Touching live wires on high voltage line.

Thread Starter

aviioo

Joined Aug 27, 2020
5
Hey everyone :)

I'm working as an automation engineer and I've always asked myself why people say, that for example birds, can sit on power-lines, because they aren't touching neutral/different potential and no current is flowing. I can't really get this explanation into my head, because technically isn't it just a parallel circuit with the power-line. I tried the circuit in a simulation tool, and according to the tool the current is in the micro/nano Ampere range. I copied the circuit below with two resistors simulating a bird and about 1km of wire.

I'm probably totally wrong, but I'd love to hear an actual explanation :)

Unbenannt.PNG
 

BobTPH

Joined Jun 5, 2013
2,538
You are correct in that the the bird is touching multiple points on the wire and the will be a potential difference between those points. But the voltage (potential difference) will be tiny, nanovolts most likely, and it does not have anything to do with the voltage between the wire and ground or any other wire. It depends only on the current carried and the resistance of the small section of wire it is touching.

Edited to add: what is wrong with your simulation is that the bird is not touching two points on the wire 100m apart! Try a couple of cm.

Bob
 

Thread Starter

aviioo

Joined Aug 27, 2020
5
You are correct in that the the bird is touching multiple points on the wire and the will be a potential difference between those points. But the voltage (potential difference) will be tiny, nanovolts most likely, and it does not have anything to do with the voltage between the wire and ground or any other wire. It depends only on the current carried and the resistance of the small section of wire it is touching.

Bob
I didn't consider this! So basically the resistance is barely a milli ohm. What exactly does the Voltage have to do with this tho, I mean yes the Voltage is basically zero between a few centimeters of wire, but current still flows, because I'm basically completing a circuit with neutral over the transformator. Or am I just thinking way too far?
 
Last edited:

ericgibbs

Joined Jan 29, 2010
10,759
hi,
The two 'feet' of the Bird are effectively in Parallel with the milliOhm length of the wire.
As the resistance of the birds body 'foot to foot' is high, only a very small current would flow via the birds body.

E
 

andrewmm

Joined Feb 25, 2011
572
Its current that kills not voltage,

But its an interesting question that got me thinking.

The birds feet are close together , and joined by a large bit of metal they are resting on,
as such , both feet of the bird are at the same voltage, and if at the same voltage , no current can flow.

Think of two resistors in parallel,
one say 10 M Ohm, the other 0.01 Ohm.

you apply a voltage across the parallel resistors say 10 V , and you see a current flow.
now you put a meter in the lead of each resistor to see how the current flows.

You see that "nothing" flows through the 10 M , and 1000A flows through the 0.01 ohm resistor.

But the ends of the 10 MOhm resistor are still 10V difference.

The bird will not feel 10V at "zero" amps.
 

Thread Starter

aviioo

Joined Aug 27, 2020
5
You are correct in that the the bird is touching multiple points on the wire and the will be a potential difference between those points. But the voltage (potential difference) will be tiny, nanovolts most likely, and it does not have anything to do with the voltage between the wire and ground or any other wire. It depends only on the current carried and the resistance of the small section of wire it is touching.

Edited to add: what is wrong with your simulation is that the bird is not touching two points on the wire 100m apart! Try a couple of cm.

Bob
Its current that kills not voltage,

But its an interesting question that got me thinking.

The birds feet are close together , and joined by a large bit of metal they are resting on,
as such , both feet of the bird are at the same voltage, and if at the same voltage , no current can flow.

Think of two resistors in parallel,
one say 10 M Ohm, the other 0.01 Ohm.

you apply a voltage across the parallel resistors say 10 V , and you see a current flow.
now you put a meter in the lead of each resistor to see how the current flows.

You see that "nothing" flows through the 10 M , and 1000A flows through the 0.01 ohm resistor.

But the ends of the 10 MOhm resistor are still 10V difference.

The bird will not feel 10V at "zero" amps.
Energy kills not Current or Voltage as far as I know. I'm just confused by the "He's on the same potential, so no current flows" because that statement isn't true, it's just a really really small amount of current, because the other resistor is almost zero
 

Alec_t

Joined Sep 17, 2013
11,637
Even a power line resistance of only 1mΩ between the bird's feet could be a problem if there's enough current flowing through the line. At 1000A there woud be 1V between the feet. 1V x 1000A = 1kW. But I suspect the bird would fly off long before it got roasted :).
 

andrewmm

Joined Feb 25, 2011
572
in general, power does not kill, its current that drives the muscles in the body and those begin driven by an outside source does the killing. Think heart stopping / starting , lungs not being able to move due to muscles contracted by the current.

