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Number of dissipator electrodes
Calculating the required number
of dissipator points is not an exact science.
One must not only dissipate the structure to be
protected; one must also dissipate the round
charge, a function of the point on earth upon
which the structure rests. Therefore dissipation
requirements are determined not only by the
structure, but also by that ground charge, i.e.
the absolute difference in potential which must
be reduced through dissipation and the rate at
which that dissipation must occur to prevent a
strike.
The ground charge is a function
of the strength and speed of the storm. (It is
possible that an elevated structure creates
somewhat of a "venturi" effect, drawing more
ground charge than that which would normally
occupy the point on earth upon which it rests.
However, to my knowledge no one has been able to
confirm or measure such an effect.)
Since a static dissipation array
must provide a low resistance path to the
atmosphere, it seems logical to provide as many
discharge points as reasonably possible. By
using a large number of points one can
compensate for any loss of efficiency from a
theoretical maximum, and spread the dissipator
elements over more of the cross-section area of
the structure.
Density of dissipator electrodes
However, one cannot emulate the
patient who, upon being told that three pills
was good for him, decided that 30 pills must be
better. Concerning the number of dissipator
electrodes, there is a point at which more is
less.
The density of the dissipator
electrodes is critical because of the
possibility of inter-point interference. If the
dissipator electrodes are held too close to one
another, the points interfere with one another's
ability to dissipate. Experimentation indicates
that the smaller the radius of the dissipator
electrodes, the closer they can be arranged
without interference.
Given moderately close spacing,
this interference only affects the dissipation
capability of any given point: not of the system
as a whole. Moderately close spacing of
extremely small radius electrodes may lead to
some inter-point interference and limited loss
of efficiency by individual points. However, it
is more than offset by providing a greater
overall number of points and greater overall
dissipating capacity.
At the extremes, too close
spacing results in the array under heavy
discharge approaching a solid surface, be it a
cylinder, plane or toroid. On the other hand, if
the dissipator points are too widely spaced, the
result is unnecessary supporting structure with
resulting excess weight, wind loading and cost.
If dissipator points cease to interfere at a
given distance, there is nothing to be gained by
increasing that distance.
Assume, for a moment, that there
is no problem of interference between dissipator
points located in close proximity to one
another. Another limiting factor arises; the
ability of the volume of atmosphere surrounding
the dissipator points to accept the charge.
Therefore, the points must not only be separated
to prevent interference, but also be separated
to provide a sufficient volume of surrounding
atmosphere to avoid "saturating" that
surrounding volume of atmosphere with charge,
Of course, this does not take
into account the effect of wind, usually present
in abundance during the conditions under which
peak dissipator discharge occurs. Wind presents
constantly renewing surrounding volumes of
atmosphere, and, if the dissipator electrodes
are sufficiently flexible, continuous movement
of the points in relation to one another
providing momentar increases in spacing.
Configuration of dissipator on structure
All objects have natural
dissipation points. On a structure, charge tends
to gather at, and dissipate from, the top of the
structure (the ultimate point) and from corners.
The most effective way to mount a dissipator in
terms of structure, weight, wind loading, cost
and aesthetics is to enhance this natural
dissipation by supporting the dissipator from
the structure itself at these natural
dissipation points, and to take advantage of any
existing grounding and bonding provisions,
particularly if the structure is a building. In
other words, the dissipator should be tailored
to the structure, not vice versa.
It was once believed critical to
effectiveness that a dissipator be the absolute
highest point on a structure. Practical
experience has proven it need not be. Indeed,
mounting a dissipator too high above the
structure in an effort to clear all
appurtenances can reduce the level of protection
by allowing charge to continue to accumulate at
the structure's natural dissipation points. In
certain applications such as electrically
floating AM broadcast towers, it is inadvisable
to install anything, including a static
dissipator, above the top of the tower since it
necessitates costly and time-consuming re-tuning
of the station.
Design flexibility crucial
A dissipator, if designed in such
a way that it need be the highest point on the
structure or mounted in any other specific
manner, interferes with available space and, by
limiting mounting flexibility, may inhibit use
of the structure for its intended purposes.
Therefore, dissipator design should offer
maximum mounting flexibility, be adaptable to
any existing structure, take advantage of
existing grounding and bonding, and should not
preclude any utility application of structure
space.
Particularly in tower
applications where weight and wind loading can
be critical, any weight and wind loading
contributed by a dissipator reduces the amount
of capacity available for revenue producing
items, i.e. antennae, etc. Therefore, it is
important to keep the weight and wind loading of
the dissipator as low as possible, consistent
with performance.
When considering the operation of
static dissipators, it is sometimes helpful to
consider the nature of static charge. It is not
a directional current requiring a superb
conductor. It is a static field. The engine
driving the system is the storm cloud charge
overhead attempting to pull the static charge
off the structure. One need only provide a low
resistance escape for that static charge to the
atmosphere. The discharge produced is in the
microampere range current, typically under 100
micro-amps.
Performance and effectiveness
Do static dissipation arrays
work? It depends on with whom you talk. In my
personal observations, the scale runs from
manufacturers (myself included) at one end who
obviously claim they do work, to theorists with
a great deal of time and reputation invested in
theories which preclude their operation at the
other end who claim the technology amounts to
voodoo pultice.
Beware of performance guarantees
or claims of 100 percent effectiveness. At our
collective present state of understanding the
lightning phenomenon and all of its variables,
we find such hyperbole to be counterproductive
and to constitute the basis for suspicion.
Besides, such clams miss the point. In our
opinion, the stated goal of static dissipation
should be to cost-effectively reduce losses due
to damage caused by lightning strikes. Claims of
absolute reliability to prevent lightning
strikes, claims which have historically been
unfounded in reality, do nothing to encourage
the legitimacy and industry perception of this
application of point discharge technology.
The real issue is cost-effective
protection. Assuming no injury to personnel, the
real issue is the cost of lightning damage and
down time. Therefore, start with inquiries to
the customers of the manufacturers to gain an
experience base. In addition to the
effectiveness of the static dissipation array
itself, other appropriate questions may relate
to:
-
availability and helpfulness
of the manufacturer in answering questions
and providing needed technical information;
-
the quality of the dissipator
and installation material, and availability
of alternate installation material to
accommodate requirements arising from the
structure upon which it is to be
mounted or from the surrounding environment
-
convenience and ease of
installation
-
installation scheduling
-
cost of static dissipation
array
-
cost of installation; and
-
manufacturer service, follow-up and
responsiveness to any problems.
As with any investigation, it is
easier to get the right answers if one asks the
right questions. The preceding will provide at
least a starting point.
In my experience and that of our
customers, static dissipation arrays can be a
cost-effective means of reducing expenditures
for damage due to lightning. A properly
designed, constructed and mounted dissipator can
help save money. But, however effective static
dissipation arrays may be, and whichever one you
choose, it should be remembered that
installation of a system is not a stand-alone
solution.
By Bruce Kaiser, President,
Lightning Master Corp.
Communications Engineering and Design December
1988 |
