Placement of lightning rods [RE-wrenches]

[R. Sparks Scott]

	Bill;

	Responding to your post about the value/purpose of lightning rods:

	Yes, a good ground is the foremost defense against lighning-induced
damage. over-voltage and over-current protection of sensitive equipment is
also desirable. A nearby lightning hit can cause significant currents to be
induced in nearby conductors and equipment not directly connected to the
subject of the strike.
	On the subject of dissipation arrays, I speak from a position of
experience. As a broadcast engineer, I must consider lightning as a serious
threat to every installation that I maintain.
	One such facility, a 650 foot FM tower situated atop a 1,200 foot hill,
has sustained repeated lightning induced losses. We have replaced a 5 bay
broadcast antenna twice due to direct lightning hits, to the tune of
$30,000 each time. In each instance, the 2" copper conductors of the
antenna were softened to the point of sagging, and soldered joints were
literally exploded apart. Additionally, the antenna deicer wiring, a four
conductor, shielded 12 gauge cable, was replaced repeatedly, as it was
acting as a fuseable link. The tower lighting and heater wiring inside of
rigid conduit running up the tower was also replaced in total once. The
building housing the equipment is very heavily grounded, and no equipment
damage inside the structure has ever been experienced.
	The last time this antenna system was replaced, we installed several
static dissipation arrays at the top of the tower, and just below the
radiating elements of the antenna. In the three years since, we have
suffered *zero* lighting damage. This kind of makes me think that the
dissipation array is doing it's job well.
	This isn't meant to be a product endorsement, but I did a quick search and
came up with a page describing one popular dissipation array, the Cortana
Stati-cat. Other dissipation devices are manufactured and distributed by
Shively Labs:

http://mwpersons.com/Cortana.html

>A lightning stroke contains far too much energy to dissipate or deflect

	This is correct, but the idea is to prevent the air in the area
surrounding the potential point of a strike from becoming ionized, which
leads to the breakdown of the insulating properties, leading to the
formation of a ground leader being sent up to meet the leader coming down
from the sky. Basically, the dissipation array acts as a lower resistance
path between the ground and air, to prevent buildup of electrons at the top
of the structure.

>It makes a lot more sense to implement good grounding, bonding, and surge
>suppression practices, which not only handles direct strike problems, but,
>also, can prevent problems with strikes remote to the structure.

	Of course, this is a part of any proper installation. Static dissapation
is not intended to be a substitute for proper grounding, but a supplemental
protection system.

	I do still have more  tips and tricks to relate, but I'll hold off until
someone expresses interest before I launch into them.

	-S

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