( That is unless you really put a lot of current through the object, then the local heating does the killing , see the comments on flames here )

https://en.wikipedia.org/wiki/Old_Sparky


I don't know about birds,
but for humans , there is a minimum perceptible current that must flow through the body to be detected, of about 1 mA AC

https://www.wikilectures.eu/w/ELECTRICITY_AND_HUMAN_BODY#:~:text=The minimum current a human,which leads to cardiac arrest.


Assuming the birds feet are say 10mm apart,
and joined by the large conductor they are sitting on,
what would the resistance be between there legs ? Im guessing much less than 1 m Ohm

You have to have a voltage difference to drive a current,
so whats the voltage between the legs,
assuming there is say 100 amps at 10 Kv flowing,
one leg would be at 10 Kv, the other at 10 Kv + or - ( 100 * 0.001 )
so there is 0.1 volt between the legs,

Assuming the body is 1 MOhm, ,
what would the current through the body be ?

I = V / R
 

andrewmm

Joined Feb 25, 2011
572
Even a power line resistance of only 1mΩ between the bird's feet could be a problem if there's enough current flowing through the line. At 1000A there woud be 1V between the feet. 1V x 1000A = 1kW. But I suspect the bird would fly off long before it got roasted :).
two things,
even with 1 V between ones feet, the current flowing through the body is 1v divide by the birds resistance, say 1 M Ohm,
i.e. next to nothing, it would not be electrocuted.


A power line dropping 1 volt per 10 mm ( the gap between the birds feet ) , would be a good heater, but not cover many miles ,
 

Thread Starter

aviioo

Joined Aug 27, 2020
5
in general, power does not kill, its current that drives the muscles in the body and those begin driven by an outside source does the killing. Think heart stopping / starting , lungs not being able to move due to muscles contracted by the current.

( That is unless you really put a lot of current through the object, then the local heating does the killing , see the comments on flames here )

https://en.wikipedia.org/wiki/Old_Sparky


I don't know about birds,
but for humans , there is a minimum perceptible current that must flow through the body to be detected, of about 1 mA AC

https://www.wikilectures.eu/w/ELECTRICITY_AND_HUMAN_BODY#:~:text=The minimum current a human,which leads to cardiac arrest.


Assuming the birds feet are say 10mm apart,
and joined by the large conductor they are sitting on,
what would the resistance be between there legs ? Im guessing much less than 1 m Ohm

You have to have a voltage difference to drive a current,
so whats the voltage between the legs,
assuming there is say 100 amps at 10 Kv flowing,
one leg would be at 10 Kv, the other at 10 Kv + or - ( 100 * 0.001 )
so there is 0.1 volt between the legs,

Assuming the body is 1 MOhm, ,
what would the current through the body be ?

I = V / R
Energy isn't Power! (Power= U*I, Energy= Power*Time), you can touch high amounts of current for a really small amount of time (ns) RCD's don't immediately trip either they take up to 0.4 seconds to break the circuit and they're still considered safe.

So basically if I'd replace the aluminum/copper wire with some sort of wire which has a much bigger resistance, the Voltage difference would be a lot higher, and I'd get a much bigger shock, right?
 

andrewmm

Joined Feb 25, 2011
572
RCD tripping time is a different question,
they have a time constant, due to the start up surge of items,
There are different grades of RCD trip times A, B , C etc. used for different environments,

Fast and slow blow fuses were aimed at the same "problem".


An introduction to RCD's I like is here,

http://www.beama.org.uk/asset/7EE14AAB-81DB-4870-A9ACFBC5C58A94A2/#:~:text='High sensitivity' RCDs, rated,current to protect the user.


Re about changing the wire,
yes, as you change the wire resistance, the voltage difference between the birds feet would change.
as the birds body resistance is relatively constant, as you increase the resistance of the wire, and put a higher voltage across the birds feet, then a higher current flows, which is what causes death.

Its just good old ohms and kirchhoff's law as to where the current flows and its current that kills.
 

Thread Starter

aviioo

Joined Aug 27, 2020
5
Energy isn't Power! (Power= U*I, Energy= Power*Time), you can touch high amounts of current for a really small amount of time (ns) RCD's don't immediately trip either they take up to 0.4 seconds to break the circuit.

So basically if I'd replace the aluminum/copper wire with some sort of wire which has a much bigger resistance, the Voltage difference would be a lot higher, and I'd get a much bigger shock, right?
RCD tripping time is a different question,
they have a time constant, due to the start up surge of items,
There are different grades of RCD trip times A, B , C etc. used for different environments,

Fast and slow blow fuses were aimed at the same "problem".


An introduction to RCD's I like is here,

http://www.beama.org.uk/asset/7EE14AAB-81DB-4870-A9ACFBC5C58A94A2/#:~:text='High sensitivity' RCDs, rated,current to protect the user.


