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Thread: Inverter grounding

  1. #1

    Default Inverter grounding

    Hi everyone

    I just happened across this form and I thought I'd stop by for some advice.

    I have a cabin out in the country where I setup an off-grid system.

    It is made up of 3x80watt solar panels, and a battery bank.

    Initially I had a simple 1000watt inverter which worked great providing there was enough sun, but without sun, charging the batteries from a generator became a problem.

    I found a solution to my problem in the form of the Xantrex Freedom HF 1000watt inverter/charger. I plan on installing it in the next few days when I get to go up to the cabin.

    My concern is grounding the unit.

    I'll describe the installation.

    The cabin is wired with standard 14/2 wiring just as it it were meant for a typical hook up. I have a breaker panel, but the main service breaker is removed and in it's place I have wiring running to the inverter. The panel has it's own grounding rod.

    The inverter I am using currently does not require a ground, I think it's too small and simple, however this new one has to be grounded.
    When I looked through the manual, one option of grounding the inverter is to connect it to the negative buss of the battery bank. I don't have a buss, I just have 1/0awg cables going from battery to battery in a parallel configuration (each is a 12V battery) would connecting the ground to the negative side of the batteries be sufficient for the grounding needs?

    I'm a bit skeptical of this setup as I have not come across it before, and I'm looking for some type of verification that this is a safe way to go about it.

    I also came across diagrams where the DC and AC grounds are bound together, but I'm not sure I need to consider this at all.

    If you take a look at the Freedom HF manual:

    http://statpower.com/documents/Inver...1_Rev-A%29.pdf

    On Page 1--6, I will have something very close to what you see, except for the car installation ie: battery isolator, alternator and starting battery, and on the other side I will not have a AC source panel, the genny will connect right into the inverter. With that diagram in mind, is it okay to tie the inverter ground to the negative side of the battery bank?

    Your expertise will be greatly appreciated.

    Thank you

    xp

  2. #2
    Join Date
    Mar 2006
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    SF Bay Area (California)
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    19,685

    Default Re: Inverter grounding

    Unfortunately, the middle 2/3'rds of the installation manual is blank on my PDF reader...

    From the owner's manual, it is a MSW type inverter and unless it says otherwise, I would not ground the "neutral" anywhere in its AC output (typically, the "main panel") has the neutral and earth ground connected together... You will need to replace that/mount the neutral bus so that it is fully isolated.

    Grounding the DC Battery Negative to earth ground, and running a ground wire to the inverter chassis ground stud is all normal.

    Basically, there are two major reasons for "earth ground". One, if any AC (hot/neutral) wire touches a piece of metal, grounding the metal prevents it from becoming "hot" and creating a shock hazard (mixer wire breaks, makes chassis "hot" and person touches mixer/kitchen faucet and gets electrocuted).

    If any hot to ground connection is made--then the breaker/fuse will open.

    The other reason for grounding is to prevent lightning/static charges from energizing part of the system.

    Grounding the negative battery post and metal chassis parts to "safety ground" is normal and a good way of protecting equipment and people.

    Because the typical MSW inverter (Modified Square Wave) does not have an "isolated" output (no isolation transformer)... If you attempt to ground both the battery input and one of the output (make a "grounded neutral")--it can create a short circuit from the battery, through the MSW inverter, to the "neutral ground"... Which will blow the power transistors inside the inverter (and/or pop fuses/beakers).

    So--Earth ground the battery, ground the inverter case, float all AC "Neutral" bus bars and you should be fine.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  3. #3

    Default Re: Inverter grounding

    Thanks Bill

    I don't have the Battery Negative terminals grounded, I will do that and ground the inverter to that.

    The AC panel has the neutrals floating, just the panel itself is earth grounded, sorry I should have specified that earlier.

    Thanks for all your help.

    xp

  4. #4
    Join Date
    Mar 2006
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    SF Bay Area (California)
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    Default Re: Inverter grounding

    xp,

    Not a problem--I try to explain what the theory is behind the issues (such as grounding) so you can make the best decision for your installation as it is difficult for me to do that from X,XXX miles away behind a keyboard.

    Regarding DC (and AC too) Grounding. You want to make sure you have only one ground connection per "circuit" so that you don't get parallel current paths (example, grounding the DC "-" at the solar panel and again grounding the Battery Bus "-" terminal). You can get parallel current flow (or lightning induced current surges) when there are multiple paths (another common multi-path ground is when you have the battery grounded and running a car or HAM radio from 12 VDC--The case is DC ground and Safety Ground, and so is the Antenna braid ground too--Running a DC ground to the radio chassis plus a safety green wire to the antenna braid ground can create such a loop).

    Regarding the solar panel ground--I am assuming that you are Earth Grounding the solar panel aluminum frame... and NOT Grounding the "-" wire from the solar array anywhere. Correct?

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  5. #5

    Default Re: Inverter grounding

    Hi Bill

    Thanks again for all your help.

    You are correct, the solar panel frames are grounded, not the negative terminal. Currently to water pipes, but once the new ground is in, I will attach them to the battery ground instead to eliminate the parallel circuit.

    With regards to per "circuit"grounding, from what you describe I am under the impression that the AC side (a circuit) should have its own ground, which is the one from the AC panel to a grounding rod.

    Now the DC would be a separate circuit and requires another ground, it did not have one before, this is what I will be adding in. Once this new ground is added, I can use it to ground all other devices on that same DC circuit (ie: switching panels from water pipes to battery ground).

