Earth

[Gunfleet]

This bulletin board is read by plenty of electrical engineers, I wonder if I could bother one for a piece of information. I have read over and over that AC earths on a boat should be bonded to DC earth (and this is the cause of much galvanic corrosion). But why? AC earth on the shoreside power supply is earthed in the normal way and the AC on the boat is connected to it. I don't doubt all the textbook writers for yacht wiring are right. But why? TIA

 

[jfkal]

Well think it is a regulation as soon as you have an Installation for AC on board. As long as you just "drag" an extension cable on board to run your power drill you are right. The moment you deal with fixed installations it is a different story. ie one of your hot cables comes loose in heavy seas and touches anything bonded into the DC ground. Next time you hook on shorepower and climb back on board the cool water in the marina will extinguish your smoldering body (RIP). I am sure someone can quote the details of whatever IEEE DIN ASI applies here ;-)

 

[vyv_cox]

No electrical engineer me, but as I understand it:

If the supply from the marina is faulty you could be in a situation that a piece of equipment in your hand is live. If you then touch something on the boat that is earthed, like say a sea-cock, you would provide a circuit that would electrocute you. Providing your own earth where the current enters the boat overcomes this potential hazard.

However, this seems to introduce more problems than it solves, from corroding various boat components to electrocuting swimmers around the boat. My own solution is to only use double-insulated appliances on board, i.e those that don't have an earth connection anyway. I have not arranged an on-board earth.

 

[ccscott49]

I tend to agree with vYv, I don't have my mains earth connected to my batteries or anodes. I also only use double insulated stuff. I am also very careful with mains connections. I don't want to be at the mercy of somebody elses bad wiring and have corrosion problems. I also have one of those earth thingys that tells me if I have no earth onshore. Can't for the life of me remember what they are called!

 

[Gunfleet]

This is very interesting. The background to my question is that I have a simple electrical supply from the shore to a couple of points on board. The connection goes through one of those blue caravan type sockets we all use and then there's about 6 inches of cable before the rcd. I use a a cheap neon tester to ensure the wires (from the pontoon socket) are in the right order and are connected before I plug anything in. I use I don't use 240v at sea. So I'm reading Mr Calder's book and thinking, so why am I introducing this great big possible galvanic problem?

 

[ccscott49]

You're already doing all that is neccessary for safety, that RCD is the thingy I was talking about, don't bother with anything else. More trouble than it's worth.

 

[rogerm]

As an ELECTRONIC (rather than Electrical) engineer I have to say that I agree with JohnM. I have a very short lead between the blue mains input socket and the consumer unit. In place of the ordinary 30ma RCD I have a 'sensitive' 10ma unit and like JohnM (and surely 80+% of us?) I don't have an on-board 240v generator that I would use at sea.

Consequently the 240v supply is gounded from the land end. While I agree it is as well to connect a checker after first connecting a marina supply to ensure their earth is sound, I certainly wouldn't dream of connecting the incoming mains earth to the 12v dc earth.

But if it really bothers you then fit a 240v mains neon indicator between the 240v earth and the 12v dc earth and mount it somewhere promient. Under normal circumstances there is no voltage across it and hence it won't glow and hence no galvanic problems. It takes about 90v, at a minute current, for the neon gas to ignite. This could only occur if a fault put mains live onto the 12v system. Its a bit silly to suppose that you would be hanging on it just then but even if you did the unbalanced current flow will pull out the RCD breaker. Thats what it is there for.

But I do recommend using a sensitive RCD as it will practically drop the breaker before you even feel anything while the usual 30ma device needs to pull quite a bit before that goes, certainly enough to floor you...
Roger

 

[vyv_cox]

Exactly what I have, except that I use a very clever little mains voltage indicator that my daughter bought me for Xmas. Instead of having to touch a conductor with the neon screwdriver that I used previously, this thing will detect current inside an insulator. So it can tell me if a wire is live, which is the live side of a socket, etc. It's about the size of a fat fountain pen and is invaluable in marinas where rcds seem to blow on a frequent basis.

 

[bryantee]

What happens if you have an inverter on board?

 

[ccscott49]

Good point that, I'll have to think about that one for a while. You could earth thaty to a bronze earth plate, as it wouldn't be a galvanic problem as long as you are not connected to the shore, as long as you have no earth leaks to any other fittings, then an RCD would find those, I think. Give me about a week to work on that one.

