First some general comments
about lightning. Lightning is a capricious, random and unpredictable event.
Its' physical characteristics include current levels sometimes in excess
of 400 kA, temperatures to 50,000 degrees F., and speeds approaching one
third the speed of light. Globally, some 2000 on-going thunderstorms cause
about 100 lightning strikes to earth each second. USA insurance company
information shows one homeowner's damage claim for every 57 lightning strikes.
Data about commercial, government, and industrial lightning-caused losses
is not available. Annually in the USA lightning causes more than 26,000
fires with damage to property (NLSI estimates) in excess of $5-6 billion.
The phenomenology of lightning strikes to earth, as presently
understood, follows an approximate behavior:
1. The downward Leaders from a thundercloud pulse towards
earth seeking out active electrical ground targets.
2. Ground-based objects (fences, trees, blades of grass,
corners of buildings, people, lightning rods, etc., etc.) emit varying
degrees of electric activity during this event. Upward Streamers are launched
from some of these objects. A few tens of meters off the ground, a "collection
zone" is established according to the intensified local electrical
3. Some Leader(s) likely will connect with some Streamer(s).
Then, the "switch" is closed and the current flows. We see lightning.
Lightning effects can be direct and/or indirect. Direct
effects are from resistive (ohmic) heating, arcing and burning. Indirect
effects are more probable. They include capacitive, inductive and magnetic
behavior. Lightning "prevention" or "protection" (in
an absolute sense) is impossible. A diminution of its consequences, together
with incremental safety improvements, can be obtained by the use of a holistic
or systematic hazard mitigation approach, described below in generic terms.
As previously stated, lightning has its own agenda; it
is entirely capricious, random, and unpredictable. Man's attempts to fit
lightning into a convenient box, with Codes and Standards to describe its
behavior, are a best guess. The system of conventional lightning rods as
commonly employed does represent the best method for providing a preferred
pathway to ground. One very important aspect of ham radio antennas is the
fact that they present themselves as being no different than lightning rods
and therefore should be installed in the same manner and according to the
standards used for lightning rod systems. Antennas are no different than
any other air terminal.
Second, lightning safety for group or large scale outdoor
events is very difficult - maybe impossible - to accomplish. Injuries at
a June 1998 rock concert at RFK stadium in Baltimore are a good example.
Some 35,000 people were there. Lightning rods were there. Still, some 13
people were badly injured by incoming lightning. In July 1998 in Las Vegas
NV, five firefighters were injured when lightning struck their fire truck.
At a soccer match in the Republic of the Congo (October 1998), 11 members
of the team were killed by lightning.
Third, the myths about lightning persist:
LIGHTNING NEVER STRIKES TWICE - (It hits the Empire State Building about 25 times a year.)
RUBBER TIRES WILL INSULATE ME FROM LIGHTNING - (It has traveled miles through spacea few inches of rubber
mean nothing at all.)
LIGHTNING CAN BE PREVENTED - (Unconfirmed/sheer advertising.)
FIRST STRIKES FROM LIGHTNING CAN BE PREDICTED - (Unconfirmed/sheer advertising.)
NEW HIGH-TECH TYPES OF LIGHTNING RODS CAN CONTROL
LIGHTNING - (Unconfirmed/sheer
Lightning Strike Regions In World
Average U.S. Thunderstorm Days/Year
On the military side, the Department of Defense Explosive
Safety Board (DDESB) has reported 88 identifiable lightning-induced explosions
in its records, with costs and deaths not calculated. DDESB was formed as
a result of the Picatinny Arsenal incident (July 1926) which killed 14 people
killed and cost $70 million. More recently in June 2001 at Buryatia Russia
the lightninginduced munitions losses exceeded 20 million rubles. Here,
fires burned for two days and were contained only after heavy rains. Three
thousand people were evacuated, seventeen people were killed. How to mitigate
the lightning hazard is the scope of this paper, with information presented
in outline form. We begin with the assumption that Safety Is the Prevailing
Directive. This leads to a prudent organizational policy which analyzes
facilities and operations so as to identify lightning vulnerability.
