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I walk to work every morning and pass under some HV power lines. On high-humidity days, there's an audible crackle/sizzle emanating from the lines. I never hear this on the way home.

I would expect this to come from condensation on the insulators, but curiously it is audible only in the center of each span. My first guess is that it's produced all along the wire but audible in the middle because that part is so much closer to the ground.

My question is: If there's enough leakage into the air that I can hear it, doesn't that represent a huge loss over miles and miles of wire?

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Yes, there's some loss, but it's usually miniscule relative to the amount of power the wires are carrying.

The sound comes from moisture on the wires themselves and in the air around them. The strength of the electric field around a narrow cylindrical conductor (e.g., any wire) is highest right at the surface of the conductor, and with high-tension transmission lines, it can reach the point where it ionizes the air itself. This is known as corona discharge. It happens all the time, but when the air and wires are dry, it doesn't produce any sound.

EDIT:

One other thought: Sometimes on very high-tension transmission lines, you'll see that each conductor is actually a cluster of 3 or more separate wires, held apart by spacers. This is an attempt to mitigate the corona losses, by increasing the effective diameter of the wire and thereby reducing the intensity of the electric field.

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  • \$\begingroup\$ Though electrons flow in a electric wire, the net charge of a wire is zero right? So how come electric field is generated outside the wire? So no Corona discharge must happen. \$\endgroup\$ – user3219492 Aug 10 '16 at 11:57
  • \$\begingroup\$ @user3219492: No, wrong. There most definitely is a net electrical charge on the wire relative to the space around it and the other conductors at the peaks of the AC waveform. Only when taken as a whole can you say that the power distribution system has no net charge. \$\endgroup\$ – Dave Tweed Aug 10 '16 at 23:09
  • \$\begingroup\$ You seem to be correct. There is Electric field felt outside a current carrying wire. ifi.unicamp.br/~assis/Found-Phys-V31-p1501-1511(2001).pdf \$\endgroup\$ – user3219492 Aug 11 '16 at 10:54

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