Electrics earthing to hull

[AlumCat]

I am building a 48 ft. aluminium sailing catamaran. I am just installing the electrics. I have a common negative post which routes it's way thu' the boat. The positive side is thru' a switch & then a 50 A breaker. I am getting a feed thru' to the hull, this is coming from the negative side, as I am able to isolate the positive side. I have taken off each negative feed & checked the current, some are zero some are 0.1 mA & some are up to 5 mA. On checking the affected instrument with a continuity check I can find no link with the hull. Yet on checking with the meter between the positive battery side & the hull I can get up to 12 V. This voltage reduces to about 8 V then starts to increase again. Am I doing something daft, can anybody help ? I have solar panels for recharging, could this affect anything ?

 

[pampas]

disconnect the common Neg and see if your are getting leakage from the now isolated Negs of each battery. If yes ditto to the Positives, Have come across the odd battery during working life where the battery cases pass a small leakage through the cases.
In all insulated circuit it pays in the long run to fuse both + and neg ,certainly for the domestic circuits.

 

[vyv_cox]

Is this the accepted way of wiring aluminium boats? I understood that having the hull as the negative return led to all sorts of unwanted corrosion problems.

 

[Graham_Wright]

Is this the accepted way of wiring aluminium boats? I understood that having the hull as the negative return led to all sorts of unwanted corrosion problems.

Don't think that is what he described.

 

[vyv_cox]

Don't think that is what he described.

Yes, could be. I am easily confused

 

[nigelmercier]

Yes, could be. I am easily confused

I didn't follow it either.

 

[fuss]

I am building a 48 ft. aluminium sailing catamaran. I am just installing the electrics. I have a common negative post which routes it's way thu' the boat. The positive side is thru' a switch & then a 50 A breaker. I am getting a feed thru' to the hull, this is coming from the negative side, as I am able to isolate the positive side. I have taken off each negative feed & checked the current, some are zero some are 0.1 mA & some are up to 5 mA. On checking the affected instrument with a continuity check I can find no link with the hull. Yet on checking with the meter between the positive battery side & the hull I can get up to 12 V. This voltage reduces to about 8 V then starts to increase again. Am I doing something daft, can anybody help ? I have solar panels for recharging, could this affect anything ?

Electrics on an aluminium boat should probably be done by someone who knows what they are doing. Normally on a new installation the electrician would have started with a concept and then wired everything up accordingly.

For example, he would have considered the following before starting....
Will the engine block be completely isolated from the hull or will the block be connected.
Will I have a fully floating system or will I have a single ground point on the hull
Is the start motor Isolated
Is the alternator isolated
Did the engine manufacturer take an aluminium boat installation into consideration when designing the engine wiring.
Is there going to be an AC system and is this going to be fully floating
Is there an AC generator, what are the earthing plans
Is there an inverter, what are the earthing plans.

The fact that you are posting on here shows that there is not a full concept and you propably will make some other mistakes that could cause hull damage in the future.

I would suggest to get someone involved who knows these things... If you want to save money, do the main bits of the installation yourself after they have agreed that what you are doing is ok.

 

[prv]

I didn't follow it either.

He quite correctly wants his electrical system fully insulated from the hull, but is finding continuity between the negative bus-bar and the structure. He wants to eliminate this, but can't find the short.

To the OP - do you have the engine installed yet? The vast majority tie the block to negative, and from there there are any number of possible leakage paths. If you believe the engine is isolated from the hull, then a good trick one is the wire armouring in hose - if the wire is exposed at each cut end and is touching the hose barbs.

Pete

 

[rogerthebodger]

I had the same type of issue when I first wired my steel boat.

You need to check all items driven from the 12volt system.

I took great effort to insulated my engine from the hull but still found a 12v neg to hull connection via the engine remote controls, which was subsequently insulated. I also found a hydraulic solenoid valve body was connected to the dc neg and was mounted on an isolated pad but still had connection via the flexible hydraulic pipes.

Only thing to do is to disconnect all 12vdc electrical items and test again. Also don't forger you could have multiple links so you just cannot disconnect test and the reconnect.

I is a very time consuming job as I found out.

IMHO on a metal boat the DC neg should NOT be connected to the hull at ANY point, but the mains earth must be through a GI.

GRP boats are different

 

[AlumCat]

Thanks for all the info & comments. The engines are in, but they are not connected to the house wiring. I have isolated the positive side. I have taken off all the neg. wires, then put them on individually. Where it shows a a live reading to the hull. I then check the item, with a continuity check for a link to the hull. There isn't one. How can that be ?

 

[nigelmercier]

He quite correctly wants his electrical system fully insulated from the hull, but is finding continuity between the negative bus-bar and the structure. He wants to eliminate this, but can't find the short...

Ah, I see.

VHF antenna? The screen will be grounded, and connected to the mast, then the rest of the boat.

Strange resistance readings can be caused by dissimilar metals in water, like a battery. I found my saildrive and keel were creating 0.7V.

 

[john_q]

I also have a steel boat where the whole vessel is Isolated ground or negative floating DC supply (e.g. the negative is not connected to sea water). Luckily the engine (Perkins 4236M) was designed to be isolated from the ground

I have a simple test circuit in place to tell me if things are not correct.

If you want a copy of the circuit diagram, PM me with your email address as the only electronic copy I have is in WORD and it is too big to attach to this forum

I have found things like the ICOM 802 SSB grounds itself to the hull (not mentioned in the manual) and that some USA VHF aerials ground to the mast and a piece of metal swarf got into a coax connector

Good luck