Mixing Earthing Systems

[Wiredup]

A friend has a steel boat in a marina. It isnt connected to the shore supply so it doesnt have any 240v on board and the batteries are isolated. Ever since the marina installation was modified from TT to TN-S there has been 0.6 v dc between the steel hull sitting in salt water and the pontoon connected to the earth on the TN-S system. 17th edition mentions about precautions re earthing setting up electrolysis.
My theory is that the steel hull by virtue of sitting in the water, is essentialy TT via the water, silt and earth below. The TN-S earth is the sheave surrounding the phase and neutral, and a small emf is being induced (induction) in the earth conductor. This emf flows back to earth via the substaion earth connection, through the ground, into the water, into the steell hull, then across the water from the hull to the pontoon, into the TN-S earth. This completes the circuit.
The result is that the sacrifical anodes on the hull have wasted away, and the elecrolysis in now attacking the steel of the hull. The hull is negative and the pontoon positive.
Has any one come across this before? Comments please!

 

[thistle2009uk]

Hi WiredUp,
Your problem of electrolysis reminded me of the recent Richard Hammond Engineering Connections on the Sydney Opera House. My memory's sketchy to say the least but I recall they solved an aircon problem similar to yours by introducing a sacrifical metal (Zinc I think) into the circuit.
Apparently Mr. Columbus or similar had the same problem with his iron hulls in the 17th century.

I'm sure this may pave the way to solving your problem too. But don't take my word on it cos it's early in the morning and I've been the benefactor of several libations.

The programmes back on soon on the National Geographic channel anyway, so you can "check it out".

Hope this might have helped and sorry if it's utter rubbish.

Good sailing anyway.

Paul

A Har Jim Lad!

 

[ian.settle1]

Hi WiredUp,
Your problem of electrolysis reminded me of the recent Richard Hammond Engineering Connections on the Sydney Opera House. My memory's sketchy to say the least but I recall they solved an aircon problem similar to yours by introducing a sacrifical metal (Zinc I think) into the circuit.
Apparently Mr. Columbus or similar had the same problem with his iron hulls in the 17th century.

I'm sure this may pave the way to solving your problem too. But don't take my word on it cos it's early in the morning and I've been the benefactor of several libations.

The programmes back on soon on the National Geographic channel anyway, so you can "check it out".

Hope this might have helped and sorry if it's utter rubbish.

Good sailing anyway.

Paul

A Har Jim Lad!

They wher wooden hulls back in them good old days.

 

[solo2771]

i wonder if there would be any way of using this for free power? lol

 

[lister]

hi, you really need to be careful when it comes to boats as your boat will have a much better path to earth than any incomming supply, it also depends if were talking about salt water or river, invertor type etc etc, the box of tricks that will probably sort you out is a " Galvonic Isolator" have a read in boat type forums and within two nights you will probably know more than i do.
The other thing to consider is it may be coming from another vessel within 150M test again at more than one location ( or untie everyone else and wait 10 mins)

 

[Wiredup]

Thanks for that. The boat is steel with no 240 mains supply, so it is not connected to the electrical installation in any way. I've measured 0.6 v dc between the boat and the pontoon which is TNS, the boat is neg and the pontoon pos. It appears that Ground Loop is the condition thats creating the voltage. I would apprecaite if anyone can describe the circuit! I think its from the water to the boat, to the pontoon, down the CPC of the TNS into the spike at the substation, into the earth, through the earth and back to the silt and up through the water again. Any comments would be appreciated!!!

 

[Graeme Harrold]

hi, you really need to be careful when it comes to boats as your boat will have a much better path to earth than any incomming supply, it also depends if were talking about salt water or river, invertor type etc etc, the box of tricks that will probably sort you out is a " Galvonic Isolator" have a read in boat type forums and within two nights you will probably know more than i do.
The other thing to consider is it may be coming from another vessel within 150M test again at more than one location ( or untie everyone else and wait 10 mins)

Have a look at this months Practical Boat Owner on p60. It tells you how they work and how to make your own!!!!! Failing that, keep replacing the zinc annode, or fit a couple more.....

Re galvanitic reaction, its usually set up because of a direct connection from the boat to the pontoon, and not through the water, so a little confused on that. If there is a PD between the hull and the pontoon, its not going anywhere until its earthed (by direct connection) to the water via the hull

 

[loz1989]

i work for a company only doing caravan sites and marinas.
Marinas should NOT be on an tn-s or tnc-s system at all. They HAVE to be on tt systems, and backed up by an RCD at the socket and at the mains incoming position. This is to stop the situation your now facing happening, and also to protect the end user.

 

[Graeme Harrold]

i work for a company only doing caravan sites and marinas.
Marinas should NOT be on an tn-s or tnc-s system at all. They HAVE to be on tt systems, and backed up by an RCD at the socket and at the mains incoming position. This is to stop the situation your now facing happening, and also to protect the end user.

Well spotted..................

 

[loz1989]

not bad for a apprentice

 

[Wiredup]

Thank you for your reply. I agree with the TT although the 17th says TNS is ok. However, if TNS is used you are mixing it with the TT of the steel hull so setting up 2 earth equipotential zones. What is the company you work for?
Do you know of any refernece materal re not using TNS in marins? Cheers Peter

 

[loz1989]

i
which marina is itconcerning? would be interesting to know?

I'm not too sure on reference, as i say im only in my 2nd year so not too up on my regs yet.
me and electrician were testing a british waterways marina last week where the hook-up's were on tnc-s earthing.we rang our boss to see if he wanted us to change it their and then, or arrange another site visit with the client as was about 8pm when we rang.

he said it had to be changed so we had to put a rod down and fit a 200A rcd/mcb there and then. LOng day

 

[Wiredup]

Albert Dock Liverpool. I think Rolec put the original installation in with TT. Then a local contractor changed it to TNS hence the corrosion. The marina keep saying its to the regs which it is, but in the regs it mentions about avoiding electrolysis. They wont believe that the TNS is the problem hence requiring some info and documents to show them the error of their ways!

 

[loz1989]

i'll have a word with my boss, see if he can dirct me to the reg reference, surprised rolec put it on tt, their usually useless with earthing, one of my managers used to work for them but got fed up of them not listening

 

[Wiredup]

I realy appreciate your input on this. I'm trying to get them to get a specialist company in such as yours. Any info would be good and I will reccommed electro-tec to sort the problem. My email is [email protected]