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8/12/2019 (159373063) Design of a Low Impedance Grounding System for Telecom Applications - Mitchell Guthrie and Alain Rousseau
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A li ti
Designof a Low Impedance
Mitchell GuthrieIndependent Engineering Consultant
Alain RousseauSEFTIM
Grounding System for Telecom
Applications
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Relate gro nding s stem impedance to
Objectives
Discuss impedance performance criteria
Discuss typical existing performance
criteria
Discuss relationship between resistance
and impedance
Identify broadband response of grounding
system components and systems
Relate grounding system impedance to
telecom applications
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f di i t d
ImpulseCurrentEffects
in the earthing
Low frequency contentenergy zone
V(t) = i(t) R + L (di/dt)
High frequency
content is important
for overvoltages
Low frequency
content is important
High frequency contentfor energy dissipated
sparkover zone system
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ImpulseCurrentPrinciples
earth
can result
into earth as a function of earth resistivity or conductors
earth decreases current density at earth / electrode
Lightning takes the lowest impedance path to remote
lowest resistance and lowest inductance paths preferred
High inductance path leads to high voltage on LPS current cannot change instantaneously through an inductance
overvoltage is created and spark over to a less inductive path
Lightning currents flow radially from the point of injection
(earth electrodes) embedded in the earth
Providing multiple paths for current injection into theinterface; reducing overvoltages
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IEC 62305 10 d d
Proposed TIA B
Grounding System Performance
Criteria Resistance-to-earth
2ndNFPA 7025 or drive electrode
DoD / DOE Explosives
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Circuit Model
Z =R +jLG +jC
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Impedance vs. Resistance
Resistance
often dominated by earth resistivity
increases with frequency due to skin effect
Inductance
influenced by geometry of conductors / electrodes
increased by bends in conductor routing
Capacitance
dominated by electrode-to-earth interface
v(t) = i(t) R + L (di/dt) + i(t) / C = i(t) Z
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Impedance Measurement Method
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300 m
Vertical Rod in 300 m soil
FREQUENCY IN Hz
vertical rod
300 .m
IMPEDANCE in
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d i t it
120 )
Vertical Rod in 300 m soil
Impedance decreases with frequency due todominate capacitance
Impedance high even in low resistivity soil (70 to
120 ). Additional experiments (data not shown)
Same behaviour found for horizontal grid in 300 msoil
3 rods in triangle in 200 .m soil : Z constant (3040 )
Earth Enhancement Compound decreases localresistivity and improves contact (decreases R and Z)
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c row foot
Crow Foot Arrangement in
Rocky SoilIM P E D AN C E in
F R E Q UE NC Y IN Hz
cro w footro ck y so il
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Effi i t li ht i t ti di
Crow Foot Arrangement in
Rocky Soil 200 m soil Low increase of impedance (Z)
Less than 20 ohms change across entirefrequency spectrum
Efficient lightning protection grounding
system
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110 m
25 meter long Horizontal Tape
FREQUENCY IN Hz
25 m horizontal tape
110 .m
IMPEDANCE in
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25 meter long Horizontal Tape
Low increase of impedance (Z)
20 ohms change from 10 Hz to 1 MHz
50 m deep well
Z = 10 at low frequency / 70 at 1MHz
Additional experiments (data not shown)
Z almost doubles for 50 meters of tape
Similar behavior from 20 m to 50 m verticalwell in 200 m soil
Buried 0.5 meters in 110 m soil
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Driven rod for cabinet
Telecom Application
3 legged tower
Equipment cabinet
Driven rod per leg
Driven rod for cabinet
Ground loop
conductor tyingtogether driven rods
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Tower Leg Impedances
South Tower Leg East Tower Leg
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Tower Test Points
South Tower Leg East Tower Leg
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Cabinet Ground Impedance
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(1MH ) C bi t 79 2 t 52
SummaryofObservations
test badgiven HF criteria
Data from tower legs similar (w/in 3 )Effect of ground rods noted @ 202 kHz
R (tower legcabinet bus) = 1 m
Bend in solid wire between copper bus bar
and grounding electrode shows up as L
Rgnd Cabinet = South tower leg = 9
Rgnd (1MHz): Cabinet =79.2; tower = 52
All 3 less than 10 @ low frequency but
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Impedance Accep tance Cri ter ia
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CaseStudies
E: Group of Chimneys
F: Metallic Tanks
A: Building with large grounding system
B: Factory Extension
C: Metal Silos D: Metallic-framed Shed
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ES 1002
Mesure 14
Nombre de points 20
80
100 1000 10000 100000 1000000
-60
ZR
X(OH
M)
60
40
20
0
-20
Z
R
X
-40
Frquence (Hz)
Case A: Building with large earthing system
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Case B: FactoryExtension
Extension of existing
Clearly a bad earthing
factory
Bad soil, mainly rocks Crow foot system
R = 150
system
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Case B: Building with a crow foot in bad soil
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Case C: Metallic Silos
Small contact surface between metal and soil
R = 15
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50
30
-20
ohms
130
120
110
100
90
80
70
60 Z
40 R
20
10
0
-10 X
-30
-40
10 100 1000 10000 100000 1000000Fre que ncy (Hz)
Case C: Silo in contact with soil
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Case D: Large shed with metallic frame
R = 4
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30
50
30
-30
Z
R
ohms
130120
110
100
90
80
70
60
40
2010
0
-10
-20 X
-40
-50
10 100 1000 10000 100000 1000000
Frequency (Hz)
Case D: Shed with metallic frame
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thi t
Case E: Group of Chimneys
Earthing on each chimney
Connected togetherby copper tape
R = 5
Good low frequency
resistance but gets
very high at frequencies
above 100 kHz bad lightning
earthing system
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50
30
-30
Frequency (Hz)
R
X
ohms
130
120110
10090
80
70
60 Z40
20
10
0-10
-20
-40
-50
-60
-70
-80
10 100 1000 10000 100000 1000000
Case E: Group of chimneys
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Case F: Metallic Tanks
Tank diameter 6 meters
Near the sea
Concrete baseimmersed in
sand/watermixture
No dedicated
earthing system
R = 1
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50
30
-30
10 100 1000 10000 100000 1000000
R
X
ohms
130120
110
100
90
80
70
60 Z40
20
10
0
-10
-20
-40
-50
-60
-70
-80
Frequency (Hz)
Case F: Metallic tank near the sea
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E l f id ti
Proposed Criteria Development
earthing system to dissipate lightning current while
minimizing overvoltages
Examples for consideration:30 from low frequency to 1 MHz10 at low frequency and 100 at high frequencies ?
To evaluate the earthing systems we will calculate
the voltage generated by the injection of a 10 kA1/20 current pulse into the earthing impedance
measurements reported earlier.
Determine equivalent resistance (RHF)
Difficult to establish a single criteria for quality of
8/12/2019 (159373063) Design of a Low Impedance Grounding System for Telecom Applications - Mitchell Guthrie and Alain Rousseau
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( 46 40 4
SummaryofResults
The mean value of the measured impedance between 63 kHz and 1 MHz
(Mean Z) computed from data provided by high frequency earth impedancetest equipment.
This value is consistent with RHF and is suggested as criterion
Z Impedance value ()
Case A Case B Case C Case D Case E Case F
RLF () 4 150 15 4 5 1RHF () 47 203 35 22 47 16
Mean Z () 46.7 184 40.7 22.9 45.5 17.9
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ProposedCriteria
10