# Question about electrically common points

Discussion in 'General Electronics Chat' started by johnmp, Dec 22, 2014.

1. ### johnmp Thread Starter New Member

Dec 14, 2014
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0
I was reading the e-book and there's something i really don't get. In this picture it shows the person not getting shocked because he's electrically common to the ground, which i understand.

In this picture it shows a guy getting shocked because he's standing in the path of the current, but isn't he pretty much in the same situation as the guy in the last picture, and isn't he electrically common to the ground as well? So I don't understand why he actually gets shocked.

Also, the book says this: "There is no such thing as voltage "on" or "at" a single point in the circuit, and so the bird contacting a single point in the above circuit has no voltage applied across its body to establish a current through it. Yes, even though they rest on two feet, both feet are touching the same wire, making them electrically common." But the book also says later that if you see a downed power line you should stand on only one foot to prevent your body from creating a circuit. Why isn't it the same case as the bird where you can stand on two feet and still be fine?

2. ### #12 Expert

Nov 30, 2010
16,655
7,294
Because wet soil has more resistance than aluminum wire. Connect 30,000 volts to wet soil and it creates a voltage gradient across the surface as the current dissipates into the planet. If it takes a 60 foot radius circle to dissipate the current, how much voltage would be applied to your feet at 30 inches apart? 500 volts per foot times 2.5 feet = 1250V

If this complete guess about voltage per foot is anywhere near correct, it's a surprise if the 500 volts across one of your feet doesn't force you to lose your composure, fall over, and get electrocuted (to death).

Edit: I just noticed, this is all depicted in the drawing, but the drawing used 2400 volts.

Last edited: Dec 22, 2014
3. ### johnmp Thread Starter New Member

Dec 14, 2014
4
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Ohhhhh so it's because the dirt has resistance. That explains it ty for the help #12

4. ### MikeML AAC Fanatic!

Oct 2, 2009
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Even the wire between the bird's two feet has some resistance. It is just that the voltage gradient between the bird's feet is only mV; no where near enough to be felt...