bonding the boat ground(240v earth) to the 12v negative terminal

[RIN]

Hi all
Following on from a recent thread about helm indicators. What does the forum think should be done? a floating 12v negative system or one bonded to the boats 240 earth system?

 

[rogerthebodger]

Hi all
Following on from a recent thread about helm indicators. What does the forum think should be done? a floating 12v negative system or one bonded to the boats 240 earth system?

Have a read of this thread

http://www.ybw.com/forums/showthread.php?456670-battery-and-shorepower-(unrelated)-questions

And my posting #6.

My view is that there must not be any connection between 12 VDC negative and mains earth.

This has been discussed at great length on the reader to reader forum over the years.

 

[superheat6k]

A lot argue against it, but there is a clear safety argument for it, and most standards that say anything will require this.

Nigel Calder provides an excellent explanation of why 'yes' in his Boatowners Mechanical & Electrical Manual.

Whereas Nigelmercier of the PBO and Scuttlebutt parishes has a good argument for not doing so.

 

[Lon nan Gruagach]

Have a read of this thread

http://www.ybw.com/forums/showthread.php?456670-battery-and-shorepower-(unrelated)-questions

And my posting #6.

My view is that there must not be any connection between 12 VDC negative and mains earth.

This has been discussed at great length on the reader to reader forum over the years.

Roger puts forward the best reason and explanation for why not to connect, it can cause problems and should never be needed.

But from a domestic wiring point of view the mains wiring regulations state that all exposed metal must be bonded to mains earth, including (and this is my favourite example) the chromed plastic drain insert in a plastic shower tray. The reason is that such a bond will fire the RCCD when you drop your hair drier while having a shower.. So, if you want to go with the dont connect -12V to mains earth school then you really ought to make sure that none of your 12V system is exposed metalwork, including the engine. its doable and also good practice.

 

[jfm]

...make sure that none of your 12V system is exposed metalwork, including the engine. its doable and also good practice.

I'm confused a bit. Are you saying it is doable to make sure your engine and machinery has no exposed metalwork? (I can't see how you could do that!)

 

[Lon nan Gruagach]

I'm confused a bit. Are you saying it is doable to make sure your engine and machinery has no exposed metalwork? (I can't see how you could do that!)

Not cover the engine, but use isolated alternator, starter and sensors, then the engine is it self isolated from the 12 V and so you can earth it without connecting -12V to mains earth.
That is if you want to follow domestic regs and safeguard yourself against stray mains electricity.

 

[kashurst]

You should find that your 12V -ve terminal is ultimately connected to your anode bonding system. From a 12/24V point of view that is necessary to provide an "earth" for your VHF radio and is also unavoidable because your shafts/drives are connected via your gearboxes to your engine, connected to your 12V -ve. (albeit potentially intermitent/poor). It is also there to provide lightning protection from all the exposed metal work on the boat. If you break that connection by removing wires, trying to isolate etc - they are still connected to your anodes through the water.

Your on board mains also needs an emergency earth in case of a fault - that too should end up connected into your anode circuit to create an emergency earth into the sea/water. What you need to consider is isolating shore power earth from boat earth as the two can be at different potentials that can create issues with your underwater metal work. So go down the galvanic isolator/transformer solution.

Trying to truly isolate the two sets of electrical systems will create lots more problems. Done badly will create even more. If you are worried about mains voltage issues fit residual current breakers (RCDs) (you should already have one) and your mains earth will be effectively redundant but there just in case.

PS be careful using building wiring regs on a boat - a boat in water is electrically speaking quite different to a building on land. Have a look at the american boat and yacht council specs or the CE approval requirements for boats.

 

[KevB]

You should find that your 12V -ve terminal is ultimately connected to your anode bonding system. From a 12/24V point of view that is necessary to provide an "earth" for your VHF radio and is also unavoidable because your shafts/drives are connected via your gearboxes to your engine, connected to your 12V -ve. (albeit potentially intermitent/poor). It is also there to provide lightning protection from all the exposed metal work on the boat. If you break that connection by removing wires, trying to isolate etc - they are still connected to your anodes through the water.

