Wireless Versus Ethernet

You're quite mistaken. Read up on "flashover".

"UK doctors have warned of the danger of lightning strikes when using mobile phones outdoors during stormy weather. .... When a person is hit by lightning, the high resistance of human skin causes the lightning charge to flow over the body - often known as an 'external flashover'.

But some of the current can flow through the body. The more that flows through, the more internal damage it causes.

Conductive materials in direct contact with the skin such as liquid or metal objects increase the risk that the current will flow through the body and therefore cause internal injury. "

This is from the BBC, but the same information can be found direct from the BMA, in whose journal this was originally published .

Mark McIntyre hath wroth:

Note that I'm not recommending isolated pieces of metal attached at random places as "shielding". I'm suggesting a full body conductive suit, made from anything from foil to armour, that will divert much of the current around the body. If lighnting "flashover" really is a surface only phenomenon, then the protective wearable shield will protect the body, rather than cook it.

The problem is that I'm not particularly thrilled with testing the concept with a real lightning bolt. No matter how conductive I make the shield, when large current flow through it, something is going to get hot or melt.

I love it. Mobile phone attracts lightning? Maybe if it had a tall antenna attached but the tiny cell phone antenna isn't going to do much.

High resistance? High resistance PREVENTS conduction. Electricity follows the path of least resistance, not most resistance. The observed flashover phenomenon is probably correct, but the explanation seems a bit lacking.

Rubbish. Go find an ohms-guesser. Pick an animal, any animal. I dead chicken will do. Peel off some skin and measure the resistance. Then, shove the ohms guesser probes into the chicken and measure the resistance. The skin is very high resistance while the body, which is loaded with conductive fluids, is much less. If anything, the skin is almost an insulator, not a conductor. Since electricity (lightning = DC) follows the path of least resistance, most of the current will go through the body, rather than though the skin.

Probably true here. If I just hold the ohms-guesser probes in my hands and measure the resistance, it will be much higher than if I covered my hands with aluminium foil gloves and made the same measurement. Adding additional points of contact will increase conductivity. However, if I bypass the conduction path between my hands with a clip lead or other conductor, almost all of the current will go through the conductor, not my hands. In other words, isolated metal pads on the body are a bad idea, but a conductive bypass will protect the wearer (somewhat).

What can I say? I'm right and the rest of the world is wrong.

Jeff Liebermann wrote: (snip)

Dry skin is a good insulator, but lightning often occurs when it is raining and skin is wet.

As I understand it, much more common than being struck directly is standing near a tall object, such as a tree. After the current goes through the tree it then goes radially through the ground around the tree. Depending on your foot position, you might find a large voltage between your feet, which then goes near your heart on the way. I believe this is even worse for cows. Stand on one foot, and not too close to the tree trunk.

-- glen

On Fri, 21 Jul 2006 10:08:39 -0700, Jeff Liebermann wrote in :

You're being so hasty that you're missing the point. Read the BBC article more carefully, if not the BMJ article itself. It's not claiming the cell phone attracts lightning -- it says the cell phone increases the harm from getting hit by lightning. Again:

Conductive materials in direct contact with the skin such as liquid or metal objects increase the risk that the current will flow through the body and therefore cause internal injury.

And:

"It is well known within the thunderstorm detection community that wearing or carrying metallic objects can increase the likelihood of injury.

"It certainly adds to the intensity of the skin damage and the article certainly amplifies that here.

More: "Lightning Strikes Teenager Listening To iPod"

Jason Bunch was mowing the lawn and listening to his iPod at his home in Castle Rock in the 3200 block of Cherry Plum Way on Sunday when he was struck.

He was taken to the hospital and was then put into the intensive care unit. Bunch recovered and was released from the hospital.

"I woke up and blood was coming out my ears," Bunch, a Douglas County High School senior, said.

Bunch has large scars on his feet and smaller burn marks on his hands.

"From where the iPod was, it damaged my hearing and it ruptured my eardrums. Where the cord was, it burned me all down my body," said Bunch. "We need to shave my head because my hair is like dreadlocks. It's all sticking together."

Again, you're missing the point, which is resistance to penetration, not surface flow.

Actually true.

Again, you're missing the point. Moisture and salts on the surface of the skin can be more conductive than the skin itself, along with the short duration, leading to the 'flashover' phenomon. See :

Direct strikes cause maximum injury because the entire charge of the lightning passes through or over the person's body. Because of the short duration of contact during a lightning strike, often not enough voltage is transferred to break the insulating effect of the skin; therefore, the charge typically passes along the surface of the body in a process known as "flashover." When the charge merely passes over the victim's body, less damage occurs.

Uh huh. :)

| I love it. Mobile phone attracts lightning? Maybe if it had a tall | antenna attached but the tiny cell phone antenna isn't going to do | much.

I heard a theory long ago that RF in the mid UHF to upper EHF range had a certain ionizing effect that could start a leader under conditions where the risk was already substantially high. This _may_ be the same condition where hair stands on end, but I didn't hear the specifics like that with this theory. The impression I got from it was that this is when lightning almost certainly will be striking somewhere nearby very soon and that the RF ionization introduces a better path or makes a leader for the full return stroke to find.

