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I've been doing a lot of reading trying to get my head around what the right grounding system is to implement for a theoretical small PV minigrid (<10kW with distribution distances <300m), isolated from an external AC grid.

  1. I understand that the metal PV array frame/roof should be grounded to prevent floating potential, as voltages can be induced on the frame by the inverters high frequency switching components.

  2. Similarly, all equipment casing should be bonded and connected to earth in case of faults, for safety purposes.

  3. And finally, the earth and neutral will be bonded at one point after the inverter (if not already done inside the inverter), and then protection using RCDs installed downstream.

The PV negative line will only be grounded if ground-loop protection is installed (and this will usually be via the inverter anyway).

What is now confusing me, is if these separate requirements should use separate earthing conductors.

These technical specifications online seem to indicate a separate earthing rod for equipment casing to the PV array, and again a separate one for the AC Neutral-Earth link (but I may have misinterpreted it). https://www.drdnepmyanmar.org/sites/drdnepmyanmar.org/files/nep-document-docs/supportdoc2_minigridtechspecs_v2.pdf

On the other hand, this document seems to indicate multiple earth electrodes, but that they are all tied via equipotential bonding/ earthing cables. https://drive.google.com/file/d/1fjRi5QlU0k92Qw_6NjTdZ4u2xhBxFX9d/view?usp=sharing

And some other sites I've read keep insisting on how there must only be one earthing rod present to avoid ground loops (although perhaps they only mean on the AC side, and the chassis/equipment grounding and PV array grounding is different)

Would anyone be able to help clear this up for me? Is it dangerous/disadvantageous if the grounding rod for all 3 of the above is connected? If the PV grounding is done via the inverter, I assume that it relies on the equipment casing to be grounded (not the AC output ground, unless this is actually tied in the inverter anyway?)?

And to add to it all, what are your thoughts on grounding the negative terminal of batteries. I saw it drawn on the datasheet for an Xtender Inverter, but again I'm unsure if is always the safest option. The first link says that grounding of DC conductors should never be done directly as this complicates the protection and inspection requirements, but if the equipment is not galvanically isolated from the AC side then virtual earthing is allowed only via the AC side in one location - does that mean that a non isolated system would already be connected to the AC ground via the inverter's AC output?

I found this forum post about grounding batteries and it seems to indicate that it is in fact safest to directly ground the negative terminal when the system is galvanically isolated (contradicting the first link), and it should be a shared ground with the AC ground, however the permission to ground the negative terminal was stated in the datasheet. https://forum.solar-electric.com/discussion/356323/should-negative-on-battery-bank-be-tied-to-ground

Perhaps this is something super complicated and will vary inverter-to-inverter, and if I saw the battery grounded on one inverter's example wiring, then that means it should be done in that particular case? When a diagram shows multiple ground symbols, does that indicate they mean using separate non-connected earthing rods, or is it just a simpler way of indicating everything being at the same earthing potential, and therefore the same rod can be used? enter image description here

Obviously this is for a system with an AC in from a grid, but in an insolated system then the stuff on the left wouldn't be there. To make it safe, then the installer would have to add their own ground stake, and tie neutral and live at the AC-out (please do correct me if I'm wrong). And in that case, again would the batteries and AC-out ground share a common earth stake?

enter image description here

Also just a side note, for a system this small, I think TN-S seems like the most straightforward. Is that right? The connection between earth and neutral being done once at the AC inverter, with an RCD between neutral and live, and then neutral, live and earth being distributed separately to consumers.

I'm thoroughly confused about all this even having been reading various articles for days...

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  • The manufacturer requirements over ride the NEC in some cases. yes array assembly’s or framework are individually earth grounded if the metal structure is not interconnected. When switching to solar the home’s primary earth ground is regularly lifted and the array’s earth ground is used 3 pole switch on a 240 split system.
    – Ed Beal
    Commented Jul 7, 2022 at 15:53
  • What DC bus/battery voltage does this system run at? (Transformer-type inverters run at 12-48VDC or so, while transformerless inverters need 300-500VDC high voltage batteries/solar strings in order to generate the AC peak voltage by directly switching the DC bus) Commented Jul 8, 2022 at 1:47

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Dirt is a terrible conductor. So having some things grounded to one earth spike, and other things grounded to another earth spike, is not a good idea. Should large amounts of fault current need to come back on earth, the dirt will limit the current and the circuit breaker (overcurrent) will not trip.

And yes, grounding and bonding is a big headache. What Europe and North America agree on is that you need to bond neutral to earth at one place. And then keep it separate everywhere else. (this is fairly obvious if an RCD is involved, since the RCD will trip if you don't lol). Therefore using separate rods that aren't connected with ground wires is foolhardy.

Europe and North America also agree the bond should be as close to the source as is reasonable. Since your inverter is the only source, neutral and ground should be bonded at its output, and at that point also tied to all earth spikes through wires of sufficiently large size per your electrical codes.

If you are running power to an outbuilding the rules may be somewhat different. North America requires separate neutral and ground on all feeders to subpanels, but still requires a local ground spike in an outbuilding. The spike supplements but does not replace the ground wire.

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  • ok great, so you're saying that all grounds should be electrically bonded (equipment casing, AC ground that is distributed in a TN-S system). Just to clarify one other thing, as you said that we bond neutral to earth at one place -- I read somewhere that neutral should be earthed at every 5th distribution pole. I assume that was for a system where the combined neutral/earth is distributed (TN-C-S system) and then the RCD would be installed at the point at which the neutral and earth are separated when at the consumers home?
    – natashaxh
    Commented Jul 12, 2022 at 15:05
  • @natashaxh When I say "one place in the system" I mean one place in your site's system. The power company has to ground more often because they have lightning to deal with! Yes, the RCD must be installed after the last neutral-ground equipotential bond, since the bond would be a ground fault. Commented Jul 12, 2022 at 18:04

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