>> 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 of course ground is not 'zero' once the current traveling down the tower
gets to it, the ground potential rises also. What you need to protect
against are the signal/power carrying conductors having a different
potential than the shields of the coax or ground where the equipment is.
>> 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.
Most distribution lines are at the TOP of the poles and get hit directly,
phone lines are usually 3/4's of the way or more up the poles... and even
more important they are not coaxial lines so the power carrying conductor is
hit which takes it directly into transformers, switches, and other
equipment.
Important Note, it is not the voltage handling capability of arresters that
matter, when the arrester turns on from a stroke it limits the voltage
across itself, that is what its purpose is! It is the current or integrated
energy handling capability of the arresting device that is important. How
you can help it is by keeping all the grounds bonded together, this keeps
the potential difference between the protected cable and the local ground
smaller which reduces the amount of current that the arrester must pass to
equalize the potential between the signal/power lead and ground.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
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