Need an understandable explanation.

Think of one of those long high-power variable resistors with a viper you screw into place. When you have a powerline down, the ground is that variable resistor, the ends being electrical earth and the powerline. If you probe two points on the axis of the resistor with a multimeter in voltage mode, you will get a voltage between them. With the voltage and current levels of powerlines (10-20 kV at kiloamps), you can easily get lethal currents between two points as little as 1/2 a foot (15cm) apart, depending on variables you don't have any control over, such as soil moisture, base conductivity, air moisture etc, so the safest bet is to assume the worst case.

Also, when it comes to downed power lines, be aware that arcing can be deceptive, it may be small puny arcs when it lays on the ground, but as soon as you get close enough, you get a "JESUS HELLO KITTY CHRIST!" arc, as this video shows (relevant part being up to about a minute or so from from the point where the link drops you):

This is why you always call 811 or your national equivalent before big digs... And why you respect the authoritae of high voltage. The instant you don't respect it, it can at least hurt you if not kill you.

/Teo.

Imagine that the wire is touching the centre of a (very large) halved onion. That is, divide the surrounding earth into concentric hemispherical shells of constant thickness.

Assuming the ground is a uniformly bad 3-D conductor, the voltage drop from shell N to N+1 goes as the resistance of shell N, which goes inversely as its surface area.

Thus dV/dr goes as 1/r**2. This assumption will be poor near the contact point, but better further away.

Cheers

Phil Hobbs

Not generally a problem for humans, but cows and horses can easily be killed since their 'feet' are father apart than ours.

And they don't wear shoes.

Except the stylish ones.

Tim

Drop a rock into a pond. You get a wave spreading out from the point of impact with an amplitude that diminishes with distance. There's your visual analogy.

I think they've covered it pretty well. For the record it is called a groun d gradient.

It also applies to lightning strikes. With those the gradient can be much h igher because of the higher voltage. It can also break down the insulation of your shoes.

If a fallen wire is only like 2,200 volts, with shoes you should be alright . The real high tension wires are of course more dangerous, but nothing lik e lightning. Those are the wires that are much higher up on bigger poles. I am not sure of their voltage, and it is not the same everywhere.

Small ground gradients can bother equipment, like if your house has a prope r copper rod in the ground for Earth grounding and the cable guy comes and grounds the cable to a cold water pipe. Each building should have only one ground but if that is deemed insufficient, all grounds should be tied toget her at the panel. The cable and phone guys should ground to a piece of cond uit but some houses have no conduit. I like conduit in basements ad garages because heavy stuff might move around and damage Romex. But then some hous es ddon't have basements or garages.

Residential distribution in our area is a nominal 7200 V.

Wet shoes can be more conductive than you'd think, and grass is often at least a bit damp.

I just got shocked by 12 V yesterday. which "shocked" me! I pressed my fingers down to push a buard under test so it would make better contact with a bed of nails tester. I must have gotten very close to the point where it would have made me bleed when I got zapped.

Jon

Also, when you get into the kilovolts range, humans are excellent conductors.

/Teo.

Thanks all for the understandable answers.

While Googling around for answers I stumbled upon quite a few YouTube videos of fearless guys repairing power lines carrying hundreds of thousands of volts by approaching them via helicopters. Here's one now:

I wonder how much they pay those guys?

Jeff

Some guys are adrenaline junkies. I can think of a couple who'd probably pay for the privilege.

Cheers

Phil "adrenaline ned not apply" Hobbs

Pretty well, AFAIK. Not so much for hazard pay I think, as it's often on-demand with long hours.

Tim

googling around suggest something 100k, with lots of free time and waiting around

doesn't seem like much, but I guess there a people fighting wars for less

-Lasse

...and fires, forest or otherwise.