Copper Pipe and Engine Are Electrified

[Thomas C King]

So, I was wiring a 12v pump for the cabin bilge when one of the wires touched a copper pipe. And the pump started pumping. I later, out of curiosity, did the same to the engine and the same thing (unsurprisingly, I guess). I assume it's the same for anything touching the hull.

 

I don't even know where to start on this one. Does this kind of thing happen often, and are there any likely places where something has been installed incorrectly? Or is it most likely a worn-out wire? (but, nothing has tripped from a short-circuit).

 

Also, on a scale of 1 - 10, how dangerous is this?

[MtB]

I'd say is was the negative wire and the switch on the bilge pump was ON (or the pump was submerged). The negative terminals of the batteries are usually connected to the hull in a single 'earth' stud.

 

Had you touched the positive wire on the copper, or any metal of the boat, you'd have blown the fuse. 

 

You did fit a fuse, didn't you? 

[Thomas C King]

4 minutes ago, MtB said:

I'd say is was the negative wire and the switch on the bilge pump was ON (or the pump was submerged). The negative terminals of the batteries are usually connected to the hull in a single 'earth' stud.

 

Had you touched the positive wire on the copper, or any metal of the boat, you'd have blown the fuse. 

 

You did fit a fuse, didn't you? 

 

So, this is expected/working as it should?

 

No fuse, it was connected to a circuit breaker. It's not connected anymore, because it was temporary. But in future, fuse + circuit breaker, or just circuit breaker is sufficient?

Edited March 28, 2022 by Thomas C King

[Jen-in-Wellies]

54 minutes ago, Thomas C King said:

No fuse, it was connected to a circuit breaker. It's not connected anymore, because it was temporary. But in future, fuse + circuit breaker, or just circuit breaker is sufficient?

Either, or. As long as the fuse, or circuit breaker blow, or trip currents are suited to the size of the wiring.

[Chewbacka]

And don’t be tempted to use the hull as a negative return for anything (like cars do) it will work electrically, but it will also greatly increase hull corrosion. 

[MtB]

2 minutes ago, Chewbacka said:

And don’t be tempted to use the hull as a negative return for anything (like cars do) it will work electrically, but it will also greatly increase hull corrosion. 

 

 

Even on a bilge pump which (hopefully) never turns on from one year to the next?

[Thomas C King]

18 hours ago, Chewbacka said:

And don’t be tempted to use the hull as a negative return for anything (like cars do) it will work electrically, but it will also greatly increase hull corrosion. 

 

Excellent, thank you. I didn't, but it was tempting.

18 hours ago, MtB said:

 

 

Even on a bilge pump which (hopefully) never turns on from one year to the next?

 

We pump out the cabin bilge every now and then, whilst we hunt down the ever-evasive source of water.

[Mad Harold]

18 hours ago, Chewbacka said:

And don’t be tempted to use the hull as a negative return for anything (like cars do) it will work electrically, but it will also greatly increase hull corrosion. 

I' m a bit confused by this. 

MTB said the battery negative is usually connected to the hull, so running the negative from an electrical fitting back to the battery negative is surely just the same as connecting the negative direct to the hull, and as Chewbacca says, increasing hull corrosion.

Or am I misunderstanding this?

 

[Thomas C King]

35 minutes ago, Mad Harold said:

I' m a bit confused by this. 

MTB said the battery negative is usually connected to the hull, so running the negative from an electrical fitting back to the battery negative is surely just the same as connecting the negative direct to the hull, and as Chewbacca says, increasing hull corrosion.

Or am I misunderstanding this?

 

 

Also, stupid question, but do the sacrificial anodes protect when running the negative back through the hull? I have no reason to do it though.

[Alan de Enfield]

8 minutes ago, Thomas C King said:

 

Also, stupid question, but do the sacrificial anodes protect when running the negative back through the hull? I have no reason to do it though.

 

Using the hull as an negative-return is not only a BSS failure point but does not comply with the Electrical wiring regs for small craft.

