Lightning Eliminators & Consultants, Inc.
6687 Arapahoe Road
Boulder, Colorado 80303-1453 USA
Ph: +1-303-447-2828 Fx: +1-303-447-8122
A Hybrid Lightning Strike Protection System
Roy B. Carpenter, Jr.
Darwin N. Sletten, PE
Revision B, August 2014
Background
The first lightning protection system (LPS) went into operation over 250 years ago with the
advent of the air terminal, a.k.a. lightning rod, attributed to Benjamin Franklin. Since that time
there have been other systems that have approached the lightning event in a different manner and
as a result offered different solution concepts depending on the users’ needs. It is important to
first discuss the lightning event itself, followed by whether a LPS should be implemented, the
types of lightning protection systems and their methodology of protection, and finally, the hybrid
approach.
The Lightning Strike Event
Lightning is the result of charge accumulation within a cloud cell exceeding the dielectric
strength of the air and, as a result, the air beneath the cloud is no longer able to act an insulator
between the charge and path to ground.
When there is no thunder cell in the area, the “fair weather” electric field is around 100 V/m.
While the charge is being generated in the cloud cell, the related electrostatic field is inducing a
charge on the earth’s surface beneath it of equal but opposite polarity. It is much like an
electrical shadow that moves with the cloud, as illustrated by figure 1. When the potential at the
cloud base reaches approximately 10
6
volts, leaders form and move that voltage toward earth in
“steps” as shown in figure 1. The stepped leaders move toward earth at velocities of between 1 x
10
5
and 3 x 10
6
meters per second. As the stepped leader approaches earth, the voltage between
that leader tip and an earthbound facility rises to very high potentials; for example, the electric
field beneath the cloud rises to become greater than 30 kV per meter of elevation above the earth.
In a matter of a few milliseconds, the electrostatic voltage on a pointed rod elevated above earth
by a mere 10 meters will rise from one hundred thousand volts to over a million volts. This
potential will cause grounded pointed objects, such as air terminals, to first break into corona and
then rapidly into upward rising streamers, as illustrated in the lower right panel of figure 1.
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Figure 1: The Charged Cloud Impact
The early stages in the formation of upward streamers occur as result of what is called the point
discharge phenomenon. The point discharge theory describes the ability for any grounded
pointed object to give off discharge current. This point discharge current, or ionization, occurs
with air terminals as well as with multipoint ionizers. Recent scientific testing has suggested that
a multipoint ionizer will make that version of an air terminals self-protecting from lightning
strikes at the early stages of the stroke development
1
.
When the leader connects with one of the upward streamers it is called a lightning stroke. This
results in a discharge/neutralization process. The induced charge on the surface of the earth
discharges or neutralizes with the rush of the induced charge through the channel formed by the
creation of the stroke. This is illustrated in figure 2.
This discharge/strike can cause significant direct structural damage in forms such as fire and
explosions. Associated with the direct damage, there can be additional damage caused by
secondary effects. These secondary effects are caused by, in part, the rapid expansion and
collapse of the electric field. This type of damage can have significant effects on nearby
electrical systems and is commonly known as an electromagnetic pulse or EMP (see LEC paper
The Secondary Effects of Lightning Activity, Rev. A - Carpenter, Lanzoni - April 2014)
Electrostatic
Shadow:
5 to 30
kV/m
Electrostatic
Shadow
10
6
V+/- 10%
High
Density
Charge
Rising
Streamers
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Figure 2: The stroke or discharge process
With all of the potential termination points for a downward stepped leader, a significant question
is “Where will that stepped leader terminate?” Since a LPS is designed to control the damage
due to the lightning event, it is important to know if a structure is at risk of naturally collecting a
strike. To that end, a great deal of research has been accomplished and recorded. It seems that
the stroke termination is a function of three definable factors.
• Random chance determines the path the leader will take until that leader reaches the
“point of discrimination” as shown in figures 3A, 3B, and 3C.
