AC Grounding

[markmcb]

Hello, I am a new member and new to sailing. What a wonderful feeling.
I have just acquired an old Cal 28 and now have a long list of projects.
At the top of the list, is installing an AC and DC electrical system. The boat came with 2 batteries (with 2 busses) connected to a DC panel, and a 30 amp shore plug that is not wired. My plan is to connect the shore plug to an AC panel with a 2-pole 30 amp breaker and 3 15 amp breakers. I will then connect a battery charger and AC outlets to this. My problem is where to ground these systems. In most places, I have read to connect the ground to the engine...but I have no engine.I do have what seems to be a through hull bolt that has a ground cable attached to it from the head's seacock. I have read not to ground to seacocks, as they may blow out and sink the boat. I suppose the DC system can just be grounded to the negative post of the battery, but what of the AC side?
I appreciate any thoughts....Thanx

[donradcliffe]

Safe Shore Power by Don Casey

[s/v Jedi]

Only connect the ground from the shore-power inlet to the ground on the outlets and the battery charger. Just like you would do in a house.

cheers,
Nick.

[David M]

Agreed, connect all your AC grounds back to each other then connect them to the ground at your shorepower source. Do not in any way connect your AC ground to your battery's negative terminal.

[markmcb]

Thank you everyone for your help. If I see this correctly, the AC grounds back to the shore power source and the DC grounds to the neg. battery post. And I guess I don't have to find some seacock or grounding plate for these systems.
Thanx again

[David M]

Correct

[CharlieJ]

Sorry to disagree David but...he is creating an AC and a DC system on the boat; albeit very simple ones. These two systems come together inside the battery charger so for safety's sake, a single conductor should be connected between the AC safety ground bus and the DC negative bus per ABYC E-11:

11.5.4.7 DC System Negative Connections
11.5.4.7.1 If an alternating current (AC) system is installed, the main AC system grounding bus shall be connected to
11.5.4.7.1.1 the engine negative terminal or the DC main negative bus on grounded DC systems...

[amarinesurveyor]

Charlie is absolutely correct! Read the article by Don Casey on safe shore power systems, it explains it pretty well. There have been quite a few deaths to swimmers in fresh water from improperly wired boats.
Brian

[goboatingnow]

European Union CE regulations allows AC ground to NOT be connected to DC ground where RCD interruptors are fitted to the incoming 220 VAC, and in practice this is how most EI boats are wired reduces the risk of impressed, galvanic corrosion.

[chala]

The following threads and links are informative readings.

Originally Posted by DeepFrz

I believe that Nigel Calder's article in "Professional Boatbuilder" has some good information on Isolation Transformers and Galvanic Isolators.

Professional BoatBuilder - April 2006/May 2006

This good article is an eye opener in regard to earthing. It shows that some countries are not able to maintain an effective segregation between AC and DC electrical circuits. It is unthinkable that AC could wander into DC wiring and definitely should never happen.
It will be interesting to imagine what would happen if the AC was to leak into the positive of a DC system, now the complete DC installation would be at the AC voltage.

[donradcliffe]

And you thought there would be a simple answer to your question...

If you want a simple, reliable system which does not damage your boat, don't connect the DC and AC grounds. There is about a one in a million chance that your battery charger may become defective so that you get a shock from your DC system. There is an even lower chance that if your boat is in fresh water at the time someone is swimming around the boat may die.

However, the good folks at the ABYC have created a standard which insures that you will be sued in that event, encouraging you to connect the two grounds and suffer corrosion damage and spurious trips from their safety equipment. They will be happy to sell you more equipment to minimize the damage, and services to sort out why their complex system doesn't work all the time.

Even the experts will admit that the ABYC system doesn't provide 100% protection, and they are hard at work devising even more complex systems to protect you against more 'what if' situations. This is no different in the land-based electrical systems, where the national electrical codes have become more complex and expensive than the tax codes. Just remember how these ABYC people make their living...

[CharlieJ]

donradcliffe-With the possibility of starting a flame war; I have to STRONGLY disagree with your characterization of the ABYC and your cavaliar approach to a personal safety issue.

First, let's discuss the ABYC. The ABYC is made up of marine industry professionals trying very hard to ensure safe boating. These professionals are volunteers and as such invest a tremendous number of manhours to try and provide a clear and concise set of standards, the dedicated purpose of which, is to ensure personnel safety on board. The standards are consensus standards; they are not generated by a bunch of bureaucrats. Each technical committee is made up of manufacturers, the USCG, the NMMA, tradespeople, consulting engineers, etc. Nobody can ram through anything and get their agenda pushed through.