Your on board mains also needs an emergency earth in case of a fault - that too should end up connected into your anode circuit to create an emergency earth into the sea/water. What you need to consider is isolating shore power earth from boat earth as the two can be at different potentials that can create issues with your underwater metal work. So go down the galvanic isolator/transformer solution.

Trying to truly isolate the two sets of electrical systems will create lots more problems. Done badly will create even more. If you are worried about mains voltage issues fit residual current breakers (RCDs) (you should already have one) and your mains earth will be effectively redundant but there just in case.

PS be careful using building wiring regs on a boat - a boat in water is electrically speaking quite different to a building on land. Have a look at the american boat and yacht council specs or the CE approval requirements for boats.

Glad you said all that as that's how mine is including the RCD and Galvanic isolator

 

[rogerthebodger]

You should find that your 12V -ve terminal is ultimately connected to your anode bonding system.
.

Why there is not necessary to connect the 12VDC -ve to the anode system and the current think is to have no bonding of metal underwater fittings at all. This is IMHO a myth and promoted you the anode suppliers. It can infact cause more problems by the possibility of allowing induced currents to flow causing more corrosion.

From a 12/24V point of view that is necessary to provide an "earth" for your VHF radio

The radio earth is and AC earth and not DC so no DC connection to VHF is needed and In any case most VHF aerial have a counterpoise coil with no DC connection

and is also unavoidable because your shafts/drives are connected via your gearboxes to your engine, connected to your 12V -ve. (albeit potentially intermitent/poor).

My shaft and prop is totally isolated from my gearbox/engine by having a flexible coupling and by cutlass bearings and PSS seal also isolate the shaft/prop from the hull which in my case is steel which the standards deal with but most people just ignore

It is also there to provide lightning protection from all the exposed metal work on the boat. If you break that connection by removing wires, trying to isolate etc - they are still connected to your anodes through the water.

Have you ever had a direct lighting strike. I have at home and it just melted the 10 mm sq earthing wire from my TV aerial and destroyed my TV which landed in my swimming pool. So I would not expect any anode bonding wire to last very long in a direct lighting strike. If you wish proper lighting protection you need to divert and lighting away from the inside of your boat and around the outside incase it blows one of your skin fittings out and sinking your boat.

Your on board mains also needs an emergency earth in case of a fault - that too should end up connected into your anode circuit to create an emergency earth into the sea/water.

Again why and emergency earth only works if you have a guaranteed earth point at the substation/transformer. What you need is a way to ensure the wired earth from the shore power supply is good. I have Neons across mains live to earth wire and mains neutral to earth wire that will show good earth and no live / neutral reverse.

What you need to consider is isolating shore power earth from boat earth as the two can be at different potentials that can create issues with your underwater metal work. So go down the galvanic isolator/transformer solution.

Unless you have a steel (conductive hull) with metal cased appliances bolted to the hull there does not need to have a boat earth as such as you are in fact inside a double insulated appliance. So where could you cone into contact with any part of the hull that will conduct electricity ( except in a metal hull boat) even the metal fittings are insulated from each other unless they are bonded which is another reason to not bond.

You only need a GI or isolating transformer if you have a metal boat or all your underwater fittings are bonded. No bonding as current thinking) no GI ot IT on a GRP boat, and no anodes for that matter except the propshaft if the shaft and prop are different metals.

Trying to truly isolate the two sets of electrical systems will create lots more problems. Done badly will create even more.

I don't see why that should be a problem, its only metal cased mains appliances the need a earth wire back to the incoming earth.
The mains and low voltage wires should be run in separate trunking anyway so there cannot be any chafe of wires to cause intercuit shorts.

If you are worried about mains voltage issues fit residual current breakers (RCDs) (you should already have one) and your mains earth will be effectively redundant but there just in case.

IMHO no mains system should be without a RCD in fact when I lived in UK my house did not have one as it was not a requirement in UK but in South Africa it was mandatory.

