How do I test that a battery charger is galvanically isolated?

[fuss]

I would like to test that my battery charger is isolated as I hear many bad stories of what happens when it isn't.

Do i just test for a connection on the device when it is off between .....
ac earth and dc negative?
ac neutral and dc negative?
ac positive and dc positive?

or do I have to do something else?

Can I do the same test for the inverter?

Thanks John

 

[VicS]

Galvanic isolation is something required with a permanent shorepower installation. The galvanic isolator is a device through which the shorepower earth connection is made.
There is short explanation HERE

It is normally only necessary if the shorepower is left connected (not necessarily in use).

The shorepower should be connected to the vessels 12 volt DC negative (as close as possible to the battery negative), anodes etc but often is not. If it is not then a galvanic isolator is not necessary.

It only applies to the shorepower connection so your inverter is not affected but one side of its output should be connected (internally) to its earth connection in order th create a "neutral". Its earth should be connected to the vessels Dc negative etc in the same way that the shore power earth is.

More detailed discussion of galvanic corrosion, galvanic isolators HERE The second page goes on to discuss the merits of isolation transformers

And another good explanation by Charles Sterling HERE

 

[fuss]

Thanks Vic,
I have a galvanic isolator, I am mainly concerned about the individual devices. Battery chargers seem to be picked out as a device that must be manufactured as galvanically isolated.

If you are concerned about your battery charger check that there is no connection between either the positive or the negative and the earth connection.

Could you expand a bit on what you wrote above. Do you only mean the AC side?
I thought that when a device is galvanically isolated then there should be no physical connection between the input (in this case AC) and the output (in this case DC).
Thanks again
John

 

[Bosun Higgs]

The worry is a path through the earth wire of the 240v supply to the 12v negative to the stern gear and anodes. My old Sterling charger didnt have such a connection and personally I made sure that the earth wire was not bonded to the 12v negative anyway. But that was my decision and I was aware that I was taking what I viewed as a small risk. So I won't recommend that you do the same.

How do you test? Use an ohm meter with one probe on the stern gear ( or the battery negative if in the water) and the other probe on the incoming earth wire with the battery charger on. Take precautions in case there is anything funny in the circuits. If the charger is isolated, there will be an infinite resistance.

But do take care - rubber gloves, dry boat etc. If you are at all unsure about working with 240v, then get a leccie in.

 

[markdj]

The shorepower earth and the DC negative do not have to be connected as long as an earth leakage trip is installed on the boat and this will prevent any stray currents as AC wiring is effectively isolated from DC.

If the DC negative and AC earth are connected, the problem only occurs if there is a high resistance to earth. In this case, stray current will seek the path of least resistance to ground which could be via your anodes and any steel piling or someone elses anodes and their DC negative to AC earth connection which is when things really kick off in style This is more of a problem in the US as they do not require earth leakage trips to be installed on their boats.

Having an earth leakage trip prevents anything more than 30 milli-amps passing through which protects your boat and yourself.

 

[Bosun Higgs]

Your reply came in whilst I was typing the above.

If you have a galvanic isolator then forget what I said about putting probes on stern gear - it would be pointless assuming everything is working.

 

[fuss]

How do you test? Use an ohm meter with one probe on the stern gear ( or the battery negative if in the water) and the other probe on the incoming earth wire with the battery charger on. Take precautions in case there is anything funny in the circuits. If the charger is isolated, there will be an infinite resistance.

So does this mean that if there is a connection between the ac earth and the dc negative at the charger, when it is switched on , then the charger is not manufactured galvanically isolated.

 

[VicS]

Could you expand a bit on what you wrote above

I deleted it, but obviously not soon enough.

The AC input, the DC output and the safety earth connection should all be isolated from each other

I don't know what you have read or been told about battery chargers in particular. The link to Smartgauge explains a potential problem with any equipment using a switch mode power supply.
A good standard of insulation is obviously required in a battery charger as it is one piece of equipment that is connected to both the shorepower supply and the craft's low voltage DC system.