By the way, if you want to participate in the review process, contact the ABYC and put your name on the mailing list as an "Interested Party". The standards that you are interested in will be sent to you as part of the review cycle and you are encouraged to comment. Your comments will be reviewed and, if worthy, they will be incorporated.

Now to your second opinion:

There is an even lower chance that if your boat is in fresh water at the time someone is swimming around the boat may die.

You are just plain wrong on this point. There have been several, documented, fresh water drownings that, upon careful examination, were found to be caused by an AC field in the fresh water. The human body is more conductive than the fresh water, a few mA flow across a 2 or 3 VAC/ft gradient, and the diaphram seizes and the person dies. All preventable.

And finally, how does connecting the vessel ground to the safety ground cause "spurious trips"? Overcurrent protection devices respond to either an out of spec load current or a short circuit L-N or L-G. The circuit breakers are either double pole (L and N) or single pole (L). They are not part of the safety ground circuit.

Chala-And your comment is exactly why there is a requirement to tie the vessel ground and the safety ground together. If AC makes its way into the DC system, AND there is a circuit back to the safety ground, an over current protection device will trip.

goboatingnow-..."impressed galvanic corrosion" is an unidentifiable term. Galvanic corrosion is caused by two dissimilar metals, electrically connected, submerged in an electrolyte. If by "impressed galvanic corrosion" you mean stray current corrosion, that is not generally a serious corrosion mechanism when dealing with AC (shore power). It is a very big deal if the stray current is DC.

Regarding RCDs: RCDs are sophisticated and somewhat robust GFCIs with a 30 mA/100 mS tripping spec. They trip when the current entering the vessel on the Line exeeds the current leaving the vessel on the Neutral exceeds 100 mA. It provides whole boat protection from lethal levels of leakage current.

[GordMay]

What CharlieJ, & amarinesurveyor said!!!

[David M]

Originally Posted by CharlieJ

Sorry to disagree David but...he is creating an AC and a DC system on the boat; albeit very simple ones. These two systems come together inside the battery charger so for safety's sake, a single conductor should be connected between the AC safety ground bus and the DC negative bus per ABYC E-11:

11.5.4.7 DC System Negative Connections
11.5.4.7.1 If an alternating current (AC) system is installed, the main AC system grounding bus shall be connected to
11.5.4.7.1.1 the engine negative terminal or the DC main negative bus on grounded DC systems...

Thanks for the correction Charlie. Previous to this, I had never heard of tying the DC and AC system together.

[donradcliffe]

Well, I know a lot more about the subject now than when I recommended Casey's article.......Just to show that the ABYC experts and their black and white safety prescriptions aren't always 100% right, here is a tragedy where two people would be alive today if the DC and AC grounds were NOT connected...the owner rewired the shore power plug to hook the hot shore lead to the boat AC ground, which was connected to the DC ground, which was connected to the water, and killed his wife and daughter when they went swimming. The Case of the Hot Marina If the boat had not had the AC ground tied to the DC ground, this fault would not have found a path into the water and the wife.

There is a database of freshwater electrocutions in marinas at http://www.qualitymarineservices.net...ent%20List.doc which is well worth reading if you have your boat in fresh water. There are indeed a few fatalities due to faults in the battery charger and lack of either an AC-DC ground connection or an adequate AC ground. However, there are as many or more fatalities listed due to faults in the AC neutral and ground circuits, where the AC ground gets energized and kills someone because there is a path to the water via the DC ground!

The only good news is that the second link was unable to discover any saltwater electrocutions--"Our studies have shown that, in salt water, the high voltage gradients required for electric shock drowning could not be established with the available fault current levels. In no cases can we attribute cause of death to electric shock drowning in salt water."

The bottom line--

1. Don't ever go swiming in or around a marina (or that aluminum houseboat) in fresh water.

2. Press at least the freshwater marinas to install RCD devices, which would have prevented many of the electrocution deaths regardless of how the AC and DC grounds were wired. The RCD would force the boatowners to solve their wiring problems if they wanted dockpower.

3. Regarding tying the AC and DC ground together, there are good arguments either way, but my vote today would probably be to get rid of the connection in salt water. I think the European solution (RCD and no connection) will save more lives and damage fewer boats than the ABYC solution (although my experience with RCD's in Europe was not good, as the Xantrex inverters do not lift their internal ground/neutral connection until they sense dock power, by which time the RCD has tripped...grrrr).

I would welcome more discussion from the ABYC defenders, and I'm not interested in flaming anyone. I was ranting a bit about electrical codes because I just discovered I have to have a Ufer in my new garage which ties the ground system to the rebar in the slab to protect the garage from lightning--we get a thunderstorm about once every 5 years here in Santa Cruz...and the old slab lasted 90 years without rebar.