Girl Needs Help!!! Grounding my Off Grid Solar PV system

KNLsolarKNLsolar Registered Users Posts: 18 ✭✭
edited March 2017 in Solar Beginners Corner #1
I have 3 100w panels with 3 12v 35amp hr batteries. Plan on increasing the size of my system so I want to be sure I do it correctly. I have 1 8' grounding rod.

Can I run the system ground to the same rod as the equipment grounds?

I have just the neg side of my battery bank hooked up to the rod. Is that sufficient for the whole system electronics grounding?

C. Do I need to have both neg and pos terminals of my batteries fused or just the positive side?

Thanks! Natalie


  • BB.BB. Super Moderators, Administrators Posts: 31,771 admin

    There is nothing "magic" about driving an 8 foot rod into the ground. They may have as high as 25 Ohms of resistance (as I recall)--And that means ~1 amp if driven by a 24 volt battery bank.

    When you tie all of the grounds (DC Battery bank negative, DC Green ground wire, AC green ground wire, AC neutral/white wire, etc. to the ground rod--That means you have a have a common connection where if there is a short circuit from (say the DC + input to your AC inverter to the inverter chassis--There is a green wire connection from the chassis to the ground rod and back to the battery bank negative--So the fuse/circuit breaker will trip (blow).

    Tying your cold water pipe to the ground rod--Helps to make sure you do not have a metal water pipe at something else other than zero volts with respect to the soil. If there is a voltage, the cold water pipes will experience electrolysis and eventually holes will form in the pipes (and other structures).

    The other usage for the ground rod is static discharge (your solar array on a roof, radio/tv antenna, etc.) can form several hundred volts or more of static charge. Also, if you have a thunder cloud overhead, thousands of volts. And of course, a lighting strike, you want the lighting to go to ground vs somewhere else (through your inverter, electronics, etc.). 25 Ohms really does not slow down lighting (there are other issues with how best to protect against lighting if you live in a lightning prone area).

    The short answers:

    A. Yes tie all of the grounds directly to the common ground rod.

    B. Typically, negative grounding the battery bank is a good thing--But not required for system operation (i.e., the system will run without a ground rod--Just like in an RV). You would also bring the green wire(s) from chassis (AC inverter Chassis, Solar Charge Controller Chassis, etc.) all to that same ground rod too. As well as your AC green wire ground and AC Neutral ground (usually the green wire is tied to the neutral bus bar in the main circuit breaker panel for the house)--Always understand what you are doing, grounding can actually get "complicated" in some situations (large genset, RVs, boats, etc.).

    C. When you have a "common ground" / "grounded return line" such as the negative terminal of your battery system, or the grounded Neutral bond in an AC system, then that is what lets you "get away" with fuses/breakers only in the + battery power (and "hot" black or red for AC systems). If you have a "floating" power system, you are supposed to have fuses in both + and - battery systems (or Hot and White/Return in AC systems). Floating systems have failure modes that can cause high current to flow through smaller branch wiring if only + branch wiring has fusing/breakers--Although, many people do "float" small solar power systems or other DC power systems (i.e., boat wiring).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • KNLsolarKNLsolar Registered Users Posts: 18 ✭✭
    Thank u for taking the time to explain. Very helpful.

    I think I am understanding this better. As far as the neg input from the panels, should that be grounded? And if so, to the same ground or should it have its own?
  • KNLsolarKNLsolar Registered Users Posts: 18 ✭✭
    I guess my concern is that if I have the panels and My array grounded to my system inside my house if lightning were to strike my array that the voltage could travel into my house and into my equipment and of course cause a problem such as fire and so on.
  • mike95490mike95490 Solar Expert Posts: 9,295 ✭✭✭✭✭
    Grounding  happens outside the house, the cables are outdoors, the common tie point is outdoors in a electrical box. and that helps keep the high voltage outdoors     will come up with a lot of pics, some are good, some not so correct.

    Powerfab top of pole PV mount | 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

    gen: ,

  • BB.BB. Super Moderators, Administrators Posts: 31,771 admin
    As Mikes says--You want to address grounding/lightning control at the wall of the building. You do not want to bring lightning into your home.

    Lightning is a high(er) frequency event. DC and low frequency AC (60 Hz house current), follows wiring very well.

    High frequency currents--You get into "impedance". Basically, the electrical conductors have both DC (low freq AC) resistance, but they also have inductance (and capacitance) which at higher frequencies impends the flow of current. Wide/flat braided wire conducts high frequency lightning much better than a single solid strand of cable.

    From another post:
    Re: Working Thread for Solar Beginner Post/FAQ

    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 (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.

