Ground wiring configuration for solar panels

rdgoodsrdgoods Registered Users Posts: 6 ✭✭
Forgive me if you've addressed this previously. I'm confused how to properly configure the ground wire between panels. I do understand that a bare copper (#6 min.) be routed down to a single common earth ground, per your direction. What I need a better understanding of is; do I connect the bare wire to each panel using the pinch connectors by screwing these to each panel frame, effectively "daisy-chaining" the panels together with the ground wire down to the common earth ground ? Also, between the last grounded panel and the earthen ground rod, can I enclose the bare wire inside a ~1/4" PVC sleeve to protect the wire without interference of ground conductance ? I'd like to conceal this along the exterior of the house as well. I'm really new to this, so thanks for endulging my ignorance !
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  • rdgoodsrdgoods Registered Users Posts: 6 ✭✭
    More about grounding:
    It occurs to me having done more research, I may have asked the above question prematurely. It's my "understanding" the proper way to do this (hopefully to code) is: attach lugs to each array panel frame, route common ground wire between all panels via lugs, down through conduit (same conduit as solar conductors ?) to common ground buss. Run ground wires from all system equipment (combiner box, charge controller, inverter, etc.) to same ground buss, then one ground home run to existing common earth ground. This is as much a question as a statement I admit, but am hoping I have a better understanding. 
    Other Questions:
    Do I also need to run a ground from my battery array to buss bar ?
    Do I need to run solar panel ground through racking as well, despite metallic connectors ?
    thanks very much for your help !

  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    Grounding is a difficult question to answer simply... There are two (major) reasons for grounding... One is to prevent electric shock (if there is a short between hot and ground, a good electrical ground will "trip" the circuit breaker rather than let the piece of metal/framework become "hot").

    Electrical code requires that these safety ground wires daisy chain from panel frame to panel frame (and even probably connect to the ground rails in an approved manner) with a single length of copper cable (splice are allowed if they are "permanent" such as a crimp connection or thermally "welded" or fused) so that a safety ground connection cannot be undone by somebody with a screwdriver and make the system unsafe.

    The second is for lightning control. The issue is that lightning does not "follow" the copper wire like normal AC and DC current does. Sharp corners (turns) in the wiring, long wire runs, etc.--And the lightning will find another path (technically a "low impedance path").

    While NEC code requires cables to be run together (power, neutral, AC green wire grounds) into the house and into the main service panel--Lightning grounds should be run separately--On the outside of the house, down exterior walls, down to a ground rod (or similar) approved grounding point on the outside of the building. You do not want to bring lightning current inside of your home as it has a nasty habit of leaving the ground wire and jumping to other points inside of the home.

    Code requires safety ground to be the size of other conductors minimum (for 14, 12, and 10 AWG). And for larger AWG of conductors, a 6 AWG minimum ground wire is OK (something up to like a 200 Amp service--I am not sure, do your own research with your electrical code).

    For Lightning grounding, from what I have read, 6 AWG is the minimum gauge of cable you want to use... 8 AWG and lighter gauge cable can be vaporized by a direct lightning hit. 6 AWG and heavier is much less likely to be fused.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rdgoodsrdgoods Registered Users Posts: 6 ✭✭
    Bill, I really appreciate your clarity regarding the importance of grounding within an electrical wiring configuration, especially for the sake of lightning shunting. In the case of AC, it seems much more straight forward, as the ground wire is accounted for in the configuration by default. In the case of DC, not so straight forward, working with only positive and negative leads, and having to account for the ground separately, hence my confusion. In that previous posting dated last evening, I had postulated what my research had derived to be the more "conventional" configuration for my off-grid solar configuration, though had posted that as as much a question as a statement. That said, do you agree with my summation ? Please punch as many holes in that as you'd like, as I'm hoping all you experts can set me straight (before I burn my house down accidentally). Thanks again for your helpful clarification ! 
  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    You try to keep the AC grounding separate from the DC grounding... And the reason is that DC battery banks run at much lower voltage than AC power... For example, a 1,500 Watt AC inverter running at 12 volt battery bus will draw:
    • 1,500 Watts * 1/0.85 AC inverter eff * 1/10.5 volts cutoff voltage = 168 Amps DC
    • 1,500 Watts * 1/120 VAC inverter output = 12.5 amps AC
    So, you can see that the DC side current is 10x the AC side current.

