(159373063) design of a low impedance grounding system for telecom applications - mitchell guthrie...

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


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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

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