When lightning strikes an asset, facility or structure
(AFS) return-stroke current will divide up among all parallel conductive
paths between attachment point and earth. Division of current will be inversely
proportional to the path impedance, Z (Z = R + XL, resistance plus inductive
reactance). The resistance term will be low assuming effectively bonded
metallic conductors. The inductance, and related inductive reactance, presented
to the total return stroke current will be determined by the combination
of all the individual inductive paths in parallel. Essentially lightning
is a current source. A given stroke will contain a given amount of charge
(coulombs = amps x seconds) that must be neutralized during the discharge
process. If the return stroke current is 50kA that is the magnitude
of the current that will flow, whether it flows through one ohm or 1000
ohms. Therefore, achieving the lowest possible impedance serves to minimize
the transient voltage developed across the path through which the current
is flowing [e(t) = I (t)R + L di/dt)].
Why Should You Protect Your Home and Your Ham Station
This year 250 people in the United States will be killed
by direct strikes of lightning. Another 500 will die in lightning caused
fires. And at least 1500 persons will be injured by lightning which causes
more damage, injuries and deaths each year than tornadoes, hurricanes or
floods. The Lightning Protection Institute notes that 95% of today's homes
are not protected against lightning surges. Most of the deaths and injuries
would not have occurred if proper lightning protection equipment was installed.
Lightning protection is designed for two objectives: it
must provide a direct path for the lightning bolt to follow to ground, and
it must prevent destruction or damage, injury or death, as it travels that
The question: Copper or Aluminum?
Both COPPER and ALUMINUM are approved by UL (Underwriters
Laboratories) for the installation of Lightning Rod and Grounding Systems.
There is little argument that copper is the material of choice because it
is a better conductor of electricity. If aluminum is used in an installation,
the cable has to be larger than the copper cable to conduct the same amount
Whenever possible, specify the use of copper in systems
because it is the better material and is physically stronger and a better
dollar value. The copper cable is smaller and thus less conspicuous and
blends in better with most architecture, such as against brick walls and
There are times when it is preferable to use aluminum materials.
Since aluminum and copper are of dis-similar metals, they tend to corrode
each other. Therefore, specify aluminum in the following exceptions:
1. If your roof were made of bare aluminum or bare galvanized
2. If your building were very light colored and the use
of the dark copper cable would be unattractive.
3. If the absolute lowest cost were the only consideration.
Aluminum Lightning Protection equipment is approved by
Underwriters Laboratories and has proved most satisfactory, but there are
a few precautions to consider when using this material:
1. Never use underground. The alkali in the soil will
destroy aluminum. Aluminum should terminate at least 1' above ground.
2. Aluminum and copper materials should not be used together
unless approved bimetallic connectors are used.
3. Aluminum should never be used where it will come in
contact with white wash, calcium, or alkali-base paint as these are most
injurious to aluminum.
4. Aluminum should never be placed where leaves or moisture
will collect and remain for a long period of time.
5. Copper grounding equipment is always used with aluminum
Typical Roof Bonding and Air Terminal Plans
In all cases, any ham radio antenna or tower should be
connected or bonded to the lightning protection system. Failure to properly
comply with this method will produce a condition with a "step voltage"
as a result of a nearby strike. In such event, the difference in potential
will be developed between two or more ground points and thus current will
flow between the two different potentials. In many cases, the radio equipment,
via the AC neutral or supplied equipment ground, is in this path thus resulting
in damage or destruction of the equipment.
Figure 1 - Roof Bonding Techniques
HOW A LIGHTNING PROTECTION SYSTEM WORKS
Figure 2 - Area Protected by Lightning Protection System
Figure 3 - Phase I of Lightning Stike
A lightning strike consists of opposite charges of electrical
energy. A negative charge or build-up occurs in the bottom part of the cloud
closest to earth and a positive charge of energy occurs directly underneath
in the ground. Separating these two opposite charges is the non-conducting
dry air belt separating cloud and earth. As the two opposite charges continue
to build up and the dry air belt becomes moist, lightning starts down toward
earth in 150 foot jagged steps or intervals. The positive ground charge
is attracted upward, utilizing the lightning protection system on the building
as an outlet.
