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Shore Power/Galvanic Corrosion


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I know this subject has been covered many times. The problem is it all gets so technical with opposing views that I am never quite sure what the real answer is (if indeed there is one). Anyway, I have shore power, it is run through an RCD but the installation is not earthed to the hull. The arrangement is basically an extension lead - the boat is not permanently plugged in. There is nothing ever attached to the shore power that is connected to the boat (other than the  battery charger which is rarely used as I rely on solar and the charger is not permanently plugged in). I use the shore power to run electric tools and the hoover when necessary and that is about it. Need I be concerned about GC?

Thanks

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I am trying to see how a line short to the hull would trip the RCD or MCB. The hull would become live but depending upon the conductance of the water there might not be enough inbalance between L & N to trip the breaker. If the breaker did not trip then you just might get a shock when stepping on or off the boat. The hull bond is there to ensure the RCD does trip.

My unqualified view would be to fit a good quality galvanic isolator WITH a monitoring device built in and bond the hull. Others may differ. I would never have anything other than a true extension lead without a hull bond.

I am much more concerned about the potential danger such a setup poses than galvanic corrosion. Taking your use as stated I dpoubt there will be much to worry about GC wise but if the usage pattern alters _______?

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38 minutes ago, lampini said:

In-line rcds are available, use at power source. swop 13a for 16a ceeform where exposed to the elements/damp environment. Best be safe!

That will protect against shorts the the earth wire but I do not see how it will stop a hull going live with a short to the hull if the water & hull coating conductivity is not high enough to give sufficient unbalance in the N &L to operate the RCD.

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29 minutes ago, Tony Brooks said:

That will protect against shorts the the earth wire but I do not see how it will stop a hull going live with a short to the hull if the water & hull coating conductivity is not high enough to give sufficient unbalance in the N &L to operate the RCD.

An IT installed on the boat would have the same issue tho.

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9 minutes ago, Robbo said:

An IT installed on the boat would have the same issue tho.

Yes without the hull bond. In my view the vital thing missing is the hull bond. After that we worry about GC and for the OP's stated use a GI with status monitor would be perfectly adequate.

Once you ensure a line to any type of conductor including the hull short can reliably connect to ground any RCD in line, on post, or in boat will be fine.

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11 minutes ago, Tony Brooks said:

Yes without the hull bond. In my view the vital thing missing is the hull bond. After that we worry about GC and for the OP's stated use a GI with status monitor would be perfectly adequate.

Once you ensure a line to any type of conductor including the hull short can reliably connect to ground any RCD in line, on post, or in boat will be fine.

But with a IT the hull isn’t bonded to the earth shore cable, so the same risk exists if the shore cable shorted on the hull.

Edited by Robbo
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Just now, Robbo said:

But with a IT the hull isn’t bonded to the earth shore cable, so the same risk exists.

No but the boat's RCD is on the side of the IT that should be bonded to the hull so it will work. I agree that if the shoreline side of the IT shorted to the hull the boat's RCD would not work and the bollard RCD also may not. This is why some advise that an IT should ideally be in a waterproof box OFF the boat.

I think that as far as the OP is concerned he has his answer. We are now getting into the complications he mentioned in hos first post.

It is all trying to balance risks. A GI without status monitoring or one with status monitoring can fail and either leave the boat unprotected against shorts or permanently connected to earth. With the Ops stated use I think a GI would be fine as long as he checks the status monitor.

An IT is unlikely to fail in that way so is probably better for long term unattended use but depending upon how it is wired a wiring fault could make the hull live so that is what the installation must guard against by either having the IT off the boat or if on the boat keeing the shoreline to IT leads as short and a swell supported as possible and ideally run in insulated trunking.

 

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If you're sure that your boat is not hull-earth bonded you need not worry about galvanic corrosion. However, as the others have said you've got bigger problems to consider. You're sitting inside a steel box floating on the water with live mains cables running from outside to inside. If a cable chaffed and your hull were to become live it wouldn't be at the same potential as earth. Anyone on board or on the bank would be fine until they stepped on or off the boat. 

I've used this analogy on this forum before, but would you be happy to use an old power tool with a metal case if you knew that the internal mains earth bond to the case had come loose and was no longer connected?

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4 minutes ago, Traveller said:

Thanks all. The wiring has always been this way and boat safety has never questioned it - not that that makes it right. I will heed your advice and run the earth to the hull.

