The Lightning Event – from NOAA
The lower part of a thundercloud could be positively or negatively charged. The upward area is
mostly positively charged with negative areas throughout the top. Interior flashes are
attempting to balance /equalize the over-all cloud potential referenced to earth below and the
stratosphere/ troposphere above. Lightning from the lower negatively charged area of the
cloud generally carries a negative charge to Earth and is called a negative flash. A discharge
from a positively charged area to Earth produces a positive flash . ….(kr)
The lower part of a thundercloud is usually negatively charged. The upward area is
usually positively charged. Lightning from the negatively charged area of the cloud
carries a relatively negative charge to relatively positive Earth. A relatively
positively charged area of Earth can produce an upward positive flash. ….(kr)
The Lightning Event
A negative strike showing charge accumulation and discharge
The Larger the Charge, the Larger the Step
Typical Step 150ft. @ 50µS per Step
( 1µS jump, 49µS pause )
Step Leader
Distance
10kV/m to 30kV/m
E - Fields
Jumping
Hemisphere
150 feet
Measured Peak Lightning Current
350kA
Maximum with 99.5%
Confidence level
300kA
Maximum with 98%
Confidence level
M. Uman (U.F. corrected)
~50% at 18 – 20 kA pk
Ref: W.C. Hart, E. W. Malone, Lightning
and Lightning Protection, EEC Press, 1979
Time to Peak Lightning Currents
Max. 10-sec
Min. 0.7-sec
0 to peak current with 96%
confidence level
M. Uman (NASA)
~1.8 – 2 uS to peak
Time
0 to peak current
Ref: W. C. Hart, E. W. Malone, Lightning and
Lightning Protection, EEC Press, 1979
Duration and Amplitude of Continuing Currents
Max. 1000A
Min.
30A
Max. 550m-sec
Min. 35m-sec
Ref: N. Clanos and E.T. Pierce, “A Ground Lightning Environment for Engineering Usage”,
Contract L.S.-28170A-3, Stanford Research Institute, CA
How a Tower Lightning Strike Damages a Communications Site
The fast rise time inductive voltage
drop across any tower during
a lightning strike is much higher than
the resistive voltage drop.
Higher instantaneous peak voltages
can damage equipment!
All Towers can be
Inductors
Communications Radio Tower
Inductance
High Peak Voltage going to Entrance Panel
360kV Peak
Strike Voltage Distribution and
cable shield potential at entry port
Distributed
Voltage
across mast
Peak Voltage on Cable
Shields ~ 28kV going to
entrance panel
360kV would arise at the top of a 40µH mast
with a relatively small 18 kA w/ a 2µS risetime
strike . The voltage would be distributed down
the mast to ground. If the cable shields were
bonded to the mast at the 8 foot level, about
28kV would be riding on shields going to the
entrance panel
Equipment Grounding with Coax Entering
from a High Entry Panel
 Grounding at bottom of the rack
creates a path for high peak
surge current to traverse the
rack, upsetting or destroying
equipment.
 Proper grounding of the
equipment rack. If coax jumper
cables enter at the top, ground
high. If they enter low, ground
low. There will be minimal
current flow through the rack.
Tower
Mag.
Fields
Current
Flow
Reverse
EMF
On Coax
BACK
EMF
Tower
Coax
150ft / 360kV
75ft / 250kV
Inductive voltage drop across entire
40uH tower with 2us rise time and
peak current of 18kA -E=Ldi/dt
Magnetic field coupling into coaxial cable from
current flow down the tower can cause a
reverse emf on the coax, opposing downward
current flow, and creating a differential voltage
between tower and coax. Coax cable insulation
could breakdown and allow an arc back to the
tower.
An additional ground kit at the tower center
brings the shield back to tower potential
reducing peak voltages and the probability of
coax breakdown
8ft / 28kV
BTS
Shelter
If all coaxial cables were grounded and pulled away
from the tower at the bottom, and brought to the
shelter at or below grade to a well designed entrance
panel with protectors:
• There would be no differential voltage between the master
ground bar and earth ground below it.
• There would be no current flow from the coax jumper
through the equipment rack to earth ground.
• All equipment would be at the same potential as earth.
• Most lightning damage problems to equipment would stop.
Unfortunately, most tower site installation plans specify a feeder cable
“pull off” from the tower at 8 feet or higher. This practice stresses
equipment, reduces lifetime, and can cause catastrophic damage.
