# High voltage power lines: helicoptor versus birds [duplicate]

As can be seen in this video, bringing a maintenance helicopter to the same potential as the high voltage power line is not without some arcing.

Why doesn't that happen to birds?

## marked as duplicate by placeholder, PeterJ, Vladimir Cravero, Leon Heller, Daniel GrilloJul 8 '14 at 17:05

• You're making an assumption that birds do roost on those lines. – placeholder Jul 8 '14 at 14:44
• I wonder if static buildup from the spinning rotors is significant under some conditions. – Phil Frost Jul 8 '14 at 15:40
• @PhilFrost I can't speak for helicopters, but on airplanes, static is enough of a problem that there is hardware to dissipate it. – Matt Young Jul 8 '14 at 15:47
• @MattYoung I know it is a problem with helicopters. Marine crews have a guy with the sole job of grounding the helicopter so it doesn't zap someone ("static man"), and if you search for "helicopter rotor static" there are all kinds of images and videos of really neat glowy rotors on landing. There's even this lineman that gets shocked by the helicopter. What I'm not sure of is if these effects play any significant role in the helicopter-vs-bird situation posed here. – Phil Frost Jul 8 '14 at 16:25
• Wait... so this question is marked as the duplicate of a duplicate? – Doktor J Oct 17 '16 at 16:21

The amount of current needed to bring a suspended object to the same potential as a conductor is proportional to the capacitance between the object and earth. Since the helicopter is a big object made of metal, and the potential of the line is very high, lots of charge has to flow in order to bring it to line potential. A bird, being much smaller and much less conductive has a much lower capacitance to earth and therefore much less current is needed to bring it to line potential. My guess is that for very high voltage lines even the bird will feel a zap.

The capacitance of an object 3m x 3m that is 10m above earth is 8pF

The capacitance of a medium sized bird (300mm x 300mm) at the same height is 100 times smaller.

EDIT - I miscalculated - it's a hundred times smaller (not 10,000) capacitance based on a flat surface of area overlapping another flat surface but the ratio is in fact bigger when you take into account that these objects are 3-dimensional and fringing occurs.

• Care to elaborate on this statement? Which formula accompanies this? I'm interested :) – Evert Jul 8 '14 at 14:54
• $C=\epsilon_0\frac{S}{d}$ – Vladimir Cravero Jul 8 '14 at 15:18

It might arc a little bit with birds, just not nearly as much. Birds have much less size and so won't store as much extra charge. You can think of every object like a capacitor. Birds are much smaller capacitors. Since the capacitance is smaller, it takes much less charge flow to increase the birds voltage to a high voltage line when compared to a helicopter. Since there's less charge flowing, any arc if at all will be much smaller/shorter in duration.

• Also, the series resistor of a bird (feathers?) is likely to be higher than that of a (mostly metal) helicopter, so the charging process will be slower. – Wouter van Ooijen Jul 8 '14 at 14:43
• @WoutervanOoijen Feet probably. Agreed though that the series resistance would make it charge slower. – horta Jul 8 '14 at 14:48
• Also note that most bird's feet are little more than bone, sinew, and skin, with very few nerves. They might not feel a thing. Do a google search for "bird feet nerves" for more info. – Adam Davis Jul 8 '14 at 16:24