Even with a RCD the earthing of metal cased appliances is not redundant as it is the earth that carries the difference in the current in the live and neutral the the RCD detects. This is due tom one of the fundamental laws of electricity Kirchhoff's circuit laws.

PS be careful using building wiring regs on a boat - a boat in water is electrically speaking quite different to a building on land. Have a look at the american boat and yacht council specs or the CE approval requirements for boats.

I do agree that building regs do not cover all the issues you have on a boat but I also find that the small boat electrical regs the VicS posts also do not cover all issues and IMHO some of the recommendations are very old fashioned and IMHO have followed the building regs too closely
.

 

[kashurst]

thats quite a reply Roger, ~ I will try and reply.
A: consider a boat in the sea away from all other boats:
1: galvanic corrosion will take place between any disimilar metals that are electrically connected to each other, that are submerged in the conductive water.
the purpose of an anode is to provide a more reactive or less noble metal that will sacrifice itself over the other less reactive metals. i.e bronze props bolted to stainless shafts. without anodes electrically connected to the shaft prop assembly, the more reactive metal element in the bronze alloy will leach out and weaken the prop - eventually going pink and brittle. So boat builders fit anodes electrically connected to the underwater metal parts at risk. This really happens its not a myth. The dissimilar metals in a electrolyte create a weak battery and current will flow.
2: out at sea there is a the possibility of lightning. the purpose of a lightning conductor is to reduce the chance of a lightning strike, and in the event of an actual strike providing a "safe" path of least resistance. On buildings a big sharp needle like object connected to earth will reduce the chance of a strike by encouraging the air around the needle to ionize as the storm goes overhead reducing the build up of potential difference from ground level to the storm cloud. It doesn't always work, but most of the time helps a lot. On wooden/fibreglass ships and boats all the external metal rails should be connected together with big wires and down to the underwater metal parts to try and create a lightning conductor and in the event of a strike provide a path of least resistance, the higher up the better. If there is a strike on the boat with a "lightning conductor " it will undoubtedly do a lot of damage and may well blow out the anode connections or a skin fitting. These however are survivable damage that the crew can plug whilst working out what still works/ calling for help with the handheld VHF they put in the microwave when the storm got near by etc.
Without a safe path the boat could be holed in more unpredictable places. e.g. blow the P brackets off which would be hard to survive.

B: consider a boat in a marina but not connected to shore power:
lightning is still a possibility, so you need a safe path (moor next to a taller boat)
Your boat is probably very close to other boats with underwater metal parts and the metal parts of the marina pontoon structure that are underwater. If you have anodes they will protect your own underwater metal parts and not do much to help or hinder your neighbours metal work or the marina. If you do not have any anodes your neighbours anodes will provide some protection through the conducting water - dissimilar metals electrically connected again. How much effect they will have - there are too many variables.

C: consider a boat tied up in a marina but connected to shore power:
you still need lightning protection
you still need some sort of anode protection
you may have an issue with the earth connection through the shore power cable.
you need maximum protection from high voltage electric shock/on board fault

in a nice shiny new marina the earth bonding and all the cables will be new and uncorroded. There will be no electrical potential difference between the sea earth potential and the land earth potential via the shore power lead so no current will flow. All the other boats in the marina will have nice working anodes, all the underwater marina metal work will be very well connected to shore power earth.
Now move on 10 years and all the wiring will be corroded, the original earth stake in the ground has probably rotted away and some boats don't have anodes fitted or working - and you decide to park your boat in this marina.....
1: if these problems exist (and they do) then if your shore power earth is not connected to your boat AC earth wiring, then no problem if you have your AC earth wiring connected to your anodes providing an escape path. Also you will have fitted a residual current detector that uses Kirchoffs law to measure the current going out equals the current going in. No earth connection at all is required. If you use an Earth leakage detector you must have the earth connection on that connected to the source earth - ie the shore power earth. So RCDs are safer in this application. You can not bother having an AC earth at all on your boat on the premise that the RCD will always work. If it doesn't thats another story
2: shore power lead earth is connecting all the other boats' underwater gear to you - your anodes may be protecting your neighbour or the marina and will dissolve faster maybe leaving you unprotected from galvanic corrosion. Solutions - use an isolation transformer so the shore power earth is not connected to your boat AC earth, or use a galvanic isolator that only connects your boat AC earth to shore power earth if a potential difference of more than 1.5V develops. Now you have shore power and either: a boat AC earth to sea water safe path or the galvanic isolator creates a path to shore power earth if a significant fault develops. Or you can assume your RCD will always work....

