Locating AC shore power grounding? (leaking stray current issue)

[Briis]

With silver cell I've measured hull potential to -1100 mV when shore power is disconnected. It's near perfect zinc protection for a steel-hulled sailing boat. BUT when I connect shore power it's -850 mV, hence just a notch below zinc protection level and thus it creates stray current corrosion on my hull when connected to 230V shore power. This means the boat itself creates stray current via some grounding leak (if it's the correct English term since it's not my first language).

I've bought a 30 amp galvanic isolator (zinc saver) to fix that, but I can't find the correct spot to install it to make the expected difference - meaning there should be almost no difference between silver cell measurement on connecting and disconnecting the shore power, it's supposed to be around -1100 mV range in both settings.

Just in case I took the yellow (green wire in US?) wire close to where the shore power comes into my boat and connected to braker, but it did not change the connected/disconnected state change via silver cell measurement. Hence I assume shore power must be grounded after that somewhere in the boat.

I've done all the visual checks on finding the grounding spot but to no avail. It's getting kind of annoying since I'm getting to the point where without stripping half of the internals inside of my boat I could not locate the shore power grounding spot on the hull.

So I am asking for the more knowledged if there's a better way of finding the grounding spot, i.e. with a tester or by any other means, i.e. seeing the potential differences in different spots of the hull by intentionally leaking some voltage to the hull to get a rough idea where it's located?

Maybe then I can find it without stripping the entire boat from the inside.

Any advice on finding that shore power ground/bonding spot highly welcome!

[s/v Jedi]

Originally Posted by Briis

With silver cell I've measured hull potential to -1100 mV when shore power is disconnected. It's near perfect zinc protection for a steel-hulled sailing boat. BUT when I connect shore power it's -850 mV, hence just a notch below zinc protection level and thus it creates stray current corrosion on my hull when connected to 230V shore power. This means the boat itself creates stray current via some grounding leak (if it's the correct English term since it's not my first language).

I've bought a 30 amp galvanic isolator (zinc saver) to fix that, but I can't find the correct spot to install it to make the expected difference - meaning there should be almost no difference between silver cell measurement on connecting and disconnecting the shore power, it's supposed to be around -1100 mV range in both settings.

Just in case I took the yellow (green wire in US?) wire close to where the shore power comes into my boat and connected to braker, but it did not change the connected/disconnected state change via silver cell measurement. Hence I assume shore power must be grounded after that somewhere in the boat.

I've done all the visual checks on finding the grounding spot but to no avail. It's getting kind of annoying since I'm getting to the point where without stripping half of the internals inside of my boat I could not locate the shore power grounding spot on the hull.

So I am asking for the more knowledged if there's a better way of finding the grounding spot, i.e. with a tester or by any other means, i.e. seeing the potential differences in different spots of the hull by intentionally leaking some voltage to the hull to get a rough idea where it's located?

Maybe then I can find it without stripping the entire boat from the inside.

Any advice on finding that shore power ground/bonding spot highly welcome!

You should have a shore power inlet from which three conductors come in: blue, brown and green/yellow striped. Here (after everything is disconnected and safe) you need to cut the green/yellow ground conductor. Now use your multimeter is resistance mode and measure between the two ends you just cut. You should have a high resistance. This is then where you insert the galvanic isolator.

[smac999]

If the boat changes when you plug it in. That means you don’t have a galvanic isolator or iso transformer. Or if you do it’s wired wrong or a problem on board bypassing it

These readings just means you are sharing the zincs with other boats and they will go faster. It doesn’t nessarly mean anything is wrong. Lots of boats don’t have isolators. If not they all simply share zincs at the dock.

If cutting the green cable at the shore inlet doesn’t stop the plug in change. Then you probably have a ground to neutral bond on the boat. That you will need to find.

[Briis]

Cheers guys for the valuable info!

Originally Posted by smac999

If cutting the green cable at the shore inlet doesn’t stop the plug in change. Then you probably have a ground to neutral bond on the boat. That you will need to find.

I reckon that's the case on my boat. Haven't found the place where it's located though. I'll dig on once the weather gets warmer!