    AC ground, 14 AWG cable would work fine. For the DC side, the + and - leads would be ~2/0 to 1/0 cable (if using the NEC tables, where are pretty conservative, but nice for solar), not including the 1.25x NEC derating for breaker/wiring continuous current derating:

    https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

    So, you don't want to "accidentally" get a DC short current on your 175 amp+ fuse/breaker circuit to hit an AC ground that may only be 14 AWG.

    So the DC wiring  and grounding is sized for DC fault currents (6 AWG up to 200 Amps is OK for grounding, I think--please confirm with code), and you can use smaller AWG grounding for the AC sides. And make one point where DC and AC ground connect together (same ground rod, same cold water pipe, etc.).

    A couple threads about Lightning:

    Off Grid Grounding Technique?
    Another Question, this time about Lightning

    Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

    And our host's FAQ:

    Lightning Protection for PV Systems

    From other past posts here, Windsun (retired admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

    Towards the end of this thread is a very nice discussion of proper generator grounding.

    If lightning is going to be an issue, then using Midnite's surge suppressors on the DC solar input to the charge controller, and on the AC output of the inverter would be a good idea:

    https://www.solar-electric.com/catalogsearch/result/?q=midnite+surge+suppressor

    Grounding for the solar array racking--There should be instructions on grounding (I am not sure here--And each vendor is probably different--But I would expect one connection to the 6 AWG ground wire would nominally be sufficient).

    Normally, the + and - from the solar array pass directly to the input of the solar charge controller PV Input terminals. The Array Frame Grounding wants to go from the array directly to a ground rod at the base of the array (outside of the home if roof mounted, to the base of the array structure if free standing). I would suggest running a 6 AWG from the array ground rod back to the house ground rod (if where the solar cables enter your home is a distance from the home's main AC ground rod/water pipe connection, you may need to drive another ground rod next the the Solar entrance, ground the remote array there, ground the surge suppressors there, and then run another 6 AWG cable to your main home ground bonding point).

    You do not want to bring the lightning into the home (even though this is "allowed" by NEC). And lightning does not like to follow 6 AWG cable very far--Perhaps a few ten's of feet. If you look at "real" lightning ground cable, it is much larger diameter overall, and is "braided" to for lower "impedance" (surface area of multiple small conductors for better "skin effect" and radio frequency current conduction).

    There is a lot of specifics for lightning (and even safety) grounding. It is difficult to give "generic" recommendations without writing a book (and I do not have enough knowledge to write that book). So, instead, addressing your specific configuration would probably be more helpful (array construction, a few distances, where the battery shed will be, is this off grid or backup for utility power, etc.).

    This is not a slam dunk project. You will have to do a bunch of reading/research, and with help from others here--Figure out the optimum design your for your needs/installation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rdgoodsrdgoods Registered Users Posts: 6 ✭✭
    Hmmm... Well, in the case of my particular configuration; this is an off-grid "solar generator" only, and enough to supply critical loads only. Based on your (most appreciated) feedback, it sounds like the overall DC side grounding needs to be entirely separate from the AC side, to include its own ground rod. Do I interpret this correctly ? I've read in other segments of this site that I do NOT want to have two separate earthen ground points, so this causes me some concern. Again, I'm here to be set straight, so...
    thanks !
  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    For lightning, you need to get the current to ground in a short/straight line (see design rules). If your array is on the same side of the house as the house ground rod, you can use that. If the array is 20 feet or more (round number) from the house rod, then drive a "local ground rod" and connect lightning ground to that rod, then connect a 6 awg cable from the local ground rod to the house ground rod (so that you still have a good AC grounding connection to pop fuses/breakers).

    Lightning does not travel far in cables before it jumps to something else... So "RF" design rules (high frequency electrical impedance for the 60 Hz low frequency "resistance" is important).

