Grounding question

VaughnVaughn Registered Users Posts: 2
I am installing an off-grid solar system at my cabin. I have been told that all components need a separate ground rod and need to be grounded individually (solar panels, load center, charger/inverter, etc). This goes against everything I thought I knew about electrical systems where a common earth ground field was used to keep all components at the same ground potential (or as close as possible). What is the approved grounding method?
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  • BB.BB. Super Moderators, Administrators Posts: 30,740 admin
    Welcome to the forum Vaughn.

    Do you have lightning strike possibilities in your area?

    What is the soil like (rock, dry/sandy, etc.)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • VaughnVaughn Registered Users Posts: 2
    Thanks for your reply. Yes, we have lightning commonly ( I'm in northwest Wisconsin). I've never had a strike close to the cabin but it could definitely happen. The soil is sandy and wet with various layers of small rock and gravel. I have driven copper clad 10' by 5/8" ground rods (3) linked with #4 bare copper that I currently have my load center grounded to. I'm not up and running yet and still in the design/implementation stage and learning.
  • BB.BB. Super Moderators, Administrators Posts: 30,740 admin
    Grounding is sort of addressing two different issues that have two different "physics" behind how they work.

    The first is the classic 120/240 VAC 60 Hz (and 12/24/48 VDC) grounding issues. More or less, your ground is a single rod near the main panel and is usually tied to a cold water pipe to keep things at "ground potential". If there is a short circuit (120 VAC to electrical box, or metal faucet, etc.), the green wire ground connection (and tied to AC neutral and DC battery bus negative) provides a current path to trip the breaker or fuse (i.e., an unground sink garbage disposal with a short to 120 VAC hot wire, would be an electrocution hazard).

    The AC main panel ground and neutral to ground bond (in one location) is then tied to a ground rod and water pipe. The ground rod does not really have good enough electrical connection to (for example) trip a 120 VAC 15 amp breaker. The maximum resistance of 25 Ohms from ground rod to earth only carries (120 VAC / 25 Ohms =) 4.8 amps... The ground rod is needed to reduce the possibilities of galvanic corrosion, and provide a ground references for some applications (some florescent lamps need a grounded metal chassis to start reliably, etc.).

    The other major need for ground rods is for lighting control. For those, ground rods need to be installed close to where the "thing" being protected is mounted. I.e., a ground rod for the main electric panel. Another ground rod for the solar array (if not close to the main panel), etc. More or less, lighting is a "radio frequency" current (albeit a very low frequency), and will not follow a 6 AWG cable for more than 10 feet, or take a right angle bend, without jumping to something else that provides a lower impedance to ground (impedance being the key term here-The distributed inductance and capacitance of the cabling and other physical issues that change impedance--such as wide sweeping radius vs sharp 90 degree turn in wiring).

    And because the ground rods and grounding also needs to address the 120/240 VAC and VDC safety ground requirements--Tying all separate ground rods together with a 6 AWG cable (home size power system) supports that short circuit/trip breaker function that is needed.

    Other issues like running solar panel grounds and DC wiring down the roof and on the outside wall to ground rod exterior to foundation (for grounding ing) and surge suppressors where the DC lines enter the home (you want to divert lightning energy "outside" of the home envelope).

    Any way, a very complex subject. Here are some links with more information:
    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.

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