The more I read about grounding the more confused I get!

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97TJ
97TJ Solar Expert Posts: 68 ✭✭
in Off Grid Solar & Battery Systems #1
I have a small enclosed porch/addition on the back of my cabin where I will be installing all of the components of my solar system. The panels will be on the roof and the charge controller, charger, batteries and inverter will be in this porch. The cabin is wired for a small generator. The current wiring all goes to a panel on the east side of the cabin which will give me about a 20' run of 12/2 to get to the inverter. All of the current wiring is grounded to a rod on the east side of the cabin. Do I ground all of my solar components to a single buss bar and then run 6 awg stranded to the ground rod currently attached to my distribution panel? Also do I use a separate ground for the solar panels only or should they also go to the current ground rod? It is all very confusing when you start reading multiple threads on the subject.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
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    Re: The more I read about grounding the more confused I get!

    If you can possibly have just one ground rod, that is preferable. My advice is set a new ground rod at the power porch and ground everything there. Remove the ground rod on the east side of the house and remove the bonding jumper in that east panel. Buy a new breaker box for the porch and bond the system there. Then run a line from the new box to the old box. The old box is now a subpanel of your new main panel.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
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    Re: The more I read about grounding the more confused I get!

    Things to watch out for:

    Wiring grounding system so that an alternate path is available which may cause other components to become energized in the event of a failure rather than having all such current directed to Earth.
    MSW type inverter. Notorious for going up in smoke if a neutral-ground bond is present on the AC.
    Positive ground panels. Rare, but will play merry heck if the whole DC system isn't set up for positive ground with proper isolation.
    Confusing lighting protection with electrical safety ground.

    Otherwise you can ground the panels to the same ground and you can connect the other component grounds together and then to the same ground.

    Yes it is all very confusing and even people who understand it sometime disagree on interpretation and application.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
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    Re: The more I read about grounding the more confused I get!

    Think of two grounding issues...

    First is "electrical safety"... Neutral, DC grounds, AC grounds, etc. should all go to one grounding point (typically the "ground rod"). If you have rocky/dry soil, then run 6 awg wires (star pattern) to the extra ground rods (don't tie AC ground to one rod and DC ground to a second rod--Keep all of your electrical grounds on the "master" ground rod/plate/etc.).

    Second is Lightning Ground. If you are in an area where lightning strikes are common, then, I would tie the solar array frames/mounting rails to together and run the wire strait down the outside of the building to a ground rod driven next to the foundation.

    I would not run a lightning ground around/over/through a building to meet at the "master" ground rod. Lightning, in general, wants to flow "outward" from the center of the building. Brining the ground towards the center/through the building may allow the lightning to jump "elsewhere" on its path to ground.

    For a "complete" safety ground, you should run a 6 awg ground wire from the "master" rod to the lightning ground rod on the other side of the building. That way, if there is an electrical short from solar +/- to solar frame ground, it will find a return path the battery back through that long 6 awg wire and trip a breaker/cause a fault/etc.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Jigme Urgyen
    Jigme Urgyen Registered Users Posts: 13 ✭
    #5
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    Re: The more I read about grounding the more confused I get!

    first, you need to check the quality of the grounding in your soil conditions etc. if you have high resistance because of particular soil type or whatever, then you will need to solve that problem first. no point putting in grounding only to find out the fault current doesn't flow to ground because of high resistance.

    There are several scenarios for grounding, all of these are based on your resistance and soil type. if you have good soil, you may only need a single pit with grounding rod for each system. if soil is poor, you might need multiple pits with multiple rods bonded together or a mesh or something like this. it all depends on the resistance.

    I have done a lot of these scenarios, where you have multiple rods going down, those have a chemical treatment, and all the rods are bonded exothemically together using 2.5 mm x 5mm copper tape, and where the tape runs through the ground, you dig a trench and bury the tape with the same chemical.

    This type of config i think is the best for either poor soil conditions, and this is the preferred type of grounding for a direct strike lightning system where you have a decent air terminal up in the air.

    as others in the thread have suggested, you want to keep system grounding and lightning strike systems separate. everywhere, but then bond the systems together near the pits. This will ensure both systems are the same potential, but there is no chance for anything to back-feed.

    one earth system will provide the grounding for the ac side and dc side of inverter, array, panel/ frames, combiner, and array GFCI, any surge arrestors in the system, both ac and dc side distribution panels, change controller and battery, any other electrical enclosures etc. There should only be a single neutral to ground bond in the system for this system. I would normally do that at the most major panel, something like service entrance, depends on where the earth down conductor terminates.

    If you are using metal conduit to run cabling from the array down to your power plant, you can use that conduit as an earth conductor. You need to bond the conduit to the combiner box at the array and bond the conduit to some grounding bus bar at the power plant. at the array side, bond everything back to the combiner, the ground path will then follow the conduit back to the termination point. this approach is ok if you look up the NEC and might save you some time and dollars.

    the other earth system will be bonded directly to your air terminal. This has to be done very nicely, or if you get a direct strike, you'll blow off the joints, or melt the cheap components. Ensure the cable or tape going from the air terminal flows downwards and takes the most direct path as possible. dont allow sharp bends at all. make sure all cables, tape, components are rated to actually carry a direct strike to ground. a lot of systems have those spiky ball things up in the air, useless, or the cables/tape is undersized. again all useless.

