Posted: 2007-03-26, 20:29 Electicity Question for someone who doesnt know anything
I was having a debate with one of my friends over certain situation.
If you are hanging from powerlines (but not touching the ground or anything else) would you get electrocuted? (imagine a squirrel upside down i guess).
And a follow up; in order for electricity to pass through you; you need to be grounded correct?
Posted: 2007-03-31, 00:24 Re: Electicity Question for someone who doesnt know anything
BigZed wrote:
I was having a debate with one of my friends over certain situation.
If you are hanging from powerlines (but not touching the ground or anything else) would you get electrocuted? (imagine a squirrel upside down i guess).
And a follow up; in order for electricity to pass through you; you need to be grounded correct?
Thanks for any help.
Can you hang from a power line without getting electrocuted? Yes, with exceptions.
Do you need to be grounded? No, all you need is a potential difference.
For example, you're hanging from the power line with one hand and reach over to another phase. You are instantly fried, but not grounded.
Another example. A cow is standing in a field and lightning strikes close by. The tremendous amount of current in the ground has a parallel path up the cow's rear legs and down it's front legs. If the current flowing through the resistance of the ground causes enough voltage difference between the cow's front and rear legs, the cow can be electrocuted.
Take that example one step further. You're still hanging on the power line with one hand. You reach up with the other hand and grab the same wire. Your body is now in parallel with the wire. If the potential difference between your hands is significant enough, current can flow through your body and kill you.
That is why even though you see birds sitting on relatively low voltage distribution lines, you rarely see them on high voltage transmission lines. The potential difference between their legs makes it too uncomfortable.
Posted: 2007-04-01, 20:51 Re: Electicity Question for someone who doesnt know anything
BigZed wrote:
And a follow up; in order for electricity to pass through you; you need to be grounded correct?
You need to complete a circuit. If one side of the circuit is grounded, and you are grounded, and you touch a conductor connected to the other pole of the circuit, current can flow through your body, in at your hand, and out at your foot, where it touches the ground.
But if neither pole of the power supply is grounded, and you touch one wire with your left hand, and the other wire with your right hand, the current will still flow from one hand to the other, and you'll still die, even if you're standing on a rubber mat 3 inches thick!
WFO wrote:
Take that example one step further. You're still hanging on the power line with one hand. You reach up with the other hand and grab the same wire. Your body is now in parallel with the wire. If the potential difference between your hands is significant enough, current can flow through your body and kill you.
But it won't be in a practical power system. Come on now! That amount of voltage drop in that length of conductor! Think about what you're writing, man! You started off OK, but you got carried away, I think.
Last edited by contrex on 2007-04-03, 16:27; edited 2 times in total
Actually, it will be.
Keep in mind I'm talking about high voltage. If you read all of my post, you'll note I said that it makes little difference at "lower" potentials (for example, less than 30 Kv). But when you get into transmission voltages (hundreds of amps at 138 to 345 Kv), the potential difference is very real.
Next time you're out, look around and see which lines the birds are sitting on.
You're not answering my point. Imagine I could fly. I fly up to a 400 Kv cable and hang on to it with my arms outstretched. Let's say my hands are a metre apart.
Let's do the sums. I'll make the numbers simple. A possible current in a 400 Kv cable might be 1,000 Amps. Now the resistance of such a cable would be a very small number of milliohms per metre. say 0.5 to 2 thousandths of 1 ohm. (Ballpark figures) The voltage drop is given by V=IxR, so the potential difference between my two hands, when I am "in parallel" with the line, as you put it, is going to be around 0.5 to 2 volts. If the voltage drop in a human-spannable length was enough to even tickle you, the power line would be monstrously inefficient. It would be too hot to touch anyway. Can't you see that?
Birds don't generally sit on 400 Kv cables because they are too high up. Gulls sometimes do, though. They often sit on 132 kV and lower voltage cables.
I reckon that it would be corona and acoustic noise that would put birds off perching on HV cables, if anything was going to.
You're right, it's a classic case of taking theory a little too far (although at 5 milliamps through the heart to cause fibrillation, and assuming he had skinned his hands raw jumping from a helicopter onto the wire, and he had a pacemaker....... ).
You know it's strange to get a different argument and suddenly see a different perspective. I had forgotten that transmission lines are usually loaded (heat-wise) to the point that nothing would touch them without asbestos gloves (which birds usually can't get). Good point.
Having worked around 345 Kv lines in substations, I can attest to the fact that the static is awesome.
I'm going to have to think about the lines being too high for the birds though. That one might be for the birds (sorry, It was too hard to pass up).
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