How do I ground an AC inverter in a RV?

Question

I am working on installing a Victron Energy Phoenix Inverter 800VA into my little teardrop RV. I am doing something men just don't do, I read the Victron Energy Phoenix Inverter manual! Appendix A starts off by saying:

The AC output is isolated from the DC input and the chassis. Local regulations may require a true neutral. In this case, one of the AC output wires must be connected to the chassis, and the chassis must be connected to a reliable ground. Inside the inverter, a provision has been made to be able to connect the neural and the chassis; the way to do this is explained below.

Please be sure to disconnect the battery when connecting the neutral to protective earth (PE).

An internal PE wire, which is used to connect the neutral and the chassis, is accessible after removing the plastic cover. A Torx T10 screwdriver is needed to loosen the four screws which hold the plastic cover.

And it concludes with this:

For both safety and functionality, I am seeking two answers:

1: It sounds to me like the neutral is floating right now. Do I need to open up the case and move the wire?

2: Regarding the ground on the inverter (seen in the next image), should I be connecting that to the chassis of the RV?

Out of pure curiosity, I have one question:

What what I have read about neutral-ground bonding, why would you NOT want the bound together? Aka, why do some generators not have them bound, it seems to only cause problems, but there has to be a reason for it, so inquiring minds want to know!

---

I left off some important info out of ignorance. Sorry about that:

This question is NOT a continuation of my question the other day. Thanks to lots of GREAT answers to that question I finally realized that 120V AC is actually complex, too complex for my simple mind. I was going to get a Victron MultiPlux 800VA until someone on FB pointed out they are not 120V, but 230V.

I have opted to keep things real simple. I will install the inverter and then use a good old extension cord to run it to whatever I want to power at the time. At times that might mean I actually have to unplug something from the shore power outlet, but that seems a lot safer, easier, and cheaper than trying to figure how how to implement my own version of a MultiPlus.

In time, if this whole idea of working out of this little RV is successful, then I will look at investing in a MultiPlus 2000, or, if it isn't, then I will be looking at getting a bigger RV where I will need at least a MultiPlus 2000, if not a bigger one. (When this is all said and done, I do want to be able to run the RV's A/C off solar. A big ask, but that is a LONG way down the road! Step one is to simply be able to power the core work things with the battery.)

Answer 1

TLDR: Do bond the inverter. Use a "generator interlock" or other DPDT switch that switches both hot and neutral between shore power and inverter. That "switch" could consist of an outlet (fed from the inverter) right next to the RV's inlet. To run on inverter simply use a 1' extension cord to jumper the outlet to the inlet.

What you don't want is TWO neutral-ground bonds in the system.

I believe we already discussed the "how to" of isolating neutral and ground when you are on shore power. It requires a large, annoying, and hard-to-fit interlock (or at the least a UL listed 30A DPDT switch, hard to find in a small form factor). The rest of this section describes the "why" - feel free to skip it if you're a believer lol.

The first problem is accidentally being THE neutral-ground bond for the whole neighborhood. The house is supposed to have its own neutral-ground bond for fault current. Suppose the house bond fails. Consider this scenario:

Suddenly, due to an internal short, the house's range is shunting 2000A from hot to ground. What's supposed to happen is it flows the 2000A via the house's neutral-ground bond to the neutral, back to the transformer, resulting in massive current flow and instant trip of the 50A breaker.

However, with the house's bond broken, the only neutral-ground path is onboard your RV. So all the range's fault current goes out ground on your 30A extension cord, to your chassis, through that FJ2 connection on the Victron, back to the panel and out the RV's neutral wire. And because of the very long wire run, only 80A flows. That could take 30 minutes to trip a 50A range breaker*. Remember all this current is moving on neutral and ground, which don't have fuses.

You could have the same problem if a neighbor's house has a working neutral-ground bond, *but has lost neutral from the utility's service drop (we field a question about that at least once a month)... so it is routing all neutral current through the dirt, to other houses and other people's neutral-ground bonds. This is not even your current, yet your redundant bond is forced to handle it because your house's bond is inop.

Ed Beal discusses another double-bonding failure mode: you plug in shore power from a site with an inoperative ground wire, and then, have a ground fault on the RV. Now, the RV chassis is energized!

I hope I haven't beaten that dead horse too much.

Do you have to bond this? Pick your poison.

The simple answer is to have your utility/inverter interlock switch neutral, i.e. since it's a 120V RV, you need a 30A DPDT switch. But I know in our earlier discussion, I tried ways to build this out of Home Depot tier electrical gear, and that was bulky, and you were already committed to a bulky "all-in-one" battery charger which was not designed for use with an inverter. There wasn't room for two bulky things.

So to sustain the simple answer, there are 2 options: go industrial-tier and find a compact, UL listed DPDT switch... or simply wire the inverter's output to an outlet right next to your RV's inlet, and install a 1' jumper cord between them, so the RV is literally plugged into either shore power or inverter.

However, if that is not an option, then pick your poison: Jumper and risk the above scenario, or DON'T jumper and then, in inverter mode, you have an "isolated system". Normally, we don't like those, for reasons.

• It means hots and neutral are floating at ??? volts above chassis ground. A transformer leak could have L1, neutral and L2 at 2400, 2520 and 2640 volts compared to ground, and appliances just aren't insulated for that. However in your RV, there's no other source since you are isolated, so it won't raise to thousands of volts; it'll simply be a question as to whether hot or neutral is near ground.
• A bolted hot-ground fault results in the system no longer being isolated, and hot is near ground and neutral is near 120V. This creates a risk for appliances which are not double-insulated, and again, neutrals are not fused.
• Hot-ground faults in appliances will not be detected. For instance, if a blender has a bolted fault to ground, we want it to flow 500A and instant-trip the breaker. On the other hand, your inverter is an inherently current-limited source, which may not even be capable of flowing 15A, let alone the 19A+++ required to trip a breaker.

So in your case, the risks of an isolated system are less serious than most. The root problem is, the armchair wild guesses of some guy on the Internet will be cold comfort when sitting in an uncomfortable chair in a Wyoming district attorney's office with your lawyer, being asked to explain how intentionally violating NEC is not manslaughter.

As such, I can only proscribe NEC-compliant solutions. Either a) find a compact DPDT switch so you can switch neutral... b) use the plug-jumper method, or just c) break out of the "fallacy of sunk costs" dump that "all-in-one panel-charger", which is bogging down your project since it's just not designed to do what you want to do.

---

---

* Breakers use an "inverse time curve" between 1x and 5x trip rating (50 and 250A). This is to be tolerant of momentary overloads like motor start, or mild short-term overloads like toaster + coffee maker. So it will trip in milliseconds at 250A, seconds at 150A, and minutes at 100A. UL only requires breakers positively trip at 135% (67.5A).

Answer 2

Grounding the neutral creates a parallel path for return , or in some cases with the neutral connected to the PE it is possible for some one standing on the earth or touching a metal fence and the trailer to get a shock because of the parallel path. I am not sure of the wiring but your inverter may only connect the ground to neutral when on battery or actually producing a separately derived voltage (code speak).