Galvanic Isolators. Isolation Transformers. SMPSU. Anodes.

CaptainBob

CaptainBob

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I currently have a GI fitted, one anode on the prop shaft (new 2 months ago) one anode on the hull (guessing 60% remaining 2 months ago) and one anode in the engine (fitted new 2 weeks ago).

I frequently run two laptops and various other items on shore power which probably use SMPSUs and someone on here mentioned that SMPSUs might cause a voltage over a GI enough to stop it being effective.

This in mind I checked by newish engine anode and found a 1mm thick grey layer which I could easily remove with my thumbnail... given these things are meant to last a season it occurred to me that this might be too much too soon? I've not checked them but does this perhaps mean my hull and prop anodes are on the way out?

I have a decent DC voltmeter and a clamp meter. Is there any way to check the DC voltage across a GI's terminals while it's on? I'm thinking I can just check the DV voltage directly using a DC voltmeter as the potential at each terminal will oscillate both sides of the GI, cancelling each other out, and that if I measure a voltage more than about 1.5V that I possibly do have a problem.... am I right? Don't want to blow up my voltmeter really!

And if it is high, is the answer an isolation transformer? If so, are they relatively easy to fit yourself? Which one is best etc?

Thank you!
 
The galvanic isolater is supposed to protect the anode(s) and other underwater metal against minor leakage to earth from the 240V AC power system. You will measure a small AC voltage across it if there is any there, the voltage won't be more than about approx 0.5V AC, this maximum voltage will mean the isolator could be conducting. You would need to detach the earth cable to the isolater and put an AC Ammeter in line. Your clamp meter would probably be too high range for a sensible reading, you would expect to see something less than the RCD trip current 25mA (?), Ideally of course zero voltage and current. It's not impossible with modern gadgets that high frequency voltages and currents could be present which you wouldn't be able to measure with what you've got.

The anode might not work well if zinc in fresh water.

The way to go in my opinion is an isolation transformer, don't bother with an expensive marine one.
 
Why would you expect the current to be less than the trip rating? Although one would hope the current was very small, its not relevant to the RCD.
Also current can be AC or DC.
An isolation transformer is probably the Rolls Royce solution if you must use mains around the boat, otherwise a double insulated, isolated battery charger/12V supply.
 
[ QUOTE ]
Why would you expect the current to be less than the trip rating? Although one would hope the current was very small, its not relevant to the RCD.
Also current can be AC or DC.
An isolation transformer is probably the Rolls Royce solution if you must use mains around the boat, otherwise a double insulated, isolated battery charger/12V supply.

[/ QUOTE ]

Running two laptops aboard all day most days so mains is fairly essential really. Isolation transformer time?
 
[ QUOTE ]
Why would you expect the current to be less than the trip rating? Although one would hope the current was very small, its not relevant to the RCD.
Also current can be AC or DC.
An isolation transformer is probably the Rolls Royce solution if you must use mains around the boat, otherwise a double insulated, isolated battery charger/12V supply.

[/ QUOTE ] Current must be less than the RCD trip current because it would trip. You could end up with a residual DC current but it's unlikely. The galvanic isolator is primarily used to protect against currents flowing from AC supply ground to boat ground (ie the sea via your engine, propeller, anodes, etc)

Look for a suitable transformer on ebay. It should be a dual wound and/or shielded transformer. I don't know the latest international standards but there are perfectly adequate surplus, used units, normally used for electronic, available.laboratories.
 
No it would not trip, its a current running down the earth line, not a residual current ie difference between live and neutral, which is what RCD's sense.
It can be DC caused by electrochemical difference between where the shore power is earthed and the marina water. There will be an AC component too, from various coupling of the live and neutral to the earth line.
Surprisingly, one of my laptop (mains) psu's does not carry the 'double insulated' (2 squares one inside the other) symbol, so the laptop chassis may be earthed?
I use a DC-DC to run my laptop on a boat.
 
