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leaders that branch down from the cloud.
These
stepped leaders move downward in
jumps of approximately 150 feet. Each set
propagates
down through the set before
it, leapfrog fashion. Stepped leaders form
tendril-like
branches from the cloud down.
These are often visible in a photograph of
a lightning
strike.
When the
stepped leaders are approximately 500 feet off the ground, the electric field intensity becomes so
strong that structures on the ground begin to break
down
electrically. They respond by shooting streamers up toward the stepped leaders.
When a streamer connects with a stepped leader, the ionized path becomes
the
channel for the main lightning discharge.
The other streamers and stepped
leaders
never mature.
For this
discussion, it doesn't matter
whether the cloud base charge is positive
or negative,
because it can vary. The entire process can occur in the opposite
direction,
Lightning
solutions
given
our current technology, nothing can be done to affect the cloud charge or
affect the propagation of stepped leaders.
To protect a structure from lightning,
something
must be done about the ground
charge and the formation of streamers
from the structure to be protected.
One of the
primary tools for lightning
control is the air terminal (a lightning rod).
In most
cases, this is mounted at the topmost point of a structure.
One of the primary
tools for lightning
control is the air
terminal. Alt air
terminals are not
created equal, nor do
they function alike.
All air terminals are not
created equal, nor do they function alike. Three schools
of thought include:
1.
A structure can be
protected by installing
air terminals that attract lightning. The
energy of the
strike can then be conveyed
through a
low resistance path to ground.
This is generically referred to as
early streamer-emitting
air
terminal technology.
2.
Conventional
air terminal technology
can be used that is designed to intercept
nearby strikes and convey the energy to
ground.
3.
Static dissipation
technology can be
used to attempt to
reduce the likelihood
of a
strike. |
A static
dissipation array consists of many air
terminals with small point radii. These
discharge
ground charge, and build up the corona to discourage
streamers.
The first
two approaches assume that
strikes are inevitable. They attempt to handle
them with minimum damage. Static
dissipation technology attempts to reduce
the incidence
of strikes. However, a well
designed static dissipation system can also
handle
direct strikes in the same manner
as conventional systems.
Spare the rod?
Conventional air terminal technology is well understood. It is described in
detail in Underwriter's Laboratories UL 96A and in National Fire Protection
Association NFPA 78. For this article, we will focus on static dissipation
techniques.
This method of lightning prevention has
two Thrusts. First, the buildup of the
ground charge can be reduced to keep it
from reaching the critical flash point.
Next, the points on the structure from
which streamers are most likely to originate
can be identified, then their formation
or their height can be delayed. Doing
this would make these streamers less likely to be the first to reach a
stepped leader. This means the
arc would form elsewhere, effectively diverting the lightning
strike
static dissipation devices function because
of the point-discharge principle. Point-discharge theory holds
that electrical
discharge from the point of an electrode
(such as a broadcast tower) into a
surrounding medium (such as the atmosphere)
will follow predictable rules of
behavior. The smaller the radius of a static
dissipation electrode (actually, the
'smaller the radius of the point at the end
of the electrode), the greater the
resulting
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A
well-designed static
dissipation system can
handle direct strikes
in the same manner
as conventional
systems.
electric field intensity and flux
density.
As the
dissipation electrode radius approaches zero, the electric field intensity
approaches infinity. These strong fields
bleed off some of the ground charge into
the atmosphere surrounding the air terminal.
This reduces ground charge accumulation. This high electric field intensity
and flux density also retards the
formation of streamers. It is difficult for the
cloud charge
or stepped leaders to pull a
streamer thru the intense corona.
This is
illustrated by comparing a sharp lightning rod with a blunt one. Assume a
sharp rod and a blunt rod sit side-by-side.
As the ground charge reaches the
two rods, the potential rises on both.
The sharp rod will tend lo break
down into corona under a relatively low potential. The blunt
rod will hold
its
charge, with ions accumulating on
the blunt end.
As the ground
potential builds, the corona builds around the sharp rod, while
the blunt
rod tends to hold its charge.
When the ground potential becomes extremely high, such as when the stepped
leaders are on their way down from the
cloud, the sharp rod's corona will build in
density and elevation. On the other hand,
when the blunt rod finally breaks down,
it does so catastrophically. The
accumulated charge jumps off of the blunt rod in
a streamer
extending well upward toward
the stepped leaders. Because the object
that throws off the best streamer is
the one
most likely
to be struck, the blunt rod is
more likely to trigger a strike than a sharp
rod.
All objects
on the ground dissipate naturally to some extent. How they dissipate
is also
related to the point-discharge principle. The ground charge is first drawn
to the top of the object (the ultimate point),
and then to the corners or other points
All objects
on the
ground dissipate
naturally to some
extent, flow they
dissipate is related to
the point-discharge
principle.
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