But then you will be risking galvanic corrosion unless you fit a GI or IT. How big that risk is no one can say without testing the difference between the then hull potential and the ground/water potential. Even so the can alter so what is unlikely  to cause damage today may be different next week/year. Even a fault on an adjacent boat could cause problems. This is why I suggested an earth bond plus GI.

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4 hours ago, blackrose said:

 

I've used this analogy on this forum before, but would you be happy to use an old power tool with a metal case if you knew that the internal mains earth bond to the case had come loose and was no longer connected?

Yes but only if it was connected to an rcd

in the case of op using an extension lead to power say a old power tool with or with out an earthed metal case how is he going to get  a shock ?

if the cable chafes and makes the hull live, then he is in no danger until he provides a path to earth. as he steps off the boat any current from live hull to earth trips the shore rcd - thats kind of the point...

Not saying that's ideal of course and when I used to use an extension lead I routed it carefully to minimise any risk of damage.

and you need to sure of the rcd so if you're in doubt either test it or fit one in line

Best solution is a proper shoreline install with gi or it and on board consumer unit and hull bond no argument on that 

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I now see boats coming in with severe galvanic corrosion on an almost weekly basis. Most of these HAVE galvanic isolators and most are marina based. 

We had a 50 year old boat in last week with minimal corrosion( and nil waterline rust), moored àt an online marina and NO 240V onboard. 

I have not yet seen a boat come in that uses an isolation transformer,  in 5 years of doing hull observations

I can only recommend spending a bit more on your boat and getting it shot blasted and either 2 packed, comasticked, or zinger ed. These all seem to protect the hulls more than bitumen and a GI

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Thanks, we are beginning to get back into the world of if, buts and maybe - which is just how I have found all threads on shore power and galvanic corrosion. That is why I framed my original post the way I did. The point is I need electricity infrequently and even if I am on board at the mooring for a few days I doubt if I would be plugged in for more that two or three hours a day and even if plugged in  it is likely that nothing would be plugged into the live plugs for much of that time. Anything that was plugged in would not be connected to the boat in any way - save for the earth between shore power and hull (when created). The exception is the battery charger which is used rarely. From what I am reading now it seems the best answer is to just rip it out but that would result in running extension leads from shore to boat!

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The simple answer to your original question is no, galvanic corrosion will not cause you a problem. GC is a slow process with these sorts of stray voltages, so even if your boat's 12v negative is bonded to your steel hull and to the earth from your shore power, so long as you are only connected to shore power for short periods, you won't notice any additional corrosion.

In any event, it will be your anode that will 'dissolve' first, before your bronze propeller, stainless steer shaft or mild steel hull suffers. 

 

 

 

 

 

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Matty is simply telling us what he has found working on hulls. That is all. What you have to grasp is that as with many electrical things there are so many ifs and buts it is simply impossible to give what I expect you would call a straight answer.

Unless you can tell us about what appliances are being used on all the boats around you, the conductivity of the water, your hull coating on all the wet surfaces, what you are tied against, and the potential difference between the mains earth terminal and the ground/water only an idiot would give definitive advice.

You are correct that all the time your boat is not connected to the shoreside earth no damage from mains induced galvanic corrosion should occur. However extrapolating from that to say that X amount of damage will occur when connected to a shoreside earth is not possible. With a 100% insulating hull coating no damage should occur. A minor scratch in such a coating will concentrate any damage in that one area so it could be bad when compared with the same damaged spread out across a whole baseplate. I you are in very conductive water close to very heavy power users damage is likely to be more than in clean water way out in the country.

A further complication is that any mains appliance on board using a switched mode power supply is likely to put a small potential on the earth line. This builds up as more are added. Typical users are computers, battery chargers, TVs etc. Other people's equipment on other boats can add to this and increases the potential. Remember this while I talk about galvanic isolators.

A GI is nothing more than two or three diodes in series with another set in parallel to them but in reverse. These diodes SHOULD be very carefully selected and mounted to ensure they can cope with the several thousands of amps a fault current may produce. If they are not then one fault incident may cause them to "blow" that is fail open circuit in which case you get no earth protection or short circuit that then gives no galvanic corrosion protection. This is why it is vital to buy a GI that has inbuilt monitoring device. Just to muddy the waters further any GI will conduct when the potential between hull and earth cable is high enough so those switched mode power supplies and poor shoreside installations can render a GI ineffective - again hence the need for monitoring so you know when its conducting.

