[TowerTalk] Grounding Coax at Tower

[David Robbins K1TTT]

> K4SAV wrote:
> > Having the cables exit the tower above ground is a really bad idea.
> > Voltage drop down the tower is very high during a strike.  Anything
> > exiting the tower above ground will see a very large voltage impressed
> > on the cable at the point it exits the tower.
> 
> 
> But how high?  And what needs to withstand that potential?
> 
> The voltage would depend on whether resistance or inductance dominates.
> A 30kA strike on a 1 ohm tower would only be 30kV over the whole length
> of the tower.. call it on the order of 5kV over a 10 foot length out of
> a 60 foot tower.
> A 30kA strike with a rise time of 2 microseconds on a 20 microhenry
> tower (1 uH/meter) would result in a voltage across 10 feet of about
> 3E-6*30E3/2E-6 or 45kV.. so for the direct strike scenario, the
> inductance dominates.

Well, I ran this through the software that I write for EPRI (www.epri.com).
(yes, I do get paid for this software work, though not for trying it for ham
tower experiments)  The software is for lightning protection design on power
lines, not for radio towers, but it will do some things on a single tower
that are interesting and that I trust.  For this case I set it up for a 120'
tall pole to approximate a tower.  With a 25 ohm driven rods ground in
average soil.  Then I hit it with a 30ka 2usec risetime stroke.

Drum roll please....

At 10' up from the ground I get a voltage of 450kv... now, this is of course
relative to the infinitely distant ground (or your power line entrance).
Relative to the earth at the base of the tower it is only about 100kv.

You should note that the system of a guyed tower is much more complicated
than what I have modeled above.  The 450kv assumes no current goes off on
any guy wires(philly or other perfect insulator guys), it all goes straight
down the tower.  If I allow for 10kv insulators on the guy wires that
probably all flashover early in the strike the voltage relative to the tower
base goes down to something under 20kv on a quick eyeball of the graphs.

For those interested in such things the estimated surge impedance (similar
to coax characteristic impedance) for this tower works out to 240ohms, most
of it is pure inductance of course.  To get an idea of the total possible
voltage, if there were no ground that would make a voltage of about 7MV at
the top of the tower, with the ground connected that is reduced to 1.7MV at
.5usec into the stroke, well before the peak current is reached.  With the
insulators flashed over that peak voltage is only 800kv, this is because
lots of the current is drained away from the tower on the wires.

Some things that mitigate the situation... the coax/control cable bundles
leaving the tower are usually much skinnier than the tower, unless you have
bundles like mine of course.  This gives them a much higher impedance than
the tower so more current goes down than out the cable bundle.  For a single
cable this can be something like a 10:1 ratio in currents.  Intermediate
supports that are conductive, like the wood poles I use, can drain more
current along the way to the shack.  And good grounding outside the shack to
your house perimeter ground should equalize the cable bundle with the shack
ground at the entrance... yes there is a big inductive loop, but that should
be shorted out at the shack end if you do it right.

Fun stuff this lightning, isn't it.

Oh, and there are lots of GOOD reasons for running cables overhead.
1. easy access to repair year round
2. no need to dig in rocky soil
3. infinite expansion capability without having to add conduit
4. no worry of water trapped in conduit
5. easier to remove old cables than pulling out of conduit
6. no need for direct burial or special flooded cables



David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net