Figure 4 - Phase II of Lightning Stike
As the negative leader stroke from the cloud continues
toward earth, the positive ground charge travels up through the Lightning
Rod System and when the negative leader stroke is about 150 feet above the
top of the protected building, the positive ground charge starts upward
to meet and neutralize the downward leader stroke.
Figure 5 - Phase III of Lightning Stike
In Figure 5, the two opposite charges are neutralized
emptying the negative charges from the cloud and dissipating the ground
charge. This all occurs in about one five thousandths of a second.
Figure 6 - Phase IV of Lightning Stike
In Figure 6, the discharge has been completed and
the negative cloud charge and the positive ground charge becomes zero.
Note: If the residence had not been equipped with a lightning
protection system, the positive ground charge would have accumulated under
or within the house. The negative cloud charge would not have been neutralized
150 feet above the residence and would have entered the building, causing
possible fire, destruction, side flashes within the building or even injury
One thing to keep in mind is that a properly installed
antenna system does contribute to overall lightning protection. This
protection is not only to the antenna but the associated radio equipment
and the structure. Failure to adequately comply with this process will leave
the associated radio equipment and or the structure at risk for extensive
THE INS AND OUTS OF LIGHTNING PROTECTION
Chances are, sooner or later, some part of your facilities
will take a lightning strike. Preparation can help guard your expensive
radio and computer equipment against damage or even destruction. Here you'll
find some tips to help keep your station running smoothly.
For Telco lines
Several companies make surge suppressors intended for phone
lines and ISDN lines. Some are in-line connectors with modular plugs. Others
are hardwired at the demark point. The main methods of protection are Series
and Shunt. Series devices typically plug directly into the line between
the equipment and the demark attempting to block incoming surges before
they reach your equipment. Shunt devices attach in parallel with the line.
They try to direct the surge away from the equipment by providing a better
path to ground. Some devices combine both methods of protection.
Some Telco equipment manufacturers offer punch block solutions,
such as the Siemon Pico Protector Module(5). These
mount right to the punch block in place of bridging clips. Other modular
devices are offered by several companies (such as Polyphasor(1), TrippLite(2), APC(3),
and Panamax(4) that include ISDN and T1 solutions.
(Note that the basic shunt device for POTS lines will work for an ISDN line
when utilized into the U interface of a Zephyr.)
For AC Power Lines
Power conditioning and backup is fast becoming a requirement
for many sensitive electronic devices including computers, audio processors,
mission critical components, any device that relies on clean power for a
CPU controlled device. Many of the home backup power devices even include
RJ jacks for network and/or modem protection. Larger commercial types of
power backup provide better protection that the smaller consumer devices.
They have a better reaction time and offer better line filtering. Some of
the very large installations tend to work in a "hot" mode where
they are constantly online and commercial power is merely maintaining a
charge on the batteries.
A good surge suppressor should have some kind of an alarm
system. This is because once a surge suppressor has done its job typically
they are no longer any good and should be replaced. The technology behind
some suppressors is that there are special components inside that get destroyed
during a major lightning strike. The deadly force of the lightning is dissipated
while components themselves are being destroyed and (ideally) not your equipment.
If a series device fails "open" it will prevent power from passing
to your device. A shunt device may fail as well but unless it shorts the
power to ground you might not know this device is no longer protecting your
equipment. This is the reason for some type of alarm or indicator informing
the user of a failed status. Better surge suppressors have a longer useful
life, but after so many lightning strikes (or one HUGE one) they may fail.
Power back up devices or Uninterruptible Power Supplies
(UPS) convert DC power supplied from batteries into AC power. Smaller units
tend to provide just enough power to ride out a short duration power outage.
The idea is to ride out these power outages till the power comes back or
allow you enough time to gracefully shut down your equipment. More sophisticated
UPS units have communication ports for monitoring their status so that equipment
can automatically shut down should commercial power be lost.
Small to medium UPS units tend to be preferred where there
is a backup generator installed, such as a transmitter site. This works
well when commercial power is lost and the UPS carries the load until the
backup generator comes online.