• The striking distance is the distance from the lightning leader at which any earth-bound
structure will generate an upward streamer as seen in Figures 3A and 3B.This radius is a
statistical quantity dependent on the current intensity (current) of the ensuing lightning
strike as shown in formula 1.1.
• The point of discrimination is the point where the force of attraction between the
stepped leader and one of the upward streamers determines which upward streamer will
generate the stroke channel between the leader and streamer. At this point, the leader is
directed, in its final step, to the upward streamer with the greatest attraction force. This is
shown in Figures 3B and 3C.
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Figure 3A: Striking Distance
Figure 3B: Point of Discrimination
Downward Leader
Striking
Distance = R
Point of
Discrimination
Upward Streamers
Striking
Distance
Downward
Leader
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Figure 3C: Completed Stroke Channel
Figure 3A shows the striking distance of an example strike with striking distance R. Everything
within the distance R from the leader tip is shaded yellow. This region is the shape of a
hemisphere. The objects within the yellow hemisphere produce streamers while objects outside
the area do not. This is true even if the structure is tall, as in the case of the tower on the left side
of the figure.
As the leader progresses, the tip of the leader moves toward the point of discrimination and the
“striking distance hemisphere” moves with it. Figure 3B portrays the leader shortly after the
point of discrimination. As you can see, the hemisphere has moved since Figure 3A and now
additional streamers are forming.
At this point of discrimination, the leader selects which will be the “winning” streamer and will
move in its direction until contact is made. This change in movement can be seen right after the
point of discrimination in Figure 3B. Figure 3C shows the final connection which is made
between the leader and the streamer which was determined at the point of discrimination.
Calculating Striking Distance
A common method to determine the striking distance is through the formula
r = 10 I
0.65
Formula 1.1
2
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Where:
r: striking distance in meters
I: peak current in kA
These steps and the striking distances vary from a low of approximately 15 meters to a high in
excess of 200 meters. However, the average length is only about 50 meters for a negative stroke,
which is the most common polarity. Positive strokes can have a striking distance of over 300
meters.
Risk Assessment
The need for a lightning protection system usually involves an analysis of risk and the following
types of loss.
1. Loss of, or risk to, human life
2. Loss of production or service to the public
3. Loss of economic value
4. Loss of cultural heritage
Both the National Fire Protection Association (NFPA) and International Electrotechnical
Commission (IEC) offer guidelines on risk assessment.
Standards
In the United States safety is the main determinate of most standards, and the National Fire
Protection Association (NFPA) is no different in this regard. NFPA 780 put safety as the most
important factor when addressing lightning protection. These standards are designed to address
safety from a position of collecting the strike and discharging it in the ground as safe as possible.
A determination of the location with the highest probability of collecting a lightning strike can be
done using a rolling sphere method which is illustrated in figure 4. This method is endorsed by
National Fire Protection Association (NFPA) which publishes the NFPA 780 Standard for
Lightning Protection in the USA. Using this method whatever part of a structure that is touched
by a sphere that is rolled all around and across the structure is subject to collect a lightning strike.
Wherever the sphere touches the structure an LPS is deployed.
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Figure 4: Rolling Sphere method of analysis
The radius of the sphere should indicate the striking distance based on the strike current. To
maintain consistency, NFPA 780 uses a radius value of 150 feet (46 m) which equates to a strike
current of approximately 35 kA. A limitation of using this method as detailed in NFPA 780 is
that the rolling sphere method does not account for shorter strike distances than 150 feet (46 m),
which would allow a strike to slip into the protected area, or competitive factors, which make
some locations more likely to collect a strike than others.
Types of LPS
There are four general types of commercially available lightning protection systems, as follows:
1. Conventional air terminal lightning protection systems – designed to collect strikes
2. Early streamer emitting air terminal lightning protection systems– designed to collect
strikes
3. Charge Transfer Systems – streamer delaying arrays that are that are designed to prevent
all possible lightning strike collection
4. Hybrid System - streamer delaying air terminals that collect strikes only when charge
transfer capacity is exceeded by dissipation requirements.