    If there is another much better direct strike system nearby, dont bother putting your own up. Just do a very good job of grounded everything together and have good surge arrestors in the combiner and at the charge controller. If your inverter has an AC input connected to the grid, put a good surge filter on there and connect it to the ground bus.

    and remember, if the ground resistance is high, nothing will work well, you want all those conductors in the ground to have the lowest resistance possible.

    good luck.
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  • Jigme Urgyen
    Jigme Urgyen Registered Users Posts: 13 ✭
    #6
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    Re: The more I read about grounding the more confused I get!

    Oh, also grounding the system side needs to take into account what you are running out of those systems. For example, if your grounding a computer room with multiple power distribution units, you will have a much more complex grounding configuration, both in terms of conductors in the ground as well as bonding.

    If you are connecting an audio system to ground that is not floating and all of the cables and devices are grounded, i.e the TRS/XLR pins are actually all connected, then you would want to keep all that audio gear separated from lights, ballasts etc. both line, neutral and grounded should be separated. If not, you'll find your desk is turning those electrical harmonics present on the lines into sound.

    This might also apply to a cable TV system that grounds via the ac plug.
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  • 97TJ
    97TJ Solar Expert Posts: 68 ✭✭
    #7
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    Re: The more I read about grounding the more confused I get!

    Thank you everyone for your help.
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  • Caneman
    Caneman Solar Expert Posts: 71 ✭✭✭✭
    #8
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    Re: The more I read about grounding the more confused I get!

    If you ground the (-) side of the battery bank, do you still have to run a ground from any device attached to dc side of the battery bank?

    For example, I run a #6 bare copper ground wire from the main (-) buss bar to a copper ground rod... then I attach an inverter to the buss bar, do I still need to run another wire to ground on the dc side of the inverter?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #9
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    Re: The more I read about grounding the more confused I get!

    No... The idea is to have a "single" ground to earth. If you have a ground at the battery and another ground on the "-" of the inverter, you have a parallel current path--both through the return lead and the ground lead. That can lead to problems (usually the ground is smaller than the battery/return lead, if the return connection is broken, then too much current will overheat the smaller earth ground wire.

    If the inverter has a metal case and/or an AC earth ground connection, then there should be a separate green wire to case/earth ground from the inverter back to the common ground connection (typically ground rod).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
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    Re: The more I read about grounding the more confused I get!
    Caneman wrote: »
    If you ground the (-) side of the battery bank, do you still have to run a ground from any device attached to dc side of the battery bank?

    For example, I run a #6 bare copper ground wire from the main (-) buss bar to a copper ground rod... then I attach an inverter to the buss bar, do I still need to run another wire to ground on the dc side of the inverter?

    Usually things do not have their negative side connected to the case they're in. So the case (if metal) should have a separate ground connection on it. If it does, you want to run a ground wire from that to the other chassis (case) grounds and on to the grounding rod. You do not want to rely on the negative wiring to also serve as a safety ground. The negative of the battery terminal is grounded so that the system doesn't "float" Voltage (have both positive and negative Voltage potential in respect to earth ground). This is similar in function to the neutral-ground bond found in most AC systems.

    That either answered your question or confused you. Which did it do?
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  • Caneman
    Caneman Solar Expert Posts: 71 ✭✭✭✭
    #11
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    Re: The more I read about grounding the more confused I get!

    LOL, just got me to think more....

    Can I have a "ground bus bar" where the (-) BB wire is attached to it, and then any chasis ground wires attached to to it as well... then this "ground bus bar" is attached to the ground rod with one wire?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
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    Re: The more I read about grounding the more confused I get!

    You can, if I understand the proposal correctly.

    Collect all ground wires together on one bus bar and then run a single, heavy wire to the actual ground rod.

    This is in fact what is done inside breaker boxes for AC: bus bars for neutral and ground (connected), single big wire going down to the earth grounding rod.
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  • Caneman
    Caneman Solar Expert Posts: 71 ✭✭✭✭
    #13
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    Re: The more I read about grounding the more confused I get!

    scary, i might just be starting to understand some of this stuff... :cool:
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
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    Re: The more I read about grounding the more confused I get!
    Caneman wrote: »
    scary, i might just be starting to understand some of this stuff... :cool:

    Don't worry; it only means you're going crazy.
    Like the rest of us. :p
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  • nasty
    nasty Solar Expert Posts: 35 ✭✭
    #15
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    Re: The more I read about grounding the more confused I get!
    BB. wrote: »
    If the inverter has a metal case and/or an AC earth ground connection, then there should be a separate green wire to case/earth ground from the inverter back to the common ground connection (typically ground rod).

    -Bill

    My inverter has a GFCI that I plug a circuit with non GFCI recepticals into. The inverter has a chassis ground lug that I run to a ground bus with my other components then to a ground rod. Is this enough? Are the recepticals protected? Off grid system no grid tie no generator.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #16
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    Re: The more I read about grounding the more confused I get!
    nasty wrote: »
    My inverter has a GFCI that I plug a circuit with non GFCI recepticals into. The inverter has a chassis ground lug that I run to a ground bus with my other components then to a ground rod. Is this enough? Are the recepticals protected? Off grid system no grid tie no generator.

