For Us Techi Guys

[JennyMorgan]

Re galvanic action, if you moor in a marina and you have a poorly installed mains system aboard then you could be causing major problems for you neighbour. I know of several boats that have needed refastening below the waterline after a spell in a marina.

[MauriceMynah]

How can this happen if the mains is completely isolated from the water?

 

[Vaughan]

16 minutes ago, MauriceMynah said:

 

How can this happen if the mains is completely isolated from the water?

 

Not just from the water, but from the boat itself. Any AC circuit must have its own earth to the shore, otherwise any leakage in the circuit will indeed, find its way into the river. We have discussed this elsewhere and it is why I don't like the use of powerful inverters.

[LondonRascal]

You see in the diagram above how this happens and how one boat can cause issues for a neighboring boat some distance away. 

Basically, (and I may be wrong) I understand it that when you have AC on a boat it is hard to earth the boat since it is in the water so some current leakage is bound to occur from somewhere. 

I do wonder though if you can isolate your boat from such by simply having a 'ground connection' such as a small metal with with a wire that is dispatched over the boat and connected to a ground point on the boat - this would then cause the riverbed to be the earth/ground source?

[Vaughan]

This is why I am of the school of thought that says an AC circuit must be kept totally separate from any other DC circuits. The engine should not be connected in any way to the AC earth and neither should any skin fittings.

Immersion heaters in calorifier tanks are therefore dodgy in this respect, and so are battery chargers unless they are marine rated.

[LondonRascal]

Vaughan, that said though can DC not also have a 'leakage' and cause electrolysis?

[Jonathan]

31 minutes ago, LondonRascal said:

I do wonder though if you can isolate your boat from such by simply having a 'ground connection' 

Robin,

The metal bits (propeller etc.) in the water act a bit like the electrodes in a battery.  The water is an electrolyte (because it is not pure distilled water) and so a potential difference is created (again exactly like a battery).  The green earth wire in your diagram "shorts out" the cell and so flattens the battery which wears away the metal anode.  A separate earth would not help.  You need to break the circuit and isolate the two.

I have just seen your post to Vaughan:  Yes, it IS DC and that is the problem.  But the earth in AC circuits accelerates it by "shorting out" the DC path (as in your diagram).  You either need to block the DC with an isolator or provide a sacrificial anode

[JennyMorgan]

A large part of the problem is the keen DIYer who's boat rarely leaves the mooring, being used more as a weekend cottage than a boat. Every comfort of home installed with little or no knowledge of marine electronics, a specialist subject if ever there was one. 

[Regulo]

9 hours ago, MauriceMynah said:

Oh and lastly lastly, if I have a battery charger permanently attached to a battery, is anything happening when it isn't plugged in?

Nope, there are safeguards in the circuitry to prevent your battery discharging into the battery charger.

Oh, did I warn you about the posts about galvanic isolators?

[Vaughan]

2 hours ago, Jonathan said:

The metal bits (propeller etc.) in the water act a bit like the electrodes in a battery.  The water is an electrolyte (because it is not pure distilled water) and so a potential difference is created (again exactly like a battery).  The green earth wire in your diagram "shorts out" the cell and so flattens the battery which wears away the metal anode.  A separate earth would not help.  You need to break the circuit and isolate the two.

I have just seen your post to Vaughan:  Yes, it IS DC and that is the problem.  But the earth in AC circuits accelerates it by "shorting out" the DC path (as in your diagram).  You either need to block the DC with an isolator or provide a sacrificial anode

Jonathan has answered this more concisely than I could!

I would just add that this is why DC circuits in a boat are wired negative earth (they didn't always used to be) as in this way you can choose to protect important fittings such as the prop shaft and rudder, due to the "direction" of the electrolytic action.

It also goes to prove why you must fit sacrificial anodes and change them regularly.

[Viking23]

11 hours ago, TheQ said:

The wiring on that telegraph pole looks like many of the undersides of model railway layouts too....

Wiring when random with no order is often called birds nests, now don't under estimate the advantage of birds nests, if you go for nicely loomed wiring, bundling up low voltage circuits with dirty mains, ie that running electric motors  and then bundle up some low voltage transducer or sensor wiring, with or without screened cables then you are asking for trouble. A spike on a mains cable can be superimposed by induction and by capacitive coupling, into an adjoining cable, the longer it is run in parallel the greater the effect of the spike. It is always best to cross cables at right sngles so there is no coupling, hence the birds nest has a benefit.

Birds nests, ie point to point, also provide the shortest cable runs

Loom like with like cables together,  keep noisy cables apart from signal cables. For example, don't bundle up your radio aerial down lead with the supply to an electrical compressor fridge, as you will get all sorts of clicks and noise on the radio as the fridge cuts in and out.

Richard

[Viking23]

Quite a lot of things going on in this thread, so I will address some points.

 

Re 240 volt boat wiring, you have to use flexible mains cable, fair enough, but the conventional 13 amp sockets and chocolate block connectors rely on a single screw "drilling" into very fine wires, not a major issue for single stranded wire, for an experiment, connect some 2.5 mm2 flexible cable into a standard 13 amp socket terminal, tighten up. then slacken, then tighten, now disconnect the wire and collect all the cut strands of copper wire, and now look at the cable, it may have lost 30-50% of the wires capacity, and you have strands of copper loose to go anywhere in the socket.

Now I regularly use boot lace ferrules, available in different sizes and colours. I tend to insert the wire and squash the ferrule onto the wire with pliers, then insert into the terminal. sufficient force is required to clamp the wire within the ferrule and always best to pull the wire with medium force, as sometimes the wire isn't clamped sufficiently, but you soon get used to that.

Now, in the past, people have soldered or tinned stranded wires before putting into a screw connector, this is really bad practice, the reason being, under compression, solder tends to flow, and eventually the screw in the terminal will become loose, loose it's connection, maybe over heat or cause sparks or even catch fire.

Richard

 

[Viking23]

Re galvanic isolators, having moved to the Thames and have now got shore power, I have fitted a galvanic isolator, I have discussed this in another thread, but, on installation day, I thought I would measure the dc voltage across the isolator, shore earth to boat earth, and I measured 150 mV DC now this voltage was blocked by the isolator, but that 150 mV would have caused a current  to flow between my anode on my outboard, and mains earth, at best destroying my anode sooner than expected, and if left unnoticed then my aluminium prop and engine leg.

I fitted a dehumidifier, and I also checked the ac component across the galvanic isolator, no significant leakage voltage.

Now, something you may not know, the galvanic isolator can block 2-4 volts dc, (approx) depending on the design, but if you have ac leakage over this, then it puts the diodes into conductive mode, such that the dc current will now start to flow, rendering the galvanic protection useless. Sometimes this can be remedied by having an ac capacitor across the terminals, but this is quite controversial as it relies on high capacitance, and no one will say what value is best. Very popular in the USA.

If you have a galvanic isolator, and you happen to trip a circuit breaker or earth trip, then you should test the galvanic isolator, as a possible high trip spike could destroy the diodes, rendering the isolator ineffective, and in case of an earth fault on the boat having no earth could be very dangerous.

Easy to test, multimeter, 9volt battery with a bulb or resistor in series to limit the current, info on youtube.

Hope it helps.

Richard