I've not tested the theory. I have had what I think was one opportunity to do so, but elected to turn radios off and leave. Lightning did hit near that location about 10 minutes later (not seconds, but minutes, even though I was seeing substantial charge buildup producing half inch arcs from the inside connector of the coax leading to the roof antenna, and the car frame. Could I have brought on an earlier hit right at me or my car had I transmitted on UHF? It was "only" 446 MHz. But I did not feel like trying it. OTOH, I had about 35 watts of power, which is well less than what the cell phones and cordless phones have. With 35 watts you do NOT want to be touching the antenna when transmitting, just to give an idea.

Hey, I'm still here (to the dismay of many usenetians).

|>When a person is hit by lightning, the high resistance of human skin |>causes the lightning charge to flow over the body - often known as an |>'external flashover'. | | High resistance? High resistance PREVENTS conduction. Electricity | follows the path of least resistance, not most resistance. The | observed flashover phenomenon is probably correct, but the explanation | seems a bit lacking.

His explanation was certainly incomplete. The working resistance often used to calculate human body current is 1000 ohms, when dry. Given a high enough voltage, however, an arc can form over the skin surface. Once that happens, the new ionized path is sufficiently low a resistance to pass the full current, but still has sufficient voltage drop to dissipate a substantial amount of power, which measured across a time interval is the energy produces a temperature rise of tens of thousands of degrees (fahrenheit or centigrade? ... who cares).

|>But some of the current can flow through the body. The more that flows |>through, the more internal damage it causes. | | Rubbish. Go find an ohms-guesser. Pick an animal, any animal. I | dead chicken will do. Peel off some skin and measure the resistance. | Then, shove the ohms guesser probes into the chicken and measure the | resistance. The skin is very high resistance while the body, which is | loaded with conductive fluids, is much less. If anything, the skin is | almost an insulator, not a conductor. Since electricity (lightning = | DC) follows the path of least resistance, most of the current will go | through the body, rather than though the skin.

Depends on the level of sweatiness, salt on the surface, rain mixed in with it, wet sweaty clothing, etc.

If you think skin is an insulator, I invite you to go swimming with an electric eel. Or maybe just swim in a pool with broken lights running at a "mere" 120 volts that were never even UL-listed for pool use in the first place. The surface of dry skin makes a poor contact. But I would not characterize it as an insulator at all. Of course wetness also makes a broad parallel path through the skin, too. But then, there is a good chance if you are out in a thunderstorm that you are not dry.

| What can I say? I'm right and the rest of the world is wrong.

One of the universal truths for everyone.

Well, the tinfoil hat was precisely that, methinks.

The point about flashover is that the current is restricted to teh skin. Probably still going to cook your skin, but not your insides. If you bring metal into contact, it alters the skin resistance and current may pass through the skin into your innards.

I'm quite surprised to find you arguing about this. I should have thought it was quite obvious you were being facetious in the first place, and equally obvious that wearing metal in a thunderstorm is stupid and dangerous. So why are you tying to defend yourself? The simple answer would have been "quite right, don't do these silly things".

You apparently didn't read the article. Its nothing to do with attracting lightning. They said that if you were struck, the phone would break the skin resistance pattern and channel current into the body. This is NOT a theoretical effect, it has several documented cases, some deceased, some wheelchair bound, some luckier.

Exactly, It prevents conduction THROUGH the skin, into the body. So instead the current flows OVER the skin, through sweat, clothing, air etc.

Sigh, what do they teach em these days? :-)

What, you reckon that the more current flows, the less damage gets done? Ok, experiment time: run 0.001amps through your skull. Any effect? Now try with 1000Amps. Come back when you get out of eternity.

Precisely. This is how the effect works....

giggle.

i think you borrowed this from Terry Prachett - on the diskworld the gods take insults or disbelief personally and the response can be spectacular.....

i am not sure if anyone is really taking this seriously - but here goes

this sounds like a good subject for a PhD thesis for a budding HV electrical engineer - any takers?

try it with a bigger safety gap to start - a scrap car might be good. Then you have room to wear a fire safety suit, but not apply the current to it (or just put some instruments inside the car and watch from a safe distance)

find a University or a manufacturer for big electrical gear such as 400 KV transformers - they often have the equipment to simulate a lightning strike and you can do repeated tests under controlled conditions. (AFAIR Farranti in Manchester UK used to)

once you have a test rig (a crash dummy if you want to simulate a person, or one of those dummy they teach doctors with?) - then start including a phone (on or off) and see what difference it makes.

The assumption here is that electrical resistance is constant - i was taught that it isnt, and tends to change depending on applied electric field (and magnetic as well? long time ago...) - anyway both points of view may be valid under different conditions....

If lightning has only DC components why aren't lightning arrestors simply a 1/4-wave shunt?. Certainly a 1/4-wave piece of copper will have much better clamping than a gas tube that needs to get up to ionization voltage first.