    I understand the parallel grounding situation you explained. I don't know how the inverter handles the DC and AC grounds at this point, and I don't recall anything from the manual along these lines, I will check again.

    It kind of sounds like I'm getting myself into that very situation through the inverter but I don't think it was built in such a manner.

    Once again, thank you for your help, 4 years ago when I put this installation up it was very hard to find help on this topic, I'm glad I found this forum.

    Regards

    xp

  6. #6
    Join Date
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    Default Re: Inverter grounding

    You are very welcome XP--I too have learned a lot from other folks on this forum.

    My person philosphy on grounding (and, like other areas of interest--some of this stuff becomes religious)--Is that you pick a really good spot for grounding (say that one 8+ foot ground rod. If you need more ground (dry soil, rocky, etc.) run #6 or larger wire out in a "star pattern" from that one ground rod to other ground rods/water pipes/etc.

    From that center ground rod--that is now your "zero volt" location (in telecom, also called the Ground Window). Run single cables to each of the other points that will need grounding. One to your solar panel frames. Another to your battery negative bus, and yet another to your AC Neutral point and common AC Green Wire ground (if you have one), etc.

    You do not aim to "isolate" your AC and DC grounds--you just aim to only have one path for current to flow...

    The problem with, for example an AC ground rod and a DC ground rod--If you get a lightning strike nearby--you will have a voltage gradent through the ground. And the two ground rods separated by XX feet will have hundreds to thousands of volts differences between them. And you would end up with your DC ground being at a different voltage vs your AC ground (and water pipes, etc.).

    Having everything referenced to a single ground at the edge of your home (per code) is a much safer setup.

    Also, I have seen grid power problems with different grounds. I had two grounds 100' apart at the side of a salt water tank (dolphins). There was 60+ VAC difference between them (I got shocked) because of AC leakage current between different pumping / power points in the facility.

    Sometimes, folks get in trouble because they have heavy DC Battery Return currents (100+ amps) and they have an incidental AC ground/neutral path that is common with the DC return. You can now start sharing the 100's of amps through the 14 awg neutral/safety ground of the AC circuit and overheat the AC wiring.

    Another issue to watch for with AC vs DC wiring (ground and Hot/Neutral). Generally, you always want to run AC wires right next to each other (Hots, Neutrals, etc.) because the AC current, if for example you run one hot lead through a box opening and run the neutral/return lead through another hole--the AC current can cause circulating currents to flow in the metal box and overheat the box itself.

    With DC, you do not have the problem...

    And then there is the lightning issue... You normally want to run the DC +/- lines in tight pairs too... If there is a nearby lightning strike, if the pairs of wires are separated by several feet--it becomes a "loop antenna" and can inject energy from the strike into your wiring system.

    Another issue--If you have metal on the roof/lightning rods--Those cables should be ran on the outside of the house--do not bring them into the center of the home. High Frequency Current (like lightning) wants to spread out as far as it can... So, two wires, one on the north corner and the other going to ground on the south corner of a home is much better than running a lightning ground down through the middle of your home to a ground rod (the lightning will tend to want to leave the central ground cable and spread out through your home).

    Obviously, a direct strike is going to take out equipment--but some basic safety practices can reduce the overall risk.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  7. #7
    Join Date
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    Location
    North Arkansas
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    Default Re: Inverter grounding

    Very good, clear answer to a contentious question. Someone should "sticky" it. Thanks.

  8. #8
    Join Date
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    Default Re: Inverter grounding

    i'll add that separate grounding points can setup what they call ground loops and all kinds of crazy stuff can result.
    NIEL

  9. #9

    Default Re: Inverter grounding

    Hi Bill

    Thanks for your assistance, over the weekend I went up to my cabin and fixed up the installation.

    I ran into a few issues, but I fixed up all the ground connections. For some reason my hardware stores don't carry grounding rods anymore, but rather plates, I believe there should be not much difference. I had rods before but they were not as long as required, so I ditched those and put the plates in, 8 feet apart connected with the heavy duty copper grounding cable.

    That cable runs to the DC circuit as well as the AC breaker panel.
    On the DC side, I have a fuse box which is grounded and to the fuse box I have the batteries and inverter grounded. On the AC side I have the household wiring.

    The inverter has it's own ground wire on the AC side that runs from its output to the AC panel. I ran into an issue where the GFCI outlet on the inverter kept tripping whenever I connected the household wiring. I tracked it down to a punctured 14/2 wire which had the ground and neutral bonded. Funny enough the other two inverters which I have used before never saw this as a problem, but I'm glad this one did and that I found it before it caused any issues.

    One question, I currently have the batteries disconnected from the ground as I was doing some troubleshooting. While troubleshooting I used a multimeter to check the Voltage between the grounding plates and the negative terminal of the battery bank, it showed 12V, I was a bit surprised, but this is a connection I'm not familiar with at all. My best guess is that it has to do with relative ground vs absolute ground, could you shed some light on this phenomenon?

    I will be investigating the lightning rods in the near future as I will be building a house with a metal roof, I am guessing lightning rods need their own grounding plates/rods and should not be connected to the same ones that the household wiring is connected to, would that be about right?

    Many many thanks for all your help Bill. The manual for the inverter does not even begin to scratch the surface of what you unveiled, and I'm glad I have it done right.

    Cheers

    xp

  10. #10
    Join Date
    Mar 2009
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    Default Re: Inverter grounding

    Plate? What sort of plate?

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