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I have buried 50 m of heavy duty 4-conductor cable for a small three-phase AC wind turbine. I grounded the pole, which sits on a small hill near my offgrid house. I also added surge protectors on all three phases at the base of the pole.

Now, how shall I best ground the cable (4th conductor)?

  • I would love that the turbine does not "bring" lightning into my house ;).
  • I don't want to mess the grounding with stray voltage, especially as I have inverters and batteries.

Initially I though I would ground the cable to pole earth AND also at the other side to the house ground.

But I read a hefty ~30 V+ difference in voltage between the two outer grounds. The soil itself is pretty different at the two ends.

I could also ground it midway to a massive metallic structure next to my house, which is itself anchored to the ground.

All this mostly proves that I am pretty clueless! What is the best practice? Should I ground the cable only on the turbine side and leave it floating the other side?

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    Local electrical code is very important. Where are you located?
    – Jeroen3
    Nov 28, 2022 at 10:27
  • Yes - @Jeroen3 in FR, middle of nowhere : not connected, nor even connectable to grid.
    – MoonCactus
    Nov 28, 2022 at 11:23
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    You probably want lightning protection on the three phases where it enters your building. How did you measure the 30V ground voltage? The voltage is not so much the concern but rather the current. If you have significant current in your earth, then that is a concern.
    – Kartman
    Nov 28, 2022 at 11:43
  • I'm following this question because I have a similar situation: solar panels 50m away from the house, inverter in the house.
    – bobflux
    Nov 28, 2022 at 13:34
  • @Kartman, you mean add a second set, or have only one at my house ? I have one at the bottom of the pole (actually dismanteld until I have better faith in what I'm doing). I fear oxidization when the voltage differs from the ground, but you're right, the current is negligible (80mV across a 1K resistor)
    – MoonCactus
    Nov 28, 2022 at 14:32

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Well, what we do in North America is establish double ground spikes at every structure or suitable place (or use superior methods such as mesh/Ufer grounds into the concrete reinforcing rod). And then bond everything into one big grounding matrix. Neutral and earth are kept separate absolutely everywhere, except one designated location at the property supply.

European electrical codes tend to go a different way because so much of European housing is multi-occupancy buildings. They tend to have earth and neutral delivered separately by the power company and they just trust the power company to supply a reliable earth. Makes sense in a Paris apartment.

Given that you seem to have a bit of an estate, much more in the American suburban/exurban style, I would advise you to go with our style of bonding. Run separate earth wires everywhere to spikes everywhere. Better is "Ufer" earthing, which ties into the steel reinforcing mesh in the concrete.

I also note that the 3-phase power coming from the windmill will be variable frequency and variable voltage, so not usable directly. I assume it will be rectified and buck/boosted to feed a battery bank and inverter. You could add a transformer in that path, which would provide some additional isolation. You could even use two transformers and "step up" the windmill voltage to the limits of the wire for transmission.

Lastly, the most important thing about lightning protection is to have a hefty earthing path which draws the lightning away from the electrical parts of the windmill. If you have metal blades, then make sure the mechanical transmission provides a very happy path to your earth spikes or Ufer at the windmill. You don't want lightning finding a useful path through the motor. You also don't want lightning finding a path through the bearings, as it will spall the surface and the metal shavings will grind things up.

While it might look silly, a lightning rod high enough to catch lightning first might be a good idea. This would need to be on the rotating part (which turns into the wind) and could be aimed at an angle somewhat to keep it from whipping into fan blades. It could even be part of the tail vane, if it uses one to turn it into the wind.

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  • Wow I really appreciate your answer, you provide lots of information and pragmatic geographical considerations ! "Neutral and earth are kept separate absolutely everywhere, except one designated location at the property supply" is key I guess. Also just read that "concrete had better conductivity than most types of soil" (wikipedia/Ufer) My turbine is a helical vertical axis with one pivot at the base so it might prove a bit difficult to attach a lightning rod to it. I could try to deporting it though. Using isolation transformers / galvanic isolation is a pretty good idea. Thanks again !!
    – MoonCactus
    Nov 29, 2022 at 10:14

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