Electricity conducting on steel boats

[Nanook]

Dear all,

I strongly suspect that when I turn on the electricity supply on my steel boat, it's conducting all round the hull as well as making the fridge and lights go. We all sleep noticeably better when the powercable's out, and feel indefinably less 'frazzled' when we wake the next morning. We also get a minor electric shock sometimes off the side of the boat, especially in hot weather.

Any advice about how to pin down the problem, and what to do? I've got a friendly boat electrician, but I'm reluctant to ring him up and say "I'm getting a bit of a funny feeling off my boat, what can you do?" I fear he may think I'm just suffering from chronic New Ageness.

The wiring was done by the previous owner, who generally seems to have made a few ropey decisions (the gas and the skin fittings all had to be redone) so he could've done pretty much anything. Suggestions? Views? With the power cable out, all is fine, so it's certainly a problem within the boat, and not coming from the water etc.

Yrs, vibrating slightly,
Nanook

 

[Gumpy]

Can you give us some more detail?
Is thare an inverter/charger, isolating transformer, etc.
Sounds to me as if the hull is not bonded to the boat wiring earth (1) or the earth is disconnected in the supply line and there is a breakdown in the insulation somewhere causing stray voltages to appear on the hull.

J

(1) hull should be bonded to boat wiring earth and -12v and seperated from the supply earth by an isolating transformer or a GI to prevent galvanic corrosion.

 

[richardandtracy]

Not an easy solution to a seemingly simple question.

It's possible that the previous owner has got something seriously muddled OR the supply is wrong.
The quickest thing I can suggest is one of these socket testers - the sort with a 3 pin plug and three lights. This should be able to tell you if the supply is OK. If faulty supply, scream blue murder at the supplier.
Then move into the boat and do the same at the boat end of the supply cable. It should all be OK there too. If not, re-wire the plug or socket to make it OK.

If it's all OK in the supply, move into the boat, and do similar checks. I have a nasty feeling that, if it gets into the boat, then you're on to a big re-wire job or have check the connections on every wire to see that each instrument etc is connected correctly. I don't envy you as it'll take forever. Best to do yourself as you'll get to see all parts of the boat (and possibly spot future problems that way) and it'll take masses of time - which would cost a fortune for an electrician to check.

Failing that, take the tightwad's option. Turn off shore power. No fuss, no time & less money! He, he.

Regards

Richard.

 

[VicS]

As a matter of urgency you should check:
The earth continuity all the way from the supply to the hull.
The "polarity" of the supply and your installation using one of the socket testers suggested by Richard (inexpensive from Screwfix or DIY stores) *
The correct operation of the shoreside RCD and your own RCD.

The simplest way of checking the rcds over and above their own test button is to wire a low wattage bulb ( but it must be more than 7.5watts) between the earth and live pins of a plug. Plug that in and the rcd should trip immediately. *

I am a great fan of DIY but in this instance due to the safety issues if you cannot find a problem quickly I would advise you to seek professional help. An electrician will have all the right equipment to check the installation properly.

BTW if you do not have a galvanic isolator then you should fit one.

* Better idea is a combined RCD and socket tester. I dont have an up to date catalogue but they were £10 from Maplin

 

[richardandtracy]

I do agree with VicS about the urgency of the matter & should have stated it myself.

If mains voltage is getting to the hull, the you could kill yourself or a swimmer near the boat - humans seem to turn into corpses with a remarkably low voltage across the water. If I remember correctly the potential is as little as 1V/m in water to stun & 1.2V/m to kill (don't rely on this - it's a pair of figures that I think I remember from years back, maybe someone can correct me if I'm wrong).

When I wired my boat, I fitted two completely independant wiring systems. There was the mains 3 wire system, using Live, Neutral & Earth wires. None of this was connected to the hull anywhere. The other electrical system was a single wire 12V system, with a -ve earth through the hull. I have to confess I was never particularly happy with the hull return & will fit a two wire 12V system if I do it again. As a secondary safety feature I had a battery charger permanently connected to the 12V system, but had to deliberately plug it into a mains socket for the 12V system to be charged. This covered against stray 240V getting into the 12V system (which is another point for you to check) if something goes wrong in the charger.

I did not fit an RCD at the time, but these days I would have a mains distribution panel with one built in.

Regards

Richard.

 

[Alex_Blackwood]

Get your friendly elctrician to check it out ASAP If you can't get him get a good qualified marine electrician. I wouldn't even start to answer your query without seeing the installation. Neither would I, with the greatest respect to all these who do answer, take anybodies word without having it checked.
Also I would advise against using shore power until you have had it checked.
Please do not take short cuts.

 

[VicS]

[ QUOTE ]
None of this was connected to the hull anywhere.

[/ QUOTE ] I think you will find that current recommendations would be that the shore earth is connected to the hull or, in the case of wood or GRP, the major metal structures such as the engine, which in many/most cases means the 12 volt negative as well,

 

[savageseadog]

I think your shocks are probably static electricity. I'd go along with others with respect to safety of your electrical installation. If other work's been botched the chances are that your electrics will be as well. Make sure the steel isn't used as an electrical return on your battery or your mains system.

 

[William_H]

Further to savageseadog comments. A static electricity shock is a very sharp zap that dissipates as it shocks you. It is not normally imediately repeatable.
A mains type shock will give a distinctive vibration or in lesser degrees a kind of "gripiness" as you lightly touch a conductive surface. The mains type shock feeling can continue or be repeated. It is best to test with the back of the hand/knuckles where this vibration or gripiness is easily felt. Often with a potential difference of several tens of volts you can detect mains leakage.

A Dc shock from 12 volts is usually only felt on salty wet hands or through a wound and it kind of stings and continues or is repeatable.

This may give you a clear idea of where the shock is coming from. good luck and be carefull olewill

 

[VicS]

[ QUOTE ]
A static electricity shock is a very sharp zap

[/ QUOTE ] Interesting though that it is something they have noticed in hot, and presumably therefore dry, weather otherwise I would have thought static was very unlikely in a damp marine environment. They do not say if these shocks are something that only occur when the shorepower is connected.

In addition to my earlier comments I would also suggest that an insulation resistance test is carried out on the installation as a whole and on individual items of mains equipment. An electrician will have the necessary tester to do that.

 

[Nanook]

Dear all,

These are all really helpful comments. I think the 'zap' I'm getting from the side of my boat is more the static kind, but the improvement in atmosphere that comes from switching the lekkie off is so marked, that I'm getting it all checked out. Nice boat electrician due in a fortnight, in the meantime, we're enjoying the pleasures of being offgrid and not paying outrageous bills to the marina. I've even got myself a lamp that is powered by dynamo....

Thanks,
Nanook

 

[VicS]

[ QUOTE ]
I'm getting it all checked out. Nice boat electrician due in a fortnight

[/ QUOTE ] A wise decision. Do let us know what he finds, perhaps not what he charges though.