Yea, 20,000 amps for a hit is going to heat things up a bit. If it is real armor with sliding plates and rivets I'd expect a few very hot spots.

The really fun part comes when one looks at the magnetic fields generated by the current flowing through on the armor. Since the current is all flowing from top to bottom there will be quite a bit of force pushing the armor in from all directions. Think electro-forming. The armor and the guy wearing it are likely to end up very thin after the hit.

Here is an example when you hit coins with this sort of a field. (The geometry is a bit different from the armor case, but the concepts are similar.)

http://205.243.100.155/frames/interesting1.html

-wolfgang

Because DC is infinite wavelength. Only AC has wavelengths.

I would expect a meltdown. I've seen what's left of a steel outdoor cabinet and tower that received a direct hit. Whether anyone inside the cabinet would have survived is rather dubious. However, it still beats getting hit without any "protection".

Actually, the electric field will tend to repel the lightning towards the outside of the conductor. That's how skin effect works. My guess(tm) is that a substantial amount of the current will be going through the ionized air surrounding the suit of armour.

I don't think the magnetic field will have much effect. What was left of the tower and cabinet looked melted, not deformed. Had there been a substantial magnetic field involved, my guess(tm) is that there would have been more mechnical deformation instead of just melting.

Wheee... 3000 to 7000 Joules at 100,000 amps.

On Sat, 22 Jul 2006 04:42:11 GMT, Jeff Liebermann wrote in :

Not necessarily. Quite a few people survive direct lightning hits. If instead the lightning hits a metal enclosure with a person inside (your suit of armor), I think the odds of survival would be lower.

Let me try again. If lightning only had energy at DC and one had an antenna receiving 2.4ghz then one could short the center conductor of the cable to the outer conductor with a big-honking (tm) piece of copper that was cut to 1/4 wavelength. This lightning protector would have much lower resistance than any gas tube.

The fact that folks don't regularly do this makes me suspect that lightning does have significant RF components.

It is not clear. The picture of the guy with ipod that was hit clearly shows horrible-looking burn marks where the earbud wires ran. Looks like any conducting protection needs a layer of Nomex below it just to be safe.

If I'm not mistaken, the magnetic field in parallel wires that are all carrying current in the same direction sucks the wires together. I would expect a long hollow tube carrying a lot of current to collapse at some point.

That site also has an amazing picture of a lightning bolt. (search for "Huge positive lightning bolt"). What people do in the name of science...

http://205.243.100.155/frames/longarc.htm

-wolfgang

Lightning is a major RF source. Those RF components cause so much damage. The frequency spectrum is:

Therefore lightning causes radio frequency interference on AM (longwave) radio. If lightning were DC, then radio frequency interference would not occur.

I most certainly did, from The Colour of Magic, succint discription of TwoFlower.

Had you thought about the fact that immediately prior to the lightning strike, both voltage and current are effectively zero in the object, then say 1e-5 seconds later, theyre a couple of hundred KV / Amps etc? Sounds like a variable current to me.

yes.

-wolfgang

That was my suspicion too, since as a kid I used to have an AM "xtal" (actually germanium diode) radio that I used to listen to lightning strikes on.

The graph above does show the RF drops off rapidly. Too bad its not log-log so we can see what the numbers are for 2.4 Ghz.

The most important take-home point I see here is that a 1/4-wave filter for 2.4Ghz constructed out of heavy copper and grounded well would be a very good thing to use in addition to a gas-discharge tube lightning arrestor.

-wolfgang

More good general lightning information here (what a url!):

-wolfgang

"Wolfgang S. Rupprecht" hath wroth:

Ok, you're 75% correct (which I guess makes me 25% wrong). Lightning does have substantial energy content at RF frequencies. That's because the spectra of any impulse includes a broad spectra of RF junk. Incidentally, that's roughly the way pulsed type UWB (ultra wide band) works. The problems are: 1. The spectra is not at any single frequency that can be tuned with a 1/4 wave section. 2. The overwhelming bulk of the energy is strictly charge transfer, which is DC current. This is why you don't see "tuned" lightning rods.

Well, the fact that you can hear lightning on any AM radio, or see it on an over the air TV signal, such indicate something. Most of the HF (high frequency) radio noise comes from approximately 50-100 lightning hits per second somewhere on the planet. That's quite a bit of RF noise, but nothing compared to the energy concentrated in a bulk charge transfer.

Yuck. I wonder if Apple will honor the warranty on the iPod.

I suspect medieval knights in shining armour probably wore something under all that metal. Googling.... Well, there's a line of sportsware that offers UnderArmour clothing:

says the armour is worn over linen or woolen clothes.

I wonder if the insulation might form a capacitor between the armour and the body?

mind. Pretend I didn't suggest it.

Anyway, my original point is that wearing armour (over an insulator) will divert most of the lightning current around the body and hopefully into the ground.

You're correct (again). The do attract which will cause the armour to collapse somewhat. Maybe lightning armour idea isn't such a great idea.

High voltage is so much fun:

of lightning photos: One of my friends sometimes takes pictures of high voltage phenomenon at his work. He mentioned something about destroying 5 digital cameras (so far).