Hi all, I've been reading the allaboutcircuits text book and the section on electrical safety and this has raised a query. There is a section about downed power lines:
https://www.allaboutcircuits.com/textbook/direct-current/chpt-3/common-sources-hazard/?cb=1576709438

And how you should stand on one foot or run so that both feet don't contact the ground at the same time. Why can't current flow into the body and out via the one leg? Its suggesting you wouldn't be shocked if only one leg contacts the ground. I get that it is better than being on two legs because in that case current can pass across the body thus reducing the resistance due to two paths. But I don't see why you wouldn't be shocked at all. Not planning on coming across a downed line any time soon but it's the theory which can be applied to many situations. Thanks for your help

 

It does but at a very low potential difference hence low voltage. We have discussed and cussed this at length. What I was taught was to keep the feet together and hop. Also not a very good idea and I don't know many people that can keep their balance on one foot for very long either. Same reason birds on a wire are not electrocuted.

 

That's a whole lot more rational recommendation than an older recommendation to hop on one foot. Ever ask someone to hop on one foot for any distance? They usually fall and catch themselves with both feet and hands touching the ground a fair distance apart.

Shuffling is another alternative that I Like. Remember that power lines don't fall in clear skies and no wind. The weather is usually inclement. The ground is wet and slippery. There is not a huge rush to get away. You would already be dead, if that were the case, but there is a need to get away in the most stable manner. Even 90-year olds can shuffle. Very few can sprint with only one foot touching the ground at a time..

 

And how you should stand on one foot or run so that both feet don't contact the ground at the same time. Why can't current flow into the body and out via the one leg?

Electrons are very smart, and they place a high value on frugality and efficiency; they don't like to waste time and effort on something that's pointless. So they're not in the habit of going wandering off down some path and then stopping at some point, saying "Aww forget it," then turning around and retracing their steps right back to their starting point. They really like to go decisively from Point A to Point B, but there has to be some motivation-- like a potential difference between A and B.

If there's no Point B, only a Point A, there's no motivation so the electrons stay put. Thus you don't get shocked if you only make contact with one foot.

 

But there is a potential difference. Ground is ground, so to speak.

https://cornhusker-power.com/safety/step-potential/

John

 

LOL here we go again. All he wanted to know was why "birds on a wire" don't get electrocuted. OB kinda danced around it pretty well. The micro-shuffle gets my vote and you would know real quick if you moved yer feet too far apart.

 

A agree. I tingle is better than being crispy.

 

I was also taught to hop with your feet together.

 

If the voltage drop where you are is 100v/foot (a random but not unreasonable value that's both dangerous and survivable), then jogging on your toes (one foot down only, front of feet only) exposes your foot to thirty volts or so. Flat-footed, it's more like 100v. Taking a big step with both feet down, it's 300v, AND the current goes through your torso, not just your foot. Standing sideways, your foot has less voltage across it, but most of us can't walk or run sideways keeping one foot in the air.

Your goal is to keep that distance minimized until you're a good safe distance away. like past something that's big and grounded. Personally I'd jog (or prance, if you will) on tiptoes.

 

I'd be dead... I think the consensus was if you were still alive, don't move! It's kinda like the guy in a minefield, "I ain't movin til someone starts shootin"!

 

I attended a presentation by some power folks that worked with real HV type of lines (500Kv). With even lower voltage power lines, their advice was "hop with both feet together". Then when you think it is far enough to be safe, "go three times farther". Running is dangerous, as it is likely your two feet will contact at the same time at some point, or you lose balance on just one foot...
The potential across a couple of feet (a step) can be thousands of volts if close to a real downed HV power line.
This fellow worked on 1/2MV lines with a steel mesh suit (chain mail suit), so I think he knows what real HV feels like...
If in a car, stay in it, you are safer there even if the power line is on the car. Even if the tires arc and burn out, the car provides a short for the HV and as long as you don't touch one end of the car to the other, you are "relative;y safe" inside. Of course, circumstances may be different in each case, but remember that over a foot or two, the voltage potential can be lethal.

 

Ok so long story short the bird isn't shocked on a power line because there is no difference in electric potential between its two feet because its standing on a conductor with little resistance. With a downed power Line there is a big voltage drop between the point of the line touching the ground and the system's ground point because there is a lot of resistance from the earth. Even though someone on two feet is a small distance across the earth there could be significant voltage drop between these two points and infact the person's body may present less resistance than the ground and the current will flow through one leg and out the other?

 

So don't be "manly" and stand with yer legs widespread. It could be hazardous to yer health.