Re about changing the wire,
yes, as you change the wire resistance, the voltage difference between the birds feet would change.
as the birds body resistance is relatively constant, as you increase the resistance of the wire, and put a higher voltage across the birds feet, then a higher current flows, which is what causes death.

Its just good old ohms and kirchhoff's law as to where the current flows and its current that kills.
It's still not just current that kills tho.

1598527457818.png
 

andrewmm

Joined Feb 25, 2011
572
I don't know why you say its not current that kills,

but I think we have answered the original question as to why birds on live wires don't get electrocuted,

"there is such a little voltage drop across their feet, that next to zero current flows"
 

BobaMosfet

Joined Jul 1, 2009
1,207
Hey everyone :)

I'm working as an automation engineer and I've always asked myself why people say, that for example birds, can sit on power-lines, because they aren't touching neutral/different potential and no current is flowing. I can't really get this explanation into my head, because technically isn't it just a parallel circuit with the power-line. I tried the circuit in a simulation tool, and according to the tool the current is in the micro/nano Ampere range. I copied the circuit below with two resistors simulating a bird and about 1km of wire.

I'm probably totally wrong, but I'd love to hear an actual explanation :)

View attachment 215738
Here is how this works- current travels _all_ paths- including going through the bird. However, because the bird is sitting on the conductor with 2 points of contact, the difference between those two points of contact (it's feet) in the conductor is virtually zero. In order for current to want to take a different path (that is, through the bird in enough quantity to hurt the bird), there must be resistance between the bird's feet in the conductor. Current prefers the path of least resistance, so most of it will travel right through the conductor the bird is sitting on.

For all intents and purposes, because the conductor is so good, and the bird is so poor, there is no difference between the value in the conductor at one foot of the bird .v. the other foot, so the net current diverted through the bird is virtually zero.
 

nsaspook

Joined Aug 27, 2009
7,587
I don't know why you say its not current that kills,

but I think we have answered the original question as to why birds on live wires don't get electrocuted,

"there is such a little voltage drop across their feet, that next to zero current flows"
I think he using the correct physics meaning of current in that current is not electrical energy or power. It's a simple measure of charge moving in response to a force that's normally an electric field potential from X source. The physiological effects of the kinetic energy (heating) transferred from the field to the body's ions and the biological disruption caused by even low levels of energy on sensitive body functions are well known constants that can be calibrated to X current as a measurement.
 
Last edited:

andrewmm

Joined Feb 25, 2011
572
Its great when he set the supply to 11 volts, and puts it to his toung, he get 40 Amps, and boy does he feel that,.
on 1 volt, he gets no amps shown, and can't feel it.

and on 20 volts he has no amps between his fingers and can't feel it,.

Sort of proves that its the amps that kills, not the volts, the volts just drive the amps,

yes you cant have one without the other,

but the current is that does the damage,
as he has shown, volts can be varied, and if no or very little current flows, then no harm.
 

nsaspook

Joined Aug 27, 2009
7,587
The ampere is a rate not a measurement of force. It's a rate of electricity but using the physics meaning of electricity means amps are NOT energy. Current is part of a system that transfers electrical energy in current electricity.
https://physics.info/electric-current/
https://simple.wikipedia.org/wiki/Electricity
Electricity is the presence and flow of electric charge. Using electricity we can transfer energy in ways that allow us to do simple chores.[1] Its best-known form is the flow of electrons through conductors such as copper wires.

The word "electricity" is sometimes used to mean "electrical energy". They are not the same thing: electricity is a transmission medium for electrical energy, like sea water is a transmission medium for wave energy. An item which allows electricity to move through it is called a conductor. Copper wires and other metal items are good conductors, allowing electricity to move through them and transmit electrical energy. Plastic is a bad conductor (also called an insulator) and does not allow much electricity to move through it so it will stop the transmission of electrical energy.
https://www.nist.gov/si-redefinition/ampere-introduction

Example:
One amp is the amount of current produced by a force of one volt acting through the resistance of one ohm.
 

andrewmm

Joined Feb 25, 2011
572
Hey everyone :)

I'm working as an automation engineer and I've always asked myself why people say, that for example birds, can sit on power-lines, because they aren't touching neutral/different potential and no current is flowing. I can't really get this explanation into my head, because technically isn't it just a parallel circuit with the power-line. I tried the circuit in a simulation tool, and according to the tool the current is in the micro/nano Ampere range. I copied the circuit below with two resistors simulating a bird and about 1km of wire.

I'm probably totally wrong, but I'd love to hear an actual explanation :)

View attachment 215738

I think we have answered the OP now ?
 
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