    You do want your DC battery bus negative safety ground connected to the "same ground rod" as your AC house wiring. But in "one point" (AC/DC grounds and such, we talk about single point grounding--For lightning and RF radio frequency grounding, we talk about multi-point grounding--The polar opposites, painfully enough). The single ground bonding point between AC and DC grounding is because you do not want high DC fault current (or even DC operational current) flowing through "incidental" connections on the AC side because will can over current and over heat AC connections that are not designed for that high of current.

    The counter of AC grounding--It is "OK" if an AC fault hits a DC ground path, because a 20 amp 120 VAC fault current is not going to do anything to a DC safety ground designed for 200 amps (AC or DC). So--For something that has both AC and DC power (your AC inverter, or your epanel for AC+DC wiring), grounding those to the DC side is fine. And because you have tied (in a single point, like the main house ground rod) any AC fault that flows through the DC side, will eventually get back to the AC side (i.e., will trip the AC branch circuit breaker).

    In summary, lightning grounds directly down to ground rod (or other, like ground plate, UFER concrete+rebar ground, etc.) and connect all of those local grounds (including the main house ground) with 6 AWG cable to provide the AC (and DC) ground to prevent big chunks of metal from becoming electrically "hot" with 12 VDC or 120/240 VAC.

    There have been lots written on grounding theory--And some are contradictory to each other (local grounding vs tieing all ground rods together with 6 AWG cable is not 100% accepted either). The only way to avoid lightning damage is to do things like--When lightning is predicted (not in the middle of a storm), have a solar array plug that you can disconnect from the side of the home and pull it 10+ feet away. Of course, that kills your solar power harvest, requires you to manually disconnect, and won't happen if you are not home.

    In the end, if you have a direct strike, you will lose equipment--We are just trying to keep the risk of injury and fire to a minimum.

    I am not a lightning engineer, and I am not designing your system--Use the knowledge you gain here (and other places) to make your own decisions and/or employ a professional (that raises its own set of questions) to help/do the work. (generic disclaimer--I do not want to see anyone get hurt).

    If you have a HAM Radio club in the area--They can be a source of information too (all of the antenna are great lightning receivers).

    Also, if you have lightning in your area--Placing lightning rods near / higher than your array (and your house) with its own grounding system can reduce the chances of direct strikes too. For me, rare to get lightning to ground strikes in my area--A building in the open plains of Utah--maybe a very good plan.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • porch13porch13 Registered Users Posts: 62 ✭✭✭
    BB. said:
    You try to keep the AC grounding separate from the DC grounding...
    Hi guys.....I'm in the same boat getting ready to build (and ground) my system.  My question is, if I tie the array ground to the cabin ground, wouldn't I be tying the AC and DC grounds together? Or....should I ground the negative side of my battery bank to a completely separate grounding rod?   I have already purchased four Midnite SPD's from NAWS.  One at the array, where the array enters the charge controller, at the AC panel, and at the battery bank.

    Thanks!
    Northern Arizona...4050 Watt off-grid system - Outback FP1 - Rolls 605 in a 48V bank
  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    That is 1/2 the quote. In general, I always say to tie ac to dc ground at one point. And all "local ground rods" together with 6awg cable.

    Sorry, the below is a quote from my earlier post, on my phone and more difficult to format:

    You do want your DC battery bus negative safety ground connected to the "same ground rod" as your AC house wiring. But in "one point" (AC/DC grounds and such, we talk about single point grounding--For lightning and RF radio frequency grounding, we talk about multi-point grounding--The polar opposites, painfully enough). The single ground bonding point between AC and DC grounding is because you do not want high DC fault current (or even DC operational current) flowing through "incidental" connections on the AC side because will can over current and over heat AC connections that are not designed for that high of current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • porch13porch13 Registered Users Posts: 62 ✭✭✭
    edited October 2016 #10
    So if I understand correctly (fingers crossed over here), I will have the negative side of my battery bank tied to the same grounding point as the AC Panel.  Separate from that, I will have multiple ground spikes around my array bonded to the 6ga bare copper wire coming off the array and continuing all the way to the charge controller.
    Northern Arizona...4050 Watt off-grid system - Outback FP1 - Rolls 605 in a 48V bank
  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    Ummm, not sure.

