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Choices and consequences of station lightning protection
Safety around lightning is a measure of acceptable risks and cost/benefit. The odds of a home
being struck change only slightly because it has antennas on the roof, dipoles in the trees, or even
the presence of a short tower. But if you are struck, the consequences of damage and injury
increase dramatically when improper or no protection is provided for personnel and equipment.
This document outlays the basic knowledge, safe design and installed equipment that are
employed in the lightning protection system of a US Coast Guard Auxiliary Radio Facility. This
should also be relevant to the Civil Air Patrol, MARS, and Amateur Radio stations who provide
emergency communications capability for public safety or the military. The presentations here
assume a basic understanding of electricity and electronics. Numerous professionals were
consulted and contracted in the design and completion of the system described here. The as built
system meets NEC and NFPA code. Assistance was also provided by several Amateur Radio
stations. See credits and links at the last page of the website. Estimated time to read the entire
document: 45min-1hr.
WARNING: Contact with high voltages from electricity or
lightning is deadly.
Prelude: Some common questions and misconceptions......
Q: When we "ground" things, doesn't that attract lightning?
Q: Then what does "attract" lightning?
or not.
A: No.
A: Any electrically conductive object, grounded
Q: What's the difference in the way lighting leaves an ungrounded versus a grounded
object?
A1 Lightning leaves an ungrounded object (to one that is) by converting enormous energy
into heat. The destructive power that is possible in this kind of event can be measured in
pounds of TNT.
A2 Once embarked on a bonded (continuous) and low-impedance path to ground,
lightning ceases to convert as much energy into heat, and dissipates into earth in a nondestructive manner.
Q: Can lightning go through anything? A: Not the actual strike, but it's powerful
magnetic fields penetrate any non-metallic structure, attaching directly to interior wiring.
Lightning can also strike any outside object, and the less conductive the object is, the more
damage usually results where lightning leaves that object. But a strike-attachment rarely
transfers directly to the interior to the interior of buildings, normally racing down the
outside of a structure to ground in the most direct path. That unfortunately starts fires on
most unprotected structures. Direct strike currents follow the skin-effect principle on
conductive materials and current remains on the outer edges of a building, car, sailboat
rigging, etc. Almost anything between lightning and the ground will be considered
"conductive" to lightning. It has after all, just burned through three miles of air to get
here, and things we think of as insulators, "aren't" to lightning.
Like the Marines, lightning is made for travel to distant places, breaking stuff, and killing
people.
Q: Is anything ever safe from lightning then? A: Yes. Equipment that is grounded and
bonded will rise and fall harmlessly with the common-mode transients from nearby strikes.
This is not protection from the (magnetically) induced-voltages or surges on AC or utility
wiring, but we will cover that...
Q: Should I disconnect and ground antenna feedlines before a storm? A: Lacking a total
system*, yes.
* Refers to a comprehensive design for the control of all entrance and exit paths for lightning.
Proper bonding and grounding with appropriate conductors, arrestors and suppression
technology, taken as a whole, make a system.
Q: Do I have to disconnect equipment power supplies before a storm? A: Same as above,
yes.
Equipment that is properly grounded and bonded inside a station is not going to be
affected by lightning strike energy on or near the system ground. This includes GPR, or
Ground Potential Rise, where ground-current can enter the station from ground and leave
(backwards) through AC wires, cable or telco lines. This high ground potential from a
strike is seeking a lower potential ground elsewhere in the system, and can destroy
equipment by using the reverse path of coax arrestor devices or any wiring. Stations who
fail to bond all equipment and neglect to create a well bonded path (with high current
carrying ability) to the main utility service entrance ground may someday experience this
destructive phenomenon.
However, the two other paths lightning takes is Electromagnetic Induction on interior
utility wiring, and Power Surges that enter the building wiring from external utility lines
(electric, phone and cable).
Q: But I thought my house wiring is grounded? A: Not for lightning! Building electrical
grounding is for one purpose only: equipment-fault protection from 120v and 240v. But
nearby lightning can cause surges of THOUSANDS OF VOLTS to enter via the power
company lines, or it may be induced onto your house wiring from a nearby or direct strike.
This energy can travel on your house wiring, and may damage or destroy anything plugged
into AC, telephone or cable lines. The home electrical ground system by itself cannot
prevent this. Neither does grounding and bonding the outer case of each piece of equipment
protect from induction on the wiring or power surges coming in from a nearby strike on
utility lines. EMI on wiring can only be controlled from careful application of surge
suppression.
Q: O.K., and I use "Surge-Protectors" on critical equipment, so I'm o.k., right? A: NO!