The coax jumper shield connection
to equipment is the largest source
of damaging surge current
High Peak Voltage going to Entrance Panel
360kV Peak
Strike Voltage Distribution and
cable shield potential at entry port
Distributed
Voltage
across mast
Peak Voltage on Cable
Shields ~ 28kV going to
entrance panel
360kV would arise at the top of a 40µH mast
with a relatively small 18 kA w/ a 2µS risetime
strike . The voltage would be distributed down
the mast to ground. If the cable shields were
bonded to the mast at the 8 foot level, about
28kV would be riding on shields going to the
entrance panel
100 kA total discharge current
73 kA propagating down the tower
27 kA divided between all coaxial cables
27kA divides itself between distribution coax cables
Lightning Strike
Current Distribution
Not all the Lightning Strike current goes toward
the equipment on the Coaxial Cable Shields.
If we consider an above average Strike event
with a 100kA fast rise time dc Pulse:
Total lightning discharge current:
Total number of coaxial cables:
100kA
18
73% of current down the tower:
27% of current on coaxial cables:
27kA / 18 (number of same size cables)
73kA
27kA
Applied current to each (one of eighteen)
coaxial cable and protector =
1.7 kA
Lightning Protectors for Coaxial Cable
Spectrum analyzer sample: actual frequency / power
distribution is based on stroke rise time)
While dc and low frequency (50 -60 Hz) current utilizes the conductor’s entire cross section,
high frequency current tends to migrate towards the conductor’s surface. A conductor’s hf
impedance ( Z / per unit length) during a lightning transient will be reduced as the
circumference is increased. Lower Z means less voltage drop, and a better lightning
grounding conductor.
Skin Effect !
Skin Effect !
Lightning current distribution at terminating end of coax cable
Velocity of propagation on shield vs. center conductor
Coaxial cable shield lightning current
Center conductor lightning current
Unequal current distribution /time causes a peak voltage differential between the
elevated shield to equipment ground, and center conductor circuitry to equipment
ground. The difference in peak voltage across the shield and center pin can cause
damaging currents to flow through sensitive equipment input circuitry.
Lightning protectors equalize this differential, and reduce lightning energy
throughput, saving equipment inputs from damage.
Antenna
LMR400
LMR200
Coaxial Cable
Protection for wireless sites
with directly connected
antenna (no dc requirement)
Required grounding points
Times-Protect Arrester
LP-BTR/W, STR(L), GTR or
LP- GPX / WBX as required
Shelter
LMR
Jumper
Tower
IDU
RF Lightning Protectors and PIM-What you need to know
In today’s wireless architecture, another important issue is Passive
Intermodulation (PIM). Passive Intermodulation distortion is generated when
two or more RF signals pass through a non-linear junction.
The below graphs provide visual illustration of this phenomenon.
Fig. 1 below shows the linear response of a proper contact,
Fig. 2 represents the behavior of a non-linear junction in the RF path.
(Reprinted with permission From Microwave Journal – May 1995 Issue)
For PIM sensitive applications, properly designed and tested
lightning protectors should be selected and installed in accordance
with required guidelines.
The below measurement is a Times Protect LP-STRL Lightning Protector designed
For low PIM with two +43dBm (20W) carriers applied to the surge side connector.
Sample was taken “off the shelf” to provide an objective evaluation.
Actual PIM measurement -180 dBc
The primary causes for PIM generation
•Dissimilar metals (galvanic action)
•Poor Surface Quality (roughness)
•Low Contact Pressure (improper torque or solder)
•Poor Contact Cleanliness (residual chemical films)
•Use of Magnetic Materials
•Changes in Temperature and current density
In-line RF lightning protection devices can
contribute to PIM interference based on one
or more of the above issues.
Antenna & ODU
Times-Protect Arrester
LP-GTR, LP-GTV, or
LP-GPX as required
Broadband protection for wireless sites
with antenna incorporated into the ODU
LMR400
LMR200
Coaxial Cable
Required ground kits
Times-Protect Arrester
LP-GTR series, LP-GPX,
LP-GTV or LP-18-400 as
required
Shelter
LMR
Jumper
Tower
IDU
Antenna
LMR Jumper
Broadband protection for wireless sites
with antenna separated from ODU
Times-Protect Arrester
LP-GTR, LP- STR, or LP-WBX series
as required
ODU
Times-Protect Arrester
LP-GTR, LP-GPX or LP-GTV as required
Required ground kits
LMR400
LMR200
Coaxial Cable
Times-Protect Arrester
LP-GTR, or LP-GPX, LP-GTV,
or LP-18-400 as required
Shelter
LMR
Jumper
Tower
IDU
Product Selection Matrix
(20-1000MHz) DC blocked N type – UHF/VHF applications – Indoor use
LP-BTR-N
SCADA, LMR, Utilities, Public Safety, Oil, Gas
LP-BTRW-N
(20-1000MHz) DC blocked N type – UHF/VHF applications – IP67 Weatherized
SCADA, LMR, Utilities, Public Safety, Oil, Gas
LP-STR-D & N Series
LP-STRL- D & N Series
DC Blocked
DC Pass
LOW PIM (800-2500MHz) DC blocked DIN & N type
- Cellular Carriers
LOW PIM (680-2200MHz) DC blocked DIN & N, type –LTE, 700MHz
Public Safety Spectrum, Cellular Carriers
Broadband
Wireless
LP-WBX-N Series (2000-6000MHz) DC blocked
LP-GPX-05
(1000-2000MHz) N, TNC & SMA type (5V) DC pass- L1, L2 & L3
Bidirectional GPS Protector
(DC-3000MHz) DC pass N type (50/210/550 Watts)
LP-GTR-N Series
LP-GTR-D Series
(DC-2500MHz) DC pass DIN type (50/210/550 Watts)
LP-18-400-N Series (DC-6000MHz) DC pass N type F&M with EZ-400 interface (150W)
LP-GTV-N Series (DC-7000MHz) DC pass N type Female /Female & Female /Male (150W)
0
MHz
1500
3000
Smart – Panel &
Grounding
Accessories
4500
6000
7000
Lightning Grounding Conductors
While dc and low frequency (50 -60 Hz) current utilizes the conductor’s entire cross section,
high frequency current tends to migrate towards the conductor’s surface. A conductor’s hf
impedance ( Z / per unit length) during a lightning transient will be reduced as the
circumference is increased. Lower Z means less voltage drop, and a better lightning
grounding conductor.