 

4 General requirements

4.1 The system type shall be either a fully insulated two-wire d.c. system or a two-wire d.c. system with a negative ground. The hull shall not be used as a current-carrying conductor. Engine-mounted wiring systems may use the engine block as the grounded conductor

 

 

[David Mack]

On 28/03/2022 at 10:39, Thomas C King said:

So, I was wiring a 12v pump for the cabin bilge when one of the wires touched a copper pipe. And the pump started pumping.

You don't say whether thit was a positive or a negative wire which touched the pipe, and whether that wire was connected to the 12V supply, the bilge pump or both.

As others have said, if the bilge pump positive was connected to the battery positive and the bilge pump switch was on, then touching the pump negative wire to a copper pipe or the engine would complete the circuit and the pump would run. If the wire had been connected to the battery positive then touching the copper pipe would have produced a blue flash and should have tripped the breaker. Any other arrangement would probably have resulted in nothing happening.

 

The battery negative should be connected to the hull, but only at a single point. Typically this happens with internal connections in the starter motor and alternator connecting battery negative to the engine block (and hence anything electrically connected to it), with an earth strap then connecting to a bolt on the engine beds, especially if you have a flexibly mounted engine where the rubber mounts will be non conductive.

 

And without wishing to be rude, if you have to ask questions about this sort of thing, then I think you should ask yourself whether you have the necessary knowledge and understanding to be messing with boat electrics.

Edited March 29, 2022 by David Mack

[Thomas C King]

29 minutes ago, David Mack said:

And without wishing to be rude, if you have to ask questions about this sort of thing, then I think you should ask yourself whether you have the necessary knowledge and understanding to be messing with boat electrics.

 

Thanks for the candour, point taken. I won't be doing anything else with the electrics (the corrosion question was out of curiosity). But if you or anyone else can recommend books etc. to get to a place where it would be safe to make small changes, e.g., light fittings (or advise that I would really need a qualification) then that'd be appreciated.

[Mad Harold]

29 minutes ago, David Mack said:

You don't say whether thit was a positive or a negative wire which touched the pipe, and whether that wire was connected to the 12V supply, the bilge pump or both.

As others have said, if the bilge pump positive was connected to the battery positive and the bilge pump switch was on, then touching the pump negative wire to a copper pipe or the engine would complete the circuit and the pump would run. If the wire had been connected to the battery positive then touching the copper pipe would have produced a blue flash and should have tripped the breaker. Any other arrangement would probably have resulted in nothing happening.

 

The battery negative should be connected to the hull, but only at a single point. Typically this happens with internal connections in the starter motor and alternator connecting battery negative to the engine block (and hence anything electrically connected to it), with an earth strap then connecting to a bolt on the engine beds, especially if you have a flexibly mounted engine where the rubber mounts will be non conductive.

 

And without wishing to be rude, if you have to ask questions about this sort of thing, then I think you should ask yourself whether you have the necessary knowledge and understanding to be messing with boat electrics.

And without wishing to be rude Mr.Mack, I and others joined CWF to learn about boating and canals by not only reading posts of interest but also asking questions.The answers received usually increased knowledge and understanding.

Thank you for your enlightenment regarding earthing, but your last paragraph did rather sound as though you were pissing on us from a great height.

 

[Thomas C King]

1 minute ago, Mad Harold said:

And without wishing to be rude Mr.Mack, I and others joined CWF to learn about boating and canals by not only reading posts of interest but also asking questions.The answers received usually increased knowledge and understanding.

Thank you for your enlightenment regarding earthing, but your last paragraph did rather sound as though you were pissing on us from a great height.

 

 

I think it was a fair point. There's a difference between asking questions and wiring things up. As an aside, it was the negative touching the copper pipe.

[BEngo]

As a matter of good practice you  should  check that any connection point is electrically dead before wiring it up. Even at 12 V.   Pull the fuse or circuit breaker and check, using a multi-meter,  that neither the positive or the negative has voltage present before connecting the wire.  If voltage is present and the circuit breaking device is out then there is an electrical fault which needs to be fixed.

