2 Safety Earthing

Copyright © Siemens AG 2008. All rights reserved.

Sector Energy PTI NCTheodor Connor

Safety earthing

01.2008For internal use only. / Copyright © Siemens AG 2008. All rights reserved.

PTD SE PTITh. Connor

Content

Introduction

Theoretical background

Soil Analysis

Design of earthing system

Measurements on earthing systems



Design of earthing systems

Why ?

How much ?

Where ?



Current flow in the soil

I

Equipotential lines

Streamlines of current

Incremental resistors



I

G

I

Diagram of surface potential along horizontal axis

Top view of surface potential

Principle of current and potential distribution

Equipotential linesand streamlines of current



ExampleHigh voltage substation

UE = RE · I

Transformer

Metalstructure

Building

Fence

Lamp post

Earthwire

Earthing system



Ground potential rise and touch voltage

Transformer

Potential control

Surface potential

Touchvoltage

Ground potential rise

Touch voltage



Vulnerable phase

479-1 © IEC: 1994

ECG

Blood pressure

Ventricular fibrillation

Effect of current on heart activity



Tolerable touch voltage

Source: HD 637

t = 3 s

Ut zul = 85 V

Ut zul = 650 V

t = 0.1 s

0,05 0,1 0,2 0,3 0,4 0,5 0,7 1 2 3 4 5 6 7 8 9 10

V1000

98765

4

3

2

100987654

Duration of current flow

Touc

h vo

ltage

UTP

s



National

Germany DIN VDE 0141Austria ÖNGreat Britain BS

Europe

pr EN 50179

HD 637⇒ DIN VDE 0101

America

ANSI IEEE 80 International

IEC TC 99

Today FuturePast

Development of earthing standards



Start of operation

Measurement of specific soil resistivity

Design

Erection

Current injection test

time

Start of project

Data collection

Main steps forearthing system design



Typical figures :

Definition : in Ω · mρ E

D2E

ER⋅

=ρ

1m1m

1m

Importance of specific soil resistivity

Marshy soil 5 to 40 Ωm

Loam, clay, humus 20 to 200 Ωm

Sand 200 to 2500 Ωm

Gravel, rock 2000 to 3000 Ωm



Measurement of soil resistivity

Currentsource

R

Electrode spacing a

ρ=2πaR

Wenner method with four electrodes



Apparent soil resistivity

Upper layer

Bottom layer

Thickness



Analysis by Rashid

0

200

400

600

800

0 5 10 15 20 25 30 35 40 45

Probe spacing in m

Soi

l res

istiv

ity in

Ohm

.m

Seq 1

Seq 2

Seq 3

Seq 4

Seq 5

Seq 6

Seq 7

Seq 8

Seq 9

Seq 10

Seq 11

Seq 12

Seq 13

Seq 14

Seq 15

Seq 16

Analysis by Siemens

0

200

400

600

800

0 5 10 15 20 25 30 35 40 45

Probe spacing in m

average

S1

S2

ExampleMeasurement of specific soil resistivity



Measurement by unexperienced Company

0

200

400

600

800

1000

0 10 20 30 40 50 60 70electrode spacing (m)

rho

( ohm

*m)

S1S2S3S4S5S6

ExampleMeasurement of specific soil resistivity

Soil ResistivityMeasurement by Siemens Expert

0

200

400

600

800

1000

0 10 20 30 40 50 60 70electrode spacing (m)

rho

( ohm

*m)

S2S3S4



Streamlines of currenthomogenous soil



Streamlines of current: higher resistivity of top layer



Streamlines of current: higher resistivity of upper layer



0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40

Sondenabstand [m]

spez

. Erd

wid

erst

and

[oh

m m

]

Measurement of specific soil resistivityPractice: Measurement line 1

Area of planned site



Area of planned site

Meas. line 1

Meas. line 2

Meas. line 3

Measurement of specific soil resistivityPractice: Measurement line 1,2,3

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40

Spacing [m]

Spec

ific

so

il re

sist

ivit

y [o

hm

m]



Specific soil resistivityDetermination of average value

∑=

⋅

= N

1n n

E1

N1

1

ρ

ρ

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40

Spacing [m]

Spec

ific

so

il re

sist

ivit

y [o

hm

m]

Seq 1

Seq 2Seq 3

Average



Specific soil resistivity – Determination of parameter of two layer model

160,00 90,00 3,50 7,05E-02

ρ top ρ bottom h Deviation factor

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40a [m]

rho

[Ohm

*m]

measured

calculated

Spacing measured calculateda [m] ρ [Ω*m] ρ [Ω*m]

0 1601 135 1592 162 1555 129 131

10 102 10520 104 9440 79 91



Determination of equivalent specific soil resistivity forhomogenous soil

Input:

Size of site Length L = 200 mWidth W = 120 m

Specific soil resistivity top layer ρt = 160 Ω * mSpecific soil resistivity bottom layer ρb = 90 Ω * mThickness of top layer h = 3,5 m

Result:

Specific soil resistivityFor homogenous equivalent soil ρres = 94 Ω * m

RE1= RE2

ρb

ρth

W

L

W

L

RE1 RE2

ρres



Practice of resistivity measurements



Basic data collection

Design according to thermal and mechanical requirements

Design to meet touch voltage requirements

Rated voltageFrequencyType of neutral earthingData of overhead lines and cablesSpecific soil resistivityLayout of site and surroundingData of generator and transformerFault duration

