Galvanic isolators

[doris]

How important is it to have one and how do they work.
I'm sure I really should know this but am deffo having a Saga moment on this one.

 

[VicS]

How important is it to have one and how do they work.
I'm sure I really should know this but am deffo having a Saga moment on this one.

If you are connected to shorepower for prolonged periods and the earth is bonded, as it should be, to the DC negative ( certain exceptions ) or the boat's anode system a GI is next to essential.

They work by blocking current from very low voltage, ie galvanic, sources but allow current from higher voltage sources to pass thus maintaining the safety features of the earth connection.

They do this with a simple circuit involving diodes but usually incorporate some extra frills

 

[theoldsalt]

How important is it to have one and how do they work.
I'm sure I really should know this but am deffo having a Saga moment on this one.

Take the trouble to "Google". There is lots of information available.

eg. http://www.yandina.com/galvanicIsolator.htm

 

[VicS]

Links:

http://www.yandina.com/galvanicIsolator.htm

http://www.power-store.com/view-item.asp?itemid=2065

http://www.smartgauge.co.uk/galv2.html

http://www.smartgauge.co.uk/galv_tran.html

http://sterling-power.com/collections/zinc-savers/products/zinc-savers-galvanic-isolators


More links:
http://www.boatus.com/seaworthy/galvanic/

http://safeshoremarine.com/

Getting in deep.... http://www.nmma.net/lib/docs/nmma/cert/techupdates/A-28_Galvanic_Isolators_standard.pdf

 

[doris]

Take the trouble to "Google". There is lots of information available.

eg. http://www.yandina.com/galvanicIsolator.htm

Ta for that.
I have had a Google and have now looked at that site as well. Trouble is that I only vague understand all of it.
To me it appears that a GI only protects me should something go wrong with the + side of my 240v system. If a boat nearby had a problem then neighbours will get stuffed regardless of having a GI, ditto should the pontoon have a problem.
Just a bit of head scratching in the deep midwinter.

Seems daft not to have one though, not a lot of money for some peace of mind.

 

[vyv_cox]

Ta for that.
I have had a Google and have now looked at that site as well. Trouble is that I only vague understand all of it.
To me it appears that a GI only protects me should something go wrong with the + side of my 240v system. If a boat nearby had a problem then neighbours will get stuffed regardless of having a GI, ditto should the pontoon have a problem.
Just a bit of head scratching in the deep midwinter.

Seems daft not to have one though, not a lot of money for some peace of mind.

I suspect you are not fully understanding it. Think back to school physics, making a battery. Two metallic plates in a solution, connect them together and a current flows (shown by a meter in the experiment).

Now, instead of the metallic plates substitute two boats. Connect them together with the earth cables of a 240 volt supply, which as Vic says are connected to the DC negative and hence to the anode and water. If both boats have good anodes, no problem. If the other boat has no anode it will be protected by your anode. If the earth from the power supply is connected to the pontoon your anode might be protecting that! Either way it will disappear at a far faster rate.

 

[Hadenough]

Sorry to hijack the thread but do you need a GI if the anode bonding is not connected to AC or DC earth?

 

[VicS]

Sorry to hijack the thread but do you need a GI if the anode bonding is not connected to AC or DC earth?

Its the connection of the AC system earth that is matters. If none of the boats systems are connected to it (apart from the AC distribution system of course) then a GI should not be necessary.

The Anodes may ( usually) or not be connected to the DC negative quite a part from the above.

The boat must have in RCD in the incoming shorepower supply if there is no bonding to the earth. The latest standards do not permit it not to be but earlier ones did provided the RCD was fitted.

Metal hulls must be earthed!

 

[VicS]

Ta for that.
I have had a Google and have now looked at that site as well. Trouble is that I only vague understand all of it.
To me it appears that a GI only protects me should something go wrong with the + side of my 240v system. If a boat nearby had a problem then neighbours will get stuffed regardless of having a GI, ditto should the pontoon have a problem.
Just a bit of head scratching in the deep midwinter.

Seems daft not to have one though, not a lot of money for some peace of mind.

It's not about things going wrong with the 240 volt system.

It's about preventing the galvanic corrosion, that Vyv describes, of vulnerable underwater bits and pieces caused by currents which the shorepower earth can conduct.

