· PDF fileContents Section I 22/799 22.1 Grounding electrodes for buildings, industrial installations and substations 22/799 22.1.1 Plate grounding 22/799 22.1.2 Pipe or rod

Contents

Section I 22/799

22.1 Grounding electrodes for buildings, industrial installations andsubstations 22/79922.1.1 Plate grounding 22/79922.1.2 Pipe or rod grounding 22/79922.1.3 Strip or round conductor grounding 22/80022.1.4 Numbers and types of grounding electrodes 22/801

22.2 Resistivity of soil (r) 22/802

22.3 Measuring the ground resistance 22/803

22.4 Metal for the grounding conductor 22/80322.4.1 Size of the grounding conductor 22/804

22.5 Jointing of grounding conductors 22/806

22.6 Maintenance of grounding stations 22/806

Section II 22/806

22.7 Grounding practices in a power generating station 22/806

22.8 Tolerable potential difference at a location 22/806

22.9 Voltage gradients 22/80622.9.1 Tolerable step voltage (Es) 22/80622.9.2 Tolerable touch voltage (Et) 22/80622.9.3 Mesh voltage or safe design voltage (Em) 22/80722.9.4 Ground potential rise (GPR) 22/80722.9.5 Transferred voltage (Etr) 22/80822.9.6 Design parameters 22/808

22.10 Determining the leakage current through a body 22/81022.10.1 Body resistance 22/81022.10.2 Ground resistance 22/811

22.11 Measuring the average resistivity of soil 22/812

22.12 Improving the performance of soil 22/81222.12.1 Conductivity 22/81222.12.2 Soil moisture and contact resistance 22/812

22.13 Determining the ground fault current 22/812

22.14 Designing a grounding grid 22/81322.14.1 Minimum size of grid conductors 22/81322.14.2 Corrosion factor 22/81322.14.3 Maximum touch and step voltages of a grounding station 22/81422.14.4 Estimating the value of ground conductor length (L) 22/816

Relevant Standards 22/821

List of formulae used 22/821

Further Reading 22/822

22 Groundingpractices

22/797

Grounding practices 22/799

SECTION I

22.1 Grounding electrodes forbuildings, industrialinstallations and substations

The following are a few types of grounding electrodescommonly used for the grounding of buildings, industrialinstallations, equipment grounding or small and medium-sized substations. Power generating stations and largeswitchyards may experience large ground fault currentsaccordingly, grounding electrodes demand a lot more safetyconsiderations and are discussed in Section II.

22.1.1 Plate grounding

Refer to Figure 22.1. The approximate resistance to groundin a uniform soil can be expressed by

RA

= 4

2

r p W (derived from Equation (22.12)) (22.1)

where

r = resistivity of soil, considered uniform in W m.A = area of each side of the plate in m2.

NoteThe minimum thickness of plate is recommended as

• For cast iron – 12.0 mm• For GI or steel – 6.3 mm• For copper – 3.15 mm

and size not less than 600 mm ¥ 600 mm

Example 22.1The resistance to ground for a 600 mm ¥ 600 mm plategrounding, considering a sandy soil, treated artificially andhaving attained an average soil resistivity of 10 Wm

R = 104

3.142 0.6 0.6¥ ¥

= 5.22 W

If the plate is 1200 mm ¥ 1200 mm then

R = 104

3.142 1.2 1.2¥ ¥

= 2.61 W

From the above the resistance to ground of a plate groundingis inversely proportional to the square root of the lineardimension ( )A of the plate. The variation in resistance withthe size of the plate is shown in Figure 22.2 considering theresistivity of soil as 10 Wm. Since the ground resistance isproportional to the resistivity of soil, there would be differentparallel curves for the ground resistance for different valuesof resistivity of soil.

22.1.2 Pipe or rod grounding

Refer to Figure 22.3. In this case, the approximateresistance to ground in a uniform soil can be expressed by

15 mm f GIwater pipe Wire mesh Cast iron

chamber cover

Funnel Inspection chamber1000 ¥ 500 ¥ 600 mm

Cementconcrete

50 mm f GIwater pipe

A homogenous layerof coke/charcoal/saltand sand

1200 ¥ 1200 ¥ 12 mmCI plate

2000mm

1200mm

NoteThe depth (2000 mm) would vary with themoisture content and quality of soil

Figure 22.1 A typical layout of a plate electrode

For r = 10 W m

Gro

und

resi

stan

ce (

W)

