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Re: [Phys-l] Lightning Rods



At 09:11 AM 6/5/2006, Mike Edmiston, you wrote:
///
(1) Is it fact or falsehood that lightning rods are sharp?

>From what I've read, Ben Franklin won the argument in this country.
///
Typical physics books that bother to discuss lightning rods say they
should be sharp.
///
(2) At the base of the clouds, is surface detail on earth all averaged
out such that lightning rods have no electrostatic effect at cloud
level? That make sense.

(3) Can lightning rods achieve any degree of discharging the cloud and
either prevent strikes or reduce their current and damage?

I suspect little affect from one building with lightning rods. But what
about a larger area with more rods?
///
Michael D. Edmiston, Ph.D.


People who appear to have relevant insights on the scale in question
are civil, naval and structural engineers, for tall buildings, tall ships
and dams etc., and electrical transmission engineers for
power network protection.

I do not have a reference to hand of the former type: so I can only
repeat that I have heard that tall buildings take hundreds of
strikes each year. The Lightning protection companies concerned
appear to use relatively sharp points at intervals on buildings of
any length, so that a square building can be expected to show at
least a conductor at each corner. There is apparently something
of an art in providing low impedance paths - so that multiple
stacked conductors can be preferred - and sharp turns avoided.

As to network protection, an included half angle of protection
of up to 60 degrees has been embodied in some three wire
plus top ground wire transmission designs.
(Knowlton, Std Handbook for EEs 9th Ed McGraww-Hill)


It is certainly the case that corona discharge is excited by
sharp conductors. If I recall, this current can typically amount
to microamps to milliamps.
A lightning discharge can be measured in kiloamps, so scaling arguments
from small scale devices are faced with a scale ratio of
(say) 2 milliamps per spike per 0.1 sq meter to 2 kiloamps per 100 sq m
so 2000 amps / 0.002 amps * 0.1m^2 / 100 m^2 = number of spikes.
This appears to call for a thousand spikes to discharge a lightning cloud
for each 100 m^2 patch of ground.


Brian Whatcott Altus OK Eureka!