[TowerTalk] Disconnecting cables and lightning (questions -- too long)

[Pete Smith]

It's lightning season again around here, and ...

Polyphaser and ICE make much of the ability of their line protectors,
properly installed, to protect against lightning hits even if the lines
remain connected throughout the event.  This is obviously relevant for
repeaters and similar systems which must remain operational.  But recently
I mentioned in an item printed in Hints and Kinks that in addition to using
a ground bulkhead, I routinely disconnect all conductors coming from
outside, in the belief that this, combined with effective grounding of the
tower, would protect against lightning coming in on those conductors.  The
editor of H&K found it necessary to add the words "only slightly," and that
raised my curiosity.

As I understand the physics from the Polyphaser book and discussion on this
reflector, when lightning strikes a tower or antenna it, and anything on
it, is immediately raised to a very high voltage.  How quickly this
dissipates, and where the current flow goes, depends on the configuration
and execution of all of the grounding provisions, including the tower base,
the house electrical service, and the station equipment.

Fine so far.  But if the base of the tower is well grounded (multiple rods
and radials, properly connected to the tower base), but it's several
hundred feet from the tower base to the house, I question whether it's
really possible to keep that ground and the house ground at the same
potential, with any reasonably-sized conductor between them.  Moreover, my
shack is on the second floor, which would make it very hard to have a good
low-impedance ground connection at the bulkhead.  Now if I disconnect all
the conductors (coax, rotor cables, etc.) and maintain a several-inch air
gap, it seems to me that a very high proportion of the strike current
should flow down the tower into ground, and that the length and
configuration of my long cables should provide sufficient inductance to
keep the voltage at the bulkhead from rising high enough to arc across to
the inside cables lying nearby.

Finally, my questions--

1.  Am I wrong?

2.  Would I be better off grounding each of the conductors coming from the
tower at the entry bulkhead, instead of letting them float, as now, even
though I know the grounding of the bulkhead is relatively poor in terms of
inductance?

3.  What about a disconnect and/or grounding panel at ground level, near
the AC service entry?  Worth doing, for the sake of the better, and more
unified ground?

4.  Some people have written here recently about cable grounding schemes
using relays to switch their cables from the shack to ground.  It seems to
me that with the close spacing of relay contacts, the ground they're
switching to better be pretty darned good, or arc-over and equipment damage
will occur regardless.  Wrong again?

Thanks for all the anticipated good advice.

 


73,  Pete Smith N4ZR
In wild, wonderful, fairly rare WEST Virginia

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