    You want each point that can receive lighting to go directly to a ground rod as soon as practical.

    You do not want, for example, a pv ground wire going from array to charge controller to ground rod. That can bring lighting energy to the controller/inside the building.

    Go from pv frame ground down to ground rod. Stop. Go from surge protector (connected to +/- pv leads) ground in metal box on outside wall down to ground rod. Etc.

    Sorry, doing on phone, so may not be clear.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • WaterWheelWaterWheel Registered Users Posts: 335 ✭✭✭
    edited October 2016 #12
    Bill,    Since you're discussing grounding give me you opinion on this grounding setup.       Panels and ground mount (100' from house) grounded using 6 ga wire to a  8' copper plated rod in the ground under panels.      This is to help keep lightening out of the house in my lightning prone hilltop location.

    Then a 6ga ground from the combiner box (mounted under panels but not touching panel mount) through the PDP (inverter and charge controller also grounds through the PDP) and then using 6ga connects to the house ground rod.      Four SPDs (2 DC and 2 AC) are on this ground.

    I had considered connecting the panel ground to the house ground but the Conext inverter manual said not to because it may trip the inverter's GIF.      Still,     sometimes I consider running a wire from the panel mount to the combiner box (house) ground and see if the GIF triggers or not.        Do you think I should?, or am I better off leaving things as they are since it would help keep lightning out of the house?      

    FYI, two close lightning strikes this year, one in the front  yard.      I had no lightning damage (except Dish antenna and wiring between dish and receiver).      Neighbors 300 yds away had major lightning damage from both hits so they were strong strikes.    I actually saw one strike in my front yard and it blurred my vision for a bit.

    Conext XW6848 with PDP, SCP, 80/600 controller, 60/150 controller and Conext battery monitor

    21 SW280 panels on Schletter ground mount

    48v Rolls 6CS 27P

  • rdgoodsrdgoods Registered Users Posts: 6 ✭✭
    So, my configuration is intended to come together as such: the panel array (5 panels broken into two 2 branches of 700w each), which will run to a DC disconnect with breakers. I was initially intending to run the ground from the panels and racking along with the pos. and neg. leads within non-metallic liquid-tight conduit down into the house and into a DC QO Sq. D disconnect box with QO DC breakers for each branch. I was intending to install this QO disconnect box on the interior wall adjacent to the other related equipment (i.e.: charge controller, combiner box, inverter, etc.). From the DC disconnect, I am wiring to the charge controller (Outback Flexmax), to a combiner box, to the battery array (4-255 a/hr.), back to the combiner box, then to the inverter (Xantrex). Each of these pieces of equipment will also have their own isolation breakers and grounds (routed to a common buss within the combiner box) then a single ground wire from this to the house AC earthen ground. The distance between these two earthen grounds would be approx. 20 ft. Please let me know if this is logical and somewhat accurate. Thanks again for your helpful feedback !
  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    WaterWheel,

    Just remember, I am not a lightning control engineer.

    To be clear, we are always floating the +/- leads of the solar panels--Never tie - panel lead to earth ground (other through a DC surge protector, if used).

    From a lightning control point of view, there is probably little reason to connect a 6 AWG cable from the mount ground rod 100' to the house ground rod after the fact. Lightning only runs for several 10's of feet down a 6 AWG. If the 6 AWG is buried in soil contact--It probably does do some lightning control. It is mostly for electrical grounding (AC and DC) that I suggest that connection (although, if it was me and I had an open trench, I would run the 6 AWG cable--More connections is almost always better, multi-point grounding, than not for lightning/RF grounding).

    For the PDP panel ground direct down to a local/outside the foundation ground rod--But then run a 6 AWG from that local ground to the house AC/DC main ground point (if I am understanding you correctly).

