Isolation Transformers

[EmeraldCoastSailor]

I recently suffered nasty damage to my iron keel from what pros at the yard are saying is no doubt caused by stray current. The damage happened fast (less than a couple of weeks as best I can tell). Based on what we know and see, the yard guys are guessing DC stray current was involved.

My boat has long had a galvanic isolator installed.

Question: will an isolation transformer prevent damage from 12 volt DC stray current? Seems like it connects into AC circuitry, perhaps similar to my simple galvanic isolator.

I guess I don't know the similarities and differences between isolation transformers and galvanic isolators. I am not confident I can cure the 12 volt source (not my boat).

[smac999]

the galvanic isolator and the isotransformer both should block DC current from the shore cord. so if your galvanic iso was working and wired currently you should not have had dc stray current comming from another boat, entering your boat via metal peice, and going back to dock via the shore cord.

the iso transformer will also block ac stray current. and has a few other advanages over a galvanic isolator

neither will prevent current from flowing in one part of the boat and out another. (this can happen even with no shore cord) they only prevent current flowing back to the dock via the shore cord.

it's also possible the boat has an onboard issue that it's creating itself and neither those things will help you.

stray current is a tough subject not many can master. I only know some of it.

[FlyingScot]

Isolation transformers are somewhat big, heavy, expensive and can be noisy. Your galvanic isolator may be broken. You can test for this.

Personally I recommend not plugging into shore power or to plug in only battery chargers and run from you inverter(s).

Your boat is the most likely cause of the stray current corrision.

[shimari]

There's another thread that is similar to this: https://www.cruisersforum.com/forums...ct-279664.html

Pay particular attention to post #5 from Boatpoker. You can get a device from Boatzincs that can test for stray current for around $140. The manual with it is very straight forward and easy to follow. I recommend you start there. If you find anything with your testing, it's time to bring in the experts.

Good luck!

[CharlieJ]

I am an ABYC Certified Corrosion Technician. It has been my experience that >95% of boats that suffer from stray current corrosion are the cause of said corrosion.

Testing for stray current sources on own boat is straightforward: use a reference cell connected to the B- bus to attain the baseline hull potential. Observe this value as every piece of DC equipment on the boat is operated (operated, not just energized to its switch), including the alternators. A significant (>60mVDC) positive swing in hull potential indicates that the device just energized is the source.

[SailingHarmonie]

Originally Posted by CharlieJ

I am an ABYC Certified Corrosion Technician. It has been my experience that >95% of boats that suffer from stray current corrosion are the cause of said corrosion.

Exactly ^^^^THIS^^^^

It is POSSIBLE that the stray current is coming from the neighbor's boat through the safety ground. It happens. It is far more likely the cause is much closer to home. An isolation transformer WILL block all externally caused stray current corrosion, but will do nothing for stray current you generate on your own boat. If your galvanic isolator is working it will do the same.

It is POSSIBLE that your galvanic isolator has failed and is letting DC current flow through the green wire on the AC Shore power system, but those two things are both fairly low probability. Again, not impossible, but not the first place I'd look.

You have to clean your own house first.

[GordMay]

A ground fault [even high resistance fault], in your bilge pump & switch wiring, is probably the single most common source of internal stray current.

[EmeraldCoastSailor]

Originally Posted by GordMay

A ground fault [even high resistance fault], in your bilge pump & switch wiring, is probably the single most common source of internal stray current.

This is something I have heard several times in my research on this mysterious situation. My shallow bilge is 100% dry 99.5% of the time. My float switch has never been in water aside from testing.

My port side neighbor's liveaboard boat on the other hand developed an internal freshwater leak a few months ago. He uses city water pressure so the leak is continuous. Every 20 - 30 minutes the bilge pump evacs a few gallons. I know for a fact the float switch is old, possibly quite old. Given my understanding of how float switches work, I infer the switch is always in water (can't get all the water out before switch shuts pump down). It is a hurricane damaged boat that hasn't moved in 5 years and likely has other issues. Interesting that my problem developed on the portside upper part of my keel shortly after the leak developed. I ordered a corrosion reference anode and plan on testing in the near future.

Two professionals that have vastly more experience than me and that know my situation in detail have indicated their best guess is stray 12 volt current from the neighbor's boat. But of course testing will be needed to figure it out for sure.

[EmeraldCoastSailor]

Originally Posted by smac999

the galvanic isolator and the isotransformer both should block DC current from the shore cord. so if your galvanic iso was working and wired currently you should not have had dc stray current comming from another boat, entering your boat via metal peice, and going back to dock via the shore cord.

the iso transformer will also block ac stray current. and has a few other advanages over a galvanic isolator

Two pros I have consulted with said that a galvanic isolator will only prevent stray DC current up to a point (strength of stray current). Both said that an iso transformer will do a better job of preventing stray current flow in situations where the current is strong. I think I am learning as I go but there are lots of opinions which I appreciate.

[SailingHarmonie]

You could get a copy of Nigel Calders book Boatowner's Mechanical and Electrical Manual. He has a detailed procedure in there for tracking down stray currents. It's either that, or hire somebody.

It's not a problem you can solve piecemeal, you have to approach it holistically. The topic takes dozens of pages to explain, anything you get here is necessarily incomplete and missing key details that might be important.

[maury]

Originally Posted by CharlieJ

I am an ABYC Certified Corrosion Technician. It has been my experience that >95% of boats that suffer from stray current corrosion are the cause of said corrosion.

Testing for stray current sources on own boat is straightforward: use a reference cell connected to the B- bus to attain the baseline hull potential. Observe this value as every piece of DC equipment on the boat is operated (operated, not just energized to its switch), including the alternators. A significant (>60mVDC) positive swing in hull potential indicates that the device just energized is the source.

Sounds important what you write, could you please clarify what you use for a “reference cell”.

[FlyingScot]

https://www.boatzincs.com/corrosion-...ode-specs.html

[maury]

Originally Posted by FlyingScot

https://www.boatzincs.com/corrosion-...ode-specs.html

Thank you, I was wondering about a tool of this nature.

[EmeraldCoastSailor]

Originally Posted by CharlieJ

I am an ABYC Certified Corrosion Technician. It has been my experience that >95% of boats that suffer from stray current corrosion are the cause of said corrosion.

Testing for stray current sources on own boat is straightforward: use a reference cell connected to the B- bus to attain the baseline hull potential. Observe this value as every piece of DC equipment on the boat is operated (operated, not just energized to its switch), including the alternators. A significant (>60mVDC) positive swing in hull potential indicates that the device just energized is the source.

I experimented with the corrosion reference electrode today. I didn't have coordination with boat owners, and my boat is on the hard getting repaired, so I was not able to test any vessel's hull potential and I wasn't able to turn off and on devices. I measured between .46 and .61 v DC with the electrode in the water and the positive test lead touching various pedestal outlets' ground. When I switched the multimeter to measure AC, the readings were between .5 and .72.

I have no idea if these readings are abnormal but I do plan on testing at other marinas to see if there is a big difference.

The education on this mysterious topic continues.

[CharlieJ]

#14
“Dock potential” of about 500mVDC is perfectly normal. The AC readings can vary significantly but are usually >500mVAC and are a function of how well the shoreside safety ground system is tied into earth and hence into the water.