Not if your "surge protector" is the standard "power strip"! The slow-clamping but highvoltage protection provided by MOV (Metal-Oxide Varistor) power strips all shunt surge
voltage directly to AC (third wire) ground.
Vital Information: Typical MOV-type surge suppressor power strips are Dangerous:
The ground system of your home electrical wiring was never designed to handle lightning
surges.
Magnetically or surge-induced transient voltage in AC wiring should only be referenced
back to neutral (normal mode), never to AC ground. Never to AC ground! Referencing
EMI to ground requires sophisticated equipment not available at retail and should never
be attempted by amateurs using equipment not designed for that connection (such as
power strips).
There are technical reasons why these can also destroy sensitive equipment and data that
relies on a zero-volt ground reference during even normal motor-starting surges, but the
most serious reason is again.... The ground system of your home was never designed to
handle lightning surges. All kinds of things are "grounded" for electrical safety that use
inferior-sized conductors on un-bonded equipment in your home. 2,000-5000v surges can
flashover, and burn or destroy many things, if not your entire home.
Surge suppression for interior AC equipment should be normal mode only, which is Lineto-Neutral return. There are power-strips available that can do this (Transtector etc). They
are never sold at retail.
Let's say a nearby strike imposes 4000v on the utility pole wires (above or below ground)
heading toward your house wiring. At your AC service panel, ground is bonded to neutral
(this is code). Most Common-Mode (Line-to-Ground and Neutral-to-Ground) transient
voltage stops right there by virtue of this bond. You could put a powerful MOV arrestor
there, before the electric panel. Your electric utility may sell these, or you can have them
installed (with utility permission). As few homes go this expensive route, we will assume
that 4,000v wave is still riding on the hot wire (Line), with the differential being to neutral,
as it always will be inside your home. Non-degrading silicone avalanching diodes * in
"normal-mode" suppressors will reference this over-voltage back to the neutral line in < 5
nanoseconds. You will see later that at the radio station, referencing surges only to back to
neutral makes all the difference in the world! This normal mode transient voltage
suppression protects against not only lightning, but against all surges that typically happen
in the house or station. Standard MOV power-strips are dangerous here, taking between 5
and 20 microseconds to begin to act, and then shunting surges very few times before they
can short and burn. Also, you will get a lesson in why bonding is important the first time
your MOV-power-strip shunts 4,000v across an unbonded piece of "AC grounded"
equipment. It can also be incredibly destructive when it short circuits high ground
potential up into your house wiring from your own ground system!
MOV-type "surge" power strips are deficient and we were all misled by big names in the
industry for decades. MOV's are dirt cheap (pennies) to make and that's one reason they
are so popular. They are not safe indoors! They may self-ignite when the MOV's eventually
fail-shorted. Never use these in a home that employs lightning protection for
communications equipment. It is possible to design a total system that uses only MOV type
suppressors and this should be done by professionals only. Never mix normal and
common-mode surge suppression on the same branch circuit.
I recommend only commercial-grade Transient Voltage Surge Suppression (TVSS) in
normal-mode at both the service-mains panel and the individual circuits serving the
equipment, or, the disconnecting of all equipment power supplies is required. Many people
choose to simply disconnect before a storm. But how many of us go around disconnecting
everything? Why not protect your whole house?
Normal mode from this side->
Common mode "Strike"
Requires TVSS
bonding & ground
<Requires
Lightning directly striking the house, antennas, or the ground nearby causes common
mode transients. Common mode is the step-potential or transient voltage between two
ground potentials. It is defended against by lightning rods, grounded masts, coaxial
feedline arrestors and complete and proper common bonding. The proper bonding is so
critical that it cannot be over-emphasized.
The normal mode voltage differential is the only kind that can exist on inside AC wiring. It
comes into the house via either power line surges, or it can be imparted directly onto the
house wiring by electromagnetic induction. Surge protection is the only defense here, and it
should be normal mode (line to neutral return only) for all interior AC-wiring systems.
Satellite, CATV and telephone line protectors (which are vital to include in your plan)
normally shunt surge voltages to ground. If you're thinking - "Which ground, AC
ground?" NO! Remember the ground system of your home was never designed to handle
lightning surges. The Telephone Company already has one protector installed. That more
or less protects their lines, not your home. I added silicon-diode protection that protects
telephone Tip to Ringer, Tip to Ground and Ringer to Ground, and installed it in the
station.
Q: Can lightning also "induce" current inside my equipment? A: No, if it is built with
metal case enclosures, common bonded, grounded, and either surge-protected or
disconnected from all external power, then only the outer "skin" of the equipment will
carry energy. That surface-effect current will not harm electronics inside bonded metal
equipment cases.