    Regarding your specific question about hard grounding the solar panel negative lead... It depends. Many solar charge controllers do not care--But a few higher end MPPT controllers measure the array current in the negative lead, and grounding the solar array, will cause the controller to not run correctly.

    Also, there is a DC Ground Fault System. It has been (sort of) required by NEC. Instead of tying DC battery negative to earth ground with a heavy copper cable, they use a 1 amp circuit breaker or fuse. The theory if there is a short to ground somewhere, the fuse/breaker will trip and turn off the solar charge controller (early arc fault protection). There is a long story behind that... Short story is I do not like this type of ground fault detection, it causes all sorts of safety issues (shock, fire, etc.).

    Anyway, the short answer is to follow the instructions that come with your charge controllers/inverters/etc. and don't do something because somebody suggested it was a good thing. There are so many different mfg and systems out there, that it is very difficult to give "generic answers" that work for everyone. Details matter. And grounding has always been a complex issue. And mixing different grounding solutions together can cause safety issues.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • KNLsolarKNLsolar Registered Users Posts: 18 ✭✭
    again thanks for this help!!

    So if I understand as far grounding my array that my panels are on it should be grounded to a separate ground than my indoor electronics, but needs to go to my outdoor ac ground rod or other ground rod. Is it affective to have the array grounded to a 8' dieode by itself or should it be tied into my homes ac ground dieode outside of my house?
  • KNLsolarKNLsolar Registered Users Posts: 18 ✭✭
    edited March 2017 #8
    > @BB. said:
    > As Mikes says--You want to address grounding/lightning control at the wall of the building. You do not want to bring lightning into your home.....

    I understand that my charge Controller should take care of the grounding of my panels electronics. However in the case of lighting strikes how can I avoid the voltage from the strike from entering my home via the panel neg & pos leads?
  • BB.BB. Super Moderators, Administrators Posts: 31,771 admin
    Where the cables enter the home, use surge suppressors. They connect between the incoming cables and the metal box/tied to your ground rod. The suppressors are designed to take excess voltage and shunt it to ground (lightning strike in area).

    The Midnite Solar ones are pretty nice:

    And they have some nice videos on the product:

    Remember that nothing (short of military Tempest standards) will prevent damage from a direct strike. The idea is to keep the people safe in the building, and reduce the damage where possible.

    If you are in an area with high risk of lightning strikes, installing lightning rods to take the strikes and direct them to earth may be for you.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • KNLsolarKNLsolar Registered Users Posts: 18 ✭✭
    Ok that makes sense. Right now where my cables come into my house I have a breaker box on the inside of the house with DC fuses on both sides. Is that still OK as long as I put a separate box on the outside of the house with the surge protectors. And I'm also assuming that the box itself on the outside of the house I would also need to ground it to the outside ground as well?
  • KNLsolarKNLsolar Registered Users Posts: 18 ✭✭
    Btw I live in southern Florida so yes lightning is a big issue
  • BB.BB. Super Moderators, Administrators Posts: 31,771 admin
    You might be able to put the surge suppressors directly in your main panel (solar electrical box, etc.). With lightning, the current is wants to "spread" out to the edges of where it can flow.

    For example, you have a solar array on the middle of the roof and bring all the wiring down into the house--The lightning energy will try to find the conductors at the farthest edge of the home (if you have lightning grounding cables down the four corners of the house, the lightning energy from your array strike will try to go "outward" to those conductors).

    You want the wiring from the solar array to go down an outside roof/wall until you hit the point that you want to bring the wiring into the home. Note, you will have to ground the solar panel frames/racking too. Again, 6 awg minimum from frame to ground rod at base of home next to foundation. If there are multiple ground rods involved, I would suggest connecting each ground rod to each other ground rod via 6 AWG cable... This helps to tie the house grounding together (all grounding has a heavy 6 awg back to the main house rod/ground/cold water pipe). That way if there is an AC short circuit (say your corded electric drill shorts to the solar array mount, then the short current can find its way back to your AC main panel and trip the breaker).

    Generally, there is a transition from out side the house to the wall (metal conduit typically), then a box with fuses/breakers, then into the home with Romex/etc. You would put the suppressor(s) right there at the box and run a (at least 6 awg) cable straight down to a ground rod (or the common house ground rod, if it is in the right area). Lightning will not like to follow the 6 awg cable for more than ~10 feet or so. And make large sweeping bends (no right angles, those are "high impedance" and the lightning current will not want to follow around the corner).

    I am not a code person--And code/requirements are regional/city specific. The above is a general idea of what you will be doing. And I am not telling you to go against code (except in some limited cases at times).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • KNLsolarKNLsolar Registered Users Posts: 18 ✭✭
    Thanks Bill!!!
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