5.22

3.48

2.61

7

6

5

4

3

2

1

0 0.5 1.0 1.5 2.00.6 0.9 1.2

Linear dimensions ( A m)( )

Figure 22.2 Variation in resistance to ground with the linear dimensionsfor a plate grounding, for the same resistivity of soil

22/800 Electrical Power Engineering Reference & Applications Handbook

R

d =

100 2

log 8 – 1 e◊◊

ÈÎÍ

˘˚

rp �

� W (22.2)

where

� = length of pipe in cm d = internal diameter of pipe in cm

NoteThe diameter, thickness and length of the pipe is recommended asfollows:

• Cast iron pipes – 100 mm internal diameter, 2.5 to 3 m long and13 mm thick. (This is a cumbersome and costlier arrangement,is not often used)

• MS pipes – 38 to 50 mm diameter, 2.5 to 3 m long (also notoften used)

• Copper or GI rods – 13, 16 or 19 mm diameter, 1.22 to 2.44 mlong.

This type of electrode grounding is more suited for a soil possessinghigh resistivity, and the electrode is required to be longer and drivendeeper into the soil to obtain a lower resistance to ground. Theapproximate variation in resistance with the length of electrode fora particular value of resistivity of soil is shown in Figure 22.4, forgeneral reference.

Example 22.2The resistance to ground of a 19 mm internal diameter pipe,2.44 m long, with r as 10 W m

R = 100 102 244

log 8 2441.9

– 1 Ohmse¥

¥¥ ¥

p

= 0.65 [loge 1027.37 – 1]

= 0.65 ¥ [6.93 – 1]

� 3.86 W

Figure 22.4 is drawn for r = 10 W m.

22.1.3 Strip or round conductor grounding

In this case the approximate resistance to ground in auniform soil can be expressed by

R

h wQ =

1002

log 2

+ Ohmse

2rp ◊ ¥ ◊�

�(22.3)

where � = length of strip or rod in cm h = depth of the strip or rod in cmaw = width of the strip or diameter of the conductor rod

in cmQ = –1 for strip and –1.3 for round conductor grounding.

a The minimum cross-sectional area of the strip or therod should be chosen according to the ground faultcurrent and its duration (Section 22.4.1 and Equation(22.4)). The minimum area of cross-section isrecommended as

• For copper strip – 25 ¥ 1.6 mm2

• For MS or GI strip – 25 ¥ 4 mm2

Figure 22.4 Approximate variation of resistance to ground with thelength of pipe or a rod electrode for a particular value of resistivity of soil

For r = 10 Wm

0 50 100 150 200 250 300244

Length of electrode (cm)R

esis

tanc

e to

gro

und

(W)

13.81

7.88

4.553.86

3.245

16

14

12

10

8

6

4

2

15 mmf GIwater pipe Wire mesh

Cast ironchamber cover

FunnelInspectionchamber1000 ¥ 500¥ 600 mm

Cementconcrete

A homogenous layerof coke/charcoal/saltand sand

1250mm

2500mm

150mm150mm

100 mm ID, CI pipe13 mm thick

Figure 22.3 A typical arrangement of a pipe electrode groundingstation


The performance of this type of electrode grounding isalmost the same as for the pipe grounding (Section 22.1.2)as is the variation in resistance to the ground with thelength of the electrode as in Figure 22.4.

Example 22.3The resistance to ground for a 100 mm ¥ 5 mm, 5 m copperstrip, buried at a depth of 1.5 m, having a soil resistivity of100 Wm

R = 100 1002 500

log 2 500150 10

– 1e

2¥¥ ¥

¥ ¥¥p

= 3.185 loge 333.33 – 1 = 3.185 ¥ 5.81 – 1

= 17.50 W

If the length of the strip is 25 m, other parameters remainingthe same, then

R = 100 1002 2500

log 2 (2500)

150 10 – 1e

2¥¥

¥¥

¥p

= 0.637 loge 8333.33 – 1

= 0.637 ¥ 9.03 – 1

= 4.75 W

NoteWe have considered a single length of electrode. If there is morethan one length the values can be obtained from BS 7430 for differentelectrode arrangements.

