Grounding the shore power line?

[Baltimore Sailor]

The shore power setup on my boat is pretty primitive-looking: there is only a two-conductor Romex-looking wire running from a standard shore power plug in the cockpit directly to a GFC-protected outlet in the cabin. There is no AC panel, nor breaker, nor even switch in the circuit.

At the very least, shouldn't there be a ground wire in there? And if so -- and I hadn't ever thought of this before -- is it kosher to ground 110v AC to the same ground as your 12v DC? I can't figure how you could do otherwise in a boat, but it seems wrong, somehow.

[policecentral]

Ground Wire

If I understand, you have only a single outlet for 110v. However, it certainly should connected to shore power connector by a three wire, grounded wire. Copper (or green) wire is ground, white wire is neutral, black wire is hot. Connect to same wires at the SP connector.

Also, you should buy a small fuse box, and put in a 15 amp fuse for 14ga wire, or a 20 amp fuse for 12 ga wire to the outlet. As you are presently connected, you have no fuse excepting the shore power supply that is probably 50-100 amp or so. This is a dangerous situation.

Also, check your shore power cable to make sure it is three wire, grounded as well. When you have completed this, you can buy an inexpensive ground tester, three light gadget to plug into your outlet to check it out. Your gfi as presently connected with no ground is useless.

You can ground to a convenient bolt on your engine -- I use a bolt on the flywheel cover. All grounds are the same, 12v, 110v.

[Bob.Griffin]

I'm not the greatest expert on shore power installations but I'd be a bit wary of connecting the 110 v shorepower ground to your 12 v ground, who knows what stray currents may occur and cause galvanic problems. As I understand it, you only need a good ground for the 110 v shore supply when you are actually plugged into shore power, and this ground MUST be the third ground wire that should run from the boat 110 outlet(s) all the way to the shore outlet you plug your shore power cable into. When you are not connected to shore power, you don't need any grounding on the boat, because the 110v circuit is dead. I agree with 'policecentral', you definitely need a fuse and a switch, preferably marine quality.

Bob (s/v Saga)

[rigspelt]

Some sources I found useful for AC wiring on boats:

http://www.boatus.com/boattech/casey/36.htm

http://seagrant.uaf.edu/bookstore/boatkeeper/electrical-safety.pdf

Also Nigel Calder's Boatowner's Mechanical and Electrical Manual and Wing's Boatowner's Illustrated Electrical Handbook.

[jhwelch]

My boat is not wired for AC, so for the times that I am at a dock I
just run an extension cord so I can use the computer or charge
a cell phone, etc.

If you only have that one outlet a cord might be less trouble than
adding a circuit breaker, etc. for the few times(?) you need electricity.

-jonathan

[SEMIJim]

I'm not the greatest expert on shore power installations but I'd be a bit wary of connecting the 110 v shorepower ground to your 12 v ground,

Where else would you connect it? It's ground. That's where it has to go.

who knows what stray currents may occur and cause galvanic problems.

There had better be none in the ground wire. If there is: Something is badly broken and the shore power connection is dangerous and should not be used under any circumstances.

Not only should you have a fuse or breaker on the boat, as close as possible to the shore power inlet, but all 120VAC fixtures should be GFCI-protected. (Something I have Yet To Do on our boat.)

Jim

[Baltimore Sailor]

My boat is not wired for AC, so for the times that I am at a dock I
just run an extension cord so I can use the computer or charge
a cell phone, etc.

If you only have that one outlet a cord might be less trouble than
adding a circuit breaker, etc. for the few times(?) you need electricity.

-jonathan

True. But the system is there on my boat, and my OCD won't let me not make it work as it should. I am helpless.

[Bob.Griffin]

Bob: I'm not the greatest expert on shore power installations but I'd be a bit wary of connecting the 110 v shorepower ground to your 12 v ground,

SEMIJim: Where else would you connect it? It's ground. That's where it has to go.

Bob: The AC ground has to be onshore where you plug in your shore power cable. Electrical generation companies go to great expense to ensure every 110 v AC service is properly grounded within their infrastructure. The ground wire is provided in the event that an electrical appliance short circuits, and instead of electrocuting the user, the short circuit should go to ground. This is another reason I would avoid grounding to an engine block, this may not be an effective ground to avoid electrocution in the event of a faulty 110v electrical device on board. BTW, I totally agree that all onboard outlets must be GFCI types that trip the power in the event of water/moisture incursion.

Bob (s/v Saga)

[High Hopes]

Hello Police Central,

GFI’s work with or without a ground connection. They work by comparing the difference between the current in the hot wire and the current in the neutral wire. These currents should be exactly the same. If they are not, then some stray current is going somewhere – like through your heart. Typically a stray current is from the hot lead to earth (ground). The neutral wire is already at or near earth potential, so it is harder to develop a stray current between the neutral wire and a ground.

Schematics of actual GFI circuits show no involvement of the green ground wire. Not only that, a GFI can be reverse wired with respect to hot and neutral, and it will still function properly.

Hi Balt,

I do not connect the ground wire to anything except the AC receptacles (which should be three pronged). This proves protection form a faulty piece of AC equipment. However, ABYC recommends connecting the AC ground to the engine (and hence the 12 volt system).

