Electrickary

[Bat21]

Sorry if this has been covered before, I have tried searching and found lots of connected posts but nothing which gives a definitive answer.

I have just bought an Aquador 25c, their is no shore power fitted and I don't think that the skin fittings are bonded, the only anodes are those on the outdrive and trim tabs.

I intend to have shore power fitted with a merlin consumer unit, galvanic isolator, circuits for immersion heater, battery charger and a couple of 13a sockets.

What earth bonding should be specified?

None?

Should all the skin fittings be bonded? If so, to the ac earth and/or to a hull anode or both?

Should the engine and drive be bonded ?

Should the 12v Neg connected to the AC earth.

Thanks

 

[PCUK]

I like to keep the AC and DC separate but others may differ.

 

[rafiki_]

You will find opposing views on this. The fittings are bonded on my Azimut, they weren't on my previous boat.

 

[David2452]

From your post I assume you will be having it professionally fitted so it depends whether the installer works to (current) BSI standards or not. If they do then they must be connected if not then it will depend on the installer's opinion. Opinions vary, standards do not, though of course they do change and this is a recent change.

 

[Viscount]

I fitted a galvanic isolator a few years ago and found an immediate improvement by way of reducing corrosion on the props. I have the -12V and the boat side of the AC earth joined at the boat side of the isolator itself the idea being I could isolate the two if I wanted to and also measure if any flow there - never have though. My fittings are non-metallic except the seacocks for the heads which are connected via plastic pipes - they are not bonded. All seems fine. In the early days of ownership, the RCCB I fitted when I got the boat tripped regularly - eventually found it was a duff RCCB.... Anyway, before I realised what the fault was, I fitted 13A outlets behind the main panel and plugged in leads from there to all mains operated equipment - i.e. charger, hot water immersion, outlets; it's an easy way of isolating to fault fund (or remove something that's got wet). Good luck.

 

[Bat21]

From your post I assume you will be having it professionally fitted so it depends whether the installer works to (current) BSI standards or not. If they do then they must be connected if not then it will depend on the installer's opinion. Opinions vary, standards do not, though of course they do change and this is a recent change.

Hi, whilst I may employ a "professional" to fit the system, I rather like to understand what the safest and most effective would be, my concern is that as their is no hull anode, the installation would be reliant upon the outdrive as a primary route to earth, is this wise?

 

[VicS]

From your post I assume you will be having it professionally fitted so it depends whether the installer works to (current) BSI standards or not. If they do then they must be connected if not then it will depend on the installer's opinion. Opinions vary, standards do not, though of course they do change and this is a recent change.

You are saying that the new standard requires all seacocks/ skinfittings to be bonded, along with the anodes and DC negative (where it is not an isolated presumably) to the shorepower earth ?

Ouch ! that is a problem because from the corrosion protection point of view it is recommended that the skin fittings etc should not be connected to the anodes, at least not to the same anodes as other other parts that require cathodic protection by anodes.

 

[David2452]

No Vic, sorry for any confusion I was referring to the final question on Extra low voltage DC & low voltage AC, should have been clearer.

 

[Keith-i]

Ouch ! that is a problem because from the corrosion protection point of view it is recommended that the skin fittings etc should not be connected to the anodes, at least not to the same anodes as other other parts that require cathodic protection by anodes.

It would be very helpful to have a brief summary of what should and should not be connected. On my boat the DC neg is connected to the AC earth with a galvanic isolator. Should there be any link between the AC earth and the bonding wire linking the main anode to the various underwater metal lumps?

I assume bronze skin fittings connected to plastic pipes do not need connecting to the main anode. What about the bronze rudder for example and the prop shaft if it has it's own shaft anode?

I realise there are two different questions above but they do get interlinked often

 

[Bat21]

So, the DC neg is connected to the AC earth at the boat side of the Galvanic isolator. The bronze skin fittings are not bonded to anything as they are isolated by their connection to plastic pipes, the outdrive and trim tabs are protected by their own anodes.

Is it that simple?

 

[VicS]

No Vic, sorry for any confusion I was referring to the final question on Extra low voltage DC & low voltage AC, should have been clearer.

Thanks for clarifying ..... had me worried there for a bit.

 

[KevB]

My DC earth and AC neg are connected. My skin fittings are bonded.

 

[boatmike]

There is either a degree of confusion here or I have my theory wrong. Please correct me if I have misunderstood but the following needs comment and knowing this forum I will soon be corrected if I am wrong
Firstly any metal part, skin fittings or anything else, is at risk of electrolytic corrosion regardless of it being connected electrically to other parts of the boat or not. The fact that plastic pipes are used does not remove this risk.
There are various schools of thought about skin fittings being bonded together or not, but the idea of doing so is to give them connection to external anodes that give them protection. My own thought is that bonding them often helps and is seldom harmful so do it.
Secondly and quite seperately adding AC systems introduces a risk of electrocution which demands that the earth be connected to seawater which is usually achieved by connecting to this bonding system on small craft that do not have a seperate external grounding plate.
Doing so puts the metal components at increased risk of galvanic corrosion when on shore supply so this AC earth is connected to the bonding system through a galvanic isolator. It follows therefore that only the AC earth should be connected through this isolator not the DC system ground.