22.1.4 Numbers and types of groundingelectrodes

1 The normal ground impedance in LV systems, isgenerally high. To achieve a ground fault current ofthe order of 11/2 to 3 times the rated current, necessaryto protect a low current system against a ground faultas discussed in Section 21.2.1, would be difficult unlessadequate measures are taken with the groundingstations to have as low a ground impedance as possible.To achieve this, the grounding stations are madeelaborate, at adequate depth, with proper chemicaltreatment and watering arrangements to ensuresufficient moisture throughout the year. This is attainedby a perforated pipe driven from ground level up tothe electrode. See Figures 22.1 and 22.3, showingtypical arrangements of a grounding station, with aplate grounding and a pipe grounding respectively.Despite this, the ground resistance may still be toohigh to meet the design parameters.

To overcome this, a number of such groundingelectrodes (two being a bare minimum to provide adouble grounding system) may be essential andconnected in parallel to achieve the required lowvalue of ground resistance. A cumulative resistanceof up to 2.5 W is considered satisfactory. However,for more effectiveness and to make the groundprotective circuit more sensitive, a resistance of upto 1 W would be better. The grounding stations maybe separated, centre to centre, by 2 m or more, to beout of each other’s resistance zone. A better gapwould be around 4.5 to 6 m, when full ground

resistance may be achieved by each electrode withoutinfringing on the resistance territory of the otherelectrodes as well as economizing on the number ofelectrodes. This use of more than one groundingstation in parallel so that each station is out of theresistance zone of the other stations, would alter theircumulative resistance, generally as follows accordingto Hand Book of Electrical Installation Practices byE. A. Reeves:

Plate grounding

• For two electrodes – 50%• For four electrodes – 25%

Pipe, rod or strip grounding

• For two electrodes – 60%• For three electrodes – 45%• For four electrodes – 35%

For more accurate calculations refer to BS 7430. Thelikely variation is illustrated in the form of the graphin Figure 22.5.

2 The resistance to ground would vary with depth ofthe electrode. A minimum depth of 1.5 m from groundto the top of the electrode is considered mandatory,and even deeper to reach damp soil.

3 A pipe or strip grounding is more effective than aplate grounding.

4 The size of the plate, the length of the pipe or the stripmay be altered to obtain a lower value of groundresistance.

5 The choice of the metal (Section 22.4) for the groundingelectrode will depend upon the corrosion factor of the

100

75

60

50

45

35

25

Pipe grounding

Plate grounding

0 1 2 3 4Number of electrodes (stations)

% R

esis

tanc

e pe

rel

ectr

ode

(sta

tion)

Figure 22.5 Likely variation in ground resistance with more thanone electrode connected in parallel


soil. But all metals are equally good and possess a lifespan of 12 years and more. For a longer working life,the thickness of the electrode may be increased asdiscussed in Section 22.4.1, GI* being a more preferredmetal for grounding purposes. Choice of the metalfor the ground electrode would, however, depend uponthe underground municipal services, such as water,sewerage and telephone lines and also the structuresand foundations of nearby buildings to save themfrom corrosion and erosion. Copper, being galvanic,under damp conditions forms a complete electrolyticcircuit between it and other metals and causescorrosion, which erodes the other metals (an effectsimilar to that discussed in Section 29.2.5, whilemaking a bimetallic joint).

6 Any of the grounding methods may be adopted forbuildings, industrial installations, equipment groundingor small and medium-sized substations, dependingupon the type and condition of the soil. A sandy soilwill be easy to dig and plate grounding will be easier,while a rocky soil will present problems in diggingand a pipe, rod or strip grounding would be easier.Similarly, dry soil will require deeper digging, wherea pipe, rod or strip grounding would be a better choice.

22.2 Resistivity of soil (rrrrr)

This will depend upon the type and quality of soil and itschemical composition, i.e. the composition of salts andminerals, content of moisture and normal rainfall duringthe year. Table 22.1 obtained from BS 7430 and Table22.2 from IEEE 80 show the likely resistivity of differenttypes of soils in ohm-metres. These are only likely valuesfor a general reference. It is recommended that where agrounding station is to be installed, the soil is tested atnearby locations and an average value of the soil resistivityis determined, as discussed in Section 22.11. The conditionof soil, such as its moisture content, temperature andcontent of salts and other minerals have a large bearingon its resistivity. Figure 22.6 illustrates the effects ofsuch factors on the resistivity of soil. While the temperatureof the soil is a fixed parameter, at a particular location ofthe grounding station the soil can be artificially treatedto improve the content of moisture and chemical composi-tion, to achieve a lower value of soil resistivity. It has

Table 22.1 Likely resistivity of soil

Climatic conditions

Type of soil Normal and high Low rainfall or desert conditionrainfall (more than (less than 250 mm a year) Underground500 mm a year) water salinity