Things to Consider About UPS Devices
Some of the smaller and older UPS units provide little
to no surge suppression. Many people buy these devices thinking that they
are also protected from power surges. Best to check the manufacturer's specifications.
Commercial AC power out of the outlet is typically a pure
sine wave. Most UPS devices do not produce a pure sine wave output but a
modified alternating square wave. Most equipment with switching power supplies
and analog power supplies can tolerate this modified sine wave but there
is some equipment that cannot. For power solutions check out products from
APC Inc.(3), Best Power(6), or
TrippLite(2) (see references at end of this document).
For RF Devices
Suppression of lightning and surges for RF follows the
same principles for dissipating surges as do Telco and power lines. Polyphasor1
has a full line of devices for RF that are well suited for coaxial lines
with consideration to power levels and system frequencies.
The ground system at the base of a tower should be bonded
to all other grounds associated with the system. This includes the AC power
ground at the service entrance, any Telco ground which should be connected
at the AC power ground point, any cable TV or satellite antenna systems,
well pumps should be bonded back to the AC power ground and therefore there
is no reliance on the AC neutral.
One other area of concern is the vertical mast protruding
through or from the top of a tower. A separate bond strap, flexible in nature
if there is a rotating system, should connect the mast to the tower at the
top of the tower. Typically the vertical mast should be fitted with a pointed
rod of some 0.5 inches in diameter protruding from the extreme uppermost
point. This then forms a proper air terminal for the structure.
Many ham antenna installations use a very unscientific
and poorly planned ground system. Typically the idea of more grounds thus
more driven ground rods and attachment points the better. This is not necessarily
correct in that one important aspect is often omitted in the ground system.
Proper grounding is essential to maintaining equipment safety. Many facilities
employ a grounding system that is very intricate. The idea is to provide
a central grounding point for all equipment to maintain a common ground.
RF lines entering buildings from antenna structures would typically have
copper strap bonded to the outer conductor and shunted to the common ground
to help divert the major surge away from the equipment. Equipment racks
in transmitter sites are also tied into this common ground system to maintain
the same potential. Dedication to proper AC and ground wiring techniques
can ensure minimal risk to one's equipment due to a surge and also can reduce
incidents of ground loops and hum in your facility.
The grounding system must address low earth impedance as
well as low resistance. A spectral study of lightning's typical impulse
reveals both a high and a low frequency content. The high frequency is associated
with an extremely fast rising "front" on the order of 10 microseconds
to peak current. The lower frequency component resides in the long, high
energy "tail" or follow-on current in the impulse. The grounding
system appears to the lightning impulse as a transmission line where wave
propagation theory applies.
A single point grounding system is achieved when all equipment
within the structure(s) are connected to a master bus bar which in turn
is bonded to the external grounding system at one point only. Earth loops
and differential rise times must be avoided. The grounding system should
be designed to reduce ac impedance and dc resistance. The shape and dimension
of the earth termination system is more important a specific value of the
earth electrode. The use of counterpoise or "crow's foot" radial
techniques can lower impedance as they allow lightning energy to diverge
as each buried conductor shares voltage gradients. Ground rings around structures
are useful. They should be connected to the facility ground. Exothermic
(welded) connectors are recommended in all circumstances.
Cathodic reactance should be considered during the site
analysis phase. Man-made earth additives and backfills are useful in difficult
soils circumstances. They should be considered on a case-by-case basis where
lowering grounding impedances are difficult an/or expensive by traditional
means. Regular physical inspections and testing should be a part of an established
preventive maintenance program.
A considerable part of lightning's current responds horizontally
when striking the ground: it is estimated that less than 15% of it penetrates
the earth (Sandia Labs, 1993)(11). As a result, low
resistance values (25 ohms per NEC) are less important than volumetric efficiencies.
Corrosion and cathodic reactance issues should be considered
during the site analysis phase. Where incompatible materials are joined,
suitable bi-metallic connectors should be adopted. Joining of aluminum down
conductors together with copper ground wires is a typical mistake.
Good station grounding techniques are essential to minimize
the damage of a lightning strike. With the proper grounding of your station
equipment and AC power ground, you stand a much better chance of surviving
a lightning strike. Instead of losing the entire station, you may only lose
one or two specific items. With adequate surge protection, the damage to
the specific item(s) will ideally be minimal (if any).