[Briis]

An update on solving that stray current issue and maybe a worthwhile lesson to any metal (steel or aluminium) Hull boat owners.

I did not manage to find the neutral to ground spot, nor did various experiments with zinc-saver device worked on reducing AC-induced stray currents. Once the shore power was connected the potential dropped -100 to 150 mV which was enough to lose zinc protection and hence stray current (read: agressive-) corrosion risk on the hull which you see surprisingly lot on metal hulls and owners often suspect wrong causes.


After lots of research I went with the best option - an isolation transformer. Mine's the Victron 7 kW isolation transformer model. Tried and tested old-school transformer-method, quite heavy, but it does what it's supposed to - it physically separates shore power from onboard power via transformer induction.

Once installed (super easy! Just find a convenient spot and put it somewhere between 3-shore power wires), and bam - all stay current issues gone! No matter how I connect or how much stuff (load) I run, electrolytic balance via silver cell measurement with the surrounding water remains the same. Hell, not even a millivolt difference! (Before it was almost hundred millivolts drop and somewhat depends on the load). So with the isolation transformer it's a magic cure right away, in the very sense! It's a joy and when you look at the simple schematic it's a pure and simple physics behind the current separation via transformer solution.

So if anyone has discovered the same or similar stray current issues via shore power, an isolation transformer is by far the easiest and the most efficient way to fix this. Unless you had luck and it does not change over 10-20 mV via shore connection don't bother with compact diode based modern devices (zinc savers etc lots of products marketed, but they all do the same). Get a proper isolation transformer instead to instantly save the near-endless bulkhead stripping hassle, voltage drop comparing, many leak tests and bilge acrobatics with wires of finding the root cause of bonding spots. In my case worth the investment in spades, IMO.

Not only you will thank yourself, but your metal hull will thank you as well

Me 2c.

[s/v Jedi]

Congratulations, you took the superior option. This is why I put an isolation transformer in all my reference diagrams: it takes care of everything, even when the cause of trouble isn’t clear.

Also, you now have much better protection for crew aboard as well as swimmers around the boat. The shore power is now a very compact system from the boat inlet to the primary winding where it powers a magnetic field and that’s where it stops. Easy to keep that compact part of the installation shielded and isolated from everything else

Originally Posted by Briis

An update on solving that stray current issue and maybe a worthwhile lesson to any metal (steel or aluminium) Hull boat owners.

I did not manage to find the neutral to ground spot, nor did various experiments with zinc-saver device worked on reducing AC-induced stray currents. Once the shore power was connected the potential dropped -100 to 150 mV which was enough to lose zinc protection and hence stray current (read: agressive-) corrosion risk on the hull which you see surprisingly lot on metal hulls and owners often suspect wrong causes.

After lots of research I went with the best option - an isolation transformer. Mine's the Victron 7 kW isolation transformer model. Tried and tested old-school transformer-method, quite heavy, but it does what it's supposed to - it physically separates shore power from onboard power via transformer induction.

Once installed (super easy! Just find a convenient spot and put it somewhere between 3-shore power wires), and bam - all stay current issues gone! No matter how I connect or how much stuff (load) I run, electrolytic balance via silver cell measurement with the surrounding water remains the same. Hell, not even a millivolt difference! (Before it was almost hundred millivolts drop and somewhat depends on the load). So with the isolation transformer it's a magic cure right away, in the very sense! It's a joy and when you look at the simple schematic it's a pure and simple physics behind the current separation via transformer solution.

So if anyone has discovered the same or similar stray current issues via shore power, an isolation transformer is by far the easiest and the most efficient way to fix this. Unless you had luck and it does not change over 10-20 mV via shore connection don't bother with compact diode based modern devices (zinc savers etc lots of products marketed, but they all do the same). Get a proper isolation transformer instead to instantly save the near-endless bulkhead stripping hassle, voltage drop comparing, many leak tests and bilge acrobatics with wires of finding the root cause of bonding spots. In my case worth the investment in spades, IMO.

Not only you will thank yourself, but your metal hull will thank you as well

Me 2c.