I used to work in an office building where smokers had to go out onto the loading dock to enjoy our cigs. The loading dock was on one side of the building as was a parking garage which overhung the loading dock. There were lightning rods on the top deck of the parking garage. On one side of the loading dock a heavy exposed (no cover) aluminum cable came from the the parking garage above and entered the concrete of the loading dock. It was pretty obvious the cable was the ground for one or more lightning rods.
In addition to leaning against the walls, some folks occasionally would hold onto the cable with one hand while smoking. These people would even hang onto the cable when thunder and lightning was nearby. I didn’t think this was a great idea, but not wanting to appear a nervous Nellie, I rarely said anything.
Luckily, I never had to observe what effect a strike to the garage would have on our cable hanger (or the rest of us). While the cable would have much less resistance than our hapless human, I’m guessing the strike wouldn’t go unnoticed. How bad would it likely be? Shock of their life? Burned hand? Worse?
it’s one of those all depends. death and burns are all possible. depends on how much electricity goes through that particular cable and how the person was touching it and other objects.
Once had the lightning rod attached to our building - and by that I mean, not just the cable but the entire rod - vaporized during a bad storm. So yeah, it could potentially be bad.
The cable may be a much better conductor than the human, but there will still be a proportionate current pass though the human. What matters is the proportion.
A simplistic analysis of just the human versus conductor. Some numbers stolen from this useful document. The bulk resistivity of a human is about 5Ωm, and you could say maybe 500Ω hand to foot. Copper is a vastly better conductor, resistivity of 1.7x10[sup]-8[/sup]Ωm. But even the heaviest lightning conductors are only about 5cm[sup]2[/sup], and start at about 1cm[sup]2[/sup]. So you get about 0.00004Ω over 1.5 metres at best, and maybe only 0.0002Ω at worst. Sounds good so far. Direct lightning strikes will deliver very significant current. 50% of strikes deliver over 28kA. So over the 1.5 metre gap from hand to ground, the voltage across the lightning conductor will be in the range roughly 1 to 6 volts. Which isn’t going to kill anything. Moving to the unusual strikes, you can get ten times that current in the last fraction of a percent of strikes, and that would see 10 to 60 volts, which does get you into the range where it might kill you. 50 V at the wrong moment will send your heart into fibrillation, so it isn’t to be ignored.
All of this depends upon the strike behaving like a simple Ohm’s Law problem, which it probably will not. The voltages involved can create all sorts of interesting and weird behaviours that mean the basic analysis is invalidated. It particularly needs the grounding system to be very well installed. If the ground is dry, or there is not a good very low resistance path in the ground, things get a lot nastier. One Ohm in the ground path and you have between 28kV and 200kV (50% of strikes) on your hand. That will kill you stone dead. Not heart stopped electrocuted (and revivable) dead, but irreversible large scale tissue damage dead. 100Ω in the ground (which could happen in very dry ground) and you will have a couple of million volts looking for a home. Then you don’t even want to be near the conductor.
Which brings up the issue of what the ground potential is. Where the lightning conductor enters the ground will be elevated to a seriously high potential. The potential difference with distance radially from the entry point can be many thousands or tens of thousands of volts per metre. Standing right next to a lightning conductor could cause you to be electrocuted via your legs. A hand on the conductor and feet even a small distance from the entry point in the ground will probably mean thousands of volts between your hand and feet - simply because of the voltage drop between the point on the ground where you stand and the actual conductor. Again, you won’t be electrocuted dead, you will be fried meat dead.
One further thing to think about - lighting conductors also get hot. They are designed for peak temperatures of 250C. That will cook your hand if you are holding onto it.
Very interesting analysis - thanks. From the description of tree damage from moisture heating/expansion from your link it seems quite possible that a human could be pretty well poached!
If it were, in general, dangerous to touch that wire, it would be installed in such a way that you couldn’t easily do so.
And to add to Francis Vaughan’s analysis, that’s assuming that lightning even strikes the lightning rod in the first place. A lightning rod’s primary purpose is not to draw lightning bolts, but to draw off smaller zaps to discharge the air before it becomes lightning.
I worked at a forging shop years ago and I went up the roof with a company that they hired to replace a lighting rod after several bolts struck the building. From the damage around the rod and to the rod itself, it was clear that if anybody had been close to the rod, they would have been killed.
It should have been installed so it was not so accessible, but I’m sure all lightning protection contractors are not created equal. Also, this was an office building so the loading dock would hardly ever be used until the smokers were exiled out of the building. The cable was definitely for lightning protection.
This isn’t quite right. While lightning rods may draw off some charge, if a strike is imminent a lightning rod is not appreciably going to reduce the chance of it occurring. It’s true that lightning rods don’t “draw lightning” in the sense of causing it to occur or reaching out to cause it to hit from some distance much further away than it normally would. However, due to its low resistance the lightning rod makes it likely that a strong streamer or leader is produced to meet one of the downward propagating steamers and causing the lightning to hit the lightning rod rather than some other nearby point.