26 Aug Outlet Tester Showing Open Ground on Inverter
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.