 

The reason for this is that it prevents you getting a surprise when the item you are connecting starts up in your hand. If you do not take these precautions what generally happens is that the item flashes up,  you shout "Flip" or similar, jerk your head back or stand quickly quickly and smack  hard into something fixed, like an exhaust manifold.  This acts as learning reinforcement.

 

N

[Tony Brooks]

3 hours ago, Mad Harold said:

I' m a bit confused by this. 

MTB said the battery negative is usually connected to the hull, so running the negative from an electrical fitting back to the battery negative is surely just the same as connecting the negative direct to the hull, and as Chewbacca says, increasing hull corrosion.

Or am I misunderstanding this?

 

 

The answer to this question. The steel the hull is made of is not all identical, for a start the welds will probably               have a different composition although still steel and the composition within the plates may vary slightly. This means that there are areas of higher and lower electrical resistance within the steel. With just one negative bond point or possibly two very close  to each other and joined by a thing low resistance copper cable there is nothing to cause the electricity to flow from that negative bond point and through the hull because there is no other way back to the battery.

 

Now, if you connect (say) the water pump negative to the hull all the electrons that pass the pump will have to flow through the hull before they can get to the negative bond point and back to the battery. They will do this until they find a part of the hull with a higher resistance and ta that point they may find it easier to leave the hull and flow though the dirty, polluted water, bypassing the higher resistance area and then back into the hull.

 

As they leave the hull they will pick up an iron molecule and transferred that to where they enter the hull again, where it will be deposited. That point it leaves will eventually pin hole or worse.

 

The second, anode question.

 

The anodes will only help if they present a lower resistance path to the electrons and that is unlikely as the negative bond  and the appliances are likely to be located between anodes on the majority of boats and even nit they are not it still wont help.

[Iain_S]

10 hours ago, Thomas C King said:

 

Thanks for the candour, point taken. I won't be doing anything else with the electrics (the corrosion question was out of curiosity). But if you or anyone else can recommend books etc. to get to a place where it would be safe to make small changes, e.g., light fittings (or advise that I would really need a qualification) then that'd be appreciated.

As well as Tony's excellent course notes, Gibbo's site   is worth a browse.

[Chewbacka]

On 28/03/2022 at 16:20, MtB said:

 

 

Even on a bilge pump which (hopefully) never turns on from one year to the next?

If everything is perfect, probably not a corrosion problem, but in the real world with damp switches (from time to time) and poor wiring - for example another item sharing the bilge pump supply, then very small currents could flow more often than you would like or realise and corrosion would then be a real risk.

[beerbeerbeerbeerbeer]

On 29/03/2022 at 12:29, Thomas C King said:

 

Thanks for the candour, point taken. I won't be doing anything else with the electrics (the corrosion question was out of curiosity). But if you or anyone else can recommend books etc. to get to a place where it would be safe to make small changes, e.g., light fittings (or advise that I would really need a qualification) then that'd be appreciated.

[Tony Brooks]

On 29/03/2022 at 12:29, Thomas C King said:

But if you or anyone else can recommend books etc. to get to a place where it would be safe to make small changes, e.g., light fittings (or advise that I would really need a qualification) then that'd be appreciated.

 

I think the major potential "got ya" for the inexperienced in working on boat electrics is believing the currant capacity shown on the cable drum, catalogue or the interweb is all you need to know for wirirng, but on narrow boats for most cable runs the volt drop the run will cause is far more important. You can find out how to calculate it in my notes or in a variety of places.

 

The next thing is to ensure that the fuse or circuit breaker is never rated higher than the cable's current rating, you can use the numbers on the drum etc. for this. A lower rated fuse is perfectly safe.

 

You now know that you must always use a positive and negative cable and that is about is safety wise as long as you don't go causing short circuits - that is all too easy to do when trying to use an ammeter. Voltmeters are inherently safe, do something odd and you may not get a reliable reading but you can't cause a short circuit. If you have correctly fuse the circuits even a short circuit is not the end of the world because it will just blow the fuse or pop the circuit breaker.

 

No you do not need a qualification to do such work but a bit of practical ability and common sense always help.