Electricity Underwater- whats safe?

[whipper_snapper]

Isn't it all about the voltage gradients experienced by the person in the water ? 1.1 Amps passing straight through you will certainly be lethal, but to achieve that you need a big voltage gradient across your body; 12V will not do it - put one hand on a 12V battery + and the other on the -ve, you will not die because you are not a good enough conductor, if you put an ammeter in series with one hand, the current measured would be minute. If you did this underwater, the only difference is that you would make better electrical contact with the terminals so the current that you experience would go up very slightly.

If there is a positive electrode at 12V relative to a negative electrode and they are separated by 12 metres of water then the gradient will be 1V/m, and a swimmer 2m tall perfectly aligned between those terminals will feel only a 2V differential between his head and his feet and will come to no harm. The fact that he is wet will not change things much. It makes little difference if it is salt water or fresh, the current through the water would be different, but the swimmer's conductivity is about the same, so the current he feels is about the same.

Have you seen those terrifying videos of people crawling along high tension power cables They survive because they don't put their body across a voltage differential. If they crawled across a bad bit of cable with a high resistance they would fry, hence the metal underwear.

 

[Graham_Wright]

[ QUOTE ]
Erm...

Bilgediver refers to "autotransformers" but these are impossible to come by, unless custom made (at least for 12V output), so it is not likely to be one of those!

[/ QUOTE ]

Just nit-picking but;- no they are easy (Farnell etc). Any mains input transformer will probably have tappings at 200v, 220v, 240v etc. So you could (if you were stupid,) tap off 20,30 or 40v if you wished and it would probably be from the high voltage end of the winding (i.e.0v generally goes to neutral). There wouldn't be much current available but, as I said, nit-picking!

 

[DrCS]

It's all about voltage gradients as whipper snapper correctly and thoroughly analyzed. In the marine environment the human body will not be affected by 12v. ( or even 220v) because the gradient won't be different enough compared to the sea water. ie, the electricity won't flow faster through the body than through the water.

In freshwater it will be lethal at 240v but again not at 12v DC. There are many reported cases of deaths in freshwater by exposed live wires or missconected "live" grounds in the water even for people close to but not touching the wire.

The 70ma you are referring to is for current flowing directly over the heart.

Your amperage for your device is irrelevant except in that it defines what your fuse size should be. put a dc fuse on the dc side that is slightly higher than your expected maximum amperage. consider risk if this is left untended of the fuse tripping and you are left without data (assuming it's a sensor). If nobody is watching this you might consider a timer and an automatically reset-able fuse, cycle power every now and again so if the fuse trips it resets.

There are special marine cables that have multiple shields including a layer of steel armor. They are meant for powering underwater, they are expensive ( 5 euros per meter? ) but perhaps for your application necessary.

Love to know more about the application

 

[CLB]

Holy thread revival!

It was 10 years ago.

 

[lpdsn]

Love to know more about the application

It must've worked or you would've read about it in the news.

PS some forumites get rather hot under the collar with people who needlessly revive ancient threads, so be warned.