Galvanic Isolators (again, sorry)

MagicalArmchair

MagicalArmchair

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Right, okay, so my electrical system is from the ark. No GI, no VSRs, no megafuse, no installed charger, no electrickery at all, something I plan to sort in full come the chilly winter months. I have trawled previous posts, so apologies if this is a repeat. Also, let me apologise for my shocking electrical notation.

vZu9eJDl.png


We plug in when we are in the marina on board to trickle charge the batteries whilst we kip on board overnight running the fridge etc. so its on for a weekend at a time. When I came out of the water last time around, I noted the engine anode had been eaten about 50% and the prop anode was just about gone and was hanging on by a whisker rattling around on the shaft (I had left it for a few weeks plugged in that year though)

Its birthday time, yippee, so I'm thinking of asking the First Mate to get an exciting GI, such as this: http://www.mdsbattery.co.uk/shop/productprofile.asp?ProductGroupID=8924

  • Where on earth on my wiring diagram would this little fellow sit?
  • Or, do I just get one of these: http://www.safeshoremarine.com/Inline isolator.htm and be done with it?
  • The FM radio on the boat is constantly wired in to the engine start battery to stop it losing its memory (I know, its a bit mad, something I will change). Will this mean more galvanic corrosion... I thought that only happened when wired to the shore...?
  • Any other nasties in the above set up?

OR should I just forget about it for now, continue sailing, stop worrying, get some new Gill sunnies (the dog ate mine this weekend sailing, damn him).
 
l'escargot said:
The GI goes in the 240v system, in the earth line, between the inlet plug and the consumer unit

Like this: http://www.safeshoremarine.com/diagram for earth wiring.pdf
Click to expand...

Personally I wouldn't be sure what "the consumer unit" is, so I would say that if you decide to connect the earth on the 240V input (not shown on your 12V schematic of course) to the engine block then the GI needs to go in series in that earth-to-block cable.

If you don't connect the 240V earth to any metal work on the boat then you don't need the GI.

Different opinons seem to exist on the Forum as to which is the best approach.

Richard
 
If all you want is to trickle charge the batteries, get a double insulated charger with no connection from the -ve output to main earth.
It's when you start 'installing' the charger and connecting its incoming mains earth to the boat that problems begin.
 
RichardS said:
Personally I wouldn't be sure what "the consumer unit" is, so I would say that if you decide to connect the earth on the 240V input (not shown on your 12V schematic of course) to the engine block then the GI needs to go in series in that earth-to-block cable.

If you don't connect the 240V earth to any metal work on the boat then you don't need the GI.

Different opinons seem to exist on the Forum as to which is the best approach.

Richard
Click to expand...

They are shown on the schematic I linked to - the consumer unit is labelled "consumer unit" and the 240v input is labelled "240v input on boat". That is the recommended installation as per the latest published wiring regs. Unless of course you meant to quote the OP?

There are other opinions often expressed on the forum about locating the GI elsewhere but they are not in accordance with those regs...
 
Not sure why you have an "earth anode" in the 12v circuit. If the anode is protecting your prop it should be bonded to the shaft, probably through the gearbox housing. It is nothing to do with the electrical circuits. If the anode is eroding it is doing its job. The engine anode will erode because it is in the raw water circuit - that is its job.

You don't say what your 240v system is. If you do not regularly use 240V then you do not need a GI. If you do have a proper 240v circuit and plug into the marina supply, then install a GI just after the input socket as shown in the literature on GIs.
 
Okay, there is no 240V circuit on the boat at all. The only 240V that goes on is to the Smart Charger that is used to juice the batteries up when we are on board (and perhaps my fan heater plugged directly in to an extension lead from the shore power), err, so a bit like the below. Ha, well it isn't Earth (Anode) I suppose, Ground then would it be? Its a black wire that goes to the anode is all I know!

l9pxgsHl.png


When I was on a swinging mooring, I had basically no eroding of my anodes, hence my worry I am doing something 'wrong' in my marina berth and should be doing something with a GI when putting the boat on charge. Bear in mind, I never leave the boat on charge, I only have it on charge whilst we are on board, and we use the boat enough to keep the batteries pretty well topped up. There is no 'consumer unit' on board, I don't know if that means something like a fitted charger (like a Xantrex Truecharger), or something that actually distributes 240V around the boat.

So if we take the below, they suggest the GI will protect me from stray current from other boats? So perhaps what I am looking for is one of these (http://www.safeshoremarine.com/Inline isolator.htm) to prevent other boats leaching my anodes when I'm plugged in... however, perhaps (like many of these things) I am overthinking it, and for the small amount of time I am plugged it, it isn't actually an issue? Or, as the boat is only connected to the charger, and not the earth at all, is it isolated anyway and I don't need a GI at all? :confused: Thanks for your help btw :encouragement:

 
l'escargot said:
They are shown on the schematic I linked to - the consumer unit is labelled "consumer unit" and the 240v input is labelled "240v input on boat". That is the recommended installation as per the latest published wiring regs. Unless of course you meant to quote the OP?