Take some time to read some of the links I posted and the sublinks they lead you too.

You cannot have looked at them let alone read them in 10 minutes.

Devote some serious time to studying them over a day or three.

 

[doris]

It's not about things going wrong with the 240 volt system.

It's about preventing the galvanic corrosion, that Vyv describes, of vulnerable underwater bits and pieces caused by currents which the shorepower earth can conduct.

Take some time to read some of the links I posted and the sublinks they lead you too.

You cannot have looked at them let alone read them in 10 minutes.

Devote some serious time to studying them over a day or three.

Spent shedloads of time over the weekend looking at stuff but will now seriously ponder those sites and your comments, all of which I thank you for. Being laid up the mo I have ample time, Ta.

 

[KellysEye]

Galvanic isolators are needed because galvanic corrosion is caused by stray current and is often called stray current corrosion. Stray current is normally a bad earth in the marina or a mix of fresh and salt water. Our boat had GIs on the earth wire of the 240 volt shore power and 12 volt house battery. Corrosion is exacerbated if the anodes are covered in Calcium and not working.

 

[VicS]

Galvanic isolators are needed because galvanic corrosion is caused by stray current and is often called stray current corrosion. Stray current is normally a bad earth in the marina

Galvanic corrosion is caused by dissimilar metals, ie metals well separated in the galvanic series, immersed in the same pool of electrolyte, namely seawater, being connected together electrically. This could be anodes on one boat and the steel hull of another, or the steel piling around the marina, connected by the earth conductor of the shorepower system.

or a mix of fresh and salt water.

Can you please explain how a mix of salt and fresh water is the cause. Surely it's just diluted seawater?

Our boat had GIs on the earth wire of the 240 volt shore power and 12 volt house battery.

Can you expalin the purpose of the GI in the 12 volt battery circuit. A GI will not block current originating from a 12 volt source

Corrosion is exacerbated if the anodes are covered in Calcium and not working.

Calcium reacts with water and will not form a covering on the anodes. Perhaps you meant to say calcium carbonate but Ca²⁺ ions are not attracted toward the anode in normal circumstances. It's the anions eg Cl^- which are attracted to the anode.
If you have calcium carbonate building up on the anodes something very odd, which requires investigation, is happening. Electrolysis perhaps.

 

[Plevier]

My boat has DZR skin fittings that are not bonded, a sail drive that is electrically isolated and has an anode, a bronze prop (FlexoFold) that is isolated from the sail drive output shaft and is claimed not to need an anode. The rudder shaft is stainless steel. The keel is a steel (or cast iron, not sure) blade fully encapsulated in GRP fairing with a lead bulb that is epoxied but undoubtedly has lead exposed underneath. The shore power earth is bonded to the DC -ve and to a keel bolt. There are no anodes other than the sail drive one.
In this situation, would a G.I. confer any benefit? I can't see it.

 

[lpdsn]

How important is it to have one and how do they work.
I'm sure I really should know this but am deffo having a Saga moment on this one.

I fitted one nearly three years ago. I bought a Safeshore one. I eventually fitted it between the shorepower E and the boat's grounded 12v -ve (based on some comments VicS made at the time). The 240V E is direct to shorepower Earth (I felt happier not having to rely on diodes for safety if/when I get electrocuted). I will make an exception for the Earth of the immersion heater in the calorifier, but there isn't one fitted at the moment (reason for the exception is concern about a galvanic path through the water system if I don't).

Anyway, the result is that my anodes seem to be lasting much longer. I didn't replace them last spring because there was well over 50% left after 12 months in the water.

 

[VicS]

My boat has DZR skin fittings that are not bonded, a sail drive that is electrically isolated and has an anode, a bronze prop (FlexoFold) that is isolated from the sail drive output shaft and is claimed not to need an anode. The rudder shaft is stainless steel. The keel is a steel (or cast iron, not sure) blade fully encapsulated in GRP fairing with a lead bulb that is epoxied but undoubtedly has lead exposed underneath. The shore power earth is bonded to the DC -ve and to a keel bolt. There are no anodes other than the sail drive one.
In this situation, would a G.I. confer any benefit? I can't see it.

No I cannot see any reason.

The bond to the keel bolt rings a bit of an alarm bell though.