Traditional method consisting of:
• High material and labor cost
• Lack of provisions for other service entries
• Theft exposure
• Very high impedance return path to
ground
• High preventative maintenance
Proposed method:
• Addresses theft issue
• Does not require external shield grounding kits
• Makes provisions for Coax/EWG/Data/DC and fiber
• Minimal labor cost
• All prep work performed at the shelter manufacturer




A: Copper strap may be visualized as an infinite number of
infinitely small wires spaced infinitely close together
B: The magnetic field surrounding each imaginary wire
C: Wire magnetic fields added vectorially
D: Magnetic field close to surface of strap, extending at edges
Note single small ground conductor on MGB above
Copper Ground Bar and conductor missing below
After the fix (?)
External grounding
after theft
The high
impedance fix
RF Protectors
Bulkhead mounted
2/0 Conductors
to ground
Traditional method:
• Requires separate Inside MGB (IMGB)
• Trapeze or other method to ground SPDs
• Performance affected by long ground wires
• High impedance IMGB ground conductor
• Single point ground by installation
• High ground loop probability
Propose method:
• All RF protectors bulkhead mounted for best surge performance
• Assembly accommodates different wall thickness
• Provisions for grounding of all protectors to the same SPG
• Low impedance ground path for lightning current
• Control of MGB potential rise due to low “L” of assembly
• Accommodates for additional equipment mounting
2 ft. separation provides
120dB isolation for
equipment at 20KA
lightning current on
down conducting plate
Equipment
Rack
Effects of 1.5 ft ground lead inductance of #1 AWG Cu wire
Voltage and energy throughput drastically increases
Bulkhead mounted SPD without added ground “L”
Surge return directly connected to protector ground
+544V, -176V
+25.6V, -9.2V
Surge
Generator
+
-
Center Pin
DUT
Su rge
+
Ground
Inductance
V oltag e M easured at O sco pe
Ce nt e
r Pin
DUT
Chass is
Ground
G en er ator
-
Voltage M easured
at O-scope
Note: The length of the grounding conductor connected to any lightning protection
device has a major effect on the protector performance as illustrated by the above
test. Leadless/Bulkhead installation (Smart Panel) for RF lightning protection devices
will eliminate the additive voltage / energy through-put to the protected equipment
Proposed Coax Protector Mounting
Lightning
Protectors
IMGB
Low Inductance
Down conductor
Large conductors for outside
Ring Ground connections
Getting down to Earth
Grounding Electrode (Rod) Sphere of Influence
Radius of Primary
Influence
Concentric Shells
– Sphere of Influence is Conical:
– Width Has Radius Equal To
Electrode Length
– Depth Is 2X Electrode
Length
2X Rod Length
INCORREC T SPACING
CORRECT SPACING
8 ft. Rod - 8 ft. Radius Influence
10 ft. Rod - 1 0ft. Radius Influenc e
Add additional ground rods:
* Decreases the distance between rods, lowering radial inductance &
reducing series ground rod inductance to “good conductivity” layers
Remove soil (sand) around horizontal radial & add conductive
backfill material
* Should remove down to partial conductive layer
Change radial to copper strap (lower inductance)
ALL OF THE ABOVE
Original Ufer Ground
Accessible Junction
Box with screw
on cover
Bare
Copper
Conductor
How good is your ground?
A tower ground system should disperse large quantities of electrons
over a wide area very quickly with minimum ground potential rise.
Multiple radials with ground rods are the most effective solution.
Ground rod spacing should be done based on how much of the rod is
in conductive soil.
