Lethal Stray Current in Water

[Buut]

Most electric equipment these days are insulated, i.e. no electric connection between outer case and any internal circuits. That's why they only have a 2pole plug (in Main Europe!). Of course it may be against relevant regulations, but it is very efficient to simply disconnect the earth wire (green/yellow) at the socket where landpower comes in. The problem is the current between the own ship and neighbouring ships. This is what eats up the anodes. The Galvanic Insulators do the same by means of two anti-parallel diodes and a capacitor across.

I never leave my boat plugged in when I go home, but most motor boats are plugged in because they have problems with water coming in and need an active bilge pump.
On my previous boat this problem was extrem, anodes were eaten in half a season. Finally I disconnected the ground wire and the problem was gone.
I shall not recommend this because it may violate some electrical safety rules, however it is efficient. After the Battery Charger all electrics are 12V DC. None of that should touch the engine body either. A boat is not a car, where all metal part are Minus. If everything is floating there is no current to neighbouring vessels or the landpower.
I hope I could tell it half way clear. Anyway once out the Harbour and unplugged this problems should not exist. If they still do exist, it is as mentioned above. There is some connection between Minus and engine block, or saildrive. It's all a bit mysterious, but as long as nothing is connected to each other it should be fine.

This is a very difficult subject with a lot of confusion. I will strongly recommend not to simply disconnect the groud wire from your shore connection on the ship. You can only do that when you are absolutely sure, that your complete ac installation is isolated from every metal part aboard the ship. (there is also no need anymore than to disconnect the ground wire anyway) Point is, that in case of a fault condition on the ship, the submerged metal parts can become live and very possibly kill a person swimming around the ship. By cutting the ground wire you also cut off the path for stray currents to be detected, so that your rcd protective device does not function, untill somthing or someone outside the boat functions as return path for the stray current. And this may kill a person. It is not just theory, it has happened.

An isolation transformer would be ideal, a zinc saver works, but the components may fail and stay undetected, which creates the same condition as cutting the ground wire.

[Salty]

Point is, that in case of a fault condition on the ship, the submerged metal parts can become live and very possibly kill a person swimming around the ship. By cutting the ground wire you also cut off the path for stray currents to be detected, so that your rcd protective device does not function, untill somthing or someone outside the boat functions as return path for the stray current. And this may kill a person. It is not just theory, it has happened.

I'd be grateful if someone could explain the mechanics of the foregoing comment, particularly in regard to the magnitude of the current flow that would need to be present in order to "kill a person swimming around the ship," thanks.

[Nigel]

I'd be grateful if someone could explain the mechanics of the foregoing comment, particularly in regard to the magnitude of the current flow that would need to be present in order to "kill a person swimming around the ship," thanks.

A 100mA RCD might not protect from a lethal shock, so I guess this is the turning point. I've also heard the "killed smimmer" tale, I think it unlikely but possible.

Can I just refute a train of thought that has entered this thread. It is not the presence of the earth connection that makes electricity dangerous, this connection is vital to return current in a fault condition. Do not remove the earth connection under any circumstances.

[Salty]

I'd be grateful if someone could explain the mechanics of the foregoing comment, particularly in regard to the magnitude of the current flow that would need to be present in order to "kill a person swimming around the ship," thanks.

A 100mA RCD might not protect from a lethal shock, so I guess this is the turning point. I've also heard the "killed smimmer" tale, I think it unlikely but possible.

Thanks Nigel, I too think it unlikely, but I wonder if there is something I've misunderstood.

Surely for an electric current to kill a person, that persons body has to be acting as a conducter from one external contact to another. An example of non conduction being a bird landing on an uninsulated overhead power cable where it sits and surveys its next meal prospects, has chat with a few of its mates and then flies off without a care in the world because it has not conducted the electricity in the cable from anywhere to anywhere else. A swimmer in the water would surely be in the same relative position as the bird. Even if they were to touch some metallic part of a boat, that part would be in contact with the water which is probably a better conducter than the swimmers body with the result being no flow of current through the swimmer. Going a step further, if the swimmer chose to hang on to a metal part on the boat and to an underwater metal part on the pontoon, because the water would be conducting arguably at a better rate than the swimmers body, again no flow of current through the body. The only possibility I can see would have to be a situation where the swimmer had one hand on a metal part on the boat and a metal part on the pontoon which was not connected to any other underwater metal, but was connected to the pontoon electrical system, and was near enough for the swimmer to be able to reach and remain in contact at both ends.
So is there a situation where individual metal parts of the pontoon, including parts both above and below the water, where those parts are not joined up with a common earth bonding system?

[The Other Woman]

Hello Salty

Just a thought if the swimming ladder was in the water and some way was earthing the boat and you climbed out of the water then grabbed the rigging to give you a pull up as we tend to do, if there is a fault on the boat and it finds its way to the rigging or rail you become the circuit WHAM .
One of the causes I have found was a faulty batter charger (sterling) letting 240v through to the battery side of the charger .

