When to ground?

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I have a off-grid, pole-mount system. System voltage is 24V. The panels are in series and apply around 60vdc to the charge controller. The entire system is 24dc, no inverter, etc. Am I required to ground the system under NEC? I did read something that any PV system over 50VDC needs to be grounded. Does the pole itself act as a grounding rod? The pole is 4 feet deep.

Thanks,
Scott

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: When to ground?

    In a case like this the panel frames & mounts would be grounded at the pole and the negative battery post would be grounded. Grounding posts should be used; a minimum of 5' deep and usually 10' for most soil conditions. I would not trust the steel pole as a grounding conductor.

    Except for the new NEC regs about DC GFI of course. But if this is a "non inspected" location I wouldn't bother with them.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,471 admin
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    Re: When to ground?

    I always like to go back to basic requirements when answering these type of questions.

    So, why ground?

    First, if you have lightning in the area, "proper" grounding can help protect equipment and people. Also, if there are tall metal structures (antenna towers, etc.), these can become charged from static electricity and grounding the structure will reduce chance of shock.

    If you have "plumbing" (water pipes, sinks, even gas pipes), you want grounding so that if somebody grabs a metal sink/heater, and a electrical device "neutral" conductor, there is no current flow/shock. With AC, we have the GFI (ground fault interrupter) that can protect against shocks (there is no practical DC GFI system--AC GFI use transformers to measure leakage to ground currents).

    And, grounded conduit/plumbing/etc. prevents a large metal object from becoming "hot" if there is a electrical short to metal in the system.

    Note that grounded systems only need 1 pole breakers/fuses for over current protection. If there is a short between a ground bonded neutral and grounded metal, there is no excess current flow. If there is a short from Hot to Grounded metal, then there is over current flow with trips the breaker fuse.

    If you have two hots (i.e., 120/240 VAC split phase power systems in North America), then you need two pole breakers which trip together to break both hot leads.

    If you have a floating power supply, you should have two pole (ganged) breakers to trip if either leg gets shorted so that all power is killed. Note that with floating power systems, if one leg gets grounded then the system effectively becomes a ground bonded neutral type system until a second short happens.

    Note there is grounding all metal (such as to the chassis of a vehicle/trailer) vs grounding to the earth. Grounding to chassis is to limit short circuit currents. Grounding to earth tends to deal with static electricity, lightning, and corrosion (DC current flow can corrode in ground pipes, ground contact metal, etc.) and creating a "zero volt" reference between various "metal things" in the human/animal contact environment.

    Note that earth grounding is not a "solid" or even very good ground. I have received electric shocks between two ground rods ~75 feet apart (around 60 VAC measured) because the earth was energized by large electrical pumps (salt water aquarium on land fill). There have been cattle kills from power line leakage to earth (four hooves on ground, current path right through heart, makes 4 legged animals more susceptible to electrocution due to energized "earth").

    Functionally, there is no need for earth grounding for a solar PV system to function correctly. And a truly "floating" power source is actually "shock resistant" (you can touch one lead and not get shocked--you need a complete circuit path, which "ground bonding" creates to get shocked). However, the one problem with a floating system--is you should setup a test/verification procedure to ensure that the system is still floating after months/years of use. It is possible for a short to a metal object to make the system "ground referenced" and nobody knows.

    The NEC uses ground bonding (really bonding all metal together, then running a wire to earth) as part of their over current / short circuit protection scheme (single pole breakers/fuses on 120 VAC circuits). And a bit for lightning control. Other countries do OK without any earth bonding at all. And those of us with old homes, still have the two blade outlets without the third grounding wire and survive.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Scottg4001
    Scottg4001 Solar Expert Posts: 47
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    Re: When to ground?

    Can someone upload a picture of how the battery negative is properly grounded. A schematic wont help me, I need to see picture of a actual setup. Thanks,