Outlet Tester Showing Open Ground on Inverter

By Carolyn Shearlock © 2014 • all rights reserved

Outlet tester showing an open ground on your inverter outlet? It might not actually be a problem.

When we bought Barefoot Gal, one of the survey items said that there was an open ground on the inverter. We’d been there as the surveyor was testing and I saw that he was using a simple outlet tester similar to the one we used on Que Tal when we plugged into shore power to make sure there wasn’t a problem with reverse polarity. All of the outlets powered by shore power tested just fine; it was only the inverter outlet that had an open ground.

If you’re not familiar with these outlet testers, here’s the one we have (on Amazon) – they’re great for telling you about dodgy shore power. Lots of different brands are available and our experience is that there’s virtually no difference between brands.

We were planning to replace the very old modified sine wave inverter that was on the boat with a new, larger, pure sine wave inverter (if you’re not familiar with the differences between modified sine wave and pure sine wave inverters, read Inverters 101). Consequently, the open ground problem didn’t worry us as we’d be sure to properly install the new inverter.

I’ve been researching various products in preparation for when we move aboard and begin working on Barefoot Gal. We’re not buying them now, I’m simply listing everything we need by project on a spreadsheet with links to the items (or a note to buy it at a local store).

So this past week, we picked out the inverter we wanted – 2000 watt Xantrax ProWatt –  and then I began looking to see what wiring, fuses and other items we’d need (Nigel Calder’s book is great for figuring out what size wire you need). This boat is wired very differently than Que Tal was – much more simply in fact. The inverter doesn’t have a remote on/off or a transfer for switching the outlets between shore power and the inverter – if you need AC power when away from shore power, you have to press the switch on the inverter and plug your cord (or power strip for several items) into it.

Thus, I felt pretty confident about Dave and I installing it ourselves. Projects like this tend to be joint for us – Dave does the drilling and mounting and I do the planning and actual wiring. NOTE: Neither one of us is a marine electrician and we would not attempt the installation ourselves if it was going to tie in to the boat’s shore power or AC (110 volt) system.

So my next step was to research the ground fault problem on an inverter – what causes it and how should the inverter be wired to not have a ground fault? I wanted to be sure we did it right, and didn’t just wire it the same as the previous person had done.

The first thing that I discovered is that lots and lots of people have this problem. There are all sorts of questions on RV forums asking about this (RVers also use inverters) as well as a few on cruising forums, and few real answers. Not helpful.

So then I started exploring manufacturers’ web sites, figuring that if this is a common problem, someone surely would have addressed it in their installation guides, troubleshooting tips or FAQs.

Before proceeding, I want to reiterate that I am NOT a marine electrician. I can tell you what I learned but I don’t know if your situation is the same as ours. It’s up to you to make sure your inverter is wired properly.

It turns out that these little testers are designed for testing house or shore power wiring that’s supplied by a power company. They are not designed for power supplied by batteries and an inverter. I got that much from a couple of places, where their explanations were rather convoluted. Then I found the Cobra page which told me exactly what I needed to know (emphasis added):

Outlet testers are intended for testing normal utility power in your home.  With normal utility power, the neutral and ground are connected together in your fuse box.  Outlet testers are designed under this assumption.

The outlet on your power inverter, however, is not configured exactly the same as in your home.  It is normal for an outlet tester to show an open ground. This does not indicate a problem with your inverter.

You may know Cobra as the maker of the first CB radio back in the 60’s, but they are now a world leader in mobile electronics for drivers, RVs and boaters. I later found a few other manufacturers who came to the same conclusion but not nearly as straight-forwardly as Cobra.

Whew. We didn’t really have a problem.

Just to recap if you are using an outlet tester:

  • If you are using inverter power, it is normal for the outlet tester to show an open ground on the inverter outlet or any boat outlets that are receiving power from the inverter.
  • If the boat is plugged into shore power, any outlets fed by shore power should NOT have an open ground when tested.

So it’s not that the outlet tester is useless on the boat, but realize that in this one situation it will falsely tell you there’s a problem.

Frankly, after investigating all this, I was surprised that the surveyor (who is accredited by all the major groups) didn’t know that the open ground wasn’t a problem. It would seem like it would come up on boat after boat.

Upon further reflection, perhaps most boats are wired so that the inverter powers the AC outlets throughout the boat and have a transfer switch (bypass) so that if there is shore power, that powers the outlets. In those cases, he may simply test the outlets once – while on shore power. Only with boats without the bypass wiring – or where there was no shore power when testing the outlets – would he see a problem when specifically testing the inverter outlet(s). I sent him an email with the information above, but haven’t had a reply.

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Comments

  1. Green is earth ground on the 12v side of a boats grounding plane that goes directly to water. unfortunately most of the time a ground wire from the battery is tied to this plane, typically at the engine. When connected to shore power the earth ground from the outlet or green wire “should” be grounded on land someplace with a ground rod and not tied to the neutral wire together in the panel. When not connected to shore power and checking the Invertor with a common outlet tester the earth ground is grounded to the invertor box and a separate wire typically runs from the from the box to the boats ground plane. The tester will malfunction or error because it sees the ground and the neutral tied together. Short version, this is OK.

  2. Not experiencing that situation, but either way, the outlet tester is a handy gadget that I wouldn’t have thought to bring along. Thanks for always providing helpful & thorough info!

