On 12/8/13 9:08 PM, Joe Subich, W4TV wrote:
Consider a direct strike to the tower? The cables being above or
below ground don't materially change the voltage waveform, at least
as selection of the suppression devices are concerned: it's kilovolts
you need to deal with.
Cables "tapped" to the tower are coming off a voltage gradient from
several hundred KV at the top of the tower to zero at ground (if the
tower is properly grounded). At 10 feet on a 100 foot tower, one had
several 10s of KV .. not easily dealt with using simple devices.
But is it that high?
The inductance of the tower is about 1 microhenry/meter, as is the
inductance of the cable.
So say you tap off 3 meters above the ground, and you have a 3 meter
horizontal run, and then, the entrance point to the shack is 1 meter
above the ground..
So we have 3 uH from the tower tap point to ground, then 6 uH on the
horizontal and vertical components of the cables, but the input to the
shack is only 1 uH from ground, so it's like a 1:6 voltage divider.
And, then, if you have a breakdown device at the entrance point, it
breaks down at say 1 kV, and there's significant voltage drop from the
tower tap point to the entrance point.
It's kind of like standing next to Van de Graaff generator charged to
500kV. If I take a spark from it, it's not like my body sees the full
500kV.
Conventional transient suppression techniques work for phone and
power lines, which run above ground for miles.
Phone and power lines are not connected to lightning rods several
times the height of the lines.
Yes, but they ARE the highest thing above the ground for a long distance.
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