Q: Can I touch my bonded/grounded equipment during a storm then? A: No. Unless you
are electrically bonded to the ground system (insulated from any other ground potential)
you could suffer burns if contact is made with both hands, a hand and foot, etc. "Step &
Touch" hazards exist on any ground system. Safest practice is don't touch a bonded
ground system during a thunderstorm. Use One-hand rule if you do.
Lightning basics: behavior across objects of differing
potential
Not all lightning is created equal. About 30% of all lightning strikes have a peak current of over
10 kA, while about 10% of all lightning strikes have a destructive current of over 50 kA. A
percent or two of strikes get over 100 kA, and strikes have been recorded with current peaks well
over 200 kA! Even the DC component of lightning creates immensely large voltage differences.
Its AC components develop astronomical voltages that saturate any earth ground.
A typical ground rod (8' X 5/8") has an impedance of about 5 ohms in good soil . Dusting off
ohm's law, we see that 5 ohms times 20 kA is 100,000 volts! Or is it? Well, we did neglect the
inductive reactance of the ground wire at the 100Khz to 100 Mhz associated with lightning. Take
notice that the power developed from just average lightning is sufficient to overload most
protection systems unless voltage-division (bonding and ground systems) are present.
The fact is that lightning will find a network of paths to ground by arcing over to whatever is at a
lower potential. Installed at the utility service entrance, either MOV or Silicon surge-suppression
can handle most differential voltage that becomes superimposed over the normal power line
voltage. Modern development in interior surge protection involves ignoring common mode
which is unable to enter U.S. buildings via power lines, in favor of normal mode protection.
Normal mode protection involves the hot to neutral lines only. Normal mode suppression never
assumes more than 6,000v because that is the level where an electric meter and all house wiring
would simply arc-over, and no more voltage than that can be carried by interior wiring. In June
0f 2004 lightning struck a home 1/2 mile from me, blowing the electric meter off the house! An
ineffective service-entry ground rod (in sandy soil) is almost surely the cause of this damage.
Common mode verses differential mode transients
Understanding the difference between normal-mode and common mode transients is key to
analyzing susceptibility to lightning damage. It is common in commercial and industrial
construction to find powerful MOV protectors guarding the utility line entrance to the structure.
These often protect against all modes (differential between each of the wires and each wire to
ground potential). On large, heavy duty HV entrance lines that's appropriate, but those high
current carrying lines do not come into a home. All common-mode (if any) will always stop at
the service panel, where neutral and ground are bonded anyway. Thereafter, voltages imposed
will be "normal-mode".
As the previous paragraph explains, newer protectors ignore common mode error in favor of
normal mode protection. Another way to distinguish differential and common mode transients,
is to think of differential mode as a transient injected with a parallel connection and common
mode as a series connection. Lightning itself (the actual strike) is common mode.
The common mode transient, or when is a ground not
ground?
The local ground connection will not stay at ground potential. If we could freeze time at the
instant a lightning strike connects, we would see that the local ground has an immense voltage at
the center of the strike due to the limits of the earth's impedance. As we move away from the
strike center there is a voltage gradient (step voltage).
Imagine further, concentric circles with the lightning stroke's connection with ground at the
center. With your feet about shoulder width apart, do you want to be standing parallel or
perpendicular the voltage gradient? The unfortunate sole who makes the wrong choice could be
killed by the strike with his two feet at vastly different potentials, while someone next to him
standing with both feet on the same voltage gradient line can walk away unscathed.
While holding this picture, we can see that for any direction one faces it is possible for lightning
to strike at a point that exposes our feet to ground connections of different potentials. Just as we
would be safe standing on one foot, our station can survive with a "single point ground" as well.
Finally, bringing the story back to equipment damage prevention, we need to assume that any
two ground sources entering a building can have a large voltage difference when lightning strikes
nearby. The common mode transient, as defined here, is a transient voltage difference between
two local ground sources. Damage happens when the ground rises to 100's of thousands of volts,
and uses our grounded equipment as a path to lower potential somewhere else!
Common mode transients can be of great concern when connecting antennas and their ground
radials to the Station ground. If the electric utility ground connection was the only ground
entering your building, there would be little problem because everything would go up and down
in voltage together, and there could be no electric discharge. But when you bring common mode
transients into a different side of the home (the radio station!), you have a major electrical
potential on your hands. Bonding direct from station ground to the utility service ground with
heavy, low impedance cable or copper strap to the service mains is a must. This bond is critical,
as the AC ground wiring in the radio room is of hopelessly high impedance to be a lightning
conductor. Lightning will force that path if you don't provide it!
