Galvanic Isolators (again, sorry)

[RichardS]

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.

Spot on!

The multimeter might have ohm ranges like x1, x1000 etc but any range should do. First hold the two tips together and either the needle will swing right to one end of the scale or the digital screen will say 0 ohms.

When you then touch the tips to the negative battery clamp (it doesn't have to be clamped on the battery but it doesn't matter if it is) and the top (large) earth pin on the charger plug either the needle will not move or the screen will say continue to say OC (or whatever it says to mean open circuit). If the needle moves in the direction it did when you touched the ends together or if the screen shows a number of ohms, then there is a connection between earth and battery -ve and, if you left the charger connected for a long time, you could have a reason for anode erosion ..... or, as Tranona rightly says, it might not be anything to do with it.

Richard

 

[VicS]

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 ...

Yup, I won't worry, sunnies it is! Its now more my academic curiosity. Thanks again

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.

Like this

The meter is on its highest resistance range, 20 Megohms, and the reading of 1 . indicates more than 20 Mohms

My other charger only has a dummy earth pin on its mains plug

 

[Tranona]

But don't forget the Mr Duff is in the business of selling anodes.

IMHO some of the advise is just wrong or at least not up to moden thinking

Read my post #11 above.

Anodes are not required for through hulls on a GRP boat for the reasons you state. However they may be required (as for the OP) where there is a yellow metal prop on a stainless shaft, or sometimes in the seawater circuit of the engine. The prop/shaft fulfils your criteria - dissimilar metals in contact in seawater.

I referred the OP to the Duff site, not because they sell anodes but because there is an explanation of galvanic action that will help in understand the purpose of anodes and how they work.

Pretty standard stuff and from what he says he has the correct arrangement to protect his prop, and has no need for a GI unless he installs shorepower.

Our club has just installed shorepower to our berths and it is mandatory that the boat is fitted with a GI and the 240v on the boat meets the latest standards if you want to use shorepower.

 

[nigelmercier]

Vic's charger looks similar, so my suggestion may be a red herring. However, I did once install a charger that had the mains earth connected to the negative, it didn't occur to me that this would be a feature so I didn't check. Big mistake.

 

[MagicalArmchair]

Like this

The meter is on its highest resistance range, 20 Megohms, and the reading of 1 . indicates more than 20 Mohms

My other charger only has a dummy earth pin on its mains plug

Hey great, thanks everyone. I'll run that test.

Vic, yours, as the resistance is 1, showing the actual resistance is greater than 20 Megohms, that means there is no connection between the two in your case and it's safe and won't leech the anodes? Or it means there is a connection there but with loads of resistance?

 

[rogerthebodger]

Like this

The meter is on its highest resistance range, 20 Megohms, and the reading of 1 . indicates more than 20 Mohms

My other charger only has a dummy earth pin on its mains plug

You could also check between the live pin and the positive and between the neutral and the negative to determine if the charger is a transformer isolated type.

I'm not surprised that reading is the case and the charger has a plastic case and it you look at the mains cable it is oval in shape and therefore most likely to be only 2 core cable.

 

[KellysEye]

>It doesn't explain why my anodes have been vanishing more quickly in the marina than out of it though

It's caused by stray current corrosion caused by a bad earth in the marina 240v supply and that needs a GI. One issue is you have two earths engine and anodes but I don't know if that requires 2 GIs.

 

[VicS]

Hey great, thanks everyone. I'll run that test.

Vic, yours, as the resistance is 1, showing the actual resistance is greater than 20 Megohms, that means there is no connection between the two in your case and it's safe and won't leech the anodes? Or it means there is a connection there but with loads of resistance?

In practice it means there is no connection although strictly speaking the meter is saying that the resistance is greater than 20 Mohms

A resistance of 20M ohms would mean that if the voltage causing any galvanic corrosion was as much as 1 volt the max current would be 0.05 μA. Its easy then to calculate the rate of loss zinc .

I make it 1 gram in 1850 years, but feel free to check the arithmetic.

 

[RichardS]

..... and it you look at the mains cable it is oval in shape and therefore most likely to be only 2 core cable.

If it was 2 core then presumably no connection at all to the earth pin in the plug.

In which case wouldn't the meter show open circuit rather than 20 meg? Or could the resistance of the plastic plug material be 20 meg?

Richard

 

[rogerthebodger]

If it was 2 core then presumably no connection at all to the earth pin in the plug.

In which case wouldn't the meter show open circuit rather than 20 meg? Or could the resistance of the plastic plug material be 20 meg?