For more information on facility grounding and techniques
refer to the references listed below.
Old Fashioned Tricks
For years, computer technicians have talked of tying knots
(usually three) in the power cords of the computers as lightning protection.
They report that in really bad lightning strikes, the power cords with knots
were totally destroyedbut the computer was perfectly fine. The only cost
incurred was for a new power cord. Not bad lightning protection for $3.99!
Others have used ferrite beads and looped the cords through
these. Some people report the same results doing the same trick with phone
Surge protection devices (SPD aka transient limiters) may
shunt current, block energy from traveling down the wire, filter certain
frequencies, clamp voltage levels, or perform a combination of these tasks.
Voltage clamping devices capable of handling extremely high amperages of
the surge, as well as reducing the extremely fast rising edge (dv/dt and
di/dt) of the transient are recommended.
Ordinary fuses and circuit breakers are not capable of
dealing with lightning-induced transients. Surge suppressors should be installed
with minimum lead lengths to their respective panels. Under fast rise time
conditions, cable inductance becomes important and high transient voltages
can be developed across long leads.
In all instances, use high quality, high speed, and self-diagnosing
protective components. Transient limiting devices may use a combination
of arc gap diverters or metal oxide varistor or silicon avalanche diode
technologies. Hybrid devices, using a combination of these techniques, are
preferred. IEEE 1100 gives good guidance here. Uninterrupted Power Supplies
(UPSs) provide battery backup in cases of power quality anomaliesbrownouts,
capacitor bank switching, outages, lightning, etc. UPSs are employed as
back-up or temporary power supplies. They should not be used in place of
dedicated SPD devices. Correct IEEE Category A installation configuration
is: AC wall outlet to SPD to UPS to equipment.
Know your clamping voltage requirements. Confirm that your
vendor's products have been tested to rigid ANSI/IEEE/ISO9000 test standards.
Avoid low-priced, bargain products which proliferate the market (caveat
Put the surge protection circuits close to the equipment.
If possible, tie the ground of the Telco line suppression unit to the common
ground or the chassis of the protected equipment. Use a very short lead
(4-6") between the suppression module and the equipment.
Establish a common ground in your facility and make the
attempt to route all your grounds to that point.
Choose power conditioning/UPS units suited for your application.
Make sure they are rated to handle the equipment they will power. Check
for features and options you will need. Verify the surge protection of the
device and make sure it is adequate. Frequently check these units for faults,
especially after a storm. Buy the best your money can buy.
Don't forget that all ham radio, TV, telephone and cable
related equipment must deal with a "double whammy". Lightning
can come from either the power line or the phone line/cable line (sometimes
BOTH!). For example: lightning can come from the phone line and try to work
its way to the power line ground (or the other way around). In the process,
it will usually destroy the gear and neighboring equipment! Protect all
lines into your equipment.
Try tying knots in your cords. Add the ferrite beads. Couldn't
hurt to try.
LIGHTNING PROTECTION DESIGNS
Mitigation of lightning consequences can be achieved by
the use of a detailed systems approach, described below in general terms.
Air Terminals: Since Benjamin
Franklin's day, lightning rods have been installed upon ordinary structures
as sacrificial attachment points, intending to conduct direct flashes to
earth. This integral air terminal design may not provide protection
for electronics, explosives, or people inside modern structures because
of flashover and transfer impedance. Inductive, magnetic and capacitive
coupling from intended lightning conductors can result in significant voltages
and currents on unintended interior power and signal conductors. Lightning
will follow the path of least impedance to ground.
Overhead shield wires and mast systems located above or
next to the structure are suggested by the US Air Force (AFI 32-1065) and
by NASA (STD E0012E) as preferred alternatives to rods in many circumstances.
These designs are termed indirect air terminals. Such methods are
intended to collect lightning above or away from the sensitive structure,
thus avoiding or reducing flashover attachment of unwanted currents and
voltages to the facility and equipments.