Warning: If you do not bond from station to service entry point, a GPR event (Ground Potential
Rise) can use your AC ground wires (assisted by those dangerous ground-referenced "surge
protectors") to destroy your equipment by drawing ground current up through the station
ground and out through the equipment in reverse along your AC wiring. The same is true of
coax arrestors, phone line protectors, etc. They all work horribly in reverse, destroying the very
equipment they were designed to protect. Bonding is the only defense against this damage,
unless you can afford high voltage isolation transformers or fiber-optic isolation from ground
entirely.
Finally, ensuring that all ground conductors are sufficiently sized to carry high current for a short
time and common bonding all equipment, will allow a ground system to prevent what would
otherwise be "violent current equalizations" during a strike. If the system can't handle the
current, destructive flashovers will occur.
Other ground sources are also troublesome. Plumbing is a very low impedance ground source
and often enters on a different side of the building than the electric utility ground. If this is the
case, care must be taken to ensure that no utilities are connected to this separate ground source. It
is however advisable to include (bond) the cold water entrance into the overall station and
building ground system. Just don't bond a lightning down conductor from the roof to anything
until it reaches it's own ground rod first. Once it does, it is both safe and code requirement to do
so.
Note: I discovered that my cable provider failed to connect their cable shield ground to my
service mains ground rod. The installer initially failed to connect any ground at all, and when I
pointed that out, he connected "his" ground to a water pipe under the kitchen. I was not aware at
the time how a lightning strike could cause a huge voltage potential between these unbonded
ground points, and destroy not only the cable modem but the computer and everything it was
connected to. That's because it was bonded to nothing, and had a separate ground from the
rest of the house!
*** Check your utilities! *** Bond everything ***
O.k., what keeps lightning from destroying stuff?
Common Bonding: Rule #1 for successful grounding.
Maintaining bonded paths of extremely low impedance from each station equipment to a
single point ground is vital in preventing the exchange of lightning energy between
equipment in the station. The station equipment individually bonds to this single point of
the station ground. No grounding system offers any protection without 100% equipment
bonding. This system, effective only when taken as a whole, is the principle known as
COMMON BONDING and the SINGLE POINT GROUND SYSTEM (SPGS).
What this means is that all station equipment is at the same potential and the equipment is
all tied together at a single point ground. That common tie-point should be immediately
adjacent to the equipment.
After connecting to that first ground rod, it must bond to the home's utility service
ground.
The bonding and ground materials, in addition to being good conductors with very low
resistance, must also have a large surface-area. This is what makes them low impedance!
For instance, 3" wide 20ga. solid copper strap (flashing) has similar resistance to a #4 solid
copper wire...but the strap has a lower inductive reactance, and is therefore lower
impedance than the solid wire. 6" strap is even better. This is important!
Bonding Materials:
5/8"x 8' copper-clad steel ground rods, heavy (#2 or #4) solid copper wire and wide copper
strapping are good example of recommended material to tie all equipment grounds,
antenna grounds, RF grounds, and the station's main service ground together. Only ULapproved connectors should be used to join ground wires, ground rods, etc. The Cadweldsystem ( exo-thermal bonding used outdoors only) can be expensive and is used in
underground work . When a ground system is properly connected, there exists almost no
electrical difference between earth ground and all radio equipment in the station, the SPGS,
all of the station's antennas, and the SPGS of the house utility service-mains. All will rise,
(float), and fall together.
The GOAL of all this common bonding is to prevent lightning from sensing a difference in
potential between any equipment in the station or it's antennas and maintaining similar
low impedance with earth ground.
Q: Is that ever possible (the perfect equi-potential ground)?
A: Only until you get struck! Example: When lightning strikes next to a station, the area
immediately around the strike-zone (including the surface of the earth) raises from zero
volts to hundreds of thousands of volts for a very brief time. "Ground" as we knew it,
ceases to be a safe place. At this same moment in time, the station ground system several
feet away feels a huge potential difference and will reference thousands of volts from the
earth. Even small values in conductor impedance will allow momentary flow of hundreds of
amps of current across the ground system. But the direction it comes from does not matter.
If the grounding system is common-bonded it will always work! Current equalization can
be violent across anything but a well designed and capable ground system. However,
drawing current up from the earth to an un-bonded system has consistently devastating
results. The bonding is the critical feature, as it allows all bonded equipment to be in the
same "boat". That boat rises equally with the lightning energy, and falls equally with it.
When properly bonded, the ground system could care less what actual voltage is present on
it.
In grounded stations with un-bonded equipment, there have been flash-overs between
separately grounded equipments, including explosive current equalization inside the air
spaces of the station!