Each of these system types is comprised of two basic subsystems, as follows:
1. The devices that are installed on top of or above the structure or area to be protected. These
devices may include single point air terminals, multipoint air terminals or arrays, shield wires,
masts, ionizers, etc. Systems using these devices offer protection from direct strikes to objects
Strike Termination Devices
Radius of
Rolling Sphere
Protected Structure
Rolling Sphere
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and structures that fall within a protected zone adjacent to and beneath the highest point of the
devices.
2. A grounding electrode system designed to provide a sufficiently low resistance connection to
earth. The lightning protection devices listed above must be bonded to the grounding system
using conductors adequately sized for lightning currents.
The grounding subsystems for all four types of lightning protection systems are essentially
identical. However, there are differences in the design and installation of the lightning protection
devices , as described below:
Conventional Air Terminal Lightning Protection Systems
A conventional air terminal lightning protection system consists of installing a suitable number
of air terminals (also called lightning rods), conducting masts or overhead shield wires above the
structures or areas to be protected. These devices are then bonded to the grounding system. The
air terminals, masts or shield wires are designed to collect incoming lightning strikes by
generating upward streamers. Installation requirements and specific information about the
protected zone can be found in NFPA 780, Standard for Lightning Protection. Note that NFPA
780 is a standard and not a national code which requires compliance.
Conventional air terminal lightning protection systems do not protect against indirect lightning
currents or induced voltages. These effects are addressed by proper bonding and the application
of surge protection devices.
Early Streamer Emission Air Terminal Lightning Protection System
An early streamer emitting (ESE) air terminal lightning protection system consists of a suitable
number of ESE air terminals above the structures or areas to be protected. These devices are
then bonded to the grounding system. ESE air terminals are designed to generate upward
streamers that launch sooner, and with a greater collection zone, than conventional lightning
rods, thus providing a more attractive point of termination and collection. Installation
requirements and specific information about the protected zone is available from the systems’
manufacturers. Early streamer emitting air terminal lightning protection systems do not protect
against indirect lightning currents or induced voltages. These effects are addressed by proper
bonding and the application of surge protection devices.
Charge Transfer Systems
A charge transfer system consists of installing a suitable number of ionizing arrays and ionizing
air terminals above the structures or areas to be protected. These arrays are then bonded to the
grounding system. The arrays and supplemental terminals are designed to avoid the termination
of incoming lightning strikes by lowering the electrostatic field thereby suppressing or delaying
the formation of upward streamers. For more technical information see LEC paper Preventing
Direct Lightning Strikes. Without the leader streamer connection, there is no strike. Installation
requirements and specific information about the protected zone is available from the systems’
manufacturers. Depending on the manufacture and product type, charge transfer systems will
have some benefit in reducing indirect lightning currents or induced voltages. Some of these
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products come with a performance guarantee. However, proper bonding and surge protection
devices should still be provided.
Hybrid Systems
In addition to these three types of systems, there have been several attempts to improve the
performance of air terminals by providing them with charge transfer capability. This capability
allows hybrid ionizers to prevent or delay the formation of upward streamers in the same way
that a full capacity charge transfer system would, but the terminal will collect the strike when the
charge transfer capacity is exceeded by dissipation requirements. There is evidence that
indicates that these devices do reduce the risk of a strike but with varying degrees. Furthermore,
no hybrid system is 100% effective at lightning prevention. When a hybrid ionizer does fail to
prevent a strike, it functions in an alternate mode as a stroke collector. Thus, the name “hybrid”
has been applied to these designs because they share some of the benefits of a charge transfer
system while performing no worse than an air terminal.