I was wrong about the VHF requiring an earth - they don't I was mixing it up with SSB transmitters that do need a sea water ground plane.
You say you have a metal hulled boat - is the hull made out of the same material as the skin fittings, P brackets etc ?
you have isolated your shaft and prop with cutlass bearings and a flexible coupling - did you remove the link connecting wire from the flexible coupling ? Is your prop made from the same material as the shaft ?

 

[rogerthebodger]

Your first statement is the one that you base all else on and the current thinking is that if there is no electrical connection between different metals in a electrolyte there will be no current flowing thus no galvanic corrosion. Hence no bonding. This applies to a GRP boat that most are these days but mot to metal (steel / Aluminium);

Once you have removed the bonding of underwater metals then there is no common point on a GRP boat to have the battery -ve connected to.

As far as connecting the AC earth to the sea in an attempt to provide an alternative path as in a domestic earth spike this is no guarantee that an effective current path will be established as you have a sea water to ground (soil) interface of unknown current carrying capacity so IMHO cannot be relied on to cause the RCD to trip.

As for the potential for the wired earth line to become faulty you are correct but if thee is a fault in this line it can easily be detected by using very cheep neon lamps that ill show an earth fault. There are also cheap plug in mains line testers that show various faults. I have one in my too kit for testing other peoples boats as mine has the neons permanently connected. It will also show when a suitcase generator does not have the correct earth to neutral connection ( most don't) required for the RCD to operate.

With no bonding and thus no mains earth to the hull of a GRP boat the and a lessor requirement for hull anodes the problem of anode usage due to anodes/underwater metal connected together and to your next door neighbour in the marina. There is also no need for a GI of IT in this situation to stop galvanic currents as there will be non as no connections of anodes to next dor boat.

Your point about a 10 year old marina with corroded earth spike also means it would be better not to rely on the ground/sea leakage current path but ensure the the wired earth is working correctly.

Now lighting lighting protection with a sky spike and heavy conductores here in a high lighting area 6000ft above sea level consists of the spike at a point and height to provide the cone of protection for all the buildings it is protected and connected to a ground spike OUTSIDE of the building so the the lighting strike is directed away from any internal wiring that may give an easier path for the lighting to floe to the ground. Now on a boat we don't have a ground spike nor generally we don't have a heavy cable running from the mast head spike to the sea OUTSIDE the hull. This is what you need if you wish to have proper lighting protection and in place of the ground spike you need an EXTERNAL plate as the ground/sea connection and not using skin fittings/anodes bonded as these are not just external and the bonding wires are internal and not big enough.

As far as SSB ground plane is concerned, yes you need one and the bigger the better but It again does not have to have DC contsct with the sea. In fact one of the best SSB ground plains is a copper mesh or tape bonded inside the hull of a GRP boat so again not useful for any other purpose.

Yes I do have a steel boat and my arrangement I considered very carefully. My DC is totally isolated from the hull and my engine that does not have isolated DC is totally isolated from the steel hull and mt coupling never had a bridging strap as when I fitted it all together I did not want bonding.

My mains incoming earth is connected to my steel hull as the hull is conductive and a number of my appliances have metal cases bolted to the hull and so need mains earth for safety. I do have a GI as I have anodes welded to ny hull but the erosion of the anodes is very low ans no corrosion on the hull so all IMHO the design is correct.

All my through hull fittings are stainless steel and currently the only yellow metal below the waterline is the leg of my bow thruster. This as an anode on the thruster and no sign of erosion to date.

