Design of a Low ImpedanceDesign of a Low Impedance Grounding System for Telecom

A li tiApplications

Mitchell GuthrieIndependent Engineering Consultant

Alain RousseauSEFTIM

ObjectivesObjectives• Discuss typical existing performance yp g p

criteria• Discuss relationship between resistanceDiscuss relationship between resistance

and impedance• Identify broadband response of grounding• Identify broadband response of grounding

system components and systemsRelate gro nding s stem impedance to• Relate grounding system impedance to telecom applications

• Discuss impedance performance criteria

Impulse Current EffectsLow frequency content

energy zone• V(t) = i(t) R + L (di/dt)• High frequency g q y

content is important for overvoltages

• Low frequency content is important f di i t dfor energy dissipated in the earthing system

High frequency content

sparkover zone systemp

Lightning Current PathsLightning Current Paths

Impulse Current PrinciplesImpulse Current Principles• Lightning takes the lowest impedance path to remote

earthearth – lowest resistance and lowest inductance paths preferred

• High inductance path leads to high voltage on LPS– current cannot change instantaneously through an inductance– current cannot change instantaneously through an inductance– overvoltage is created and spark over to a less inductive path

can result • Lightning currents flow radially from the point of injection g g y p j

into earth as a function of earth resistivity or conductors (earth electrodes) embedded in the earth

• Providing multiple paths for current injection into the g p p jearth decreases current density at earth / electrode interface; reducing overvoltages

Grounding System Performance Criteria

• Resistance-to-earth– 5 Ω typical for Telecom applications – Can be as low as 1 - 2 Ω– NFPA 70 – 25 Ω or drive 2nd electrode– DoD / DOE Explosives – <25 Ω

IEC 62305 3 10 Ω d d– IEC 62305-3 - < 10 Ω recommended– NFPA 780 / UL 96A /– no specification

Proposed TIA 607 B• Proposed TIA-607-B– Grounding electrode: conductor … for the purpose of

providing a low impedance path to earthproviding a low impedance path to earth

Circuit Model

LjR ω+ LjRZ ω+=CjG ω+

Impedance vs ResistanceImpedance vs. Resistance• v(t) = i(t) R + L (di/dt) + i(t) / C = i(t) Z• 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

C it• Capacitance– dominated by electrode-to-earth interface

Impedance Measurement MethodImpedance Measurement Method

Vertical Rod in 300 Ωm soilVertical Rod in 300 Ωm soil IMPEDANCE in Ω

vertical rod 300 Ω m

FREQUENCY IN Hz

300 Ω.m

Vertical Rod in 300 Ωm soilVertical Rod in 300 Ωm soil• Impedance decreases with frequency due to

d i t itdominate capacitance• Impedance high even in low resistivity soil (70 to

120 Ω)120 Ω).• Additional experiments (data not shown)

Same behaviour found for horizontal grid in 300 Ωm– Same behaviour found for horizontal grid in 300 Ωm soil

– 3 rods in triangle in 200 Ω.m soil : Z ≈ constant (30 –40 Ω)40 Ω)

– Earth Enhancement Compound decreases local resistivity and improves contact (decreases R and Z)

Crow Foot Arrangement in Rocky Soil

IMPEDANCE in Ω

crow footFREQUENCY IN Hz

crow footrocky soil

Crow Foot Arrangement in Rocky Soil

• 200 Ωm soil • Low increase of impedance (Z)• Less than 20 ohms change across entire• Less than 20 ohms change across entire

frequency spectrumEffi i t li ht i t ti di• Efficient lightning protection grounding system

25 meter long Horizontal Tape25 meter long Horizontal Tape IMPEDANCE in Ω

25 m horizontal tape 110 Ω m

FREQUENCY IN Hz

110 Ω.m

25 meter long Horizontal Tape25 meter long Horizontal Tape• Buried 0.5 meters in 110 Ωm soil • Low increase of impedance (Z)• 20 ohms change from 10 Hz to 1 MHzg• Additional experiments (data not shown)

