[TowerTalk] Power lines, hawks, and fire ignition (slightly off-topic)

[Jim Lux]

At 12:45 PM 7/20/2004 -0500, Keith Dutson wrote:
>I saw that on the news this morning and figured it must be a joke.  Or,
>maybe the political news is getting so stale that they dreamed up another
>sensational story. :)
>
>AFAIK it takes about one million volts to jump an inch arc in air at STP.
>However, once a path is established, such as ionized air, the required
>potential drops drastically.  Perhaps the bird hit one line and left a trail
>of blood in the air to a second line whereby the arc could form.
>
>Keith

In a uniform field gap, it takes about 71 kV to jump an inch. A uniform 
field gap is closely approximated by electrodes where the radius of 
curvature is much greater than the spacing (say, two 12" spheres separated 
by an inch)

Needle point gaps are roughly 1/3 the voltage of a uniform field gap.

Lower air density reduces the breakdown voltage roughly in proportion.
Air density is reduced either by altitude or higher temperatures.  A good 
rule of thumb is that the air pressure is reduced by half for every 18,000 
ft elevation change.
High humidity increases(!) the breakdown voltage (but reduces the surface 
resistance, so creepage breakdown is more likely)

Spark Breakdown cannot occur below about 300V in air, no matter how close 
the electrodes are, or how low the pressure is.