Downed power line and walking near it

Thread Starter

bootloader9800

Joined Jan 12, 2021
79
Hi Al. I once remember reading in a textbook by Nelms that when one encounters a downed power line:
1) You should not walk with your feet spread apart but rather with feet close to each other.
2) Power could be travelling inside the earth even if you are like 50 feet away.

Can someone please explain/add as to why u shouldn't walk in long strides. Ty
 

MrSalts

Joined Apr 2, 2020
1,850
Hi Al. I once remember reading in a textbook by Nelms that when one encounters a downed power line:
1) You should not walk with your feet spread apart but rather with feet close to each other.
2) Power could be travelling inside the earth even if you are like 50 feet away.

Can someone please explain/add as to why u shouldn't walk in long strides. Ty
Because your legs make a parallel current path with the ground. If your legs are lower resistance than current traveling though the ground, you can get current flow up one leg and down the other. The bigger your steps, the larger the voltage difference.
Homework, assume the resistance up one leg of a crow and down the other is 50,000 ohms. If the crow sitting on a 50,000V power line made of 000-gauge copper cable, how much current goes through the crow's legs if his feet are 1, 2 and 5 cm apart. How much electrical heat is generated by the current flow in the crow's legs at each spacing
 

crutschow

Joined Mar 14, 2008
30,150
why u shouldn't walk in long strides
When a power line is shorted to ground, there is a voltage gradient radiating away from the wire in all directions due to the resistance of the ground, so the amount of voltage between your legs depends upon the distance between your legs.
Thus a longer stride would cause more voltage between your legs, and a greater chance of getting a lethal shock.
 

Thread Starter

bootloader9800

Joined Jan 12, 2021
79
Because your legs make a parallel current path with the ground. If your legs are lower resistance than current traveling though the ground, you can get current flow up one leg and down the other. The bigger your steps, the larger the voltage difference.
Homework, assume the resistance up one leg of a crow and down the other is 50,000 ohms. If the crow sitting on a 50,000V power line made of 000-gauge copper cable, how much current goes through the crow's legs if his feet are 1, 2 and 5 cm apart. How much electrical heat is generated by the current flow in the crow's legs at each spacing
I would say, you would have a totally well grilled crow to begin with.

Resistance = Resistivity * ( 1/ Area of cross section of the 000-gauge copper wire)
50,000 ohms = Resistivity * ( 1/ 85.0288 mm^2) [ Area of cross section of 000-gauge copper wire is 85.0288 mm^2]
Resistivity = 4,251,440 ohm-mm^2

1 cm apart:
Resistance = 4,251,440 ohm-mm^2 * ( 10 mm / 85.0288 mm^2) = 500K Ohms
Current = Voltage / Resistance = 50,000V / 500K Ohms = 0.1 A
Power = (V^2) / Resistance = 5K Watts

2 cm apart:
Resistance = 4,251,440 ohm-mm^2 * ( 20 mm / 85.0288 mm^2) = 1 M Ohms
Current = Voltage / Resistance = 50,000V / 1M Ohms = 0.05 A
Power = (V^2) / Resistance = 2.5K Watts

5 cm apart:
Resistance = 4,251,440 ohm-mm^2 * ( 50 mm / 85.0288 mm^2) = 2.5 M Ohms
Current = Voltage / Resistance = 50,000V / 2.5M Ohms = 0.02 A
Power = (V^2) / Resistance = 1K Watt
 
Last edited:

MrSalts

Joined Apr 2, 2020
1,850
I would say, you would have a totally well grilled crow to begin with.

Resistance = Resistivity * ( 1/ Area of cross section of the 000-gauge copper wire)
50,000 ohms = Resistivity * ( 1/ 85.0288 mm^2) [ Area of cross section of 000-gauge copper wire is 85.0288 mm^2]
Resistivity = 4,251,440 ohm-mm^2

1 cm apart:
Resistance = 4,251,440 ohm-mm^2 * ( 10 mm / 85.0288 mm^2) = 500K Ohms
Current = Voltage / Resistance = 50,000V / 500K Ohms = 0.1 A
Power = (V^2) / Resistance = 5K Watts

2 cm apart:
Resistance = 4,251,440 ohm-mm^2 * ( 20 mm / 85.0288 mm^2) = 1 M Ohms
Current = Voltage / Resistance = 50,000V / 1M Ohms = 0.05 A
Power = (V^2) / Resistance = 2.5K Watts

5 cm apart:
Resistance = 4,251,440 ohm-mm^2 * ( 50 mm / 85.0288 mm^2) = 2.5 M Ohms
Current = Voltage / Resistance = 50,000V / 2.5M Ohms = 0.02 A
Power = (V^2) / Resistance = 1K Watt
You have to remember that the the crows feet (legs) are in parallel with the copper power line.
 

nsaspook

Joined Aug 27, 2009
9,993
Ground Potential Rise is not a static condition with a downed power line.

What seems to be safe at one second, could be a death trap the next. That chain link fence suddenly became very hot. Electrical energy takes ALL paths, not just the one of least resistance.
 
Last edited:

SamR

Joined Mar 19, 2019
4,304
Two proposed methods of escape. Hopping with the feet together or running such that only one foot hits the ground at a time. Or, unless absolutely necessary to move for safety, stand your ground until the power is off. You have already survived so stick it out instead of falling trying to hop or run and killing yourself after you already made it this far!
 

nsaspook

Joined Aug 27, 2009
9,993
Two proposed methods of escape. Hopping with the feet together or running such that only one foot hits the ground at a time. Or, unless absolutely necessary to move for safety, stand your ground until the power is off. You have already survived so stick it out instead of falling trying to hop or run and killing yourself after you already made it this far!
It's pretty hard to stand still if you see someone zapped.

 
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