not operate grounded PV modules together with this product. Only ground the mounting frames of the PV modules."

    I have seen line drawings where the ground wire connects to a grounding rod and then goes to the inverter, and I have seen some drawings where the ground wire from the panels goes straight to the inverter.

    A little helpful input would be appreciated.

  • #2
    it sounds like just grounding the frames is the thing to do. but i've never liked the idea of running the charge from a lighting strike, into the main panel, to give everything in the house, the kiss of death. Simply grounding at the array, can leave you with the potential for a "Gradient Voltage" in case of a fault or lighting strike.
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

    Comment


    • #3
      Originally posted by stumblebee View Post
      I know that I am misunderstanding, but in the inverter manual it says, "Do not operate grounded PV modules together with this product. Only ground the mounting frames of the PV modules."
      "Do not operate grounded PV modules" means do not use modules that have the wires connected to ground. in other words, don't connect the + or - wire for the module to ground. And don't use any modules (if there are any still being made) that have that kind of connection built into them.

      I have seen line drawings where the ground wire connects to a grounding rod and then goes to the inverter, and I have seen some drawings where the ground wire from the panels goes straight to the inverter.

      A little helpful input would be appreciated.
      These are roof-mount on your home?
      The answer might depend on whether they're roof-mounted or not.

      If you're looking for lightning protection, probably a separate lightning protection system is what you need.

      Comment


      • #4
        Here's a bit of an article about lightning and grounding.
        https://electrical-engineering-porta...htning-current
        your local code officer can tell you what they are looking for.

        These sheep were under a tree, when lightning hit the tree and traveled thru the ground it was growing on. The sheep's feet and body were better conductors then the dirt, so they were electrocuted.

        Look for information about "Voltage step gradient" or "Gradient Voltage grounding", "Ground Gradient"
        http://www.esgroundingsolutions.com/...uch-potential/

        sheep-killed-by-lightning.jpg

        step-potential.jpg

        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

        solar: http://tinyurl.com/LMR-Solar
        gen: http://tinyurl.com/LMR-Lister

        Comment


        • #5
          I'm not an electrician, but I've worked on a lot of electrical equipment to hopefully give some advice and maybe someone will correct my advice if it is wrong.

          I think the issue here is you're confusing equipment ground with system ground:

          Equipment Ground:
          The NEC requires that all exposed metal parts (e.g. racking, conduit, enclosures) of PV systems (regardless of voltage) must be grounded (690.43). This equipment ground is accomplished through the electrical interconnection (or bonding) of all those metal parts with the earth. Basically: “If it’s metal, it needs to be grounded.”

          Grounding is designed to keep installers, and anyone else who has to service the system in the future, from coming into contact with electrical current. Any metal or potentially conductive materials that are likely to be energized (having electrical current run through it) in the system must be grounded.

          System Ground:
          The National Electrical Code (NEC) requires that all PV Systems over 50 V have one current-carrying conductor connected to ground (690.41). The connection between that conductor (either the positive or negative DC conductor as well as the neutral conductor if the system has an AC component) and the earth is the system ground.

          I'd imagine most of us run less than 50V DC, so we would run a floating system with no conductor ground.

          See the following links for more details:
          https://www.solarpowerworldonline.co...well-grounded/
          https://www.solarpaneltalk.com/forum...794#post382794

          My recommendation is that you securely ground the module frames, rails, and support frames with at least 6GA and using irreversible crimped lugs. Those would go to a proper grounding rod, near the PVs. You will also ground any metal boxes that sit between the PVs and the inverter. Finally, the Inverter and home wiring will share their own grounding rod for the 120/240V AC system. While the DC lines will be floating and ungrounded, I recommend that you put a lightning arrester and possibly a DC Surge arrester on the incoming DC line, so a lightning strike doesn't blow your equipment sky high.

          A lightning arrester provides a ground path for lightning to travel, but doesn't necessarily suppress all of the voltage. A Surge Arrester would help remove the additional voltage from the system.


          Comment


          • #6
            Originally posted by Mike90250
            These sheep were under a tree, when lightning hit the tree and traveled thru the ground
            it was growing on. The sheep's feet and body were better conductors then the dirt, so they were electrocuted.
            The tree is looking pretty good, what is shown. A hit on a tree on the corner of my lot,
            produced a circle of kindling wood of about 100 foot radius, no sign of the tree. I
            wonder if it is possible to determine the path through the body of the fatal current?
            Bruce Roe

            Comment


            • #7
              Originally posted by Mike90250 View Post
              These sheep were under a tree, when lightning hit the tree and traveled thru the ground it was growing on. The sheep's feet and body were better conductors then the dirt, so they were electrocuted.
              They don't have to be better conductors than dirt for them to conduct electricity.