In a proper system design, during a direct strike the bonded equipment itself might feel
thousands or hundreds of thousands of volts, but will carry very little current due to the
common-bonding and single point ground. As we discussed above however, lightning will
create massive transient voltages on it's own, and current transfer will always happen
between parts of the ground system during a strike. Subsequently, the bonded equipment
cases and bonding conductors could carry some equalization currents. Sufficiently sized
conductors must be able to carry the expected amount of current for their safety
purpose. Always err on the side of "large surface areas" there. Nothing you do is more
important than this bonding of all equipment to a single point ground! With proper
bonding, equipment might survive even with no ground at all. Without proper bonding, no
ground system is good enough to keep lightning from causing damage.
Although unfortunate, some damage can result in any high current situation. While
unlikely, a direct strike of major magnitude will find the weak points of any system, and
could cause damage in spite of a good design. I will add though, this could be compounded
by poor housekeeping in the station. Loose conductors scattered behind equipment for
instance, could cause high voltage from a strike to flash-over and damage otherwise
protected equipment. So remember to keep your station bonding and lightning ground
system as uncluttered as possible. Check at least annually for tightness of mechanical
joints.
Before we go any further, don't be discouraged and think lightning is unmanageable. It is
manageable. Even the worst magnitude of strike possible would do substantially less
damage to a protected station.
The best protection from lightning is the Common-Bonded, Single-Point Ground System.
There is simply put, no safer way to operate. Even when ground systems are carrying
massive amounts of energy, they are still doing their job. Floating that energy across an
equipotential system is a happier place to be.
2. External grounds: a better place for lightning!
Lightning does not bypass tall trees, antenna masts, dipoles, or the roof and chimney of a
house in favor of coming into the house or station first. It just doesn't happen. What does
happen to stations that improperly bond or ground, is that lightning starts down trees,
antennas, chimneys, etc., and then discovers ungrounded and / or unbonded electrically
conductive equipment inside the station. It reaches this unbonded equipment two ways:
from magnetic and capacitive coupling, and via the return-stroke current from ground.
Case in point below:
Outside feedlines were disconnected near the base of a grounded tower and lay next each
other on the grass. These open feedlines are still connected inside the station to ungrounded
and unbonded equipment. One antenna accidentally remains fully connected but like the
rest, has no shield-grounding or coax arrestor system. All equipment is grounded only by
virtue of being connected to AC power, and most is unplugged at the time. None of the
equipment is common bonded. With the exception of the one connected antenna this time,
the station in this condition had survived tower strikes in the past with minimal or no
damage. Disconnecting antennas outside and unplugging AC power inside was the
lightning protection plan.
A MAJOR strike occurs, exploding the one connected antenna and destroying the radio it
is connected to.
In the station, violent flash-overs from radios to computers, even disconnected equipment
occurs. While some of this massive lightning energy is ruining equipment on it's way to
and from the ground, most parts of the house wiring are being inductively charged to the
point of damaging or destroying most things plugged into those circuits. It's a very
destructive event, and some minor damages take months to be discovered.
It could have been worse, but not much. One of the return-strokes shot rifle-caliber holes
through unbonded equipment, induced EMI that tore up 12" holes in tile flooring where a
lone test cable hung, and disintegrated 12vdc converters wherever they were plugged in.
The operator had the misfortune to witness these events from inside the station. Thank
God, he wasn't touching anything when it happened.
While this case was somewhat rare in it's severity, even the worst lightning possible can still
be mitigated, and most lightning damage can be avoided completely.
Now what if your rooftop antennas, tower, and all equipment are properly grounded and
bonded?
Lightning will direct most of it's energy straight to ground outside the station, with
minimal if any damage to the antennas themselves. Can some lightning energy come inside
via the coax feedlines?
Yes. But it will be less than what travels a lower impedance path of the antenna's own
common ground rods. Shield grounding the coax at the tower top, tower base, and entrance
to the station is a must. Finally, the coax feedlines require either Lightning Arrestors or
they must be disconnected and shorted to common ground. Not all coax lightning arrestors
are created equal. There is one type made by Industrial Communication Engineering (ICE)
that uses a multi-attack system which they claim has never been defeated by lightning. Still,
the failure of other manufacturer's lightning arrestors are often claimed when the real
cause of lightning damage was due to improper or nonexistent bonding, poor grounding,
failure to shield-ground, and/or lack of proper surge protection. Even a single-mode gas
tube arrestor can do the job it is designed for just fine. It can't work miracles, and failure
to shield-ground the coax before the arrestor will destroy the arrestor.