Resistivity of soil

W m W m W m W m

(Likely value) (Likely range of values)

Alluvium and lighter clays, such as sandy 5 a a 1 to 5and/or muddy soil

Clays (excluding alluvium) 10 5 to 20 10 to 100 –

Marls like keuper marl such as marble 20 10 to 30 5 to 300 –

Porous limestone like chalk 50 30 to 100 – –

Porous sandstone such as keuper sandstone 100 30 to 300 – –and clay shales

Quartzite’s compact and crystalline limestone 300 100 to 1000 – –such as carboniferous marble

Clay slates and slaty shales 1000 300 to 3000 1000 upwards 30 to 100

Granite 1000 – – –

Fossil slates, schists, gneiss, igneous rocks 2000 1000 upwards – –

Based on BS 7430a Depends upon the water level of the locality

*Apparently GI seems to be the best metal as a grounding electrode.But if part of the zinc coating of the metal is chipped due to poorcoating or due to any other reason, the metal is rendered prone torapid corrosion and erosion and may fail with passage of time.Some users therefore prefer to use bare MS conductor rather thanGI.

Table 22.2 Range of soil resistivity

Type of soil Average resistivityWm

Wet organic soil 10Moist soil 102

Dry soil 103

Bedrock 104

Based on IEEE-80


been found that the resistivity of soil can be reduced by15–90% by a chemical treatment with the following salts

∑ Normal salt (NaCl) and amixture of salt and soft coke

∑ Magnesium sulphate (MgSO4)∑ Copper sulphate (CuSO4)∑ Calcium chloride (CaCl2)∑ Sodium carbonate (Na2CO3)

Refer to Figures 22.1 and 22.3, illustrating a normalarrangement of grounding stations with provision forchemical or salt treatment. The salts used need not be indirect contact with the electrode.

22.3 Measuring the groundresistance

The above tables can give only a general idea of thetheoretical value of resistivity of the soil at a particularsite for the purpose of design work. The exact resistanceof the grounding station must be determined at the siteof installation to support theoretical assumptions and thegrounding conditions adjusted, if necessary, to obtainthe required ground resistance. The resistance of agrounding station can be measured with the help of aground tester, which generates a constant voltage foraccurate measurement. The tester has two potential andone current probe. The procedure of measurement isillustrated in Figure 22.7.

One of the potential probes A is drilled into the groundat about 25 m from the grounding station G, whoseresistance is to be measured. The second probe B is placedbetween the two. The current lead of the meter is connectedto the grounding station. The meter will indicate someresistance, which may be noted. Two more readings arealso taken by shifting the centre probe B by almost 3 mon either side of the original location. For an accuratevalue of the ground resistance, the values obtained mustbe same. If they are not, the probe B is still within theresistance area of the grounding station G. Shift awayprobe A by another 6 m or so and place probe B betweenG and A, and repeat the test. If the three readings are

now the same, consider this as the actual ground resistanceof station G, otherwise shift probe A farther away until aconstant reading is obtained.

The same test can also be conducted with the help ofa battery, voltmeter and an ammeter, as illustrated inFigure 22.8. The voltmeter must now indicate the samereading at all three locations. When V becomes constant,read the current I. Then the ground resistance

R VIg = W

To measure ground resistance promptly and accuratelymanual and automatic, analogue as well as digital groundresistance testers (or ground testers) are available.Depending upon application one can choose an analogueor digital tester.

22.4 Metal for the groundingconductor

Copper, aluminium, steel and galvanized iron are themost widely used metals for the purpose of grounding.

Economical and mostcommonly used salts

More common salts

¸˝˛

¸˝˛

Currentterminal

Groundtester

GB1 B2B A Ground

level

Potentialprobes

� 3 m � 3 m

� 25 m

Groundstation

Figure 22.7 Measuring the ground resistance with the help of aground tester

Res

istiv

ity o

f so

il ( W

-m) 103

102

10

10 10 20 30%

Content of moisture(a)

Res

istiv

ity o

f so

i l ( W

-m) 103

102

10

1–20 0 20 40 60∞C

Soil temperature(b)

Res

istiv

ity o

f so

il (W

-m) 103

102

10

10 5 10 15 20%

Effect of salt(c)

Figure 22.6 Effect of moisture content, temperature and salt on the resistivity of soil


Choice of any of them will depend upon availability andeconomics in addition to the climatic conditions (corrosioneffect) at the site of installation. In Table 22.3 we providea brief comparison of these metals for the most appropriatechoice of the metal for the required application.