 

[Gunfleet]

Dunno NM

 

[MapisM]

I\'m scared!

I never bothered about these issues, since neither the 220V circuit nor the 24V on my boat gave me any problem so far.
On the other hand, I do use the genny during cruises (mainly to avoid using the washing machine in marinas or bays).
Did I understand correctly that in these conditions the risk of galvanic corrosion is much higher?
If so, how could this be checked? ...and solved, if any problem actually exist.

 

[Gunfleet]

Re: I\'m scared!

I'm not qualified to answer. However, you'll find a bit of info on this subject in Nigel Calder's books.

 

[petery]

This piece on galvanic corrosion comes from the Adverc web site and deals with other than safety aspects. I suppose it could be just a sales pitch but it seems to make sense!!

------------------------------------------

When a vessel is connected to a mains (230v AC) shore supply, the earth conductor should be taken to the same single earth point on the hull as the DC negative. The other end of the earth conductor is taken, via the generating companies distribution grid, to a sub-station where it is physically bonded down to planet earth. If a second vessel is then connected to the shore mains supply adjacent to the first vessel, a circuit exists between the two vessels via the common earth connection to the sub-station and through conductive water.

Alternatively, if the vessel connected to the shore mains supply is adjacent to a metal object (ie a jetty) that is driven to planet earth, again a circuit has been completed. If the potential between the two vessels or one vessel and the jetty is slightly different, a current will flow and corrosion can occur. This is galvanic corrosion. This type of corrosion can be prevented by fitting an isolation transformer in the mains supply immediately after the input plug on the vessel. Alternatively, a galvanic isolator can be fitted in series with the AC earth conductor to block low voltage DC galvanic current flow, whilst permitting the passage of alternating current associated with the earth conductor

 

[halcyon]

Ok it's a long time ago back in the 80's sorting out Sealine. From memory the reason for connecting dc neg to ac earth was due to earth leakage breakers. These measured any current down the ac earth line, thus any mains on to the dc circuit would go to earth and trip the breaker. Now we use RCD breakers, these measure live to neutral current differance, not needing earth now.
In the 80's the as decided never to connect DC neg to AC earth, this allowed a circuit to be created, ac earth to marina, down earth spike into ground, through earth to seabed or marina pile, through the sea water, through skin-fitting, through bonding circuit to earth point, through to AC earth. By breaking this link it stopped all corrosion, except when we had silly problems, normally a screw, one used to fasten battery charger which had a earthed cased, went through a bulk head, found a dc cable, went through the live cable insulation and just touched the conductor. In less than a month a pair of legs had been distroyed, via the above funny circuit, si it does work.

The RCD on the baot will protect the boat, the one on the marina should protect the shore side, you need one on the genny, or use a change-over system, invertes should have a RCD in my view, if I remember somewere above 80/100 volt is concidered as harmfull.

I'll make some enquiries next time I'm on the phone to Kidderminster, see if there is any changes to meet current RCD requirements.

Keep you posted.

Brian

 

[Gunfleet]

Thanks Brian NM

 

[LadyInBed]

Re: clever little indicator

Sounds like a device I have. Get them in DIY stores.
This one is called a MEET Detector (Multi-functional Electrical Electronic Tester) Model # MS-58ACM. It's about the size of a large marker pen and used to detect pipes and wires in the wall before you drill, checks fuses, detects microwave leakage and indicates which wire is live and which is neutral.

 

[jfkal]

Right that thingy will then trip for no apparent reason. The Neon indicator will not detect the 0.xx V needed to start corrosion. Why make it complicated. Bond it in and use a galvanic isolator. Better save than sorry. Besides would not trust any of those ground fault tripping devices which have been in the marina or the boat for more than a few month, might not trip anymore (happy smoldering)

 

[ccscott49]

As one final thing, you can do whatever you think is the correct way of doing it. But! I will not be connecting any of my mains, I have genny and inverter aswell as shore supply, to any anodes, negatives on my 24 volt, (three independant systems) or any other DC circuits. When in port I check the earth and have an rcd, which has never tripped, yes it is working, so I will just check my wiring and connections and not worry about stray current corrosion of any underwater components and be careful to turn off any mains, before working on it. I don't worry about it like so many others do.

 

[halcyon]

You mean a couple of diodes then.

Brian