Commercial-grade surge suppression is still required for the station wiring, or in it's
absence, all equipment must be unplugged. All this information, taken as a whole, provides
solid protection.
I use rooftop masts for UHF/VHF-marine, and tree-supported horizontal wire antennas for
HF-aeronautical. All of my rooftop antennas are bonded together and to ground rods with
#2 solid copper wire. The antenna grounding is of course common-bonded to the station's
ground system after those down conductors first connect to their own 8' ground rods. All
coax feedlines enter the station from underground, are shield- grounded and then
protected by ICE lightning arrestors. These arrestors are mounted to the station Master
Ground Bus, which is 4' above the first ground rods of the grounding system. While this
provides protection from a direct strike, grounding, bonding and arrestors do not prevent
induction of lightning energy on the home or station electrical wiring. Without surge
protection at both the house service entrance and the local load-center or power-supplies*,
you must disconnect the AC power supplies to all equipment in order to protect them from
electro-magnetic induction and it's very destructive energy.
* Polyphaser Corp and it's parent Transtector, as one example, makes devices to protect
individual power supplies and whole-house circuits by TVSS (Transient Voltage Surge
Suppression). Links to these companies and others are available at the end of this website.
3. Radio Frequency (RF) Ground Systems
Radio Frequency (RF) grounding is not related to lightning or electrical safety per se,
except that if RF energy leaks onto the coax shielding because of poor tuning, high SWR in
antenna systems, etc., there is both a safety issue and poor efficiency. More importantly, if
the RF ground system is not bonded to the lightning protection system, the entire SPGS
could be rendered useless. Serious damage could result.
RF ground systems and lightning ground systems must be bonded together, no
exceptions. *
Some types of transmitting antennas require ground radials (buried copper wire paths in a
360-degree pattern) in order to effectively radiate. My antennas derive no benefit from RF
radials so I do not use them. But all transmitting stations should have an RF Ground at the
transmitter. This can be provided by the station AC ground wiring, but that has a lot of
shortfalls. A good transmitter RF ground system should be integral and dual-purpose with
the station grounding system.
As most stations provide, I installed an extremely low impedance RF Ground System. I
took the further precaution of using multiple RF ground rods connected in close parallel
circuits to each other. This prevents a high frequency phenomenon known as "1/4 wave
resonance". Whenever an RF ground wire is 1/4 wavelength of the transmit antenna, the
inductive resistance of that RF ground wire raises dramatically. In effect, it becomes no
ground at all and has serious safety consequences. That must be avoided by transmitter to
RF ground lengths shorter than 1/4 wavelength, and/or connecting two or more RF ground
rods in parallel.
Whether a transmit station requires antenna ground radials, or just the transmitter RF
ground system as mine does, the following always applies:
An RF ground system and lightning ground system must be bonded together, no
exceptions. *
Make sure that any down conductors from roof lightning rods and masts reach their own
ground rod first before bonding to the rest of the RF and station ground system. Down
conductors will carry the direct energy of an entire lightning strike and you do not wish to
share this with your station before it reaches ground!
Notice we did not dwell too much on ground rods. Don't push them through buried
utilities, put at least one and preferably three for each down conductor (at least their sumdepths-apart in a "Y" pattern), and don't assume you are getting a "real good ground"
unless you have it tested professionally. The reason we don't spend more time on
grounding here is two-fold: 1. Everyone knows that we must have ground rods (this wasn't
intended for beginners to lightning protection) and 2. The bonding and surge protection
are simply more important. That, I fear is grossly misunderstood, and it is perhaps the #1
reason why lightning is so destructive to so many well-intentioned people, who thought they
were protected.
We keep stressing why it is vital to maintain bonding of all equipment
anywhere near a grounding system or it's connectors - because no
grounding method is low enough impedance to prevent lightning from
creating transient voltage rises.
Lightning, which is a current pulse, contains a broad spectrum of frequencies-the center of the
power spectrum is about 4.5 kHz, with the upper limit reaching into the MHz range. Its peak
return-stroke current is extremely large (10's of thousands of amperes), typically lasting for a
hundred microseconds, or so. As the return-stroke current pulse flows through the resistance of
the earth it produces a very large transient potential gradient across the ground. This potentially
lethal gradient-nominally 1,000 volts per meter-is known as step voltage (if you "step" across it,
it will kill you). However, even when the current is flowing in a substantial metallic conductor
(i.e., one having a very low value of dc resistance) very large transient voltages are developed
along the conductor. Although resistance may be very low, e.g., less than 10 ohms, the
inductance (L) of the conductor (nominally 1.5 micro henrys per meter of conductor length)
times the very high rate of change (di/dt) of the current pulse produces transient voltages
reaching 100's of thousands of volts, or higher (V = I*R + L* di/dt). Low dc-resistance to ground
will certainly help entice lightning to join a ground system. Keeping lightning there is a more
difficult task.