              When you put two conductors in parallel the current is split between them. If one has higher resistance than the other, it will get less current, but it will still get some current.

              People say electricity takes "the path of least resistance" - but that's not really true. Really it takes all paths - it's just more/most of the current goes through the least resistance.

              So back to the sheep - if they were twice as resistive as the path through the dirt they'd still carry 1/3 of the current while the dirt carried 2/3. With a lightning bolt even 1/3 or even 1/100th of the amps from a strike is probably enough to kill.

              Comment


              • #8
                Originally posted by stumblebee View Post
                A little background - I do not have an electrical background, but I am in the process of installing a ground mount system with (25) 305 watt panels, in two strings, connected to an SMA Sunny Boy 7.0 transformerless string inverter. I did re-wire all the outlets and lights when we renovated our house, so I do have some experience in straightforward electrical work.

                I live in an area where coal is king and electric rates are very low, so there is little-to-no solar being installed, and no installers nearby, and electricians are not familiar with solar. Hence, I am doing it myself. I did re-wire all the outlets and lights when we renovated our house, so I do have some experience is straightforward electrical work. When I pulled the permit for the solar, the local inspector did not require a drawing, and told me straight out that he has no experience with solar, so I am on my own to ensure that the panels and inverter are wired correctly.

                I feel pretty confident in everything except for the grounding. I have found so much information and so many drawings online that seem to conflict and I am a little paranoid about frying my equipment.

                My panels are getting mounted on Ironridge XR rails, and bonded with UFO connectors. Each of the rails will have a grounding lug with a 6 gauge wire connecting them. Here is where I get a little fuzzy. Should that 6ga wire then be connected to a grounding rod in the earth, or should it go back to the inverter and get connected to the equipment grounding terminal? I know that I am misunderstanding, but in the inverter manual it says, "Do not operate grounded PV modules together with this product. Only ground the mounting frames of the PV modules."

                I have seen line drawings where the ground wire connects to a grounding rod and then goes to the inverter, and I have seen some drawings where the ground wire from the panels goes straight to the inverter.

                A little helpful input would be appreciated.

                Grounding of the panels (not tying to + or -) just means the frames. You didn't say if this was a ground mounted system or roof. For ground mounted You should have a heavy gauge ground connected to 1 or more ground rods, where your solar panel (frames), micro-inverters or string inverter out by the panels, all array equipment all gets grounded to. You should also tie the ground from your utility panel ground system to this ground as well so you don't have any large ground potential differences.

                Roof mounted pretty much the same, but you don't really need to run them all to the same ground. For example if your roof panels run down to a few ground rods on the side of your house, and your string inverter is near your utility panel, you can ground the inverter to the utility ground system, you don't have to run 70 feet of bare ground from the inverter to the panel rods. But you do want to combine the utility panel ground and the new ground rods with a ground wire to keep the difference in potential down. If its a small house and everything can be short runs to the existing ground system, that's fine too, but with new ground rods at least you know the status of the system, if its an old house who knows what the original ground rod/s look like.

                For either if you have a string inverter you can get away with just grounding the metal bits of the array to its own ground system, then the inverter to the utility ground system. But I have seen inverters that tie the the panel DC to ground one way or another. I work for a utility, so I like to see all grounds working together. I have seen some interesting stuff happen when not. Like the person above said, DC surge lightning and surge suppressors at one or preferably both ends of the DC run if its long.
                Attached Files
                Last edited by Pir8radio; 11-26-2019, 08:38 PM.

                Comment


                • #9
                  Originally posted by Ninety-9 SE-L View Post
                  System Ground:
                  The National Electrical Code (NEC) requires that all PV Systems over 50 V have one current-carrying conductor connected to ground (690.41). The connection between that conductor (either the positive or negative DC conductor as well as the neutral conductor if the system has an AC component) and the earth is the system ground.

                  I'd imagine most of us run less than 50V DC, so we would run a floating system with no conductor ground.
                  The OP is talking a grid-tie system. (SMA Sunnyboy inverter)
                  It is going to be well over 50V DC.But it doesn't require one of the DC wires to be grounded.
                  I think it would fall under 690.41(A)(4) "Ungrounded PV arrays"

                  (Note that "Ungrounded PV arrays" doesn't mean that the metal frames, etc aren't connected to a ground rod. That's a separate thing. See 690.43 "Equipment Grounding and Bonding")
                  Last edited by foo1bar; 11-26-2019, 10:41 PM.

                  Comment


                  • #10
                    The National Electrical Code (NEC) requires that all PV Systems over 50 V have one current-carrying conductor connected to ground (690.41).
                    Just curious can someone post a link to the NEC 2014+ that states this for 50 VDC+? I don't believe that is true.
                    Last edited by Pir8radio; 11-27-2019, 12:19 AM.

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