– Z almost doubles for 50 meters of tapeZ almost doubles for 50 meters of tape– Similar behavior from 20 m to 50 m vertical

well in 200 Ωm soil 50 m deep well – Z = 10 Ω at low frequency / 70 Ω at 1MHz

Telecom ApplicationTelecom Application

• 3 legged tower• Equipment cabinet• Equipment cabinet• Driven rod per leg• Driven rod for cabinet• Driven rod for cabinet• Ground loop

conductor tyingconductor tying together driven rods

Tower Leg ImpedancesTower Leg ImpedancesSouth Tower Leg East Tower Leg

Tower Test PointsTower Test Points

South Tower Leg East Tower Legg

Cabinet Ground ImpedanceCabinet Ground Impedance

Summary of ObservationsSummary of Observations• Data from tower legs similar (w/in 3 Ω)g ( )

Effect of ground rods noted @ 202 kHz

• R (tower leg – cabinet bus) = 1 mΩ( g )• Bend in solid wire between copper bus bar

and grounding electrode shows up as Land grounding electrode shows up as L• Rgnd Cabinet = South tower leg = 9 Ω

R d (1MH ) C bi t 79 2 t 52Ω• Rgnd (1MHz): Cabinet =79.2; tower = 52Ω• All 3 less than 10 Ω @ low frequency but

test “bad” given HF criteria

Impedance Acceptance Criteriap p

Case StudiesCase Studies• A: Building with large grounding systemg g g g y• B: Factory Extension• C: Metal Silos• C: Metal Silos• D: Metallic-framed Shed• E: Group of Chimneys• F: Metallic Tanks

ES 1002 Mesure 14

Nombre de points 20

80

60

20

40

)

Z

0

20

100 1000 10000 100000 1000000

Z R

X(O

HM

)

R

X

-20

-60

-40

Fréquence (Hz)

Case A: Building with large earthing systemFréquence (Hz)

Case B: Factory Extension

• Extension of existing• Extension of existingfactory

• Bad soil, mainly rocks• Crow foot system• R = 150 Ω• Clearly a bad earthing• Clearly a bad earthing

system

Case B: Building with a crow foot in bad soilCase B: Building with a crow foot in bad soil

Case C: Metallic Silos

• Small contact surface between metal and soil

Case C: Metallic Silos

• Small contact surface between metal and soil• R = 15 Ω

130

ohms

8090

100110120

4050607080

R

Z

-100

102030

R

X

-40-30-2010

10 100 1000 10000 100000 1000000

X

Case C: Silo in contact with soil

Frequency (Hz)

Case C: Silo in contact with soil

Case D: Large shed with metallic frame

• R = 4 Ω

g

• R = 4 Ω

130

ohms

8090

100110120

304050607080

R

Z

-100

102030 R

X

-50-40-30-20

10 100 1000 10000 100000 1000000

X

Case D: Shed with metallic frame

Frequency (Hz)

Case D: Shed with metallic frame

Case E: Group of Chimneys

• Earthing on each chimney

Case E: Group of Chimneys

Earthing on each chimney• Connected together

by copper tapey pp p• R = 5 Ω• Good low frequencyresistance but gets very high at frequencies above 100 kHz• bad lightning

thi tearthing system

120130

ohms

708090

100110120

102030405060

R

Z

50-40-30-20-10

010

X

-80-70-60-50

10 100 1000 10000 100000 1000000Frequency (Hz)

Case E: Group of chimneys

Frequency (Hz)

Case F: Metallic Tanks

• Tank diameter 6 meters

Case F: Metallic Tanks

• Tank diameter 6 meters• Near the sea• Concrete base

immersed insand/watermixture

• No dedicatedearthing systemg y

• R = 1 Ω

130

ohms

8090

100110120

203040506070

R

Z

-30-20-10

01020

X

-80-70-60-50-40

10 100 1000 10000 100000 1000000

Case F: Metallic tank near the sea

10 100 1000 10000 100000 1000000Frequency (Hz)

Proposed Criteria Development• Difficult to establish a single criteria for quality of

Proposed Criteria Development

earthing system to dissipate lightning current while minimizing overvoltagesE l f id ti• Examples for consideration:– 30 Ω from low frequency to 1 MHz

10 Ω at low frequency and 100 Ω at high frequencies ?– 10 Ω at low frequency and 100 Ω at high frequencies ?• To evaluate the earthing systems we will calculate

the voltage generated by the injection of a 10 kAthe voltage generated by the injection of a 10 kA 1/20 current pulse into the earthing impedance measurements reported earlier.p

