Ground
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?
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?