    Yes. You have a GFCI outlet that is protected (grounded) so anything connected to it would be.
    You really do not want to put multiple GFCI's in series (one fed from a previous one); they tend to get "confused" and falsely trip.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #17
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    Re: The more I read about grounding the more confused I get!

    Yes--The green wire to the chassis is good.

    AC GFI outlets do not really need a "green" wire to earth ground to work correctly. It is simply a transformer wrapped around the Hot/Neutral pair. As long as the current goes out the hot and returns via the neutral, the "total current flow" measured by the current transformer is zero--And all is OK. If there is a net current flow that is >~0.005 amps (somebody touching Hot to earth ground for example), then the GFI trips.

    But it does get a bit more complex. Assuming you have a TSW (true sine wave) type inverter--Most of them are basically built with transformer isolated construction. So there is really no "hot" or neutral connection. If you touch one lead to earth ground, no current will flow because there is no return path (like touching one terminal of a floating battery).

    With TSW inverters that are permanently installed (such as a home/cabin), "Code" requires bonding the AC Neutral (White Wire) to earth ground. This is done so that we only need one circuit breaker (or fuse) in the "hot/black or red" circuits. You short Hot to ground, it pops the breaker. You short Neutral to ground, nothing dangerous happens.

    Note that bonding "Neutral" to earth ground "de-floats" the inverter output. And it is now possible to touch the "hot" lead to earth ground (i.e., a water pipe) and get a shock.

    Technically, floating power supplies are pretty safe because you cannot get a shock to earth ground. However, floating circuits should have two breakers--One on Hot and one on Neutral. Also, "real" floating supplies have to be tested to ensure that one leg is not shorted to earth (floating AC supplies are sometimes used in manufacturing to build/test equipment before UL highpot testing is performed to protect workers during assembly).

    AC GFI outlets are usually designed to break both Hot and Neutral legs (from what I have read) to protect against 5 mAmp current flow in either leg.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #18
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    Re: The more I read about grounding the more confused I get!

    Bill;

    If we're talking about an inverter with a built-in GFCI surely they took the neutral-ground bond issue into account and made the necessary connections so that adding the earth ground to the grounding lug would make it behave like a standard household AC set-up?

    Or then again maybe not; you never know what they'll do. :p

    If you were adding a GFCI on to an inverter output it would be just as you describe. Or at least it should be. :roll:
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #19
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    Re: The more I read about grounding the more confused I get!

    I have no idea what the inverter folks did... As always, read the manual for proper grounding instructions. There are enough differences between inverters/models/manufacturers that it is impossible to give an exact generic answer.

    Should add that the inverter green wire ground usually needs to be very heavy cable... Remember, that the DC input current can be 10x that of the AC output current (12VDC in, 120VAC out). And you could have a DC or AC short inside the case, so the inverter case green wire needs to be heavy enough to blow the DC fuse for the inverter (may be on the order of 100-300 DC input amps of fusing for larger inverters).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #20
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    Re: The more I read about grounding the more confused I get!

    I have the same problem, Bill; don't know what they did inside. One of the reasons I'm not keen on built-in GFCI. That and it tends to give a false sense of security (GFCI outlets are automatically assumed to be "safer" than standard outlets).

    Either way, it should work. The grounding of the chassis is the really important thing as you said. And there I'm assuming that also provides ground for the AC outlet. Hmm. My neighbour has one of these ... where's my screwdriver? :p
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  • nasty
    nasty Solar Expert Posts: 35 ✭✭
    #21
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    Re: The more I read about grounding the more confused I get!
    Cariboocoot wrote: »
    Bill;

    If we're talking about an inverter with a built-in GFCI surely they took the neutral-ground bond issue into account and made the necessary connections so that adding the earth ground to the grounding lug would make it behave like a standard household AC set-up?

    Or then again maybe not; you never know what they'll do. :p

    If you were adding a GFCI on to an inverter output it would be just as you describe. Or at least it should be. :roll:

    The inverter is a Xantrex Prosine true sine wave. The manual says that models equipped with a GFCI outlet do not need to do any additional wiring for input/output protection. Just thought I would confirm this after reading this post. Thanks for the replies.
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  • Jigme Urgyen
    Jigme Urgyen Registered Users Posts: 13 ✭
    #22
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    Re: The more I read about grounding the more confused I get!

    Is the GFCI on the output side of the AC? Or is it like the Outback Flexpower units, the GFCI is on the DC side of the system, and it breaks the PV+ input if tripped?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #23
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    Re: The more I read about grounding the more confused I get!
    Jigme Urgyen wrote: »
    Is the GFCI on the output side of the AC? Or is it like the Outback Flexpower units, the GFCI is on the DC side of the system, and it breaks the PV+ input if tripped?

    Built-in to the inverter on the AC out.
    Bill and I were just speculating as to how exactly they have the wiring set up. Well, I was anyway. :p
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