TOWER OWNERS NEED TO
BE HELD ACCOUNTABLE, BY LAW, FOR PRIVATE PROPERTY DAMAGE
There is a 50% probability that a lightning strike will be
approximately 30kA. (Anderson & Eriksson 1980) If the
self-inductance of the earth is estimated very conservatively at
.5x10-6H, and lightning takes the form of a pulse which has a
typical rise time of 2x10-6S, then from the equation, V=Ldi/dt;
the estimated Ground Potential Rise (GPR) of a 30kA strike will
be 7.5kV. Values of GPR could easily triple upwards of 25kV for
higher current lightning strikes or strikes passing through
higher inductance.
Towers attract
lightning strikes. The taller the tower the higher the
probability that lightning will strike the tower. Towers that
are over 1000 feet in height can generally expect to be struck
by lightning four or more times in a single year in much of the
United States. The probability of a lightning strike to a tower
can be specifically calculated using NFPA 780 and knowing the
location and height of the tower.
When Lightning strikes
a tower its grounding system elevates in voltage as the current
passes into the earth through the tower system ground. This
phenomenon is known as lightning induced Ground Potential Rise (GPR).
This GPR represents waves of voltage that ripple out in circle
type patterns away from the tower base. If the tower is not
properly grounded to dissipate lightning strike energy, the
higher frequency energy will travel on the surface of the earth
for considerable distances as it spreads out. Towers or attached
guy wire supports that are located within a few hundred feet of
private property can represent a very serious personnel safety
hazard and can cause much damage to equipment.
Equipment damage to
private property from tower lightning strikes is widespread
throughout the United States and the world for that matter. The
scenario is always the same: "A tower is constructed and
shortly thereafter damage begins to occur to equipment on nearby
private property. The property owner suspects that it might have
something to do with that recently erected tower. The property
owner contacts the tower owner that unequivocally states that it
cannot be their tower that is the problem, and the tower owners
have the money and lawyers to wear down any private property
owner to eventually try to find alternate solutions for
themselves.
There are alternate
solutions such as using fiber optic cable instead of copper
cable for communications and possible very fast acting shunting
devices to protect equipment that use SAD Technology. However,
sometimes even shunting devices are not good enough and that the
ultimate solution requires isolation devices that are quite
expensive. Some damage just cannot be resolved by the private
property owner and they live with repetitive damage during
lightning season from a tower owner that denies that they are
the instigator of the trouble.
So what can a
responsible tower owner do to reduce the lightning induced GPR
that his tower is causing to the nearby private property owners?
The tower owner must demand that the tower grounding system be
designed to properly dissipate lightning strike energy. To
properly dissipate lightning strike energy requires division and
control. This is an absolute must for success, because of the
magnitude of the current and the resulting surge impedance of
any single dissipation path.
Ten radials connected
to a ground ring bonded to a tower, will divide lightning
current up into ten smaller segments. This will help ensure that
the lightning will more likely follow the designated paths for
dissipation into the earth and lower the resulting GPR to the
adjacent private property. This is an absolute must for success,
because of the magnitude of the current and the resulting surge
impedance of any single dissipation path. Ten radials connected
to a ground ring bonded to a tower, will divide lightning
current up into ten smaller segments. This will help ensure that
the lightning will more likely follow the designated paths for
dissipation into the earth and lower the resulting GPR to the
adjacent private property owner.
The maximum length of
these ten radials is approximately 25 m (80 feet). Longer length
radials will offer little dissipation improvement, because the
lightning strike energy will not remain on the radials for much
over 25 m (80 feet). In very limited spaces, the recommended
minimum grounding system is at least 60 m (200 feet) of buried
bare ground conducting wire composed of five radials, each 12 m
(40 feet) in length.
A greatly improved
copper wire grounding system can be easily achieved by the use
of conducting cement placed around the radials at the time of
installation. The cement will harden into concrete, both
protecting the grounding system (giving it many years of
additional life), and making the system a much better (lower)
ground resistance.
LPGI & Affiliates
962 Coronado Drive
Sedalia, CO 80135-8303
303-688-5800
Fax: 303-688-5551
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