Lightning and Diving

[miketsp]

Possibly true but it doesn't explain that when there is lightening and you are immersed you don't feel the electric tingle. However, when you are touching an object such as a boat you definitely feel an electrical shock. When your body is immersed and not touching a ground to complete a circuit it is the same as a bird sitting on an electric wire.

Remember that even when there is no flash a current flow may be taking place. If the boat is properly bonded and there is some highest placed metallic point source, a significant ionic interchange may be taking place with a charged atmosphere. Depending on your distance from the earthing plate there will be potential difference. If you are wet you will feel even very small voltage differences.

As for not feeling it submerged, I remember reading several articles where submerged divers felt progressively worse shocks as they neared the surface during their deco stops.

 

[miketsp]

..snip..
When your body is immersed and not touching a ground to complete a circuit it is the same as a bird sitting on an electric wire.

Just to clarify why there is no comparison between a bird sitting on a power line and an immersed diver and to demonstrate that a diver in the water is subject to a voltage drop I did a quick calculation of the safe depth from an overhead strike.

When a current I enters the earth through an earth electrode, due to its hemispherical base, the current flows radially outward.

Assume r is the radius of the current source in cm.

If p is the resistivity of the conducting mass, the resistance offered by a hemispherical shell of thickness dx at a radial distance x from the current source is given by

dR=p * dx/(2 * pi * x * x)
so by integration the resistance up to a depth of r1
R = p * (1 - r/r1) / (2 * pi)

If we then assume that a current I enters the source electrode, the potential drop up to the shell radius of r1 is calculated from ohms law:
V = I * R

The resistivity of sea water is typically 20 ohms-cm +/-20%
(just for the record pure distilled water can be as high as 18Mohm-cm)

I assumed a 60cm electrode radius.

I assumed a 2 meter tall diver with body radially aligned to the current source.

I took 48 volts as being the highest acceptable voltage across the diver's body, head to toe. If you think that seems low, wet your hands with salt water and hold a telephone pair (I take no responsibility for the consequences).

The real unknown is then the current intensity of the source. So for the purposes of this calculation I used 2 values, 1000Amps and 10000Amps.

In the first case the diver would need to be radially more than 8meters from the electrode and in the 2nd case more than 27 meters.

The Excel attached shows the calculation.

 

[Dragon2115]

...it doesn't electrocute anyone because they are not grounded (or because it chooses not to, who knows).


...On the question of being on the surface, I think the likelihood of being struck would be extremely low because your profile sticking above the water in so low. Lightning works in a counter intuitive way. It doesn't 'strike down' ionised particles discharge up to the cloud and then the visible lightning follows the 'path' back 'down' to the grounded object. This accumulation of particles tends to gather around (or ionise around) objects with some height and often with irregular features, say an antenna or electric pole. I don't think, and I may be wrong on this, lightning just strikes the ground in an open field, at least not very often.

Stan

That's a little bit off. The reason they would get shocked when touching the ground plane of the boat during a lightening strike is because the connection of the vessels ground plane to earth ground (the water) wasn't good enough to conduct that much current fast enough. If it were then they would have been fine. That is the principle behind a Faraday cage. Believe it or not there is still a small amount of resistance between the ground plane of the boat and the earth ground. At low voltages this isn't a problem, but during a lightening strike this (and the salinity of the water, or more importantly, the lack of salinity) will cause a voltage to be developed in the ground plane of the boat. This is what blows out the electronics and causes injuries.

As for the ionizing of objects, that is correct. But that doesn't mean current is flowing from the ground up to the cloud. What is happening is that the ionization is being caused by the massive imbalance generated by such a large electrical charge being in close proximity but not yet able to jump the air gap to ground. Under these conditions a phenomenon called a plasma streamer is generated and rises up from various objects. Up until a few years ago nobody knew about them and consequently assumptions were made about how earth connecting lightening worked. Recently plasma streamers have been caught on film and have opened up a lot of new thinking on the mechanics of lightening. One thing they are still trying to understand is why one plasma streamer connects and another seemingly better one doesn't.

Anyway, the main charge is built up inside the cloud due to water, ice, and dust particles rubbing against each other in the violent up and down drafts of the storm. Because nature abhores an imbalance and will do whatever it takes to eliminate it, the charge is always looking for a place to go that is at a lower electrical potential that will either eliminate or reduce the imbalance. That is why some lightening discharges take place solely within the cloud and never reach the ground. Sheet lightening is a good example. What is happening here is the various differences in potential within the cloud are equalizing with each other. This main charge is still looking for a way to ground but as yet still has insuffiucient potential to jump the gap to ground.

As for being in the water during a strike, MikeTSP is spot on, the charge will dissipate with the square of the distance over the entire three dimensional hemisphere surrounding the strike point. As long as you aren't too close to the strike point you should be ok.