Electrical earthing in a steel boat

[mikewade]

I have spent some time googling but haven't found a full answer so I wonder if the panel can point me in the right direction.
My boat is a steel van de stadt 34, 20 years old and I have just rewired the 12v side.
I note that presently the 240Vac installed seems to be twin core & no earth and so I want to start repacing the 240vAC side using an RCD and galvanic isolator & three core cable.
I use shore power if available for short bursts of battery charging if she sits for a couple of weeks & through the winter will use a dehumidifier.My question is simply what do I earth the new AC supply to as it appears that earthing to the hull is a no-no! is this correct?
Is this a problem in a steel boat? my knowledge of electrics is less than zero so any help will be gratefully received.

 

[VicS]

Roger Shaw is probably the man to answer but the standard you should read is ISO 13297

 

[ianabc]

Steel and electrics....

We have found:

- you need two tinned wires for 12 volts

- a ground is needed, possibly as in most ocean going steel boat two 12 volt grounds directly to a welded threded stud on gthe hull in a dry place

- having a silver half cell volt meter to measure the voltage from the hull to the sea is good, three should be a potential depending on the metal, test rusty steel on docks and zinc protected steel to establish a base line and then a base reading for own boat, keep a record

- I have seen a blinking light system on a Coast guard vessel that was used to establish if a ground fault existed, if the light stoped blinking then there is a fault...a good system not used on any small steel boat that I have seen....

- If there is no proper ground from the battery systems to the hull then the breakers will be tripped by a hull to appliance fault

- Single breakers in the 12 volt system are good, perhaps a breaker in both the pos and neg side of the 12 volt system is better ( we have double breakers in the 12 volt panel)

- Use caution when ladvised by others, ask if they have what they describe installed in their boat, skin in the game helps...

- Nigel Calder has some very good advice in his books, bui they are for fibreglass boats.

 

[jason -and the arguenauts]

I'm not an electrical engineer so I cant recommend a solution - just tell you what I have done. Do what I describe at your own risk.

In the UK your 240v supply will incorporate an earth so I would keep the mains earth and the boat 12v earth completely seperate. My de-humidifier works on the end of the cable from the shore supply just like you would use an extension lead at home. Ditto the battery charger.

This avoids the risk of your hull becoming the sacrificial anode for the pontoon system. But it does in theory put you at risk should an electrical fault cause the charger or dehumidifier to make the hull live. That is a risk I take because I belive the probability of this happening is small whilst the problems of being the anode for the pontoon is very real. And I cant see how the hull could become live without tripping out the shore supply

I have no doubt that this solution is a no no with the regs and the H&S nazis.

P.S. I would add that my boats are / have been plastic hulls but I dont really see that that makes a lot of difference.

 

[Bilgediver]

Roger Shaw is probably the man to answer but the standard you should read is ISO 13297

Definitely read this as it fully explains when and when not required to interconnect DC and AC earth.

 

[fuss]

This standard does not seem to explain it fully, at least not in my opinion, clearly.
My boat is aluminium and so has similar problems.
It is wired up as follows.
The hull is not an earth for either dc or ac.
All dc devices are fully isolated with a negative back to the battery negative.
The ac ground (yellow wire) is ground to the negative on the dc battery and a wire runs from this dc negative to an external anode that is fully isolated from the hull, this brings everything down to the same potential.
I am not saying that this is correct, but it is the way that all is wired on my boat and sofar I have had no problems.

 

[VicS]

an external anode that is fully isolated from the hull,

What is it connected to then apart from the AC earth and the DC negative.
What's its purpose in life if it is insulated from the hull?

 

[PCUK]

On my 21 year old De-Groot the DC is earthed to the hull and AC isolated and run back into the mains supply earth. No problems ever encountered.

 

[mikewade]

Hello Guys and thanks for all the input which I think has helped to clarify the situation somewhat. For interest, I also talked to Safeshore Marine as I will use one of their galvanic isolators and this is what they emailed to me. It seems the route to go.

"I would consider it essential you bond the a.c. earth to metal hull to ensure safety. Bonding the d.c. negative to hull is beneficial to reduce corrosion as it tends to "hold" stray voltages down to one potential. This is of course entirely up to your own discretion but is now the generally accepted installation proceedure. In most cases the dc negative will end up finding a bond to the metal hull via engine block wiring or appliance wiring : Easily checked by using a multimeter on "low ohms" to check for continuity. It is advisable however to fit a bonding cable directly from the battery negative terminal to a near convenient point on the hull. This will eliminate any potential created by voltage drop between the negative terminal & the engine block via the starter cable etc. Re The ISO all our isolators comply to the 5000a test & minimum fault current of the GI70 series is adequate for all UK marina supplies ( Usually maximum of 35 amps)."

Thanks again for all the input. I knew I could count on some informative answers

 

[Gumpy]

I have spent some time googling but haven't found a full answer so I wonder if the panel can point me in the right direction.
My boat is a steel van de stadt 34, 20 years old and I have just rewired the 12v side.
I note that presently the 240Vac installed seems to be twin core & no earth and so I want to start repacing the 240vAC side using an RCD and galvanic isolator & three core cable.
I use shore power if available for short bursts of battery charging if she sits for a couple of weeks & through the winter will use a dehumidifier.My question is simply what do I earth the new AC supply to as it appears that earthing to the hull is a no-no! is this correct?
Is this a problem in a steel boat? my knowledge of electrics is less than zero so any help will be gratefully received.

OK, IMO an many others and also several sets of regulations, there is only one way to do it.
The earth MUST be connected to the hull at the same or as near as possible to where the 12v negative is connected.
This then gives you the problem of isolating the incoming earth when connected to a shore line this can be done with an iso transformer or a Galvanic isolator your choice. Personally I use an isolating transformer.

If you do not connect the earth of your system to the hull then there is the chance that the live could touch the hull and the hull could then go live which may kill anyone in the water or stepping on and off your boat.
I have lived aboard steel boats for over 15years using shore power a lot and have never had a problem with galvanic corrosion so it does work.