In poor conductivity earth, a copper strap radial would provide lower
inductance and more surface area in contact with the soil than a
circular conductor, also lowering the conductor/soil interface
resistance to allow more electron dispersal.
Since a typical Fall of Potential ground “resistance” (impedance) meter
uses a 90 – 310 Hz signal source*, and most of the fast transient strike
energy centers around 10 kHz, the meter reading does not represent a
true impedance range of the ground system during a lightning strike.
Increased GPR could occur at the MGB.
A >5 Ohm F.O.P. reading is considered essential to keep HF transient
caused GPR at a minimum.
* Review 2 Previous Slides
Radial and Ground Rod System
Radials with ground rods
extending out from the tower
base, form a fast transient low
“resistance” ground system for
a single point ground coaxial
cable entry panel.
When rods are placed
along
radials with other rods, a capacitive plate
is simulated for a more efficient transfer
of energy into the earth.
Rooftop Ground Considerations
Rooftop equipment is subject to high peak potentials until current
flow down the grounding conductors reduces (“drains”) the voltage.
Lower inductance / impedance ground conductor values reduce
equipment damage exposure.
•Local “Ground” is accepted to mean the building foundation Ufer
effect, external buried ground loops and structural lightning
system ground rods, all interconnected to the electrical ground at
the power company main entrance panel.
•Some ground systems keep the structural lightning protection
system ground rods separate from the building ground.
•A “Single Point Ground” Entry Panel is essential to maintain equal
potential between equipment racks and master ground bar during
the high potential event.
Rooftop Ground Considerations
Lightning Rods (6)
Coaxial Cables to equipment
Perimeter rooftop ground conductors for
structural protection system with additional
conductors bonding cellular antenna
support
Lightning Rod structural protection
system down-conductors (4)
Cellular Antenna
GPS Antenna
Times “Smart
Panel” Cable
Entry port to
equipment
Antenna support
& entry panel
ground bond to
building steel
Separate ground down–conductor for
antenna structure and entry port bond
Equipment ground preferences:
• Marginal Bond to structural protection or
separate down conductor
• Good Bond to structural protection and
additional separate down conductor
• Better Single bond to structural steel
• Best Combine all three methods
Ground Loop / Rods
around building
Electrical
Ground
Ground Loop / Rods
Water /Sewer Ground
There is no “ground” on the rooftop during a lightning strike. The peak potential rises
and falls (over time) due to the overall inductance / impedance / resistance of all
downward grounding conductors in matrix.
Times Protect Smart-Panel
Can be utilized in rooftop sites and stand alone tower site applications
Production Unit
Rooftop Ground Considerations -Summary
There is no “ground” on the rooftop during a lightning strike.
All equipment grounding conductors must be applied to an
isolated single point ground bar bonded to below preferences
Equipment ground preferences:
• Marginal Bond to structural protection
ground conductors or separate down
conductor
• Good Bond to structural protection and
additional separate down conductor
• Better Single bond to structural steel
• Best Combine all three methods
• Rebar can be used in addition to any or all of
the above methods if local codes permit
The Times Protect Smart-Panel is
recommended as single point ground
Presentation Summary
•Peak Lightning Transients ( hf ac component) can cause more equipment
damage than the dc component.
•Every conductor (including a tower) is an inductor with a series voltage drop
during a fast rise time increase in current.
•The coaxial cable shield(s) from the tower is the largest carrier of damaging
lightning current towards the connected equipment.
•Due to typical “unbalanced” characteristics The coaxial cable shield
propagates hf current from the strike faster than the center conductor.
•Strike current from the “unbalanced” propagation characteristics of coaxial
cable create a voltage differential across center pin to shield at the equipment
end. This differential voltage forces current through the equipment input.
•Coaxial Lightning Protectors stop the damage caused by differential currents,
and must be grounded with as short a ground conductor as possible. Direct
mounting/bonding to grounded entry panel is best.
Presentation Summary - Continued
•“Skin Effect” should be considered when sizing and routing lightning
ground conductors. Remember the 8” radius bend rule.
•A larger circumference ground conductor = a lower inductance
per unit length = less di/dt voltage drop = a better ground conductor,
essential at the coaxial cable entry to shelter.
•A ground system disperses electrons that are dissipated in the earth body.
A ground system requires a good (?) transient response to rapidly disperse
electrons from a lightning event.
•Radials and rods equally extending out from the tower base, with attention
to geometry, will yield a good (?) transient response when a 5 Ohm impedance
is achieved when measured with a F.O.P. meter. When a reading is doubtful,
change the rod layout and measure it again! Clamp-on meters are useful
when there is no room for rods and for verification purposes.
•There is no “ground” on a building roof top during a strike. Make a
“Single Point” ground, bond all equipment grounds together, and find the
best (review slide) low inductance path to earth.
Thank you and keep looking up!
Questions – Comments?