Unconventional air terminal designs which claim the elimination
or redirecting of lightning (charge transfer or dissipater arrays) or lightning
preferential capture (early streamer emitters) deserve a very skeptical
reception. Peer-reviewed studies which have dismissed these claims include:
NASA/Navy Tall Tower Study, 1975; R.H. Golde "Lightning" 1977;
FAA Airport Study 1989; T. Horvath "Computation of Lightning Protection"
1991; D. MacKerras et al, IEE Proc-Sci Meas. Technol, V. 144, No. 1 1997;
National Lightning Safety Institute "Royal Thai Air Force Study"
1997; A. Mousa "IEEE Trans. Power Delivery, V. 13, No. 4 1998 among
Lightning Rods: In Franklin's
day, lightning rods conducted current away from buildings to earth. Lightning
rods, now known as air terminals, are believed to send Streamers upward
at varying distances and times according to shape, height and other factors.
Different designs of air terminals may be employed according to different
protection requirements. For example, the utility industry prefers overhead
shielding wires for electrical substations. In some cases, no use whatsoever
of air terminals is appropriate (example: munitions bunkers). Air terminals
do not provide for safety to modern electronics within structures alone.
Additional and specific steps as outlined in this writing is required.
Air terminal design may alter Streamer behavior. In equivalent
e-fields, a blunt pointed rod is seen to behave differently than a sharp
pointed rod. Faraday Cage and overhead shield designs produce yet other
effects. Air terminal design and performance is a controversial and unresolved
issue. Commercial claims of the "elimination" of lightning deserve
a skeptical reception. Further research and testing is on-going in order
to understand more fully the behavior of various air terminals.
should be installed in a safe manner through a known route, outside of the
structure. Gradual bends (min. eight inch radius) should be adopted to avoid
flashover problems. Building steel may be used in place of down conductors
where practical as a beneficial part of the earth electrode subsystem.
Shielding is an additional line of defense against induced
effects. It prevents the higher frequency electromagnetic noise from interfering
with the desired signal. It is accomplished by isolation of the signal wires
from the source of noise.
Bonding: This assures that
unrelated conductive objects are at the same electrical potential. A very
important point: Without Bonding, lightning protection will not work.
All metallic conductors entering structures (ex. AC power lines, gas and
water pipes, data and coaxial signal lines, conduits and piping, roll up
doors, personnel metal door frames, hand railings, etc.) should be electrically
referenced to the same ground. Connector bonding should be welded and not
mechanical wherever possible, especially in below-grade locations, since
mechanical bonds are subject to corrosion and physical damage. Frequent
inspection and resistance measuring (maximum 5 milliohms per FAA and National
Weather Service standards) of connectors to assure continuity is recommended.
Lightning has its own agenda and may cause damage despite
application of best efforts, including those described above. Any comprehensive
approach for protection should be site-specific to attain maximum efficiencies.
In order to mitigate the hazard, systematic attention to details of grounding,
bonding, shielding, air terminals, surge protection devices, maintenance,
and employment of risk management principles is recommended. '
RECOMMENDED GROUNDING GUIDELINES
Prominent lightning engineers and major technical codes
and standards agree as to proper grounding guidelines. Here are summaries
of those generally accepted designs.
1. From Golde, Lightning, Academic Press, NY, 1977,
vol. 2, chapter 19 by H. Baatz, Stuttgart, Germany, p. 611:
"Equalization of potentials should be effected for
all metallic installations. For lightning protection of a structure it is
of greater importance than the earthing resistance...
The best way for equalization of potentials utilizes a
suitable earthing system in the form of a ring or foundation earth. The
down conductors are bonded to such a ring earth; additional earth electrodes
may be unnecessary"
2. From Sunde, Earth Conduction Effects in Transmission
Systems, Van Nostrand, NY, 1949, p. 66:
"Adequate grounding generally requires that the resistance
of the ground, at the frequency in question, be small compared to the impedance
of the circuit in which it is connected. By this criterion, it may be permissible
in some instances to have a ground of high resistance, several thousand
ohms, as in the case of "electrostatic" apparatus ground, the
impedance to ground of insulated apparatus cases being ordinarily quite
high. In other [situations], however, a resistance of only a few ohms may
be required for effective grounding."
