Earthing a wooden boat

We have 3 power sources on board. 24 volt batteries, 1500 watt inverter and 240 volt shore power

Shore power carries its own earthing, but what is the best way to provide a 'proper' earth for the inverter and batteries? The only thing that occured to me was to maybe use the brass seacock, but I don't want to electrocute anyone!

 

I don't know all about wiring in the UK but this may apply and be of help. In the US the inverter should have a lug on it to ground the case and as far as I know will combine the neutral and safety ground when it takes over as AC power source so you should be covered there if you have installed it according to the manufacturer's instructions. If you have a propulsion engine and the block is grounded and you do not have a drive saver or other insulator installed on the propeller shaft your DC side will be earthed that way. What is left is whether to connect the DC negative to the grounding conductor on the AC side. It is required in the US to avoid stray AC current getting on the DC side without tripping a breaker. The residual current detectors used in the UK minimize that possibility.

If it was mine I would have the required RCD breaker, connect the DC negative and green ground busses, bond the metal underwater fittings and connect that to the DC negative buss. Also consider a galvanic isolator if you do not already have an isolation transformer.

When it is all done check your hull potential to insure you have the right amount of anodes (-550 to -600 using ag/agcl reference cell). If you overprotect it is bad for the wood, underprotect bad for the metal.

What you probably really need is a marine electrician to come have a look and lay it all out for you.

 

Good advice from Bglad, except for the hull potential measurement which is no job for the average boat owner.

Where shore power enters the boat I would install a RCD breaker like Siemens 5SM3-111/6 that trips at 10 mA. and a galvanic isolator if connecting AC and DC ground is mandatory.

 

This outfit sells a reference cell for checking the hull potential if you have a multimeter to use with it (http://boatzincs.com/corrosion-reference-electrode.html). The kit comes with instructions on how to use it along with information on proper hull potentials for various materials. I keep one in my tool bag

 

Of course you do, Bglad. But you and I are not "the average boat owner".

 

Good referrence there, Bglad, thanks!

 

Actually the RCD and ISO are basically the same as in the US as far as electrical grounding (earthing) goes. There should be only one common ground (earth) on a wooden (or any) boat. Normally the engine block is used as the common ground. You do not want to use a through hull fitting or other fitting that is in contact with the water. This introduces stray currents into the water causing stray current corrosion.

 

I concur Ike, but the original poster supplied very little information about the equipment he has and immersed metal objects present. In fact we don't even know whether or not his boat has propulsion....

 

You're right. We need more info.

 

....importantly for the owner to understand, whatever the equipment on the vessel, do NOT use the boats bonding system as the DC return for any piece of equipment, the Neg of the battery and the associated neg returns are where all equipment MUST be attached. Sure they are eventually joined together, but the current must never be allowed to flow in the bonding system.

 

Good point. If that happens you get the stray current corrosion I was talking about.

 

and that should include any RF Grounding System, for any MF/HF Radio System aboard.

 

Interesting discussion. So, if the wooden boat has a motor and prop shaft, anodes on the keel and battery earth terminated on the block, what is the particular consideration?

Hope the OP got his questions answered..

 

I was laying low on this one due to the lack of info from the op but since you asked

The particular consideration is that metal parts in a hull touching the water are a path back to ground. If they are not earthed aboard it is possible to get shocked or worse if stray 110 current is running around. If all the metal parts aboard including the underwater fittings are earthed then the breaker on the affected circuit will trip when there is stray current.

Opinions abound whether to earth or bond metal underwater fittings but the bottom line it is the safest thing for people to have it that way. It is also the safest thing for the metal too but that is where the arguments begin. What is well known is that if all the underwater metal parts are bonded then they should stay very near the same voltage and the current should flow out the protective anode(s) the least noble part of the system. If the metal parts are not bonded then game on when stray current is in play.

Earthing at the engine block is ok for a simple system with just two or three connectoins but better to install buss bars in the vicinity of the various metal parts and connect the engine there too treating it like another metal fixture in the hull.

 

Thanks BGlad. This could go on - probably deserves a thread of it's own and a referee.