For many years the UK and EU had NO standards that GI builders were required to meet so there is a real possibility that some of Matty's boats had GIs that had failed under fault conditions, had no monitoring, and were never tested so they were no better than a length of wire.

Those diodes cause some voltdrop so as long as the voltdrop across the diodes is higher than the difference in potential between hull and earth cable they will not conduct but when a fault occurs they do and thus protect you. For many years GIs uses sets of two diodes but now they tend to use three so they can isolate a higher galvanic voltage. It would not surprise me to find that many of the hulls Matty is talking about have two diode (four in total) GIs and they have been driven into conduction by modern onboard equipment.

If you (which you are not) going to leave a boat plugged into a shore supply for long periods when you are away then in my view an Isolation Transformer is the safest way to go. However you are not in that position. With your usage pattern on many moorings all over the country any galvanic damage could well be all but insignificant without any form of protection but we can not be sure. I come back to what I said originally. Based on what I know and on your stated use a hull bond is vital for safety and to minimise the danger this brings from galvanic corrosion use a modern GI with monitoring built in. Otherwise the only way to be as sure as possible about minimising the potential corrosion is to use an IT but in your case, and only your case,  I think that is overkill.

My boat is a 12V only boat with a small inverter to plug chargers into. That is all and I use an extension lead on the odd occasion I need mains power either from a generator or shoreline.

Edited by Tony Brooks
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If you are only connected to shore power for an hour or two occasionally, investing in a galvanic isolator is not economically justifiable. All it will do, if it is not faulty, is to prevent an insignificant amount of degradation to your anode. 

Your anode might be a tiny bit thinner when the time comes to replace it, but a few hours connected to shore power will not be enough to significantly reduce its useful life.

If your anodes 'fiz' when you connect to shore power you should change marinas!

Forget isolation transformers.: They are very expensive, can consume a significant amount of electricity just by being powered up and to avoid bringing shore power onto your vessel, should be mounted ashore. If they can be justified at all, it would only be by owners of boats that are frequently/permanently connected to shore power.

 

 

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I think the above reply is potentially misleading but am happy to be proved wrong.

Anodes are used to protect against the corrosion caused by dissimilar immersed metals forming a cell with the anode as one pole of that cell thereby forcing the anode to corrode rather than the steel or alloy metals in the hull fittings. They are not used to protect against the type of corrosion we are talking about although in certain circumstances they might.

Anodes also have a somewhat limited area of protection with about 2 metres all round being often quoted.

The corroding electricity will leave the hull at whatever point that gives it the lowest resistance path through the water to the land or whatever is electrically connected to the land like a metal jetty or piling. There is no way of forecasting where that point will be. If it just happens to be an anode then all Bargebuilder says will be true but this is unlikely. Think about the area of the anodes on a typical narrowboat and then think about the total wetted area. That will give some idea about the chances of an anode protecting against this type of corrosion.

An anode is not some kind of "electricity magnet" that pulls the current in the hull that causes THIS type of corrosion to itself. so it cannot protect against it. The current will flow where it wants to and I would suggest that the weld to the hull and then the joint between anode materiel and steel bar would slightly raise the electrical resistance of the path through the anode and make the anode protecting against this type of corrosion less likely.

Over and above that except in the very worse cases I doubt the corrosive current this topic is about will be enough to make an anode fizz to the naked eye, even it the current did happen to be leaving the anode. A much more likely explanation for fizzing anodes is being is salt or brackish water with magnesium anode material. A trip to Liverpool with a pure 12V boat is enough to prove that.

  • Greenie 1
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Hi Tony,

You are wise to question my logic, but:

Anodes on a steel vessel should be spaced such that all the steel is protected and in my experience this is usually the case.

The anode is more noble, so if the galvanic circuit is between the vessel and the pile, the galvanic action will be dominated by loss of aluminium or magnesium over steel, all the time that either is in the locality, even if the steel was uncoated, which of course, it wouldn't be.

The weld between the stud and the anode would not have a significant effect because the current will be too tiny: Tiny increases in resistance would only be a significant factor at high currents. 

I would be horrified if there was a large potential difference between the pontoon and the boats hull. 

I was trying to keep this simple and I still insist that a few hours use of shore power could not possibly justify the cost of a gi or an si in terms of potential damage that might be done to the steel of the hull.

 

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