Nigel Mercier, you may remember suffered a compelte loss of his sail drive. Different engine without saildrive isolation but isolated from DC negative

The main cause IIRC was a wiring fault from DC neg to engineblock (and of course the SD)

contributing cause was probably the DC neg to keel bolt connection

Not suggesting you disconnect it but do be careful not to compromise the SD isolation

 

[BabaYaga]

My boat has DZR skin fittings that are not bonded, a sail drive that is electrically isolated and has an anode, a bronze prop (FlexoFold) that is isolated from the sail drive output shaft and is claimed not to need an anode. The rudder shaft is stainless steel. The keel is a steel (or cast iron, not sure) blade fully encapsulated in GRP fairing with a lead bulb that is epoxied but undoubtedly has lead exposed underneath. The shore power earth is bonded to the DC -ve and to a keel bolt. There are no anodes other than the sail drive one.
In this situation, would a G.I. confer any benefit? I can't see it.

It sounds from this description as if the the DC system is totally isolated from the water or at least has a very poor connection (keel bolt to fully encapsulated keel).
If so, what is the purpose of connecting the shore power earth to DC -ve? Isn't the idea to provide an alternative path to ground (the sea) if the earth line to shore is faulty?

 

[Plevier]

Vic
The SD is physically insulated from the engine by plastic washers and bushes. The engine is grounded.
I do recall Nigel's problems but I don't see this system as comparable.

No I cannot see any reason.

The bond to the keel bolt rings a bit of an alarm bell though.

Nigel Mercier, you may remember suffered a compelte loss of his sail drive. Different engine without saildrive isolation but isolated from DC negative

The main cause IIRC was a wiring fault from DC neg to engineblock (and of course the SD)

contributing cause was probably the DC neg to keel bolt connection

Not suggesting you disconnect it but do be careful not to compromise the SD isolation

 

[Plevier]

Yes it's an interesting point. However if you think the supply earth and the DC neg should be linked, which seems to be the present view, what are you suggesting I should do (or Dehler should have done)? Put a copper plate on the hull?
I personally think that in a grp boat with a simple shore power system (charger, calorifier, one mains socket) and with an RCD the arguments for not linking the systems and not having a "water earth"are as good or better (so does my mate who is a well qualified sparks.)
Someone will say you can't rely on an RCD. Well I do at home, like many other houses I have a 2 wire supply, no supplier earth. I have an earth rod - but you can't rely on that to blow the supply fuse/breaker, nor can you rely on the water path to blow the supply to a boat!
House regulations have changed round too - you used to have to bond all extraneous metalwork in a bathroom for safety, now in the latest regs the system is not to bond it. Not an exact comparison, but it shows that the accepted wisdom can change.
.

It sounds from this description as if the the DC system is totally isolated from the water or at least has a very poor connection (keel bolt to fully encapsulated keel).
If so, what is the purpose of connecting the shore power earth to DC -ve? Isn't the idea to provide an alternative path to ground (the sea) if the earth line to shore is faulty?

 

[VicS]

Vic
The SD is physically insulated from the engine by plastic washers and bushes. The engine is grounded.
I do recall Nigel's problems but I don't see this system as comparable.

it was the DC neg connection to the keel that rang an alarm bell ... but only a quiet one,

Your situation is comparable in that in both cases the saildrive is isolated from the DC negative, just by different methods. In your case by insulating it from the engine. In Nigel's case by insulating the whole engine and saildrive from DC neg.

The primary cause of Nigel's problem was a fault on the wiring which then connected the block to DC neg. In your case a fault that connects your SD to the block or DC neg could have the same result. Hence the warning in your engine manual.

 

[BabaYaga]

Yes it's an interesting point. However if you think the supply earth and the DC neg should be linked, which seems to be the present view, what are you suggesting I should do (or Dehler should have done)? Put a copper plate on the hull?
I personally think that in a grp boat with a simple shore power system (charger, calorifier, one mains socket) and with an RCD the arguments for not linking the systems and not having a "water earth"are as good or better (so does my mate who is a well qualified sparks.)

Yes, I think a copper plate would be an option. Either that, or as you say, not linking the systems at all ( I don't, my shore power is also very simple). But I just think that linking AC earth to DC negative without providing a good path to earth (sea) means going half way, but with no real benefit.