 

[Bat21]

Please allow me to add to your confusion, as I understand it, underwater metal fittings in a grp hull are not at risk unless they are connected in some way to a conductor. By bonding you are providing a conductor and placing them at risk.

The Galvanic isolator, isolates the boats AC earth from the shore earth unless it detects electrical activity in which case it connects to ground. ( I may have this the wrong way round)

The bit I don't understand is connecting the DC neg to the AC earth. surely if you connect the 12v neg to the AC earth you are effectively providing an ac earth path through the boats machinery to the water.

 

[boatmike]

Please allow me to add to your confusion, as I understand it, underwater metal fittings in a grp hull are not at risk unless they are connected in some way to a conductor. By bonding you are providing a conductor and placing them at risk.

The Galvanic isolator, isolates the boats AC earth from the shore earth unless it detects electrical activity in which case it connects to ground. ( I may have this the wrong way round)

The bit I don't understand is connecting the DC neg to the AC earth. surely if you connect the 12v neg to the AC earth you are effectively providing an ac earth path through the boats machinery to the water.

With respect I think you are adding to the confusion indeed! If metal components were not susceptable to electrolysis without connection to other metal parts protecting them would be easy and anodes unnecessary. Electrolysis happens when stray currents flow in an electrolyte (salt water in this case) between two metals with different potential. Zinc in particular being near one end of the galvanic scale is particularly susceptable and a lump of zinc dangled on a string over the edge of a steel pontoon will quickly fizz away without any direct connection to the pontoon.
The Galvanic Isolator insulates the vessels AC earth (which when on shore supply is connected to shore earth directly) from the vessels seawater ground unless there is a short to earth when it will indeed allow the current to pass.
Therefore regardless of any DC grounding or bonding there should always be a proper grounding of AC systems to seawater. The purpose of the isolator is to "isolate" this earth from the seawater unless a current of significant amperage flows, therefore avoiding stray earth leakage adding to any other galvanic problems that exist. My point is that IF the DC system is bonded to the vessels skin fittings etc this connection should be made seperately and directly, not through the isolator. The preferred system on larger GRP vessels is to have a seperate AC hull grounding plate, but where this does not exist the DC ground to hull fittings suffices

 

[Bat21]

Now I'm even more confused!

You are talking about galvanic action from stray currents outside the vessel but, bonding the fittings could facilitate electrolytic activity from within the vessel would it not?

I have looked at quite a few wiring diagrams haven't seen any with an AC earth other than via the shore lead or effectively the dc neg. I guess if you complicate things by having an AC source on board such as generators or inverters then a dedicated AC earth would be required.

 

[boatmike]

There are a lot of technical sites on the internet on this subject. I would suggest you look up galvanic isolation and electrolysis as you don't seem to understand either very well. As a quick answer to your questions however you need to ground both on board generators and shore supplies to a grounding plate of some kind on the hull. Relying on the shoreside earth is not just a very suspect philosophy, it could get you killed.........

As I said before, the subject of bonding causes some confusion and there are various opinions regarding the requirement to bond all skin fittings. The actual protection it gives is often not a great deal if they are a long way away from the nearest anode and if your fittings are bronze or non metallic its probably a waste of time. There is no way however that it can "facilitate galvanic action from within the vessel" and in fact this is a pretty meaningless concept. You bond things to an anode to protect them from electrolytic corrosion from outside the vessel not within.

As I said....plenty of articles on the net.......

 

[Bat21]

I actually do understand and with respect you confirm what I understood.

To quote you, "if your fittings are bronze or non metallic its probably a waste of time"

I do however struggle with the rational of connecting dc neg to ac earth, surely this gives the potential of shorting any dc power source to any ac appliance.

 

[Keith-i]

I think the theory behind connecting dc neg to AC earth is a form of safety bonding similar to your bathroom at home. It means that if an AC cable chafes and makes a metal component live then it will trip the fuse as that component would provide a source to earth sufficient to blow the fuse. This assumes the metal parts are connected to dc neg which in the case of your engine and other major lumps of metal, they normally are.

I disagree about isolated lumps of metal suspended in seawater being at high risk of corrosion. They may well corrode due to impurities in them, but it would be exponentially exacerbated if that component has a low impedance electrical link to a more noble metal. Zinc is pretty much in the middle of the galvanic scale for the types of metal we have. High to low - magnesium, aluminium, zinc, iron, tin.

So then, should the main anode be bonded to the dc neg and therefore AC earth or not?