3. From Horvath, Computation of Lightning Protection,
Research Studies Press, London, 1991, p. 20:
"The earthing of the lightning protection system distributes
the lightning current in the soil without causing dangerous potential differences.
For this purpose the most effective earthing encloses the object to be protected.
The potential increases on the earthing and on all earthed metal parts of
the object relative to the zero potential at a distant point. It may reach
a very high value but it does not cause any danger if the potential differences
inside the object to be protected are limited. Potential equalization is
realized by the bonding of all extended metal objects."
4. From Hasse, Overvoltage Protection of Low Voltage
Systems, Peter Peregrinus Press, London, 1992, p. 56.
''Complete lightning protection potential equalization
is the fundamental basis for the realization of internal lightning protection;
that is the lightning overvoltage protection for the electrical and also
the electronic data transmission facilities and devices in buildings. In
the event of a lightning stroke, the potential of all installations in the
affected building (including live conductors in the electrical systems with
arrestors) will be increased to a value equivalent to that arising in the
earthing system -- no dangerous overvoltages will be generated in the system
Nowadays lightning protection potential equalization is
considered indispensable. It ensures the connection of all metal supply
lines entering a building, including power and communication cables, to
the lightning protection and earthing system by direct junctions across
disconnection spark gaps, or arrestors in the case of live conductors."
5. From IEEE Emerald Book, Powering and Grounding Sensitive
Electronic Equipment, IEEE Std 1100-1992, IEEE, NY, 1995, p. 216:
"It is important to ensure that low-impedance grounding
and bonding connections exist among the telephone and data equipment, the
ac power system's electrical safety-grounding system, and the building grounding
electrode system. This recommendation is in addition to any made grounding
electrodes, such as the lightning ground ring. Failure to observe any part
of this grounding requirement may result in hazardous potential being developed
between the telephone (data) equipment and other grounded items that personnel
may be near or might simultaneously contact."
6. From International Standard IEC 1024-1, Protection
of Structures Against Lightning, International ElectroTechnical Commission,
Geneva, 1991, p. 23:
"In order to disperse the lightning current into the
earth without causing dangerous overvoltages, the shape and dimensions of
the earth-termination system are more important than a specific value of
the resistance of the earth electrode. However, in general, a low earth
resistance is recommended.
From the viewpoint of lightning protection, a single integrated
structure earth termination is preferable and is suitable for all purposes
(i.e. lightning protection, low voltage power systems, telecommunication
Earth termination systems which must be separated for other
reasons should be connected to the integrated one by equipotential bonding"
7. From FAA-STD-019b, Lightning Protection, Grounding,
Bonding, and Shielding Requirements for Facilities, Federal Aviation Administration,
Washington DC, 1990, p. 20:
"The protection of electronic equipment against potential
differences and static charge build up shall be provided by interconnecting
all non-current carrying metal objects to an electronic multi-point ground
system that is effectively connected to the earth electrode system."
8. From MIL-STD-188-124B, Grounding, Bonding and Shielding,
Department of Defense, Washington DC, 1992, p. 6 and p. 8:
"The facility ground system forms a direct path of
known low voltage impedance between earth and the various power and communications
equipments. This effectively minimizes voltage differentials on the ground
plane which exceed a value that will produce noise or interference to communications
"The resistance to earth of the earth electrode subsystem
should not exceed 10 ohms at fixed permanent facilities." (p. 8)
9. From MIL-STD-1542B (USAF), Electromagnetic Compatibility
and Grounding Requirements for Space Systems Facilities, Department of Defense,
Washington DC, 1991, p. 19:
"This Standard, MIL-HDBK-419, and MIL-STD-188-124
do not recommend the use of deep wells for the achievement of lower impedance
to earth. Deep wells achieve low dc resistance, but have very small benefit
in reducing ac impedance. The objective of the earth electrode subsystem
is to reduce ac and dc potentials between and within equipment. If deep
wells are utilized as a part of the earth electrode subsystem grounding
net, the other portion of the facility ground network shall be connected
10. From National Electrical Code, NEC-70-1996, National
Fire Protection Association, Quincy MA, 1996, Article 250 - Grounding, p.