The ionizers presented in this paper include the Spline Ball Ionizer
®
(SBI
®
), the Spline Ball
Terminal
®
(SBT
®
) and the Streamer Delaying Air Terminal (SDAT). These all comply with
existing NFPA and Underwriters Laboratories (UL) standards and are a low cost option to the
DAS.
Spline Ball Ionizer and Spline Ball Terminal
The LEC SBI® and SBT® were developed as optimized hybrid air terminals. Figures 10 and 11
illustrate the two configurations. Both the SBI and SBT provide the required point spacing to
maximize the ionization current. At the same time, they provide a point oriented at least every 5
degrees in azimuth for the full 360 degrees in azimuth and a full 225 degrees in elevation. As a
result, there is no direction from which a leader can approach that will not have a collective
points oriented directly toward it and many backup points close by. The SBT differentiates itself
from the SBI by a reduced number and shorter length of points thus it has a slight reduction in
performance but is ideal solution for upgrading an existing lightning rod system. The SBI is the
ideal solution for towers, large structures or vessels.
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Figure 10: Deployed SBI
Figure 11: Deployed SBTs
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Both the SBI and SBT have been reviewed by Underwriters Laboratories and have been listed as
terminals, usable as such in any NFPA-780-based lightning protection where rod-type terminals
are specified.
SDAT Systems
Similar to a SBIs and SBTs is the Streamer Delaying Air Terminal (SDAT). It works as a hybrid
system with the goal of preventing the majority of the strikes while collecting the strikes that it
can no longer prevent. The differentiating factor in the SDAT system is the number of points
and the size of the wire. While the SBI and SBT use optimum point spacing the SDAT uses
substantially more points essentially increasing the ionizing wire by a factor of 10. In theory this
approached would lead to better result but it fails to address the issue of point spacing or
crowding (point interference).
Figure 12: Deployed SDAT
The SDAT has been reviewed by Underwriters Laboratories and have been listed as terminals,
usable as such in any NFPA-780-based lightning protection where rod-type terminals are
specified.
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Comparison of Multipoint Ionizers
The separation distance between points has been shown to be a significant factor in the charge
transfer discharge current given off by each point
3
.
LEC has conducted tests to determine the separation distance required to provide the optimal
discharge (or ionization) current.
Further investigation by LEC into the ability of multipoint ionizers to generate ionization current
resulted in a test of LEC’s Spline Ball Terminal (SBT), Lightning Master’s PP-31A (a fine point
ionizer) and a 3/8” diameter lightning rod. The results of this test are shown in figure 5.
Figure 5: Comparison of Charge Transfer Ionization Capability
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This test, performed at the LEC testing lab, shows that an LEC SBT has about 50% greater
charge transfer current than the PP-31A fine point ionizer (similar to the SDAT) and over 2 times
the charge transfer current than an air terminal when immersed in an electric field of 75 kV/m.
This test indicates that multiple points ionize better than a single point and that proper point
spacing is a factor when considering a multipoint terminal.
Major differences in the multipoint ionizers are the number of and spacing of the ionizing points.
There are well over 100 ionizing points in the PP-31A which are closely spaced in a
configuration similar to that of a shaving brush. The LEC SBT has a limited number of points
spaced at 6” which has been shown to be an optimal spacing for the highest per point ionization
current. As the chart above shows, there is a much greater ionization current for the greater
spaced SBT over the PP-31A device, even though it has fewer points.
Multipoint (hybrid) ionizers will collect a strike, similar to a lightning rod, when they cannot
dissipate (transfer) the charge fast enough. As the leader approaches, the terminal may
eventually go into ion saturation and produce streamers and a subsequent stroke channel. When
they produce streamers, some of them (when properly configured) become very efficient
collectors.