Richard

The meter in VicS post is showing open circuit as a reading of "1" on the resistance means a resistance of infinity. 20 megohm is just the maximum reading it can display anything over is displayed so "1" or overrange

 

[RichardS]

The meter in VicS post is showing open circuit as a reading of "1" on the resistance means a resistance of infinity. 20 megohm is just the maximum reading it can display anything over is displayed so "1" or overrange

That's interesting. My multimeter shows OC when the probes are not touching anything and just continues to show OC if you touch the probes against things which are not connected.

My other meter which I keep on the boat is a dial with a needle which you have to mechanically zero! But then again, it is 40 years old!

Thanks.

Richard

 

[Tranona]

>It doesn't explain why my anodes have been vanishing more quickly in the marina than out of it though

It's caused by stray current corrosion caused by a bad earth in the marina 240v supply and that needs a GI. One issue is you have two earths engine and anodes but I don't know if that requires 2 GIs.

This (as usual) is absolute nonsense. You clearly do not understand how a GI works. How can he possibly install one when he does not have a 240v supply in his boat? GIs are nothing to do with the engine, nor anodes but solely for the 240v supply.

 

[VicS]

You could also check between the live pin and the positive and between the neutral and the negative to determine if the charger is a transformer isolated type.

I'm not surprised that reading is the case and the charger has a plastic case and it you look at the mains cable it is oval in shape and therefore most likely to be only 2 core cable.

You are too clever Roger. I was hoping no one would notice that! ......... but the point of the photo was to illustrate and clarify how to take the reading

 

[VicS]

>It doesn't explain why my anodes have been vanishing more quickly in the marina than out of it though

It's caused by stray current corrosion caused by a bad earth in the marina 240v supply and that needs a GI. One issue is you have two earths engine and anodes but I don't know if that requires 2 GIs.

A GI wont offer any protection against " stray currents " originating from 240 volt sources or even 12 volt sources for that matter. It will only stop current from "galvanic sources" which are less that around 1.4 volt It operates by blocking the current from these sources flowing through the shorepower earth connection.

The "two earths" you refer to are irrelevant. A GI is installed in series with the shorepower earth connection, in practice between the incoming connector and the consumer unit or common grounding point.

 

[rogerthebodger]

You are too clever Roger. I was hoping no one would notice that! ......... but the point of the photo was to illustrate and clarify how to take the reading

Sorry Vic about that and yes I understood your reasoning.

Interesting on Richards comment about the reading on his meter . I wonder if an auto ranging meter indicated an open circuit differently. My main meter is similar to yours, must check my auto ranger that I don't use much.

 

[VicS]

Sorry Vic about that and yes I understood your reasoning.

Interesting on Richards comment about the reading on his meter . I wonder if an auto ranging meter indicated an open circuit differently. My main meter is similar to yours, must check my auto ranger that I don't use much.

Somebody lent me an autoranging meter to check something out a few weeks back... I used it on an ohms range too but I cannot remember what it displayed when OC. It was a Fluke, not a cheapie like mine, so my hands were shaking so much I used it and returned it as quickly as possible.

 

[MagicalArmchair]

Thanks all, it's been genuinely fascinating and has dispelled many myths and half formed worries for me (KellysEye, I was under the same illusion as you re stray currents and the ground points on my boat). And Vic, Roger, that is my exact charger that Vic tested and mine has the same Oval cable meaning I have no earth issues to worry about (apart from it cooking me of course as it has no earth ). Thanks again for all your expert help, what an extraordinary place this forum is, I can only hope I can help others in the way you help me.

 

[BabaYaga]

A GI wont offer any protection against " stray currents " originating from 240 volt sources or even 12 volt sources for that matter. It will only stop current from "galvanic sources" which are less that around 1.4 volt It operates by blocking the current from these sources flowing through the shorepower earth connection.

My understanding is that a GI, apart from galvanic sources of current (different metals in water), also can offer some protection against stray currents that originates from a difference in potential, a volt or two, between the shore power earth (copper rod in ground) and the sea.

 

[KellysEye]

>This (as usual) is absolute nonsense. You clearly do not understand how a GI works. How can he possibly install one when he does not have a 240v supply in his boat?

The OP said "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 it is you are talking absolute nonsense. If he uses shore power even occasionally he needs a GI on the earth of the cable he uses.

 

[Tranona]

>This (as usual) is absolute nonsense. You clearly do not understand how a GI works. How can he possibly install one when he does not have a 240v supply in his boat?

The OP said "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 it is you are talking absolute nonsense. If he uses shore power even occasionally he needs a GI on the earth of the cable he uses.

He does indeed if he has a 240v circuit in his boat, but it is nothing to do with the stray currents. (see Vics post above also responding to your misinformation). If he just uses a trickle charger that is unconnected to a built in circuit in his boat he will not need a GI. You seem confused about what GIs do with your suggestion that he might need 2 and that it is anything to do with the engine or the anodes.

No point in you confusing the OP when his query has been answered correctly already.