These two different schools of thought are both based on failure mode analysis. The ABYC approach recognizes that any equipment connecting the AC to the 12 volt system – such as a charger – can cause a problem. The charger could short and cause the 12 volt ground to be hot with 110 Volts. If this happens, either the circuit breaker or the GFI will trip.

The other school of thought (not really mine) argues that if you ground the AC system to the 12 volt system, then any device that uses the 12 volt power now provides a ground point for electrocution should an AC appliance fault. There are more things to short to lurking about on the boat. If the 12 volt system is truly isolated, there can be no shock.

This method provides advantages in lightening protection systems and requires that the rigging also be isolated from the AC and 12 volt supplies. Stray marina currents, including mis-wiring of other boats, will not cause galvanic corrosion. Galvanic isolators are not even used. Through hulls are not bonded and therefore not subject to galvanic corrosion.

In defense of ABYC, it takes less work to check for a proper installation. The other scheme requires one to make sure that three systems are truly isolated (AC, 12 volt, rigging/sea water). If they are not, it can be a non-trivial task hunting down the problem. System isolation should be checked periodically as a maintenance item.

If you really want to do it right, you can buy an isolation transformer and have the best of all worlds. With one of these, you can hold the black wire, or white wire, and jump into the sea. Just don’t grab both of them at the same time. You won’t need galvanic isolators, galvanic erosion rates will be greatly reduced, and you will be in a position to install a modern lightning protection system. These transformers are now available with isolation test circuits built into them so they will tell you when something is amiss.

Bob,

Regarding electric company infrastructre, ground is typically a copper stake in the ground next to utility poles with transformers on them. It really is the earth. In marinas, it is earth under the water.

Steve

P.S. Balt, shouldn’t it be CDO with the letters in the right order the way they are supposed to be?

[policecentral]

Ground

Sorry to cause so much discussion, but "ground is ground is ground". Period. Power coming from the power plant is grounded, as is the supply line going to your house, as is properly installed shore power at the point it leaves land to the dock, and as it enters your boat and is grounded to the engine. These various points simply put a ground point close by the end use, your outlet. Stray currents will not come through your ground wire -- they will be "grounded out" via all the ground points above. And attaching all grounds to your engine insures that your shaft, zincs and propeller will all be "commonly grounded" and your engine will not be a "floating" (unconnected) ground -- thus you minimize electrolysis.

The is no such thing as a "separate" or "isolated" ground. It is all the same: "ground" (from whence it takes its name).

Regarding using an extension cord, we all have done that. But just be aware that it could be fused by shore power at pretty high amps -- if you have a short circuit that would potentially burn up the extension cord and could start a fire before it trips the fuse or breaker. Use Caution!


JSM

[policecentral]

GFIs

To SteveM,

Thanks for info on GFI, as I don't know much about them.

I have never used them, and have not been convinced that I ever will. Lots of people seem to like them, but I never felt the urge to put them in anywhere.

JSM

[SEMIJim]

The AC ground has to be onshore where you plug in your shore power cable.

Yes. But just as with electrical appliances and tools that are not "double-insulated": The metal bits in your boat should be grounded so as to prevent them becoming energized.

Electrical generation companies go to great expense to ensure every 110 v AC service is properly grounded within their infrastructure.

No they don't. Electricity supply companies' responsibility for the wiring ends at the service entrance. (Usually where the meter is located.) Just as telephone service providers usually terminate their responsibility at a point called a "NID" (network interface device), aka: "demarc" (for demarcation point). Back to electrical service: What is supplied to your meter, and thence to the distribution panel (aka: fuse or breaker box) in U.S. residential situations is almost always single-phase, center-tapped 240VAC. There are three wires: L1, L2 and neutral. Neutral is nominally at ground potential, but it is not ground and is not to be treated as such. True ground is supplied by a separate ground rod, usually driven near either the meter or the distribution panel. Ground and neutral are commonly bonded inside the distribution panel. (But still: Neutral is not to be treated as ground, and vice-versa.)

Business/commercial systems are usually quite a bit more complicated. There's no way to describe them as simply as common residential service, because there are just too many options/variables. But still: The power company does not supply "ground."

N.B.: I am not a licensed electrician, nor do I play one on T.V.

Jim

[Bob.Griffin]

I think Steve Markowski hit the nail on the head, for easier troubleshooting, better to keep all 3 electrical systems totally separated (12v dc, 110v ac and rigging etc grounding) and/or use an isolation transformer.

Bob (s/v Saga)

[SEMIJim]

Hello Police Central,

GFI’s work with or without a ground connection.

But the little test button won't

Jim

[SEMIJim]

Sorry to cause so much discussion, but "ground is ground is ground". Period. Power coming from the power plant is grounded, as is the supply line going to your house,

All incorrect. The power running in the lines up in the air, or buried in the ground (or under water, sometimes), is not "grounded." And, while it could be said to be "referenced to ground," what the power company leads to a customer premises is not "ground," per se. Even when there is a conductor that may be at the same potential as the ground, that line is actually called "neutral" and, as I stated earlier: Is not, nor can it be treated as, ground.

The is no such thing as a "separate" or "isolated" ground. It is all the same: "ground" (from whence it takes its name).

Wellll... that's not strictly true. But, for the purposes of this discussion: Yeah.

Reminder: I am not a licensed electrician, nor do I play one on T.V.

Jim