Using the SBI,
SBT, SDAT in Standards-Based Systems
Standards such as NFPA-780, UL96A, NAV FAC DM4, and Army 385-100 are based on the use
of a single point lightning rod known as the air terminal or the stroke collector. However, since
UL has listed the SBI
®
, SBT
®
and SDAT these assemblies can be used in place of the single-
point terminal. In most cases, they can be used as a direct replacement. The SBT
®
and SDAT
are designed to fit into the conventional lightning rod mounting plate.
Figure 12 illustrates a typical NFPA-780 building protection system that has been converted to a
hybrid stroke protection system. Model SBT hybrids are used in the required locations around
the periphery. SBI or SBT hybrids can be used in the required locations down the middle of the
building.
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Figure 12: SBT’s Deployed in Standard 780 Layout
Choosing a System
When assessing the three biggest risk factors
1. Loss of, or risk to, human life
2. Loss of production or service to the public
3. Loss of economic value
The cost of lightning related damages may be significant. This may be especially true in an
oil/gas facility or chemical processing plant where a strike can cause a substantial loss of product
and equipment. In these cases, the risk to human life cannot be ignored.
If there is an appreciable economic and or safety loss associated with lightning activity, it may be
worthwhile to implement a no-strike, Charge Transfer type, LPS. However, since the cost of this
type of system can be high, a cost-benefit analysis should be performed. If the cost-benefit
analysis shows that a no-strike LPS is unwarranted, a lower cost hybrid LPS may provide the
ideal solution.
A hybrid LPS functions on the same principles as a no-strike LPS but cannot prevent a direct
strike to the protected area or structure under all conditions. It will, however, reduce the number
of terminations and thereby increase safety and lower the damaging effects on the electrical and
electronic equipment.
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Grounding
If the lightning stroke is allowed to seek its own path to ground, it may cause significant direct
damage to the stricken object and damage due to secondary effects. An LPS is designed to
provide the lowest impedance path to ground for the lightning current.
For a conventional LPS, the termination device provides a generator of the upward streamer and
a highly conductive location for the lightning current to begin its path to ground when it is
struck.
If there is not a low impedance path to ground provided by the ground conductor, the high
current values will take the lowest resistance path to ground which may be through the structure
causing significant personal and property damage.
Since the lightning current must transfer current through the earth, a low impedance connection
to earth must be established. This earth connection must be able to spread the current out over a
large area of earth to prevent localized high current values. To do this a low earth resistance
must be obtained and maintained or a low resistance ground grid must be installed.
Surge protection
Surge protection devices usually deal with the secondary effects of the strike. These effects
result in earth current transients, atmospheric transients, electromagnetic pulses and the bound
charge phenomenon. The most common of these secondary effects cause problems associated
with over voltages on copper wires. Proper grounding design and application is also essential in
minimizing this threat.
Conclusion
In the industry of lightning protection, options must be considered to determine what system best
fits the user’s needs. Understanding the lightning event, the motivations for LPS implementation
and the types of systems all play a significant role in ensuring the user’s goals are met and an
unintended event is prevented. A hybrid lightning protection system can meet the basic
requirements of the user over a traditional system as well as provide an economic advantage over
a full capacity charge transfer system.
Based on a cost benefit analysis, in many commercial or industrial applications, these hybrid
lightning protection systems are the preferred cost effective lightning protection option.
A properly designed and installed standards-based system that includes the use of the LEC
hybrid terminals will provide two modes of protection:
1. A stroke prevention mode that reduces the risk of a strike to the protected facility using
the size and number of SBI/SBTs and or SDATs in proportion to the size of the facility.
2. A stroke collector-diverter system that is far superior to any system now in use because it
collects strokes entering the “protected” area from any direction and angle.
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References
1. The Case for Using Blunt-Tipped Lightning Rods as Strike Receptors, C. B. Moore et al,
Journal of Applied Meteorology, Vol. 42.
2. Eq. 5 Pp. 560, Lightning, Volume 2, Lightning Protection, edited by Golde 1977
3. Atmospheric Electricity, J. Alan Chalmers, p 249, 2
nd
edition, Pergamon Press
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