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Power lines

by david90
Tags: lines, power
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david90
#1
Apr21-04, 04:39 PM
P: 303
when workers work on transimssion lines, they first use a rod to energerize themselves to avoid getting shock. But how come a bird doesn't get shock when it sits on the line without safely energizing itself first?

Does current from transmission line flow to the helicopter when it is in the air? The copter is not touching anything so why does current flow?
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Ebolamonk3y
#2
Apr21-04, 08:15 PM
P: 204
no grounding. current cannot flow. equal potential...

Also helicopter is made of metal... electric field likes to reside on the outside of conductors...
chroot
#3
Apr21-04, 11:42 PM
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Very little current flows, just enough to equalize the potentials. Then the current stops.

Helicopters produce a lot of static electricity that birds do not. Notably, the blade cutting through the air produce quite a bit.

- Warren

TeV
#4
Apr22-04, 07:07 AM
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Power lines

The current (very small one though) in the bird flows all the time as it sits on a power line.This is becouse power line works in AC regime.
Power line Electric field and corresponding voltage with respect to the neutral oscillate at rate of 50 or 60 Hz.The same happens with a bird's body and the current flowing truogh the bird that sits on it is called capactive current.Higher the voltage , frequency ,capacity higher the current.You will not see a big bird like ravens to sit on ultra high voltage power line (V>300 kV).Not just becouse it will be shocked when he approach to sit (?) or becouse current would exceed perception treshold,or a spourios corona discharges in strong E-field, but becouse most birds can sense EM fields and "know" what is not good for them so they will not even attempt sitting in that situation (clever beasts).

Quote Quote by david90

Does current from transmission line flow to the helicopter when it is in the air? The copter is not touching anything so why does current flow?
Ha?In what possition with respect to power line the copter isDoes it touch a power line by a hanging wire or something or is it just in vicity of it ?

TeV
zhana
#5
May3-04, 04:46 AM
P: 11
I was told by my professor current can't flow into birds on power lines becouse of the same potential.Closed circuit path is needed for current to flow.
TeV
#6
May3-04, 05:57 AM
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Quote Quote by zhana
I was told by my professor current can't flow into birds on power lines becouse of the same potential.Closed circuit path is needed for current to flow.
Your profesor is quite right as long as constant (DC) potential is concerned.However,most of power lines today carry AC electricity (50 or 60 Hz).Assuming bird's legs conductive enough,you don't have to be "a rocket scientist" to evaluate current legs-body to bird steadily sitting on overhead power line.The simple formula for capacitive current (rms value) in AC circuit with pure sine wave is:
[tex]I=U\omega C[/tex]
where
[tex]U[/tex]..is potential difference (voltage)
[tex]C[/tex]..electrical capacity
[tex]\omega =2\pi f[/tex]..and f denotes frequency in Hz.
Example.Usually high voltage power lines are arranged in 3 phase system and if say nominal voltage is 380 kV,the voltage in the formula above is 220 kV due to line-phase conversion factor [tex]\sqrt{3}[/tex].
Furherly,let f=60 Hz and C~10pF (my estimation for a capacity of elevated bird of a crow size with respect to ground).
The formula points out I~0.8 mA.Not a much I guess,but if you have on mind that according to some refference I just dug out humans have threshold of finger perception current as low as 0.4 mA,be sure that crows can notice even less than that!One way or another,seems they don't like such electrotherapy and avoid approaching higher field levels.
turin
#7
May3-04, 12:31 PM
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Another consideration is the spacing between differently charged lines vs. size of object touching one of the lines. "Physical" contact is in some ways an archaic notion. In other words, just because an object does not "touch" a conductor at some different potential does not mean that the object cannot be forced to some different potential. There is a potential field produced around and between the conductors that has a gradient. The closer the conductors or the larger the object, the larger the potential difference will be from one side of the object to the other. A steady state current cannot flow through the object because, eventually, charge will build up in certain regions of the object. However, 500 mA can be deadly, and it doesn't take a very long duration of current to disrupt the heart. The point is that the transient can also kill.
zhana
#8
May4-04, 11:05 AM
P: 11
Thanks both of you for the enlightening,
Tev:I appreciate your answer becouse we are going to learn AC circuit basics on class next week!I showed your post to the professor and he said:"That's right,I totally forgot about birds' capacity,and the current is offered virtually closed path to flow through the capacity"He added if really high potentials -DC included too-are involved electrical discharge in the air might occur carrying small current.It is called corona.I see you mentioned this in your first post,but you neglected the effect for the sake of simplicity.

turin:Proffesor made a comment on this part of your post:" The closer the conductors or the larger the object, the larger the potential difference will be from one side of the object to the other".
He said that induced currents inside birds' bodies placed in electromagnetic fields of power lines are always present.Judging by fields shape in considered situation and size of the birds,the currents of that kind should be generally noticably smaller than in previous situation where the bird physically stands on AC power line.
turin
#9
May4-04, 12:39 PM
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zhana,
I was unclear. I meant that contact with both conductors is not necessary for transient current. But, I intended the consideration for contact with one of the conductors (so that the charge has a path from the conductor to the object).
TeV
#10
May5-04, 06:01 AM
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Yes zhana,I deliberatelly neglected "air discharges /spurious corona discharges" to be as simple as I could be for you.Reality isn't simple that much.
Surely,the air gas isn't good insolator any more in very strong E-fields and higher the potentials of the system of conductors more the effects and problems in that direction are to be expected.On other hand,I don't think that at voltage level I did the quick calculation for a crow , the transient in moment when crow attempt to sit could kill it.For something like that more average energy ,charge,and rms current is needed ( peak current itself is of smaller relevance I think). These instant shocks however,could be very annoying though.Few years ago,I played with 500 kV Van de Graaf generator (electrostatic DC machine) in the lab and could draw 50 cm long spark to hand held metal object.Prickling shock in my fist I clearly felt..
I never gave a serious thought before (payed attention) to the fact why the crows I see in suborb of my town never sit on 380kV power lines whereas they like to sit in nearby tall trees.Maybe the reason is very simple one.When they get too close to the energized HV conductor they disturb E-field configuration and corona suddently breaks out?
The corrona is accompanied by *hiss* sound which is warning to the bird :"Don't get any closer!".Birds are not deaf.I have a parrot at home .It hears better then me.

regards


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