Do current rules require a galvanic isolator on new build boats?

[VicS]

>A GI offers no protection from leakage current from shorepower supplies , or for that matter from defective 12/24 volt installations. Leakage currents from these, and the electrolysis they could cause, is a different matter altogether and potentially more serious because the GI offers no protection.


Again that is wrong which is why metal boat that have a GI don't get stray current corrosion and those that haven't got one have been known to get corrosion from (as I mentioned) shore power, normally caused by a bad earth, and where salt and fresh water mix. We also had a GI on our 12v system.

I wouldn't bother to fit one to a GRP boat but everyone to their own.

No its not wrong.

The whole point of a GI is that it blocks current from very low voltage sources, such as that arising from "dissimilar metals", to prevent galvanic corrosion, in particular rapid loss of zinc from the anodes.On the other hand it allows current from higher voltage sources such as the AC mains power to pass so that safety protection is maintained. Circuit breakers will trip, or fuses will blow in the event of a live to earth fault. RCDs remain fully effective too.

Current from 12/24 volt sources will also be passed as the cutoff point is usually for voltages below about 1.3 volts, although they do vary, so they offer no protection from electrolysis caused by obscure faults on DC systems

I cannot imagine what what you where doing with a GI on the dc system unless you used it as the bond between the DC negative and the shorepower earth as shown in Fig 2 in my diagram that Roger refers to.

GIs are fitted to GRP boats connected for lengthy periods to a shorepower supply to prevent rapid loss of zinc from the anodes due to current from "galvanic sources" being conducted by the shorepower earth....... As I have already said that is why they are sometimes called zinc savers. The only time there is no need to fit one is when the shorepower earth is not bonded to the anodes.

 

[VicS]

Is there a point at which a GI is worth it? My new (to me) boat is about to get a minimalistic shore power set up, but I only expect to stay I'm marinas and on shore power for about 30 days, spread over the year. The rest of the time she'll be sat on a trot, or anchored.

The anodes are only at risk while the boat is plugged into a shorepower supply...... (that is plugged in whether or not the supply is actually in use).

Furthermore the risk to the anodes only exists if the shorepower earth is connected to them. If you are using something like the camping and caravanning site mains supply units they wont be so there will be no problem .

If you do install a proper fixed shorepower ssytem and bond the shorepower earth to the DC negative , anodes, etc then assess the situation at the end of the first year . If you find your anodes are disappearing faster than they ought to then fit the GI.

Unlikely that you will need a GI for only 30 days per year but you never know.

 

[rogerthebodger]

" The only time there is no need to fit one is when the shorepower earth is not bonded to the anodes. "

I totally agree with that statement. But, other than its specified in the standards, why is it necessary to bond the shore power earth to the anodes on a GRP boat in the first place providing the shore power earth in intact and connected to the metal case of any mains powered equipment when the is an equipment lead earth wire. Double insulated equipment do not have an earth wire in their power lead so must be "totally safe"

 

[Marsupial]

" The only time there is no need to fit one is when the shorepower earth is not bonded to the anodes. "

I totally agree with that statement. But, other than its specified in the standards, why is it necessary to bond the shore power earth to the anodes on a GRP boat in the first place providing the shore power earth in intact and connected to the metal case of any mains powered equipment when the is an equipment lead earth wire. Double insulated equipment do not have an earth wire in their power lead so must be "totally safe"

as far as I can tell the anodes get bonded to the shore power earth by default when the earth is bonded to big chunks of metal in the boat; in practical terms it becomes virtually impossible to isolate the DC neg from the AC earth.

The double insulated (DI) debate was going on 30 years ago, the promoters of DI always claimed that the Live Negative Earth (3 wire) system was inherently MORE dangerous than the Live and Neutral (2 wire) system - they may have a point, statistically they do.

Many years ago a friend of mine kept a GRP boat on a swinging mooring half a mile off shore, he used it one weekend then returned to it the following weekend to discover that the propeller had disappeared - well most of it had, the only fault we could find was a bare 12v live wire in some sea water in the bilge by the stern gland, he never did say what it was supposed to be connected to. We assume that the propeller had fizzed away all week, the shaft was OK the P bracket was OK the anode was gone the batteries were flat.

 

[rogerthebodger]

" shore power earth by default when the earth is bonded to big chunks of metal in the boat "

"By default" why should it be again IMHO keep DC neg and mains earth separate. On a metal boat mains earth must be connected to the hull but not to the 12v neg.

To make a battery (that's what a boat in sea water with different metals is) you need 2 or more different metals immersed in an electrolyte (sea water) electrically connected. Get rid of the electrical connection and you have eliminated most of the electrolyte action. On a GRP boat most of the underwater metal is not connected anyway and any that is not isolated you make the same metal of fit anodes to those fittings only.

I even replaced by bronze propeller with a fabricated stainless for do just that

In your friend's case if the 12 neg had not been connected to the metal underwater that loss of propeller would not have happened. This is why it is recommended buy most metal boat owners do not connect 12v neg to the hull

http://s1335.photobucket.com/user/Distantshaws/media/IMGP2132_zps6c26766d.jpg.html?sort=3&o=394

click prev and or next to see other anodes this was 3 years after launching.

The whole build process is also on photobucket if anyone is interested.