On 8/8/15 9:40 PM, JVarney wrote:
Jim Lux wrote: >>I'm curious about why "surface area" (in TIA-222 for
ground conductors)<<
This article from Times Microwave explains it: lightning is a pulse with a
very fast rise time and acts like RF, which means inductance and impedance
come into play. That would explain the surface area reference in TIA code
regarding 2/0 radial conductors. The discussion/diagram on Pg 14 and 15
loosely illustrates the radial/star system that the TIA code describes.
rise time is on the order of 1 microsecond, typically.
Surface area doesn't affect inductance very much, but it does affect
impedance.
Consider a 33 foot (10 meter) long piece of AWG 10 copper wire.
It has an inductance of 10 uH, and a DC resistance of about 10 mOhm.
For a 20kA stroke current, rising to that level in 1 microsecond, the
di/dt 20kA/microsecond..
The voltage drop due to inductance would be 10 uH * 20kA/1us or 200 kV.
The voltage drop due to resistance would be 20kA*10mOhm or 200 Volts..
Yeah, the ac resistance of that AWG10 is higher than the DC resistance,
but probably by a factor of 10.. so 2kV vs 200kV.
(skin depth in copper at 1MHz is 2.5 mils, compare to AWG10 diameter of
100 mils. A ring 2.5mils thick with a diameter of 100mils is 785 square
mil, the area of a 100mil circle is 7854 square mil.. in reality the
current penetrates deeper..)
So the *dominant* factor is the inductance, and the shape or surface
area of the conductor doesn't affect that very much.
http://www.timesmicrowave.com/documents/resources/protectbrochure.pdf
No where near a Terajoule of energy.<<
You're right, I don't know where I got that. Should be a gigajoule. Thanks
for correcting me.
73 Jim K6OK
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