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My 5000W pure-sine inverter has its metal shell grounded via lug to a #6 copper wire that goes to ground about 4 ft. away. The leads go to a combiner box that is grounded to the same ground. Besides that, I have an additional ground outside where my breaker box is located in the cottage. That wire is attacked to the grounding bar of the box.
All outlets, including the two 110v outlets on the inverter read "open ground" with my Klein Receptacle Tester.
The cottage is wired for a 220v, 6500w generator with a NEMA L14-30P plug for the generator, carrying two 110v lines, two neutrals and a ground wire. The generator is on rubber wheels, yet when I connect the generator to the cottage, the receptacle tester indicates it is wired properly without "open ground" even before I put an additional ground to the fuse box in the cottage. As a novice, I have no understanding of what is causing this. Is it the inverter?

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  • Have you opened up an outlet to make sure it does have a ground wire attached to it? – JPhi1618 Aug 26 '19 at 18:43
  • Do you want to use the generator for portable service? Also, what make and model is your inverter, and how is the generator connected to this system? (Through a conventional ATS, or to an AC input on the inverter?) – ThreePhaseEel Aug 27 '19 at 1:58
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You have faithfully built out your grounding system and kept it separate from the neutral system, which is better than a lot of people do. It sounds like you've done this work to Code. But what you never did is fit the neutral-ground bond in your main panel.

A house has exactly one neutral-ground bond

And the problem is, your generator already has one of them.

Neutral is not ground. Neutral carries the ordinary return current that the circuit needs. Ground carries only fault current during problems.

However, neutral is tied to ground in exactly one location. That's a very big deal. The point of the N-G bond is to assure that your electrical system isn't floating at thousands of volts compared to actual earth. Because the insulation in appliances isn't rated for thousands of volts.

But if you have two of them, that is very bad. The ordinary current flowing on neutral will then "split" at one of the bonds, and some of it will travel over the ground wires to the other bond. That means safety ground isn't safety ground anymore, it's a working neutral wire. Certain failures can then cause grounds to be energized at mains voltage - which is the last thing you want.

And the neutral-ground bond (1) is always at a grounding rod point. You don't tie ground to earth at junction 1 then have a neutral-ground bond at junction 2.

Right now, your neutral-ground bond only happens through the generator. That means it's not at the same place as the grounding electrode. That's bad.

Put the neutral-ground bond where it belongs - in the main panel where the grounding electrode comes in - but then you'll have two of them when you connect the generator, and that won't do.

  • The cheap, but at least Code, answer is to remove the neutral-ground bonding in the generator, and then put it back when you unhook the generator to use it as a portable generator (where it needs to ground its outlets).

  • The cheap, non-Code answer is to remove the ground wire from the NEMA 14 inlet where you plug in your generator.

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The North American wiring scheme has neutral and ground connected together at the breaker box (note: I got this wrong the first time). The idea is that neutral is always close in voltage to ground, while ground is a safety ground. At least the cheap outlet testers that I know of just check for a voltage between power and ground, although a fancier one might check for continuity between neutral and ground.

So, your generator connects neutral and ground internally so that its outlets "looks like" the outlets in a house; your inverter does not, presumably for more flexibility, and -- apparently -- neither does your panel.

That's the "why".

As for "what to do": I do not know the most acceptable way to proceed here, although I expect that you should connect your box properly. I suspect that having multiple ground rods without a conductor between them is wrong, but I'm not up on electrical codes.

So, if it were me, I'd talk to an electrician, or take a deep dive into the local building codes.

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  • Minor correction - ground and neutral are tied together in the main breaker panel after the electric meter. There is no ground wire coming from the electric supply to your house. – JPhi1618 Aug 26 '19 at 18:49
  • @JPhi1618 More than a minor correction, then -- that means that his cottage is wired incorrectly, and that once it's fixed the problem should go away. I'm trying to decide if I should do a major edit to my answer, or stand aside and ask you to give your own (obviously better) answer. – TimWescott Aug 26 '19 at 19:09
  • I'd like some clarification on the question, really. From my interpretation, when a generator is used, the outlet tester is fine, but when the inverter is used, it says open ground... But i'm not sure how that would happen since ground and neutral should be bonded in the cottages panel. – JPhi1618 Aug 26 '19 at 19:11
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    I'm deducing that ground and neutral aren't bonded in the cottage's panel. But yes, that's a question for the OP. – TimWescott Aug 26 '19 at 19:20
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What the grounding wire does do for safety is it provides a fault path. When something that should not be energized becomes energized, the grounding wire provides a low-resistance path back to the source. This causes an over-current breaker to trip and de-energizes the circuit. In order for this to work there must be a low-resistance connection between the grounding wire and the neutral. Enter the ground-neutral bond.

The terminology is a little unfortunate because ground -- earth, soil, dirt -- is actually a poor conductor. Yet the third wire provided in electrical systems for safety is called the grounding (or ground) wire. The connection of the grounding wire to terra firma doesn't do a great deal for safety in ordinary faults; it seems to be related more to extreme faults or lightning.

While the grounding pin in the receptacles on a portable generator or inverter is almost universally connected to the chassis of the unit, ground-neutral bonding is hit or miss. Some units have a built-in bond, others don't, many are not clearly marked, and a few may have some manual or automatic switching means to open or close the bond at times.

In mains-connected systems the ground-neutral bond is a screw or jumper wire installed between the neutral bus bar and the grounding system in the first panel downstream from a utility electric meter. In independent systems like boats, RVs, and off-grid cottages it gets a little more nuanced. Especially so in the case of boats and RVs, which sometimes are independent but sometimes are grid-powered. We'll ignore that since the cottage at hand is permanently off-grid.

In all systems it's important that there be only one ground-neutral bond in the whole system and that it be sized adequately to trip any of the overcurrent breakers in the system. Probably all that you need to do is install a jumper wire between the neutral bus bar and the grounding bus bar in your breaker panel. If you can add a clear photo of the wiring inside the panel we can double-check.

Technically with the ground-neutral bond connected in the main panel it should be disconnected in the generator. The reason is that return currents to the generator will cause a voltage difference between the generator chassis and grounded structures in the cottage. If somehow a person were to lay one hand on the generator and the other on a grounded structure of the cottage while the generator was powering a significant load, there could be a possibility of being shocked. It's hard to quantify the risk, so I can't say whether it's low enough to ignore.

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