    For DC grounding... Tying the main DC negative bus to the local house AC/DC common ground is what I prefer. Many charge controllers choose to make "arc fault" protection at the solar array by placing a small 1-5 amp circuit breaker between Battery Negative (at the controller, or elsewhere) and the green wire safety ground. If the 1-5 amp breaker/fuse pops, then the charge controller is turned off (or the 1 amp breaker trips the slaved 60-80 amp breaker across the solar panel +/- leads). If you bond the DC negative bus to the AC/Ground Rod, you will "jumper" the 1-5 amp fuse/breaker and bypass the "ground fault/arc fault" detection. Your choice how you wish to handle this. One poster here with a lot of experience really like this setup (he has had many systems survive nearby lightning strikes). Me, I don't like a fuse between negative battery ground and green wire safety ground--This is a violation of every safety system out there.

    Only other thing to suggest are lightning rods near the solar array--Get the strikes to hit there, vs the solar array.

    Rdgoods, do you have a chance of lightning strikes to/near your home in your area?

    Running greenwire and +/- is what NEC does, and what was done for my GT array (15 years ago, before a learned/cared--But we really do not have lightning ground strikes in our area (hills, trees, utility lines, and I am in a valley).

    Personally, I would run 6 AWG from array straight down to a ground rod at base of array. And run the +/- separately. If above ground, you might want to use metal conduit (shielding) to keep lightning "off of the" panel wiring. I am not a code guy--But you might be required to run metal conduit through the wall to the disconnect is inside the building. then PVC is allowed after (cannot run pvc conduit into building if no outside disconnect--I think).

    For electrical safety (AC and DC), I would run a 6 AWG between the two ground points (20' run). This keeps people from getting shocked if (for example) there is a short to the array frame (AC or DC)... A ground rod does not have low enough resistance to earth to "trip" the AC/DC protective breakers. The fault current would run through the 6 AWG interconnect.

    My suggestion is to keep the lightning energy away from any other wiring (DC or AC). Connecting your AC/DC/Green Wire grounds inside the panel, then to the AC house ground rod/cold water pipe is fine.

    Assuming I understand your questions/configuration correctly.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • WaterWheelWaterWheel Registered Users Posts: 335 ✭✭✭
    Thanks Bill,      I'll add another ground rod a few feet from  the PDP panel.       The PDP is already grounded to the house ground but it's 50' of 6ga wire away from the house ground. 

    Pounding a 8' long grounding rod into hard clay is a lot of work. 

    Conext XW6848 with PDP, SCP, 80/600 controller, 60/150 controller and Conext battery monitor

    21 SW280 panels on Schletter ground mount

    48v Rolls 6CS 27P

  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    edited October 2016 #16
    Yea, I have seen folks use a medium (or a bit larger, to fit the rod) rotary hammer (set to hammer), rent one? Get a post/rod driver adapter.

    http://www.homedepot.com/p/Bosch-5-8-in-and-3-4-in-Hammer-Steel-SDS-max-Ground-Rod-Driver/203719979

    Wait until the ground is softened up from winter rains (if that is an issue where you live).

    -Bill


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • WaterWheelWaterWheel Registered Users Posts: 335 ✭✭✭
    Thanks Bill,     I never considered a hammer drill.      The last ground rod took me several long sessions with a 2 lb hammer and very sore arms.

    Conext XW6848 with PDP, SCP, 80/600 controller, 60/150 controller and Conext battery monitor

    21 SW280 panels on Schletter ground mount

    48v Rolls 6CS 27P

  • strumanstruman Registered Users Posts: 1
    edited January 6 #18
    Hi Bill,
    I know this is an old post, but it came up in my google search as I'm trying to figure out grounding for my ground mount.  I'm a DIY and this is my first project :-)

    Stage: I've built the arrays and ran a long run of DC wires and bare ground in PVC, and am at the stage of wiring into my inverters and then connecting into the house panel.

    I have a few grounding questions if I you have the time.  I've read online for hours and as you've pointed out - a lot of conflicting info....

