Grounding the AC circuit

[jaschrumpf]

The AC "connection" in my 1975 Capital Yachts Newport 28 is just a 12/2 line (I'd say it was ROMEX) that leads directly to a standard outlet. No circuit breaker in the path anywhere, just straight from the shore power plug in the cockpit to that outlet.

I'm going to put in a very basic 3-switch AC panel, and the question suddenly popped into my mind: Do I need to ground this in the boat somewhere?

In a house, of course, you ground the breaker box to the plumbing, or to a ground rod. Do you need to do the same in a boat, or is the ground coming in from the shore power pole all that's necessary. It somehow doesn't seem right to have a 12V DC and 120V AC circuit led to the same ground.

Am I right that all that's necessary is the shore-connected ground?

Thanks!

[eherlihy]

Discussed previously here

[Hartley18]

It's not really.. and it's a good question.

For the set-up you describe (a simple 3-switch panel) you run active, neutral and earth wires to each new outlet with the switch in the active line. At the panel you'll need a 'neutral' bar (supply and all other neutral's connected) and an 'earth' bar (supply and all other AC earths connected).

Connect one earth wire from the AC panel to the main grounding point on the boat using as short a length of wire possible at least the same size as the active/neutral wires - preferably a size bigger. Not at the DC panel... the main grounding point. It's typically on either a keel boat or an engine stud.

A GFI in the supply line is a good idea.

Good luck with your project!

[eherlihy]

Originally Posted by hellosailor

A "proper" light duty AC system would mean a shore power cable, a ground fault interrupt (GFI) safety device, adding an AC breaker to the panel, and running some AC wiring and receptacles on the boat. Couple of hundred bucks and one long day or casual weekend to install it all if you are familiar with it. More expensive if you are hiring labor.

Well that was 9/10ths.... but you're right. And this is a good question.

You can, and should, connect the AC system on your boat to the boat's bonding, or grounding system. However, if you do, the AC ground will provide a direct electrical connection between the boat's ground, and anything else directly connected to the ground wire (other boats, the dock, etc.). This creates a great environment for galvanic corrosion. In addition, if you use a single pole breaker (like most homes), in the case of a shore power fault (reverse polarity - more common than many realize, or two prong power cord incorrectly inserted), the entire ground system (AC and DC) could become hot, causing a hazard for people on board, and in the water!

Thus: a proper AC system on a boat should include two pole circuit breakers and, optionally, a isolation transformer.

[Hartley18]

Originally Posted by eherlihy

In addition, if you use a single pole breaker (like most homes), in the case of a shore power fault (reverse polarity - more common than many realize, or two prong power cord incorrectly inserted), the entire ground system (AC and DC) could become hot, causing a hazard for people on board, and in the water!

I've never been convinced about this... The ground wire is exactly that - a ground wire. It is not connected to the earth or neutral anywhere in the entire circuit and so could not possibly become live if someone plugged something in the wrong way.

Where it helps is, if you install a GFI in the AC supply to your boat, a ground wire guarantees that the GFI will trip by providing a path to a good "earth" that's not part of someone's body.

Originally Posted by eherlihy

Thus: a proper AC system on a boat should include two pole circuit breakers and, optionally, a isolation transformer.

An isolation transformer is probably overkill unless you plan to cruise to other countries - but otherwise, agreed 100%.

[peikenberry]

You need to take a look at Basic Electricity AC Circuits New Boatbuilders Home Page - Basic Electricity Page 7 - AC Circuits