Earth Leakage Issue

[duncan_ellison]

Hi Guys, this is an issue in a dock in EUROPE that I have been asked to weigh in on.

Several of the boats at one of the docks here have been reporting massive erosion of their anodes and propellors. Seems to be limited to 4 or 5 boats in a certain area. In one case, new anodes were fitted and the boat returned to its dock only to be hauled out again 10 days later for other for other reasons. Anodes were found to be already badly eroded having host around 25% of their mass.

Boats and docks are all wired European style 220V 32A. I used a commercial socket tester that checks for reverse polarity etc on all the dock sockets and found them to be good.

I then made up an adaptor with a male and female 32A plug and socket with the earth wire broken out and a multimeter interposed in the earth wire, then went down the dock putting this in between the boat cable and the dock outlet. All showed 0.00A, but one suspicious boat showed 0.48A AC, nearly half an Amp!

The results have implications, so I wanted to just sanity check my method. I can't think of any situation, for any reason why there would be current flow on the earth wire to the dock unless there is an issue on the boat.

My first thought is possibly a hot water heater with a bad element on that specific boat, but further testing is obviously required.

My question is, is this an absolute indication of a smoking gun or could there be another explanation for my results before I kick off a whole army of pointing fingers??

[smac999]

unless you have a mismatch of current on the hot and netreal, which you can’t in Europe due to rcds, The ground current is not from that boat. It is entering via the water and leaving via the ground cable. Using the boat as a path back to ground. It’s from the dock or another boat.


It’s pretty normal for me to see 50-200ma ac on most boats. 0.5a is pretty high though . I have seen as high as 15a leaving from a boat. . The boat beside it was taking in 2a of it and its props were destroyed.

Pretty sure ac current is not going to destroy zincs.

[duncan_ellison]

Great points, thanks.

The dock electrical system is <5 years old and all outlets have individual RCDs, so you are right that a L/N imbalance would be causing nuisance tripping and would be complaints about that.

I've made up a breakout cable, so I'll check further. I have an ampclamp, it's not super sensitive, but I figure that a leakage of this magnitude should give some indication.

But either way a 0.5A earth current is pretty bad right?

[boatpoker]

While AC current can cause corrosion it happens so slowly that you likely wouldn't notice it in your lifetime.

Sounds like someone is leaking DC current which can eat props and shaft in a matter of days. You need one of these to be absolutely sure and track down the issue ... https://boatzincs.com/cre/

[Kazamaran]

Sounds like DC current leak. Likely from a breach in the ground wire on someone’s boat; bilge pump, AC pump motor, etc, any 12 volt source in or near a wet bilge. Each vessel needs a galvanic isolator to block low voltage on and off their vessel. The dock is basically a battery. Galvanic effects of stray current will likely be more severe in boats at the end of the pier. Some low voltage stray current is creeping into the earth ground on the dock. .5 amps is profound. Doesn’t take much! Best bet is to unplug from the post until resolved. All sorts of ways to trap the current. Galvanic isolators and isolation transformers are the go-to’s. Simple method is to simply wire in two back to back bridge diodes into your ground wire before your breaker panel. That will block the transmission from the earth ground onto your boat and also keep any issues you may have on your boat. You can do that for about 30 bucks and a couple of beers. Or…you can go out and purchase a sexy 500 dollar galvanic isolator or a several thousand dollar isolation transformer. Each of which isolate stray dc current by, “wait for it….diodes! (and some other stuff). But the principle is exactly the same. Tell your friends to UN PLUG from the dock until resolved! Stray DC into the earth ground is nasty stuff! Good luck……….

[Statistical]

Originally Posted by duncan_ellison

Great points, thanks.

The dock electrical system is <5 years old and all outlets have individual RCDs, so you are right that a L/N imbalance would be causing nuisance tripping and would be complaints about that.

I've made up a breakout cable, so I'll check further. I have an ampclamp, it's not super sensitive, but I figure that a leakage of this magnitude should give some indication.