Determination of relevant fault currents

Selection of material and cross section

Determination of impedance to earth

Determination of ground potential rise

Selection of mesh width

Selection of equivalent measures

Design of earthing system



Thermal design

tF

0,06 0,08 0,1 0,2 0,4 0,6 0,8 1 2 4 6 8 10

2000A/mm 2

1000800600

400300

200150

1008060

40

2010

G

s

Short term design – long term design

Earth electrodes and earthing conductors

MaterialSteel bar / hot deep galvanizedCopper bar / tinned

A IK

t=+

+lnΘΘ

f

i

β

β

12

4

3

10 16 25 35 50 70 95 120 150 185 240 300

A mm²

2000

A 1500

1000800

600

400

ID 300

200

150

10080

60

40



Determination of minimum cross-section

IK 1 = 4 0 k A

q = 1 9 8 m m 2

c o p p e r

q = 1 4 0 m m 2

c o p p e r

q = 1 4 0 m m 2

c o p p e r

4 0 0 k Ve q u ip m e n t

IK 1 = 2 5 k A

q = 1 2 4 m m 2

c o p p e r

q = 8 7 m m 2

c o p p e r

q = 8 7 m m 2

c o p p e r

6 0 k V e q u ip m e n tq = 7 0 m m 2

c o p p e rq = 7 0 m m 2

c o p p e r



Thermal design and mechanical stress

Type of conductorrodtapestranded wire

Type of connection clampedscrewedwelded

Mechanical stress



Thermal design and corrosion

Electrochemical corrosion

copperreinforced concrete

hot deep galvanized steel

Aggressive environment



RRod = 1,5 ΩL

1,5R ERod

ρ⋅≈

L3R E

Stripρ⋅

≈

Dρ⋅⋅

≈π

ERing

3R

LD2R EE

Meshρρ

+⋅

≈

D2R E

Plate ⋅=ρ

DR E

Sphere ⋅=πρ

RStrip = 3,0 Ω

RRing = 3,0 Ω

RMesh = 2,1 Ω

RPlate = 1,6 Ω

RSphere= 1,0 Ω

L=100 m D=32 m ρE=100 Ωm

Resistances to earth



Practial stepsto fulfilltouch voltage requirments

acc HD637

Global Earthing System

Flow chart for earthgrid design



Contact resistanceRH → high

RH

RF

Local isolation RF → high

RT

Equipotential bonding RT ≈ 0

RT

Potential control RT → small

Measuresto keep touch voltages within limits



GIS

Main earthing conductor

Potential control

Further earth grid

Earth rod

Control cubicalLightningprotection

Foundation earth electrode

Earthing system componentsExample switchgear building



Current distribution



Decisive currents, voltages and impedances

R e fe r e n c e e a r th

P h a s e c o n d u c to r

E a r th w ir e

E a r th in g s y s te m

R e fe r e n c e e a r th

F o r e q u a l e a r th w ir eto w e r fo o t in g im p e d a n c e so f th e o v e r h e a d l in e s

E q u iv a le n t c i r c u i t

( 1 - r E ) 3 I 0

( 1 - r E ) 3 I 0

3 I 0

R E TR E T R E S U E

I T r I F

I R S

I T r

I F 3 I 0

I E

R E SI R S

Z ∞Z ∞

U E

I F = 3 I 0 + I T r

I E = r E . ( I F - I T r )U E = I E . Z E

1 Z E = — — — — — — — — 1 + n 1 R E S Z ∞



Practice: Earth electrodes



Practice: Gas insulated switchgear



Practice: Cable sealing end



Practice: Fence earthing



Practice: Substation in the dessert



Design according to IEEE Standard 80

tKE i

tol =LIKEE mmeshtouch =≤

tolmesh EE ≤



Remote rodTested structure

V

Principle of fall of potential method

A



Practical results

Tower southInfluence of remote current electrode

0

0,5

1

1,5

2

2,5

3

0 10 20 30 40 50 60 70 80a (m)

appa

rent

impe

danc

e (

ohm

)Measurement according to instruction manual



A

Current source

Earth wire

Remote earthingTested substation

V

Principle of current injection test



Earthing measurement with cable



Elimination of interferencePolarity reversal method

A ABC C

Test current

UC

UTest

UTestUA

UB

222

2 CBA

Test UUUU −+

=



Elimination of interferenceBeat frequency method

US

Umax

Umin

( )2

maxmin UUUTest

−=

( )2

maxmin UUUTest

+=

TestS UU ≤TestS UU >



Elimination of interferenceBeat frequency method

TestS UU ≤TestS UU >

Umax

Umin

US

UTest

Umax

Umin

US

UTest



Elimination of interference

Switching device

Transformer orEmergency diesel generator

230 V AC

Surge arrestor

Test Line



Switching device



Example infeed at MV cable feeder



Example current source



Example: Earth potential rise, site layout



Measured earth potential rise



Example: Measured touch voltages and potential differences



Measured touch voltages



Measured potential differences



Example: Measured transferred potentials



Measured transferred potentials



Earthing system design toolDiagram of surface potential



160 m140 m120 m100 m80 m60 m40 m20 m0 m

1.000 V

900 V

800 V

700 V

600 V

500 V

400 V

300 V

200 V

100 V

0 V 160 m140 m120 m100 m80 m60 m40 m20 m0 m

1.000 V

900 V

800 V

700 V

600 V

500 V

400 V

300 V

200 V

100 V

0 V

Earthing system design toolSurface potential along horizontal line



30 m25 m20 m15 m10 m5 m0 m

1.000 V

900 V

800 V

700 V

600 V

500 V

400 V

300 V

200 V

100 V

0 V

Earthing system design tool Effectiveness of potential control



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