So, despite the big emphasis on achieving a very low resistance ground, the inductive effect
predominates, resulting in transient voltages significantly higher than those attributed to dc
resistance of the grounding system. A lightning grounding system must be capable of
accommodating extremely high peak currents, and present low values of resistance and
inductance (recall that we consider this total desired effect to be "low impedance").
When grounding system resistance is tested, the test equipment operates at a very low frequency.
The result, which may look quite low, will actually be just the dc resistance component. Huge
(i.e., deadly and damaging) transient voltages will still be developed across the conductor while
return-stroke current is flowing on it.
Finally, consider a ten-meter section of heavy copper conductor connected to an earth ground at
one end only. For lightning protection, two systems are bonded to it, one at each end of the
section. The dc resistance between the two points is measured to be ten milliohms; the
inductance is 15 micro henrys. A 50th percentile lightning return stroke of 24-25 kA, with a
current rate-of rise of 40 kA/microsecond, flows through the conductor. Peak current times dc
resistance produces approximately a paltry 240 V peak between the two "grounded" points.
However, the peak transient voltage resulting from the conductor's inductance is 600,000 volts!
The two supposedly grounded systems are 600 kV apart, albeit only for a brief interval of time.
Equipment damage and serious injury or death are definite possibilities, hence the reason for
using single-point grounding and bonding of all near-field equipment. When long runs of
conductor are required between station single point ground and AC service mains ground,
antenna ground, etc., the only way to avoid deadly flash-overs and violent current equalization is
to use many low-impedance grounding rods and multiple parallel paths with all equipment
bonded together. This way, the system floats more or less equally, and is also capable of
carrying brief periods of the inevitable high current associated with a direct lightning strike.
I hope readers here come to appreciate that neither component (grounding or bonding) can exist
by themselves. Before the final page and conclusions, you may examine the schematics that
represent my specific requirements at Oceana Radio. Your station will certainly have
requirements unique to your individual conditions, but there will be commonality among all
stations that use only a total system defense against lightning. Some stations actually prefer to
toss feedlines out the window before a storm. In those cases, only mast grounding (external) and
surge protection (internal) would be required. But let those operators sink a rod under the home
for RF purposes, and....BANG....without bonding, they just drew a hundred thousand volts up
from ground into a transmitter or tuner with lots of nearby metal objects. All of it will be
destroyed, and a nearby person possibly electrocuted from a private fireworks display right
inside their home.
Conclusions, sample equipment, links, and credits
The service mains (utility entrance) ground is the "single point ground system" for your
home's wiring. Phone and cable service must also ground at this same entry point.* A
radio facility/station has special low impedance ground requirements that home wiring
does not. The station, if not located at the AC service entry, must have immediately
adjacent to it's equipment center, it's own single point ground system. Lightning arrestors
installed at the station must be on this ground panel. All station equipment must use low
impedance conductors to individually bond to this station single point ground. This is one
place you should use at least 3" wide copper strap! Discard all forms of "braided" cable.
Braid is something a girl does to her hair. It is not a good RF ground and it's a terrible
conductor for lightning energy.
You may call this a bulkhead, master ground bus, ground window, etc., but it will be the
single point at which all bonding in the station connects. And it must be as close to the
station equipment as possible! Then, some part of the station grounding system must bond
to the home's utility service entrance ground. That is a critical bond, and it is challenging
when the station single point ground is a long distance from the service-entry.
Unfortunately, recall that you cannot maintain low impedance by a long run of wire no
matter how heavy the wire is. If your station equipment is more than a few feet from the
service mains ground rod, the voltage-differences along the bonding conductor will be
significant to the massive energy of lightning. This is minimized by choosing the largest
surface-area conductor you can get, and installing intermediate ground rods along the path
to the utility service entrance ground. The better the bonding, the stronger the ground
system gets. This is rephrased below, because it is so often ignored, to the destruction of
equipment:
In house-wiring, the typical #12 or #14 ground wire at AC outlets all go to the service
panel (where they also bond to the neutral bus), and then to the service entry single point
ground rod. Fine for equipment safety, but totally unacceptable for lightning protection.
That permits a massive electrical potential between any part of your house wiring and the
potential at the service-mains ground rod. Why? Because your house wiring was never
designed to handle lightning energy! But, because your house wiring comes into your
station equipment, it is therefore critical that we bond the station single point ground
system directly to the utility entrance ground! It's a code requirement (when the station
ground is separate from the utilities entrance), and yet so often overlooked by amateurs
who learn the hard way about GPR damage.