I did have a bronze prop on a stainless steel shaft and I had shaft anodes which did show an amount of erosion after 4 years. at what point I added another shaft anode. I have also fabricated a new stainless steel prop and hope this will reduce the shaft anode erosion.

The mains cables are in separate trunking.

I don't know if you don't know Kirchoff's law or don't understand its application here but yes Kirchoff's law that the sum of currents into and away from a point must be equal so if there is a difference in the live and neutral at the RCD the difference must go sum where and the only place it could and should go is along the earth line. If there is a sea/ground path and no incoming earth yes it could pass through you but is best to not have that and insure the incoming earth is correctly working so that that case would not occur and without a sea/ground path a shock may not happen as no current would flow. and I agree that the RCD would not trip in that case hence the need for a proper tested incoming earth line on a boat.

I think the standards writers used too much of the building regs when compiling the boating regs and did not go back to basic principles which is what I tried to do.

All fo now bead time here in deepest dark Africa.

 

[kashurst]

Sounds to me that your boat, that you designed and built, will have excellent resistance to galvanic effects as you have used metals of very similar reactivity - I understand now why your anodes last a long time and arguably you probably don't need any at all - assuming the reactivity of stainless steel parts is the same (or close enough) as the steel hull. As your boat is steel you also have a pretty good faraday cage protecting everything inside from lightning too (unless the lightning comes in through the window - never going to happen)

the rest of us though are stuck with the hotch potch of very different materials and have to protect them and the boat as best we can.

I suspect though your boat is unique - any chance of you posting some pictures of it?

 

[rogerthebodger]

With regard to galvanic corrosion as you no doubt know need 4 things. 2 dissimilar metals, an electrolytic and a current path to have galvanic corrosion. Now remove the current path and no galvanic corrosion, hence remove bonding.

Easy on a GRP boat not on a steel boat. So even with a hotch potch of very different materials if they are insulated from each other (easier said than done) but on a GRP hull possible you eliminate galvanic corrosion and the rest follows.

Pics of building boat on my photobucket page link in my signature

Similar to this. This design I think was based on the design of my boat that the hull was designed and built in late 1970's but never finished until I purchased it in 2005 finished her off and launched in august 2009.

http://www.lavranosyachtdesign.co.nz/cy_highlat.htm

 

[kashurst]

that is a serious boat! where have you / do you plan on sailing too?

 

[Lon nan Gruagach]

Am I missing something, all this talk of galvanic corrosion and dissimilar metals, what ever happened to plain old rust? Isnt that why anodes are fitted in the first place, to prevent corrosion of the most reactive metal underwater even if it is the only metal underwater.

And as for stainless not bothering plain steel, when was the last time you left a steel knife on a wet stainless draining board, and cleaned the rust stain of afterwards...

 

[l'escargot]

Am I missing something, all this talk of galvanic corrosion and dissimilar metals, what ever happened to plain old rust? Isnt that why anodes are fitted in the first place, to prevent corrosion of the most reactive metal underwater even if it is the only metal underwater...

I think you are missing something, it is in the name - anode - they are fitted to prevent galvanic corrosion between two dissimilar metals in an electrolyte (sea water). Nothing to do with preventing rust.

 

[Lon nan Gruagach]

I think you are missing something, it is in the name - anode - they are fitted to prevent galvanic corrosion between two dissimilar metals in an electrolyte (sea water). Nothing to do with preventing rust.

Nahh, its called anode because it becomes the anode (preferably) thereby preventing corrosion of any other metal.

 

[l'escargot]

Nahh, its called anode because it becomes the anode (preferably) thereby preventing corrosion of any other metal.

Preventing "galvanic" corrosion, but not oxidisation (rust)...

 

[Lon nan Gruagach]

Preventing "galvanic" corrosion, but not oxidisation (rust)...

https://en.wikipedia.org/wiki/Galvanic_anode

 

[l'escargot]

https://en.wikipedia.org/wiki/Galvanic_anode

Perhaps you should read it - tells you all about "galvanic" corrosion...