Discuss 24v supply in boat in the Electrical Engineering Chat area at ElectriciansForums.net

aaroncoffey29

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My dad is a fisherman and his boat has a 24v supply.
Was wondering if anyone could help me with this.
He has said that when he connects a 24v light to the live that comes into the light fitting and then puts the neutral to the engine bay off the boat that it's lighting up the bulb and it shouldn't be doing this.
He wants me to go over and have a look I've never worked on anything like this as I'm domestic.
If anyone has any experience and could give me any advice it would be appreciated.

Thanks
 
TL;DR
Boat engine is lighting a 24v light up when connected live connected to live coming in and neutral touched to engine cover

James

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sounds like everything is working as expected,

DC supply (replace the word Live with +24v)
(replace the word Neutral with ground or 0v or -ve)

24v to one side of a light, 0v to other side via the engine bay (presumably metal and bonded to -ve on battery)

the light is supposed to light up under these circumstances or am I missing something here?
 

aaroncoffey29

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Thanks I'm not too clued up on this side off things tbh and still learning. He has said that the engine itself has earth relays and that it shouldn't be doing that. I thought that it should light up when he does this though as you have stated.

He has said that he thinks it's called Electrolytic corrosion, and it's happening because off this problem.

He has said at the minute he has put these connections to the engine on a male and female plug and disconnects them when he is back in the harbour.

I was reading up a bit on the electrolytic corrosion and if I'm right it could be coming from any off the supplies as it is caused by a stray current.

I'll see if I can test some things when I'm over to see if it's loosing any current through these.

Thanks for the replies and helping with the correct terms 👍
 

pirate

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As @James said, it sounds like everything is working as it should...I would expect a switch to be installed to turn the light on and off. if you connect one side to + and other side to - the light should come on, so I don't see the problem...how else would you connect it? OK, many boats have distribution boards where lighting and other connections are made and the - side is connected to the battery-bank, but the battery is also connected to the engine block, usually via the starter motor. Electrolytic degradation usually refers to the problem of underwater metallic parts corroding in seawater and is usually countered by fitting zinc plates to the hull, and "bonded" with cables to provide sacrificial protection.
 

Lucien Nunes

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Unlike car DC systems, many marine installations have insulated return, so that the negative side of the circuit is not connected to the hull. Sometimes, however, it is necessary to use an engine with a non-isolated starter motor or other 'earth-return' component. In this case double-pole switching might be used to isolate the negative from the block when possible and hence from the hull, or there might be deliberate isolation between block and hull.

If such a system is installed here, the OP's dad is correct in thinking that the lamp should not normally light to a hull earth (although it might to the engine block) which suggests that that the negative has become spuriously grounded somewhere. A single connection from negative to hull won't necessarily promote electrolytic corrosion but multiple connections, with current passing between them can, and it can accelerate corrosion of electrical fittings.

I would advise caution about making a diagnosis or repair of this type of installation unless you properly understand what is going on. There is a situation that can arise with incorrectly configured partially-insulated setups, where alternator and starter common negative currents are diverted along metallic pathways connecting engine block to hull or to system negative, including Morse cables, fuel lines and instrument grounds, causing damage or fire. It is essential to know what is supposed to be insulated from what, before altering any negative connections.

For comparison, my boat has block-return engine electrics but insulated return elsewhere. I have a solid connection from block to system negative and from system negative to hull, which prevents starter current passing through the flexible exhaust pipe, Morse cables and cardan shaft bearings (my mounts, cooling and fuel lines are all insulating). No other component has a deliberate negative connection to the hull so there would not normally be any current flowing through the steel, although a lamp will obviously light to a hull earth.
 
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Unlike car DC systems, many marine installations have insulated return, so that the negative side of the circuit is not connected to the hull. Sometimes, however, it is necessary to use an engine with a non-isolated starter motor or other 'earth-return' component. In this case double-pole switching might be used to isolate the negative from the block when possible and hence from the hull, or there might be deliberate isolation between block and hull.

If such a system is installed here, the OP's dad is correct in thinking that the lamp should not normally light to a hull earth (although it might to the engine block) which suggests that that the negative has become spuriously grounded somewhere. A single connection from negative to hull won't necessarily promote electrolytic corrosion but multiple connections, with current passing between them can, and it can accelerate corrosion of electrical fittings.

I would advise caution about making a diagnosis or repair of this type of installation unless you properly understand what is going on. There is a situation that can arise with incorrectly configured partially-insulated setups, where alternator and starter common negative currents are diverted along metallic pathways connecting engine block to hull or to system negative, including Morse cables, fuel lines and instrument grounds, causing damage or fire. It is essential to know what is supposed to be insulated from what, before altering any negative connections.

For comparison, my boat has block-return engine electrics but insulated return elsewhere. I have a solid connection from block to system negative and from system negative to hull, which prevents starter current passing through the flexible exhaust pipe, Morse cables and cardan shaft bearings (my mounts, cooling and fuel lines are all insulating). No other component has a deliberate negative connection to the hull so there would not normally be any current flowing through the steel, although a lamp will obviously light to a hull earth.
Interesting

A friends canal boat has the leisure battery DC negative attached to the hull and also the 230v neutral-earth link from the inverter, he has anodes that get replaced every 5 years or so and a galvanic isolator for when connected to sure power, but he is a constant cruiser so is never on sure power, is this in-correct then his boat safety check ain't mentioned it

Got another friend who has a boat, not sure if there DC negative and 230v is bonded to the hull, not seen her boat in as much detail, but will ask
 

Lucien Nunes

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The 230V installation earth should be connected to the hull, the last thing you want is earthed appliances at a different potential to your surroundings. Hence the need for the galvanic isolator which blocks small electrolytic potentials circulating DC via the shoreline, but allows the earthing to serve the AC system. Some installations use a 230-230 isolating transformer which is a more thorough way to achieve a similar result. Isolating the DC system completely from the hull is optional, although the hull should not be used to carry return current from the loads.
 
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The 230V installation earth should be connected to the hull, the last thing you want is earthed appliances at a different potential to your surroundings. Hence the need for the galvanic isolator which blocks small electrolytic potentials circulating DC via the shoreline, but allows the earthing to serve the AC system. Some installations use a 230-230 isolating transformer which is a more thorough way to achieve a similar result. Isolating the DC system completely from the hull is optional, although the hull should not be used to carry return current from the loads.
So the DC negative can still be bonded to the hull, but the hull cannot be used as a return path like it is in a car
 

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