There are other opinions often expressed on the forum about locating the GI elsewhere but they are not in accordance with those regs...
Click to expand...

Sorry Escargot .... my phrasing was rather clumsy. I was trying to say that I looked at your diagram but "consumer unit" didn't mean anything to me so I expressed the same thing as you have shown in your diagram ..... but in words.

I wasn't trying to suggest a different way to connect the GI .... rather the fact that if the OP decides not to connect the 240V incoming earth to the engine block then he doesn't need the GI at all. I see that other posters have now picked up the same sentiments.

Richard
 
l'escargot said:
Then you don't need a GI. It is to prevent stray current going to earth via the mains supply, which you don't have.
Click to expand...

Thanks Richard, Snaily, I'll get those Gill sunnies (so the dog can chew them up) and forget the GI. It doesn't explain why my anodes have been vanishing more quickly in the marina than out of it though - I'll take a look when I get hauled out mid season/end of the season and not get so worried about leaving her plugged in to shore power (if there is no bond, there can be no problem).
 
On that video it is shown and stated that if no electrical connection is in place between the 2 different metals no current will flow, so why in a GRP boat when the underwater fittings are mostly insulated from each other and therefore not current flow thus no galvanic corrosion can occur does the anode suppliers and the GI suppliers always insist on interconnections the isolated under fitting.

Well it so they can sell more anodes or GI's.

There will be cases where anodes and GI's are needed and in my case having a metal hulled boat thus a conductive hull so all underwater metal will be interconnected. If there are dissimilar metals anodes will be needed so on a steel boat keep all underwater metals the same where possible and the same with aluminium boats the same.

With GI's I have one as having a conductive hull a short to the hull could occur and thus a dangerous possibility but on a GRP boat with no interconnection the likelihood of a short to a metal part is less likely IMHO.

This has been discussed numerous times and like anchors different views are posted.

INHO you cannot have a universal rule in this matter as boats and usages differ so much and the requirements in anodes and GI's need to be considered on the exact situation of the individual boats and users.
 
MagicalArmchair said:
Well, the charger is earthed of course, but that is pushing amps to the battery through only two leads, pos and neg... so no earth gets to the boats system... so I can stop worrying... right...?
Click to expand...

You could put a multimeter on the ohms setting across the negative 12V wire and the earth pin on the plug to see if the two are connected. With it all unplugged of course. :)

Richard
 
MagicalArmchair said:
Well, the charger is earthed of course, but that is pushing amps to the battery through only two leads, pos and neg... so no earth gets to the boats system... so I can stop worrying... right...?
Click to expand...
Are you sure it's earthed. A lot of electrics these days are double insulated and not earthed. Even if it is and you don't leave it on constantly for weeks a time, just go out and buy your new sunnies...:)
 
Its one of these jobbos: https://www.amazon.co.uk/Ring-Battery-Charger-Professional-Remote/dp/B000VZ8R9K so I'm not sure if it is earthed or not? Richard, so err, set to ohms (30 ohms?), put one tip on the 12V+ pillar of the battery, and one on the earth wire coming from the shore socket (that's a female three pin, so in the earth pin at the top of the socket...?)? What would I be looking for here, 1 resistance? 0 resistance?... Excuse my shocking (pun intended!) lack of electrical knowledge :o...

Yup, I won't worry, sunnies it is! Its now more my academic curiosity. :confused: Thanks again
 
There was a discussion a little while ago about the possibility of mains power becoming connected to the battery positive or negative.

I made a statement that a marine battery charger should have a mains isolation transformer but I understand from VicS that not many of the current type are like that so if that one is of that type there could be other problems also so more technical info is needed including how and if there is earthing in the mains power wire.

Does it have a metal case and what kind of power plug does it have 2 pin o 3 pin and if 3 pin is there a wire connected to the earth pin in the plug.
 
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It is not unusual for anode erosion to vary according to location, nor for it to be higher in marinas. That is usually because of stray currents with all the metal around the boat, but a GI does not deal with it.

It may well be worth either beefing up your anode with a larger one, or using a hanging anode when in the marina.

Worth having a look at the MG Duff website for background on anodes.
 
MagicalArmchair said:
...Richard, so err, set to ohms (30 ohms?), put one tip on the 12V+ pillar of the battery, and one on the earth wire coming from the shore socket (that's a female three pin, so in the earth pin at the top of the socket...?)? What would I be looking for here, 1 resistance? 0 resistance?... Excuse my shocking (pun intended!) lack of electrical knowledge :o...
Click to expand...

No. One tip on the end of the lead that goes from the charger to the negative battery terminal and the other on the earth pin of the battery charger mains plug. If they are not connected, the resistance will be the maximum that the meter is set at.
 
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