    System info:
    4 strings
    ~15kw
    ground mount ~225' from inverters (two) that are next to house panel and PG&E meter
    Due to inverter input voltage limits, I could not combine wires at the array with a combiner box, so just a pass through box and ran four + and four - DC wires in PVC conduit - so each string runs the whole distance...
    I also ran a single, bare copper 6awg wire in the same PVC conduit between arrays and inverters
    I've connected all my racking and panels with a 6awg bare copper to a grounding rod driven 10' deep

    I assumed the following but now am questioning my grounding plan...
    1. connect the (bare) 6awg copper wire (that runs the full ~225' distance in the PVC conduit with the PV + and -) to (a) the array grounding rod and (b) the inverter(x2).  (there are designated grounding screw holes in the inverters body on the DC input side).  Is this correct?  After reading your posts above, it sounds like I could have grounded the inverter body(s) to the house ground or another driven rod versus running 225" of copper wire...
    2. I'm also reading that I need to connect my array grounding to the house grounding rod...is that correct?

    And if it helps, grounding guidance for my 7600MTLP-US begins on page 17 of the pdf
    http://https//lithium-car-battery.3dcartstores.com/assets/images/4-6kW-Solar-Inverter.pdf

    Thank you so much for your time
    Steve
  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    Steve,

    There are two major reasons for grounding... One is to "short out" a power source and prevent "energizing" a metal object (like a solar panel rack, well casing, etc.). For those, generally a 6 AWG wire run from the power source to the remote power location(s) is a good start.

    So, for grounding your AC inverter, etc., bonding its chassis ground to a local ground rod/cold water pipe and "everything else". Combining green wire safety ground and a DC battery bus ground (i.e., negative grounding the battery bus--typically) in once place (at the local ground rod, again typically) to make sure that any short circuits to "metal" (racks, electrical panel metal boxes, kitchen sink, cold/hot water plumbing, gas appliances). If there is a short, this will ensure that the breaker/fuses protecting the energy/current source will be safely tripped.

    One reason to run separate DC safety grounds is that, large lead acid battery banks supply much higher current to loads (AC inverter, battery chargers) than most AC loads and sources (12 volts @ 100 amps = 120 volts @ 10 amps = 1,200 Watts of energy). You need to check code, but 6 AWG is good for upwards of 200 Amps (guess, I do not have an NEC code book)--And 6 AWG cable fuses at ~600 Amps. Large DC battery banks can easily output 1,000s of Amperes or more.

    Another reason for grounding is lightning suppression. Short, straight runs from (for example) solar array down to a ground rod outside the building foundation (don't run lightning grounds from the "roof" down the "middle of the building" to a ground rod/cold water pipe). Lightning tends to seek the "edge" of a building. Sharp corners in wiring tends to "encourage" lightning to take a different route (rather than taking a 90 degree cable bend), and lightning will only follow 6 AWG cable for a few 10s of feet before it finds a "better" path.

    So "longer" ground bonding runs (225 feet from array frame to building) does not really "do much" for lightning control... It is to lessen the chance of a structure becoming "hot" and electrocuting somebody (i.e., you walk up to the frame in long wet grass, and get "bit".

    There are other reasons for grounding (some florescent fixtures need ground rod to fixture chassis for better starting, some gas flame detection systems need ground bonding and grounded AC Neutral bonding, and there is the issue of corrosion with "energized" metal and earth).

    The whole issue of ground bonding an AC neutral (typical North American safety ground/green wire/AC white wire neutral) gets complicated. Add in an AC genset, and that "PSW/TSW" AC inverters can typically support a Neutral/Earth/DC battery bank white wire ground bond, and MSW type AC inverters should never (typically) have a White wire + DC battery bank ground + safety ground--MSW inverters are typically have their AC output "shorted" if AC "white" and DC return or connected to ground.

    You, typically, want to make a bonded AC Neutral to Green Wire/ground rod safety ground in one location. If you bond the AC Neutral to Ground in several locations (AC main panel is typical in North America, many larger AC inverters and AC gensets have Neutral bonds internally connected when wired from the factory)--These multiple grounds can cause your load currents to flow both in the AC Neutral wiring and your Green Wire ground wiring--Not wanted. Also it has caused damage to AC genset internal wiring/voltage regulation, and would trip GFI (Ground Fault Interrupters/RCD for Europe) because of parallel AC return current paths.

    Anyway... Grounding is a complex subject. Lightning makes it even more so.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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