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Owen Duffy · April 4th 07, 09:24 PM posted to rec.radio.amateur.antenna

KE5MMJZ wrote in news:slrnf180co.7j9.spam-
:

....
ercted a tower it would probably increase the odds
of lightning striking either it or a nearby object,
and while my house might be protected (or at least
I would be the one choosing to take the risk), what
about the neighbors? Even if the strike would be
directly on my tower, from what I've read there
could be enough induced voltage/current to damage
the 'unprotected' appliances of my neighbors, and
they might blame me for their losses (or even if
it doesn't hit the tower, they may still see it
as the cause of a nearby strike).
Has anyone heard of any such cases or am I
just worrying about the highly improbable?

Frank,

The existence of a tall conductor provides a measure of protection to
nearby structures. The zone is often defined by a "ball" of radius equal
to the height of the tall structure that is rolled on the ground to rest
against the tall structure, and structures between the contact with the
ground and tower, and under the ball are considered protected by the tall
structure, ie that lightning is much more likely to strike the tall
object. In this way, a tower often protects the nearby equipment hut from
direct strikes.

Having said that, there are two other effects you must consider:
- the way in which the discharge current on the tower is dealth with; and
- the fields in the nearby area from the discharge current.

The usual approach is to try to shunt as much of the discharge current to
ground. Nevertheless, some will enter your premises and you should pay
attention to equipotential bonding and single point earthing techniques
to minimise the voltages impressed across equipment interfaces, voltage
that may cause damage.

It is not just about equipment, personal safety is more important, but
provided you use earthing conductors sufficient to survive the lightning
discharges (and people of do not) and equipotential bonding, then
personal protection is fairly easily achieved.

Even if you have dealt with minimising interface voltages in your own
premises, the discharge current may give rise to a surge on the power
lines or metallic water services that couple to your neighbours relative
to each other or telecommunications services. It is conceivable that a
discharge to your tower might not cause damage to your installation but
could damage appliance in neighbouring premises... so I wouldn't call
your tower as protecting neighbours.

Owen

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