22.4.1 Size of the grounding conductor

This is a matter of system design and is different for LVand HV systems, as discussed above. The main criterionwhen determining the size of the ground conductor is tosustain the rated short-time ground fault current of thesystem for the required duration, without damage to orpermanent deformation of the ground conductor and tolimit its temperature rise within permissible limits. Itwill also limit the voltage drop within 55 volts betweenany two grounded points with which a human body maycome into contact. However, for all practical purposes,the minimum size of conductor as determined below fora required fault level will generally be adequate to limit

A

V

Switch Battery

GB1 B2

B A Ground

level

Potentialprobes

� 3 m � 3 m

� 25 m

Groundstation

Figure 22.8 Measuring the ground resistance with the help of anammeter and a voltmeter

a This occurs when two dissimilar metals in an electrolyte have a metallic tie between them. There is a flow of electricity between the anodicand cathodic metal surfaces, generated by the local cells set between dissimilar metals. One metal becomes an anode and the other a cathodeand causes an anodic reaction which represents acquisition of charges by the corroding metal. The anode corrodes and protects the cathode,as current flows through the electrolyte between them.

No. Characteristics

1 Conductivity (%)

2 Resistance tocorrosion

3 Galvanic effecta

4 Approximate costconsiderations (%)

Copper1

100(for annealed copper)

High. Being cathodicwith respect to othermetals, which may beburied in the vicinity

Copper is a galvanicmetal and causescorrosion, in the presenceof moisture, in nearbymetals, such as cablesheathes, steel structureand water, gas or drainpipes, buried in itsvicinity. With all suchmetals, it forms acomplete electrolyticcircuit and corrodesthem. Tinning may giveprotection against itsgalvanic effects but thisis an expensiveproposition

100

Aluminium2

61(for EC grade aluminium)

Highly corrosive and is,therefore not preferredfor undergroundconnections or groundelectrodes. For surfaceconnections, however,where it is less corrosiveand highly conductive,compared to steel or steelalloys it is preferred

Steel3

30–40(for copper-clad steel core)

Corrosive. Copper-cladsteel may be used toovercome this deficiency

Galvanized iron4

8.5(for Zn-coated steel)

High, and is extensivelyused for groundconnections and grids

Table 22.3 Comparison of grounding metals

10 15Therefore most appropriateand economical

These are not galvanic but become anodic in the vicinity of copper and erode

50


the voltage drop within the safe limits. For more detailsrefer to IEC 62271-200.

The surface ground conductor can be of aluminium,GI or copper, as discussed earlier. A humid or a chemicallycontaminated location is corroding in nature. Aluminiumhas a rapid reaction and is fast corroding. At such locations,use of GI or copper conductor would be more appropriate.Table 22.4 suggests the ground conductor sizes foraluminium conductor power cables for small and medium-rating feeders when aluminium is used for the groundconductor. For a GI ground conductor, this size may beroughly doubled.

For large feeders and HV systems the ground faultcurrent would be controlled naturally through the groundcircuit impedance and a smaller ground conductor maysuffice.

NoteIn LV systems, where the neutral is grounded, the neutral as well asthe ground conductor may have to carry unbalanced currents up tohalf the rating of the line currents due to single-phase loads. Aground conductor should also be rated for the same size as theneutral, irrespective of the setting of the relay.

Now Equation (22.4) as suggested by BS 7430 willapply, which is based on our discussions in Section 21.2.1,where the ground system is normally predetermined forthree times the rated current of the circuit for an HRCfuse-protected system or one and a half times for anover-current release-protected system:

SI

kt =

g(22.4)

whereS = cross-sectional area of a bare ground conductor in

mm2.Ig = r.m.s. value of the ground fault current in amperesk = r.m.s. current density in A/mm2. This will depend

upon the material of the conductor and its maximumpermissible temperature. For more common metalsit may have the following values, assuming the initialtemperature of the conductor to be 40∞C.Copper = 205 A/mm2, assuming the final

temperature to be 395∞CAluminium = 126 A/mm2, assuming the final

temperature to be 325∞CSteel or GI = 80 A/mm2, assuming the final

temperature to be 500∞C t = duration of fault in seconds (operating time of the

protective device).

NoteFor other grounding materials, or hazardous locations requiring amuch lower end temperature, refer to BS 7430.

