Galvanic Isolator

[DavidGrieves]

Hi All

I need to fit a galvanic isolator to my boat. What is the amp rating for ie 70, 90 or 100amps? And how do I calculate which one I need?

Thanks for any advice / help.
David

 

[VicS]

No less than the main fuse or mcb trip rating on your incoming shorepower supply.

 

[DavidGrieves]

Thanks VicS

That makes sense! Am I right in thinking, if the isolator is the same as the incoming suply it can cope with the amps if there is a fault onboard and needs to dump the power to earth?

 

[VicS]

In simple terms yes that is right.

 

[jimmy_the_builder]

There's an interesting article here which suggests that if you have switched-mode power supplies on your boat, the galvanic isolator is effectively rendered useless, and your only remedy is an isolation transformer.

Cheers
Jimmy

 

[davidwf]

There's an interesting article here which suggests that if you have switched-mode power supplies on your boat, the galvanic isolator is effectively rendered useless, and your only remedy is an isolation transformer.

Cheers
Jimmy

I'm not sure the diagram on page 2 is correct, putting capacitors from Live to earth on an AC circuit will cause current to flow and trip the RCD. I think they have the diagram wrong, shouldn't the RF suppression capacitors should be on the DC output to earth ?

 

[savageseadog]

There's an interesting article here which suggests that if you have switched-mode power supplies on your boat, the galvanic isolator is effectively rendered useless, and your only remedy is an isolation transformer.

Cheers
Jimmy

Normally the capacitors form a filter circuit together with inductors. The inductors will reduce the earth current considerably. The capacitors are usually very small in value as well which means any current would potentially very small. There is however an easy path for HF noise to pass from inside the equipment to eath. Without some sensible measurements it's difficult to say how significant this will be. It would be possible to construct a filter to connect between the boats earth system and the galvanic isolator which would solve the problem.
Here's a typical mains filter circuit

I do tend to agree with the thrust of argument in the article regarding the benefits of transformers over galvanic isolators.

 

[rogerthebodger]

There's an interesting article here which suggests that if you have switched-mode power supplies on your boat, the galvanic isolator is effectively rendered useless, and your only remedy is an isolation transformer.

There was a discussion about a little time ago and that is possibly true if the GI is positioned between the incoming earth line and the boat earth line, but if the GI is positioned between the boat earth line and the boat earthing point. Any switch mode power supply HF earth leakage would then not pass through the GI but any mains supply leakage to boat hull would still go the mains earth and the GI would stop any Galvanic current passing through the hull.

VicS had a sketch of the setup up cannot locate it at the mo.

 

[VicS]

VicS had a sketch of the setup up cannot locate it at the mo.

I can though

Fig 1 is the conventional arrangement

Fig 2 is what is put up for debate

 

[mattnj]

There's an interesting article here which suggests that if you have switched-mode power supplies on your boat, the galvanic isolator is effectively rendered useless, and your only remedy is an isolation transformer.

Cheers
Jimmy

You can get them with status LEDs, which shows when they are passing any current, worth the extra few quid IMO, the LEDs are always on on mine when on the Genny, but not when on shorepower and switched mode charger is in action.

 

[mlthomas]

Hi David
They are quite easy to fit too, here is what we did
www.yachtconosur.com/galvanic_isolator.html
we are in Queens if you need anything
Mark