And
then there was the lightning rod
May 9.2011 -- This
technical treatise on lightning rods (air terminals) came from
Mr. Amir Rizk's writings and the company Lightning
Electrotechnologies, Inc.
In 2000, a paper was published in the Journal of Applied
Meteorology entitled “Lightning Rod Improvement Studies”.
This paper marked a turning point in the history of lightning
strike protection. Although the analysis of the data has evolved
considerably since this publication, the experimental design and
data collection continues to improve.
Field studies showed that lightning rods exposed to real
lightning whether sharp rods, moderately blunt rods or very
blunt rods, placed at the same height in the same location,
would interact differently and produce different results. The
sharp rods were never struck whereas moderately blunt rods were
struck most frequently. Some criticism of the work was that the
data was not statistically significant and this is true when the
rods are grouped in terms of their geometric shape alone.
However if the rods are grouped in terms of their propensity to
produce electric discharges under the slow varying electric
field that precedes a lightning strike, the numbers make a very
strong case.
However,
in the American field experiments the 'largest' most blunt rod
(51 mm) were never struck either. It was observed that these
larger blunter rods produced even more charges during the slowly
varying electric field than the moderately blunt rod (19 mm).
This could only be explained by the presence of water droplets
on the surface of the larger rods, which acted like conducting
protrusions with small radii of curvature that caused them to
produce charge at rates comparable to sharp rods. The same will
be true of just about any object that is exposed to rain and has
a surface area large enough to collect raindrops. The presence
of raindrops on the surface of a conducting object, under high
voltage, causes the water droplets to elongate and typically has
an electric effect comparable to covering the object with
needles.
The
moderately blunt rods which were struck the most were small
enough (19 mm diameter and thus about 14% of the surface area of
the 51mm rod) such that it was unlikely for a water droplet to
settle on their surface. Furthermore the height used in the
field tests, 6 m masts, produced space potentials (effective
applied voltages) very close to the level where the 19 mm rod
would just begin to produce discharges under the slowly varying
field. Had the tests been conducted with mast heights of say 10
m, they would not have detected any difference between the rods
tested since they all would have been raging with similar
discharge activity. Alternatively, had they chosen a test height
of one meter, none of the rods would have produced discharges,
and again there would have been no observable difference.
The
mechanism of space charge shielding goes beyond simple rods but
can also explain much of the seemingly mysterious behavior of
lightning. For example, the reason tall towers are sometimes hit
below the top may simply be that discharge activity from the
tower top during the slowly varying electric field, shields the
tower top in the same way as was observed with the sharp rods.
And so when the descending leader arrives, a lower part of the
tower is more prone to producing the upward connecting leader as
compared to the tower top. But it will only occur if the slowly
varying field reaches critical values for sufficient durations
relative to the timing of the arrival of the descending leader.
It is predictable but sporadic.
There
is absolutely nothing that can be done to influence any aspect
of the descending stepped leader. However it is possible to
influence a grounded structure's propensity to produce upward
leaders and from where on the structure such leaders would
emanate. The taller the structure and the more charge associated
with the particular descending stepped lightning leader, the
more the length of the upward leader from the structure can
grow. So for a low structure with a large foot print, the
probability of being struck will not be greatly influenced by
the building's ability to launch leaders. However, for tall
slender towers, their ability to launch long leaders is the
reason they are struck so frequently. Influencing the
possibility of producing upward leaders from a structure will
thus have a much greater effect on taller more slender
structures as compared to low massive ones.
For
very tall towers, in the 100 meter range and more, the
possibility of another kind of lightning strike comes into play.
These structures can generate their own lightning. An upward
flash is a lightning strike where the upward connecting leader
is created without there having been any descending leader to
which to connect. It is true that such strikes can be initiated
by nearby flashes but the distinction is that these upward
flashes do not connect with any descending lightning leader but
rather, move towards localized charge centers in the clouds.
Return strokes of comparable magnitudes to downward lightning
result.
It
is with these structures that the most can be done to influence
the probability of a direct strike. The production of charges at
the tops of these very tall towers can shield them enough to
raise the required space potential for upward leader initiation
to the point of significantly suppressing their occurrence. It
is not clear however that this effect can be significantly
enhanced through the use of devices that use sharp conductive
points. Any conducting protruding object or even the edges or
corners of such tall structures would also produce charges via
the same mechanism, streamers and in similar rates. Thus, the
effect is mostly determined by the particular nature or waveform
produced by the slowly varying field.
When
Ben Franklin first proposed mounting needle sharp iron rods at
the tops of buildings, he didn't think that they would be hit by
lightning. He initially thought that such rods would discharge
the clouds and so his choice of using sharp tipped rods had no
connection with the mechanism which he had accidentally
discovered, i.e., upward leader initiation. Had Ben used any
grounded conducting object that did not include a protruding
point, it would have been slightly less prone to discharge
activity and performed better as an initiator of upward leaders,
with fewer failures. Franklin's genius was to mount any grounded
metallic object above a structure and to adapt his views to what
he observed rather than proposed.
So for the last 250 years we have been using a device, the sharp
tipped lightning rod, in a situation for which it is perfectly
unsuited. Of course the effect of rain limits the performance of
any other shape which is why this detail has eluded us for so
long. In addition, the effect becomes more pronounced with
increasing height, as we continue to build taller structures and
thus the more these effects become apparent.
Interestingly
Nicola Tesla nearly solved the problem. He somehow recognized
that the sharp tipped rod's propensity for discharges was a bad
thing. And so he designed a device, with the intent of resisting
the production of electric discharges. However what Tesla did
not recognize is that when exposed to rain and covered in water
droplets, like almost all objects, his device produced
discharges comparable to a sharp rod and thus function as a
ordinary rod and not as Tesla had intended. With a clear
understanding of how a lightning rod is supposed to work, and
with full consideration of the effects of space charge
shielding, Lightning Electrotechnologies Inc has developed the
first ever lightning rods based in peer reviewed science.
The
F-SAT (Field Sensitive Air Terminal) A passive
electrode/lightning rod that resists the production of electric
discharges under intense electric fields and under rain and thus
never self protects via space charge shielding and always
maintains the maximum attractive area for its position. When
properly placed, lightning protection systems employing the
F-SAT ensures that any upward leaders emanating from a structure
will be from the F-SAT and not below or nearby. The F-SAT is
suitable wherever lightning protection is needed and can be used
to enhance an NFPA 780 compliant system.
Other installation methodology options using currently employed
and recommended practices for power lines can be applied to any
structure with the possibility of significant savings and
without compromising security. The F-SAT is not a one
size-fits-all solution. A 60m high building corner does not
experience the same electric field intensification as a 60m high
slender tower or a low 20m building. F-SATs come in a number of
sizes designed to meet these varying conditions.
It
is important to note that the F-SAT does not offer an enhanced
zone of protection as compared to any conducting object; it
merely prevents the type of degradation to a lightning rod’s
attractive area described above. Particularly under rain, which
is closely associated with lightning, all conducting objects
except the F-SAT are subject to that kind of degradation.
Need a lightning consultant to protect your building or tower
correctly with the latest in air terminal design for your
structure? If you do then LPGI & Affiliates is your answer.