Now fully Charged
Mick

[The Other Woman]

UMMM

Don't get me wrong I do agree that anybody swimming in the water would become part of the circuit IE not be earthed (like when you get out of the car and get an electric shock because you earth to the floor) and as you say you can grab hold of live terminals as long as your insulated.

What I was saying if you have a fault on your wiring its quite easy for it to spread round the boat  forstay connects to plate which connects to pulpit which connects to safety rail.
It only need a fault on a lets say aerial or nav light  or any fitting that someone as wired, like a car using the metal as an earth (as we all know everything should have its own earth) to make a circuit.

Has your 36 got any rust on the keel? And is it rust or is it galvanic circuit?

The mast as bonding to the keel for lightening not earthing so if there is a leak theres a circuit.

Mick

[MarkTheBike]

I tend to agree that it's unlikely for a swimmer to get a shock in salt water but I have a vague memory from 50yr old physics lessons that a body is less conductive than salt water but more conductive than fresh water. Something to do with free ions (or whatever). Could that be the origin of the tale? In a lake perhaps?

[Kibo]

I live in the USA and sail the Great Lakes. Electric shock drowning (ESD) is very real in fresh water marinas. Swimming is absolutely prohibited in any marina where shore power is available. I am British and was very sceptical initially....

http://www.boatus.com/seaworthy/magazine/2013/july/electric-shock-drowning-explained.asp

[Salty]

Thanks Kibo, the article goes a long way towards explaining the mechanics of the problem, though I still do not follow it entirely. Going back to the analogy in an earlier posting of birds sat on high voltage overhead power cables where they suffer no ill effects, I still have difficulty in understanding why this should affect a swimmer who is swimming freely and touching nothing other than the water surrounding them. For current to go through their body, there surely must be a potential difference between the point of entry and the point of exit, otherwise there would be no flow, and like the birds on the power cable, no ill effect. However, once the swimmer touches something else such as the bed of the lake or river, or is in fairly close proximity to something where a sufficiently large enough potential difference is present, then there is a real danger.
Interesting to know that current will flow more readily through the body when immersed in fresh water than when immersed in salty or brackish water.
As for shore power leakage causing erosion of under water metal parts of my boat, then perhaps (tongue in cheek) I can recommend the advantages of being on a swinging mooring where there is no electricity at the berth other than what you make from solar or wind generators.

[Yngmar]

Going back to the analogy in an earlier posting of birds sat on high voltage overhead power cables where they suffer no ill effects, I still have difficulty in understanding why this should affect a swimmer who is swimming freely and touching nothing other than the water surrounding them. For current to go through their body, there surely must be a potential difference between the point of entry and the point of exit, otherwise there would be no flow, and like the birds on the power cable, no ill effect.

Quite right! And there is a potential difference when a live wire dangles off a boat. When I was taught this back in Germany, it was called a Spannungstrichter (just look at the illustration, it really explains it all). The online dictionary translates this into "potential gradient", which sounds about right.

So the potential on the point where the boat has a wire dangling into the water would be 240V or something similarly dangerous. The potential of the ground (at shore, in the water, etc.) would be 0V. Between the two, a current flows and voltage drops, creating this potential gradient, meaning between the 240V point and the 0V area (a few meters away most likely), there is a gradient of decreasing voltages between 240V and 0V. So for example about 2m away from the cable, the voltage in the water relative to ground might be 90V (just an example, I do not know the actual numbers and it would depend on several factors).

This again, is harmless if you're small, like a fish, because you're only 30cm long and therefore you are only bridging a few volts between the ends of your body. If you're a bit longer, say around 1.8m, you are bridging a longer distance and therefore a higher potential difference between the ends of your body. This is what makes it dangerous, and that's what the illustration above depicts on land - if you keep your feet together, the potential difference between them is much smaller than if you stand with legs spread, or worse, start doing push-ups (or swim directly towards/away from the dangling wire).

[Nigel]

I agree that a live wire dangling in the water could be lethal, but earthing the battery negative or exposed metal will not prevent this. If you have a mains operated appliance then it must be earthed or double-insulated. In the unlikely event that this equipment is also connected to the outside of the hull, it should be earthed.

Also note that seawater has a resistivity ρ at 20 °C of 0.2 Ωm; a 30mA RCD trips at a resistance of 230V/30mA or about 7.5KΩ

[Kibo]

Thanks Salty. I think others have added to the clarification and I'm no electrical engineer so I'll leave it to those with more expertise.

I am investigating my keel corrosion issue and am actually hoping it is a specific marina issue since we now live aboards 5 months of the year and spend most of our time on the hook or swinging on a mooring !!

[Salty]

I didn't reply to this before, but thanks Yngmar and Nigel, that all makes sense.