Grounding System on a small boat

Re: Grounding System on a small boat

Lazza,

As you know, grounding is a big/complex issue... And doing it wrong can let out magic smoke and other problems.

So--If the boat's AC power system is shore powered with a transfer switch to the local inverter... Really need to know the brand/model of inverter and how your transfer switch system is setup/works.

First, the usual warnings:

• Never directly connect the output of a standard off-grid inverter to AC mains.
• Most TSW inverters can have an output wire grounded (for neutral). And most MSW inverters cannot. However, there is enough variability in design (and safety issues) that you need to read the manual for your particular inverter. A mistake, at the very least, will usually ruin your inverter.
• Most shore power/utilities ground reference their neutral ground. And, it is usually a good idea not to ground reference the neutral again in your vehicle/boat. Note, in North America, we do ground reference the transformer at the pole, and again at the entrance to the home. It does have its issues--But that is the "way it is done" per code. Grounding a neutral again in the boat can raise issues with redundant current paths (neutral and green wire ground in parallel). If one wire is broken, it may not be obvious--and can cause a smaller wire to carry over it rated current in some cases.

So, to your case. Grounding the Neutral is done so that all largish metal objects (plumbing, sinks, stoves, refrigerators, etc.) are always at the same potential as "earth". If you grab something in one hand and place the other on a sink (for example), you should not get shocked. Double insulated appliances (blenders, drills, mixers, etc.) do not have exposed metal that can become "electrofied" unless there are TWO (unlikely) FAILURES in the device's electrical wiring. So, some appliances only have the two wire connections instead of three.

The normal way to add a second level of protection is to install a GFI (Ground Fault Interrupter) outlet or breaker on circuits that are near water/outside/etc. If there is a current flow from Hot to "elsewhere" (ground, etc.), then the GFI will open and prevent (hopefully) dangerous current flow.

The other reasons for grounding include lightning protection (important on a boat) and to ensure that any short from "hot" to earth will trip a breaker (instead of making that metal object hot).

So--back to your needs. If you have shore power, you should not make a hard neutral to earth bond. If you have a transfer switch, a two pole transfer switch will allow you to ground reference the inverter's output (assuming TSW or other inverter that supports earth reference). The transfer switch will switch both Hot and Neutral wiring. Hot from shore to inverter. And the Neutral--which will be ground reference (if shore) or ground referenced (if local inverter).

In general, with a small inverter it is of questionable value if it is important to ground reference its AC output. With most MSW inverters, you cannot ground reference their output (i.e., share ground with battery "-" and ground reference an AC output). With TSW (and isolated MSW) inverters, you can either let the output float (which in itself, a pretty good safety feature--you cannot get a short with a single connection to AC "Hot") or grounding/making a "neutral" output (common ground inside the boat).

Sorry for the long and confusing post. Questions? Avoid me?

-Bill