But either way a 0.5A earth current is pretty bad right?

The issue is that the leaking boat won't necessarily be the one showing any imbalance on the shorepower connection.

Boat A is leaking DC into the water it seeks a ground, it enters boat B through underwater metal connection to DC ground bus to AC ground bus to shore power connection. You measure current on boat B connection but it is caused by Boat A.

The first recommendation would be do the affected boats have galvanic isolators on shorepower connection. If not they should. If they do then check that they are still good.

It is good to try and track down the issue but boats are always coming and going the first step should be to ensure the boats are protected with galvanic isolator.

[boatpoker]

Originally Posted by Statistical

The issue is that the leaking boat won't necessarily be the one showing any imbalance on the shorepower connection.

Boat A is leaking DC into the water it seeks a ground, it enters boat B through underwater metal connection to DC ground bus to AC ground bus to shore power connection. You measure current on boat B connection but it is caused by Boat A.

The first recommendation would be do the affected boats have galvanic isolators on shorepower connection. If not they should. If they do then check that they are still good.

It is good to try and track down the issue but boats are always coming and going the first step should be to ensure the boats are protected with galvanic isolator.

A galvanic isolator is an absolute miniumum but it does not protect one from ones own boat.

In every corrosion survey I have ever done the subject vessel was the cause of its own problem.

[Statistical]

Originally Posted by boatpoker

A galvanic isolator is an absolute miniumum but it does not protect one from ones own boat.

In every corrosion survey I have ever done the subject vessel was the cause of its own problem.

Agreed it is often self inflicted however in the OP story four boats all at the same time went from normal zinc consumption to super rapid zinc consumption. Maybe they did all develop a stray current issue at the exact same time but that would be one hell of a coincidence.

[duncan_ellison]

So a bit more input to this equation:

I went all around the dock unplugging and checking the earth current on each boat. All showed exactly 0.00mA except for two adjacent boats which were 485mA and 270mA respectively.

I checked AC and DC and the current values were approximately the same, which was a bit of a surprise.

I then had each of the offending boat owners switch off the on-boat main breaker, which I imagine is two pole (i.e. not switching earth) and this made no difference to the readings in either case. But .... pulling the power cord at the boat inlet point zeroed both readings.

I then monitor the current on one boat whilst completely unplugging the other to see if it was current flowing between the boats, the 270mA stayed roughtly the same and the 480mA dropped to around 380mA.

Still not sure what to make of that, but I've advised both owners to unplug from shore power and use solar instead.

The guy who lost 20% zinc in 10 days from the skeg /P bracket zincs replaced them 10 days ago and will dive on them tomorrow to see it there have deteriorated. Obviously, I've advised them to go somewhere else and not enter the water in the marina

[janis]

Originally Posted by boatpoker

A galvanic isolator is an absolute miniumum but it does not protect one from ones own boat.

In every corrosion survey I have ever done the subject vessel was the cause of its own problem.

My boat currently has only two wire shore power connection (live and neutral). Does that mean it's protected from galvanic corrosion even without galvanic isolator?

[CharlieJ]

@janis #10:
No. It means that without a safety ground wire returning fault current to the source (a transformer on land) that your boat is unsafe.

[janis]

Originally Posted by CharlieJ

@janis #10:
No. It means that without a safety ground wire returning fault current to the source (a transformer on land) that your boat is unsafe.

I do realise risks involved due to missing earth cable but my question was more related to galvanic corrosion. I was wondering if I understand it right that earth cable is needed for galvanic corrosion to occur due to connected shore power?

[CharlieJ]

#12
Galvanic corrosion will occur whether or not you have a safety ground wire. The cathodic protection current produced by your sacrificial anodes will not leave the boat.

You really need a safety ground wire.

[boatpoker]

Originally Posted by janis

My boat currently has only two wire shore power connection (live and neutral). Does that mean it's protected from galvanic corrosion even without galvanic isolator?

I concur with Charlie. You have a dangerous situation.