  3. Tommy Molnar says:

    Interesting to stumble on this topic here. I’ve had a Cobra inverter in two of our travel trailers for 20 years. I first ran into the “Open Ground” years ago and called Cobra to ask about it, along with the fact that my multi-meter showed less than 120 volts when testing our outlets. They explained the part about the open ground, and then added that your everyday multi-meter is not designed to measure inverter power. A different type of multi-meter is needed, and I don’t remember the type because I decided it really didn’t matter as everything we use 120 for in our trailer works just fine.

  4. I keep it simple. An outlet box with a cutoff for shore power. This is mostly used to run the battery charger when the boat is on its trailer in the shelter. I plug directly into the inverter when away from shore power. This mixed grounding thing is a real source of problems and galvanic action. I’d rather steer clear safe and simple. Of course I have a simple trailerable 30 footer that I use to camp cruse for 2-3 months at a time. Everything is 12v except the drill charger. I think of it as off the grid living.

  5. Carolyn,

    I am not a fan of Xantrex products. It is likely too late for my perspective to help. The failure rate is higher than alternatives and the customer service in warranty leaves a great deal to be desired.

    Unless you have separate outlets for inverter power you are tying into the boats AC system and there are very important considerations to get right, including ground-neutral bonding at the inverter when inverting and NO on-board bonding when on shore power. Isolation transformers, for those who choose that route, make things yet more complex.

    I do laud your DIY approach.

    The quote from Cobra is not strictly connect. In general for household power neutral-ground bonding happens at the step-down power transformer that serves a number of homes and not in the fuse-box. Some counties still provide for bonding in the fuse box but that can lead to other safety issues. In fairness some depends on ground resistance (coastal areas with high water table are different than places built on granite).

    The fundamental need is to provide ground-neutral bonding at the point of supply. For shore power that happens at the step-down transformer that feeds the marina. When you are running off an inverter or generator it happens at the device or–if you use one–at the output of an isolation transformer. It is important to be able to switch out the on-board bonding when connecting to shore power and critical to set up your switching so it is sailor-proof.

    In the end I agree with the surveyor that there is an issue with your installation. It is assuredly not the fault of the inverter but there is a shortfall in the installation.

    In addition, since the Honda generators were brought up in comments those generators do not bond neutral and ground and that is most assuredly a safety problem on board. There are a number of ways to manage that but blaming the outlet tester is not one. Honda generators are designed that way because the expectation is that the installation will manage that issue.

  6. Dave Skolnick, our inverter does NOT tie into the boat’s AC system. It’s solely off the batteries and does not feed the AC outlets. You have to plug into the inverter, not the boat’s AC outlets. There is no load switching. I thought I had made that clear in the article. It is a very simple setup.

  7. Then it is nominally okay but be careful around damp places (galley and head) where you would usually expect GFCI outlets. Your independent inverter doesn’t have GFCI and GFCI wouldn’t work anyway without ground-neutral bonding.

    NEC, NFPA, CE, and ABYC are not rules for their own sakes. They are intended to keep us safe. Please be careful.

  8. I came across this thread while looking for answers to a different question I have… but after reading this I wanted to comment based on my experience and research on a similar ground issue on my boat…

    Basically, the boat’s AC power system MUST have a ground-neutral bond at all times, but only one such bond must exist..
    When attached to Shore Power, there is an expectation that the marina/shore power system is bonded elsewhere, which provides the bond that the boat needs. So on shore power, you don’t need any sort of internal ground/neutral bond, and actually you should NOT have one.
    However, once you remove the vessel’s system from shore power, you no longer have the shore power’s bond either. Each independent power source must have a neutral-ground bond. For a generator, many generators do not have an internal bond, especially portable ones, because they are designed for a variety of installations, and the installer/user needs to ensure that an external bond is added. Honda’s EU series generators are an example of this, where they are not internally bonded, because in some installations there is already an external bond (consider a home standby generator attached to the home’s service panel). On a boat you need to create that bond otherwise you are not properly protected.
    In my case, we have a Northern Lights genset on board and it was not bonded. When it was running we blew a few light bulbs on the outlets, popped the GFCI a few times, and had a faint reverse polarity light (not fully bright). Completing the bond AT the genset solved all these problems.
    Most if/not all, combination charger/inverters also have internal bonds, because when the internal transfer switch cuts over from external power to inverter power, the inverter is the source of power and the shore power’s bond (whether the boat is still connected to shore power or not) is no longer applicable to the load side of the Inverter.
    The ProWatt and many other Inverters that are again intended for portable/multi-purpose uses do not have internal bonds, because there’s a chance that they will be connected to a load or set of loads (House, RV, Boat, etc) where an existing bond is already wired. In your case since you are simply plugging an appliance directly into the outlet on the Inverter, it’s not as big of an issue, but technically, to follow the rules and provide full safety, the ground and neutral *should* be bonded. If you were hard wire that inverter to a transfer switch to feed the rest of the boat, you absolutely NEED to ensure there’s a neutral/ground bond that is in place whenever the inverter is the source of power.

    So in short:
    You MUST have a bond to comply with code and protect from harm..
    Shore power connections provide that bond as long as you your loads are actually being powered by that same shore power.
    Inverters that don’t have internal bonding need to have external bonding added.
    Generators that don’t have internal bonding need to have external bonding added.
    Only one of these bonds should be connected at any given time (ie: no internal bonds connected if you are using shore power to feed your loads.)

    Hopefully that helps.

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