• Determine equivalent resistance (RHF)

Summary of ResultsSummary of ResultsZ Impedance value (Ω)

Case A Case B Case C Case D Case E Case F

RLF (Ω) 4 150 15 4 5 1

RHF (Ω) 47 203 35 22 47 16

( ) 46 184 40 22 9 4 1 9Mean Z (Ω) 46.7 184 40.7 22.9 45.5 17.9

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

Proposed Criteria• Based on RHF (or Zmean)

Proposed Criteria

HF ( )

RHF ≤ 10 Ω : very good earthing10 Ω < R ≤ 30 Ω : good earthing10 Ω < RHF ≤ 30 Ω : good earthing30 Ω < RHF ≤ 40 Ω : acceptable earthing40 Ω < RHF : bad earthing

• Quality of earthing is based on Q y gexperience and has been presented to the scientific community (ICLP SIPDA)the scientific community (ICLP, SIPDA)

Application of the Proposed C i iCriteria

Z Impedance value (Ω)

Case A Case B Case C Case D Case E Case FRLF (Ω) 4 150 15 4 5 1RLF (Ω) 4 150 15 4 5 1RHF (Ω) 47 203 35 22 47 16

Mean Z (Ω) 46.7 184 40.7 22.9 45.5 17.9

• Earthing systems for A, B and E are considered bad

eq. length (m) 24 102 18 11 24 8

Earthing systems for A, B and E are considered bad earthing systems

• Case C is acceptable• Cases D and F are good earthing systems• Cases D and F are good earthing systems.

Summary

Earthing Impedance FactorsEarthing Impedance Factors

LjRZ ω+

CjGLjRZ ω+

=• R is function of material used for grounding system

ft li ibl b t fl t d t b tt th d t hi h

CjG

– often negligible but flat conductor better than round at high frequency (skin effect) given same cross sectional area

• G - earth conductance: related to earth resistivity and t t i t b t th l t d d ilcontact resistance between earth electrode and soil

– may be increased through use of additives to increase contact between soil and earth conductors.

Earthing Impedance FactorsEarthing Impedance FactorsLjRZ ω+=

• L – Inductance of earthing system hardware

CjGZ ω+

L Inductance of earthing system hardware– reduced by use of shorter multiple conductors instead of single

one of equivalent total length

C C it b t th d thi• C – Capacitance between earth and earthing electrodes– reduced by increasing earth contact area (plates and flat– reduced by increasing earth contact area (plates and flat

conductors better than round conductors)– reduced by increasing contact between electrode and earth such

as through earth enhancing materialas through earth enhancing material

Summary and ConclusionsSummary and Conclusions• High frequency behavior of an earthing g q y g

system is different from low frequencyZ ≠ R !Z ≠ R !

• Microsecond rise time of lightning current pulse leads to high frequency componentspulse leads to high frequency components

• High frequency lightning current content l d l blleads to overvoltage problems

Summary and ConclusionsSummary and ConclusionsEarthing system impedance reduced by:

– Lowering resistance of electrode(s) and associated conductors (typically negligible)Lowering inductance of electrode(s) and associated– Lowering inductance of electrode(s) and associated conductors

– Lowering resistance-to-earthg– Increasing capacitance of earth-electrode interface

Increasing length of electrodes can reduce g gresistance-to-earth but resulting effectiveness offset by increased inductance (impedance)

Summary and ConclusionsSummary and Conclusions• Long earthing conductors good for R but g g g

not Z– Multiple paths better for high frequency p p g q y

response• Flat conductors and plates increase p

capacitance and reduce impedance• Earthing enhancing compounds can helpEarthing enhancing compounds can help

reduce R and Z

Summary and ConclusionsSummary and Conclusions• Engineering criteria in standards are better than

citing earthing resistance req irementciting earthing resistance requirement• Devices exist for measurement of high

frequency response of grounding systemsfrequency response of grounding systems• Recommendations available for standards /

specifications incorporating impedance values p p g pfor earthing systems– Average value of impedance between 60 kHz and 1

MHz (5 kHz not high enough to identify critical value)MHz (5 kHz not high enough to identify critical value)– High Frequency Resistance based on 10 kA impulse(RHF < 40 Ω)

Thank You

QUESTIONS ?