Safe Depth for a Lightning Strike

[spacemanspiff1974]

I've had a few of my sailing friends get their electronics fried from a strike. Anyone have any experience with a dive computer getting fried?

 

[Desert_Diver]

I agree with all of your post, but ....... if you look at Perrone's post just after yours you will find that he doesn't believe that the lightning was hitting right above him.

lightning strikes can be 100,000 amps. Fibrillation or stoppage of the heart can be trigger by as little as 0.05amps, and 1A through the body has a very high likelyhood of causing a fatality. This means that you need an attenuation factor on the order of 100,000 to 2 million to be safe. The current flow through your body is driven by the differential potential, which in turn is driving by both the resistance of the water (obviously much less for freshwater) and the distance from the strike. If you say the danger goes down as the inverse of cube of distance from the strike I'll believe you --- I could even easily be convinced that the effect on the body decays as the 4th power of distance.

What's missing is the scale factor on distance. Clearly, a high conductivity material like seawater attenuates/disperses the strike in shorter distance than does a low conductivity (high resistance) material like freshwater. Another factor not yet mentioned is that a lightning strike is a very high frequency event, and things like inductance and skin effect will alter the dispersion pattern.

It is conceivable to me, just barely, that 20' of saltwater would be enough protection from a strike directly above, but there is no way that 20' of freshwater is going to attenuate the current sufficiently.

I don't want to glaze anybody's eyes but there are some things that I'd like to say. You are correct that small currents directly through the heart can kill. However, the dielecltric constant of the skin/ BCD/wet suit is going to require HIGH voltages to be over come so that the current can flow internally to the body.

Even if the water offers a relatively low resistance, the voltage drop (E = I*R) for a large current (100,000 amps in your example) will be huge. Assume just one ohm of resistance to 15 to 20 feet of fresh water in Perrone's anecdotal incident (which is an unrealistically low resistance), the water has attenuated the voltage by 100 KV before it encounters the resistance of the skin/wet suit/BCD. I'm not sure what value of voltage to assign to the bolt of electricity but it is going to be greatly dissapated prior to hitting Perrone. And remember, the current density is being spread in a hemisperical manner, which means that only a small percentage of the current is now proximate to Perrone. This guy is cutting down the odds at a phenomenal rate. Without better data, I still inclined to believe his story.

And I stand by my original position of declining to test my theory.

Art

 

[Desert_Diver]

I've had a few of my sailing friends get their electronics fried from a strike. Anyone have any experience with a dive computer getting fried?

We are now discussing a different phenomenon. Prior messages were about lightning strikes on or near a diver and current being conducted by the water. What you describe does not require a direct hit.

Again, I apologize for boring the rest of you. A lightning bolt is just a flow of charge (moving charge is a current) which sets up a magnetic field around the the path of the current. That magnetic field can induce a current in another conductor that the magnetic field intersects. This is the principal that allows transformers to work.

The lightning bolt current sets up the magnetic field. The magnetic field induces a proportionate current flow in any conductor within the effect of the lightening induced magnetic field. The currents are short but can be of huge magnitude. Most portable electronics are designed to work in very low current environments. So the induced current spike smokes delicate components in your computer.

Now that I've bored you into catatonia, the lecture is over.

Art

 

[Charlie99]

I don't want to glaze anybody's eyes but there are some things that I'd like to say. You are correct that small currents directly through the heart can kill. However, the dielecltric constant of the skin/ BCD/wet suit is going to require HIGH voltages to be over come so that the current can flow internally to the body.
.......
Even if the water offers a relatively low resistance, the voltage drop (E = I*R) for a large current (100,000 amps in your example) will be huge. Assume just one ohm of resistance to 15 to 20 feet of fresh water in Perrone's anecdotal incident (which is an unrealistically low resistance), the water has attenuated the voltage by 100 KV before it encounters the resistance of the skin/wet suit/BCD. I'm not sure what value of voltage to assign to the bolt of electricity but it is going to be greatly dissapated prior to hitting Perrone.

The dielectric constant only affects the capacitance, which is not very relevant here. The rest of your analysis is similarly far off base. Even someone not familiar with basic electronics can see that your logic is faulty in that, per your analysis, low resistance sea water would deliver a bigger jolt to the diver than does high resistance fresh water. Real life observations show just the opposite.

If by "dielectric constant" you really mean the insulating effect of neoprene, don't forget to take into account that the neoprene is saturated with water, greatly reducing the resistance.

 

[Desert_Diver]

The dielectric constant only affects the capacitance, which is not very relevant here. The rest of your analysis is similarly far off base. Even someone not familiar with basic electronics can see that your logic is faulty in that, per your analysis, low resistance sea water would deliver a bigger jolt to the diver than does high resistance fresh water. Real life observations show just the opposite.

If by "dielectric constant" you really mean the insulating effect of neoprene, don't forget to take into account that the neoprene is saturated with water, greatly reducing the resistance.

You are correct. I was typing too fast and not carefully proofing my message. I did indeed say dielectric constant when in fact I meant dielectric breakdown volatage. I erred.

Faulty logic? Sea water? I think if you'll check Perrone Ford's post, he was diving in fresh water. Not as good an insulator as distilled water, but certainly better than sea water.

I apologize for my error in fact and my assumption that this was just a friendly discussion rather than a competition with the object of scoring debating points. I bow before your superior technical knowledge, debating skills and incredible ego.

Art

 

[Charlie99]

I think if you'll check Perrone Ford's post, he was diving in fresh water. Not as good an insulator as distilled water, but certainly better than sea water.

The point I'm trying to make is that the insulating properties of freshwater EXPANDS the area in which a lightning strike will be lethal, as compared to the lethal zone of the same strike in lower resistance seawater. I am so insistent because the fact that you are at increased risk in freshwater is important knowledge for divers.

Perhaps I misread your post, but it seemed that you were implying that high resistance water (i.e. freshwater) would dissipate the strike in a shorter distance, and that therefore divers would be safer in freshwater than in seawater, given the same distance from a lightning strike.

If you read PerroneFord's post again, you will see that he doesn't believe that the lightning was hitting directly above him, with the 20' of freshwater protecting him from harm:

All three "hits" were while we were in a freshwater spring doing a safety stop between 15 and 20 feet. I suspect if the storm was right over us, we would have had some real issues. We surfaced less than 5 minutes after feeling the last hit, and the storm appeared to be some distance away.

 

[Mako Mark]

I have seen lightning flashes while uw, but thankfully never felt any current.

 

[Laruuk]

I got lost at dielectic currents and voltage lost ...

 

[Jon C]

Okay, I just spoke with NOAA, MIT, UCLA and my uncle Tim...All agree the safest depth underwater during a lightning strike is: 8-10 feet below your buddy.

Glad I could help...