When it's a long distance from the station ground to the service entrance ground,
maintaining low impedance along the whole distance is impossible. But you can make a
much lower impedance path than your house wiring provides, and that is vital. I used
frequent references to earth-ground along the path (lots of ground rods). This prevents
violent current equalization from a direct lightning strike (or high EMI and transients
from a nearby strike) and the effects of cumulative impedance along the bonding path. It
also gives that long path very high current handling ability, and that's good, because the
voltage differences caused by lightning will be large. Bonding to the service-entrance
ground is vital to controlling Ground Potential Rise (GPR) from damaging equipment.
GPR occurs when a direct or nearby strike raises and saturates the ground potential so
highly that current tries to flow up into your station ground and out through utility wires
and/or coax feedlines to anywhere that the ground potential is lower. Fast-acting low
impedance grounding helps, but current will not choose your house wiring in a GPR
situation if you obey the bonding rules with a direct, high-current capable bonding path to
the utility mains ground rod. You will have safely provided a much lower impedance path
than the telephone, cable and AC wiring can offer. Failure to shield-ground coax at the
main station ground rod closest to the station SPGS can encourage the coax to feed GPR
backwards through the arrestors to some distant point of the ground system of lower
potential.
The goal is to make the entire ground system feel the same (equipotential) to lightning
energy. Heavy gage (#2 or #4) solid copper wire and heavy gage wide copper strapping (6")
and many ground rods are the tools for the job. This keeps the designed ground paths of
such low potential and high current-carrying capacity, that once on board it, lightning will
make no diversions on it's way to harmless dissipation into the earth. That's it. There is no
more we can do than to make it easy for lightning to go away once it happens.
Air Terminals (Lightning rods) on the roof of a structure are required every 20' of roofridge and within 2' of all roof edges for a structure to be protected from lightning.
Grounded and bonded masts fulfill similar protection (with the 20' horizontal limits and a
45 degree downward cone of protection). But if air terminals are added they must share the
same bonding. Metal gutters and flashing must also be bonded to prevent flashover around
the structure. Buy, borrow, or check-out the NEC-70, NFPA-780 and books on grounding.
All this information, taken as a whole, provides solid protection for personnel and
equipment safety.
* If you discover that any utility service (cable, phone, etc) is not bonded to the same single
point ground rod (normally found very close to your electric meter) call the offending utility
right away, or if you are qualified, fix it yourself.
Coax lightning arrestors, shield grounding, wire-to-ground-rod clamp, copper
strap
Gas-tube only. Effective, but...
choice.
Multi-mode-suppression, multi-strike. My
Not the most efficient. To ground your cable
strapping
shield, carefully cut/peel outer insulation and
minimum for
wrap copper- foil or use a commercial shield
connecting to ground.
ground that clamps on exposed outer shield.
Wire-to-rod clamp
#2 or #4 fits here
Use 5/8" Rod only
6" copper
is
Links to Lightning Protection Information:
http://www.rbs2.com/fire.htm#anchor777777
Fire Hazards of Surge Suppressors
http://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/Bonding-NotGrounding~20040426.php
Short discussion of why common bonding is more important than grounding. In other words,
why bonding everything is more important than fretting over miniscule ohms of resistance in
ground rod references.
http://www.harger.com/ The BEST source of Station Grounding supplies.
http://www.lightningsafety.com/
http://personal.isla.net/ice/
http://www.arraysolutions.com/Products/ice/index.html
Major dealer for Industrial Communications Engineering (best coax lightning arrestors)
http://www.polyphaser.com/
http://www.nexteklightning.com/index.html
http://www.zerosurge.com/HTML/mode2.html
Good explanation of why Mode-2 (common mode) protection is wrong for interior surge
protection.
http://www.elec-toolbox.com/usefulinfo/lightprot.htm
http://www.packetradio.com/grounds.htm
http://www.astrosurf.com/lombry/qsl-lightning-protection.htm
http://www.glenmartin.com/catalog/lightning.htm
Good examples, but Home Depot & Lowe's carry almost the same grounding accessories.
If you do decide on Air Terminals, there are new studies that prove Ben Franklin wrong about
the pointed tips...it took 230 years to discover that blunt-tip lightning rods are much more
efficient at delivering lightning bolts to ground.
http://www.lightningstorm.com/tux/jsp/login/index.jsp
Free lightning detection (30min delay on the half hour and hour). Determine lightning activity in
your area.