120 & p. 144:
"Systems and circuit conductors are grounded to limit
voltages due to lightning, line surges, or unintentional contact with high
voltage lines, and to stabilize the voltage to ground during normal operation.
Equipment grounding conductors are bonded to the system grounded conductor
to provide a low impedance path for fault current that will facilitate the
operation of overcurrent devices under ground-fault conditions." (p.
"Metal Underground Water Pipe. A metal underground
water pipe in direct contact with the earth for 10 ft. (3.05 m) or more
(including any metal well casing effectively bonded to the pipe) and electrically
continuous (or made electrically continuous by bonding around insulating
joints or sections or insulating pipe) to the points of connection of the
grounding electrode conductor and the bonding conductors. Continuity of
the grounding path or the bonding connection to interior piping shall not
rely on water meters or filtering devices and similar equipment. A metal
underground water pipe shall be supplemented by an additional electrode
of a type specified in Section 250-81 or in Section 250-83. The supplemental
electrode shall be permitted to be bonded to the grounding electrode conductor,
the grounded service-entrance conductor, the grounded service raceway, or
any grounded service enclosure." (p. 145)
11. From MIL-HDBK-419A, Grounding, Bonding, and Shielding
for Electronic Equipments and Facilities, Department of Defense, Washington
DC, 1987, p. 1-2, p. 1-6, p.1-102 and p. 1-173:
"The value of 10 ohms earth electrode resistance recommended
in Section 220.127.116.11a represents a carefully considered compromise between
overall fault and lightning protection requirements and the estimated relative
cost of achieving the resistance in typical situations." (p. 1-2)
"At fixed C-E facilities, the earth electrode subsystem
should exhibit a resistance to earth of 10 ohms or less." (p.1-6)
"All metallic pipes and tubes (and conduits) and their
supports should be electrically continuous and are to be bonded to the facility
ground system at least at one point." (p. 1-102)
"Water pipes and conduit should be connected to the
earth electrode subsystem to prevent ground currents from entering the structure."
Lightning safety should be practiced by all people during
thunderstorms. Preparedness includes: get indoors or in a car; avoid water
and all metal objects; get off the high ground; avoid solitary trees; stay
off the telephone. If caught outdoors during nearby lightning, adopt the
Lightning Safety Position (LSP). LSP means staying away from other people,
taking off all metal objects, crouching with feet together, head bowed,
and placing hands on ears to reduce acoustic shock.
Measuring lightning's distance is easy. Use the "Flash/Bang"
(F/B) technique. For every count of five from the time of seeing the lightning
stroke to hearing the associated thunder, lightning is one mile away. A
F/B of 10 = 2 miles; a F/B of 20 = 4 miles, etc. Since the distance from
Strike A to Strike B to Strike C can be as much as 5-8 miles. Be conservative
and suspend activities when you first hear thunder, if possible. Do not
resume outdoor activities until 20 minutes has past from the last observable
thunder or lightning.
Organizations should adopt a Lightning Safety Policy and
integrate it into their overall safety plan.
CODES & STANDARDS
The marketplace abounds with exaggerated claims of product
perfection. Frequently referenced codes and installation standards are incomplete,
out dated and promulgated by commercial interests. On the other hand IEC,
IEEE, MIL-STD, FAA, NASA and similar documents are supported by background
engineering, the peer-review process, and are technical in nature.
It is important that all of the above subjects be considered
in a lightning safety analysis. There is no Utopia in lightning protection.
Lightning may ignore every defense man can conceive. A systematic hazard
mitigation approach to lightning safety is a prudent course of action.
NOTE: 1, 2, 3, 4, 5, 6, 7
- This information is only provided as a courtesy as a guideline to various
surge protection methods/philosophies and possible sources of vendors and
products. There is no guarantee as to manufacturer's claims of levels of
protection to any peripheral equipment. Customer assumes all risks and
liability associated with these third party devices outside the limits
of the warranties of these devices.
11 - www.sandia.gov
This article will be published in "Proceedings
of the 2003, Southeastern VHF Society Conference. pp 101 - 117, ARRL