You'll probably find the earth of the output is connected to the earth of the input. Most isolating transformers are like this. If so it does SFA to protect from galvanic corrosion. Most of these things are designed to protect from electric shock, not galvanic electrolysis.
The smatgauge website is useful on this subject.
 
No you wont!
It comes unwired, so you have to do it yourself.
I asked Airlink to make them available as there wasn't a suitable cheap transformer on the market and I didn't want to pay "marine" prices.
You can use it on the pontoon or on the boat its up to you.
In truth the safest place for a transformer is right next to the supply.
BTW I don't get anything for the recommendation as I have no connection with Airlink other than as a customer.
 
[ QUOTE ]
and someone on here mentioned that SMPSUs might cause a voltage over a GI enough to stop it being effective.

[/ QUOTE ]
I have mentioned that a couple of times recently but only repeating some info that someone else posted 2 or three weeks ago. I doubt now if I could find the original post, the search system being what it is.
The solution apparently is to put a capacitor across the GI but the value of the capacitor will depend upon the frequency of the current which as I understand it comes from the SMPSU itself and is therefore a higher frequency that the AC mains.

I dont think I am happy enough about it to suggest a way in which it is possible to determine if the GI is in "conduct mode" or not. Presumably a small AC current from the above will indicate that it is, but it may be very small. Similarly a small DC current would indicate a current of "galvanic" origin that would be causing wastage of the anodes again it may be very small.

Regarding earths and isolation transformers. The shorepower earth comes no further than the transformer. However one side of the secondary has to be earthed to the "ships earth" in order to make a "neutral" on board.

It is described in towards the end of this section in tb Training
 
Now found where the problem with SMPSUs and GIs was mentioned

HERE

with a link to an explanation on the Smartgauge website

(just to prove it is not something I have invented or imagined /forums/images/graemlins/smile.gif )
 
Cheers for the link Vic.

I'm still none the wiser as to how to determine if I'm currently having currents in my system due to galvanic action... without going underwater in my trunks.

What if I bought a cheap anode, stuck it on the end of a bit of wire, put it inline with an ammeter, put the anode over the side, and touch the other end against various parts of my boat (engine/prop shaft etc) would that work?
 
[ QUOTE ]
I'm still none the wiser as to how to determine if I'm currently having currents in my system due to galvanic action

[/ QUOTE ] Not a direct answer to that question as such . Instead to determine if there are AC currents that might be (combined with the galvanic effects) be causing the GI to conduct.

Two items on the Yandina website HERE and HERE suggest measuring any AC voltage across the GI.
If I read them correctly the AC volts should ideally be zero. Below 0.5 volts then the GI will not be conducting but normally one should looking for less than 0.25 volts.
 
Thank you Vic. Will go have a read....

Hmmm. Two laptops, battery charger (for the boat batts) and Eberspacher on:

0.76V AC

As I switched them off one by one the volts dropped to 0.75V AC with just the Eberspacher running.

Powering that down now also...

So now with absolutely everything off I'm showing 0.716V AC

Seems odd that it would be conducting when EVERYTHING is off.

DC volts across it is zero.

I'm confused as to how it can be conducting a DC current when there's no DV volts over it - which there should be across the diodes no?

/forums/images/graemlins/frown.gif
 
I suppose it implies that there is a small AC leakage from something else.

Did you actualy unplug the various items? Open the various circuit breakers?

Switch off the shorepower main circuit breaker?

Dont tell me. You've unplugged it all from the pontoon and you've still got 0.7 volts. /forums/images/graemlins/frown.gif
 
LOL nope. Tried that just for good measure and it drops to 0.026V AC.

And you're correct that if I switch off the main circuit breaker it also drops to 0.026V AC.

But yes, I turned off and unplugged everything I could.

I can't unplug my battery charger but it has an isolation switch which I've turned off.

At a bit of a loss as to where it's coming from. Will ask the people who installed the shore power and GI and report back....
 
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