Example 22.4Consider an LV power distribution system having the mainincoming feeder rated for 400 A and the outgoing feedersrated up to 200 A. To calculate the main ground conductorsize, assume that the system is protected through HRC fuses.Then, based on the previous assumptions (Table 21.2)

Ground fault current, Ig = 3 ¥ 400

= 1200 A

and interrupting time of 400 A HRC fuses, referring tocharacteristic curves of Figure 21.4 � 60 seconds

\ Ground conductor size for an aluminium conductor

S = 1200126

60

� 74 mm2

i.e. 25 mm ¥ 3 mm ¢¢ ¥¢¢

ÊËÁ

ˆ¯

1 18

or any other cross-section of an equivalent area.If the conductor is of Gl then,

S = 120080

60

or � 116 mm2

or 25 mm ¥ 5 mm (1 )14¢¢ ¥ ¢¢

or any other cross-section of an equivalent area.It could similarly be calculated for the individual outgoing

circuits, or considered equivalent to half the cable size beingused to feed the circuit.

Example 22.5If a distribution system is fed from a 1600 kVA, 11 kV/415 V,transformer, then

I r = 1600 10003 415

A¥¥

= 2225 A

If the system is protected through over-current releases, thenapplying the same assumptions as before:

Ground fault current, Ig = 1.5 ¥ 2225 A

and the maximum tripping time at this current, referring tocharacteristics curves of Figure 21.3

= 370 seconds

\ Ground conductor size for an aluminium conductor

S = 1.5 2225126

370¥

= 509.5 mm2

or 100 mm ¥ 5 mm

or 100 mm ¥ 6 mm

or any other cross-section of an equivalent area.If the conductor is of Gl then

Table 22.4 Size of aluminium ground conductor for differentsizes of power cables for a grounding system

Sr. no. Power cable size Ground conductor size

1 Up to 25 mm2 Same as the cable size2 Above 25–50 mm2 25 mm2

3 Above 50 mm2 Approximately half the size of themain cable size. Say, for a 400mm2 main cable, a groundconductor of 185 mm2, will beadequate


S = 1.5 222580

370¥

� 802.5 mm2

or 80 mm ¥ 10 mm or

any other cross-section of an equivalent area.

22.5 Jointing of groundingconductors

As discussed in Section 29.2.5, jointing of two differentmetals (copper being one) causes electrolysis at the joints,leading to corrosion and failure of the joint. To avoidthis, it is recommended that the same procedure be adoptedas discussed in Section 29.2. Where the electrode andthe connecting ground strip are of the same metal, thenthe joints are riveted or welded with the same metal aftermaking the surface. Soldering is not recommended.

22.6 Maintenance of groundingstations

To ensure that a grounding station has not deterioratedand its ground resistance has not increased due to soildepletion it is mandatory to carry out a few checksperiodically to ascertain the resistance of the groundingstation. If the ground resistance is found more than itwas designed for, it is possible that by proper moisteningof the soil or by adding more salts or chemicals to thegrounding pit, the desired level of ground resistance isachieved once more. If not, then additional groundingstations may have to be installed to obtain the originallevel of the ground resistance.

SECTION II

22.7 Grounding practices in a powergenerating station

This is a vast subject, on which extensive research hasbeen done by many authors and field engineers over theyears. The grounding stations in such areas are normallyspread over the entire station, and sometimes may evenextend beyond its boundary to achieve the desired results.Here we discuss briefly, the basic criteria behind theelaborate requirement of a grounding system in a powerstation and its design considerations.

The magnitude of ground voltage in such areas in theevent of a ground fault is very high, due to high systemvoltage. On a ground fault, the ground path resistancemay become a source of a high potential gradient acrossthe grounding conductors at a particular location. It maybecome high enough to prove fatal to a human operatorcoming into contact with it. To limit this potentialdifference at all locations within a tolerable value andachieve an equipotential distribution of a ground conductor

over the station is the basic criterion on which is basedthe design of a grounding system for a power generatingstation. Our discussion is also applicable to outdoorswitchyards and large substations. For detailed working,it is advisable to refer to IEEE-80.

The following basic data are important for the designof such a grounding system.

22.8 Tolerable potential difference ata location

We discussed in Section 21.1.1 the maximum tolerablecurrents through a human body and their duration. Thepotential difference in a ground conductor at any pointwhere a human body may come into contact with it duringthe course of a ground fault should be such that theresultant current through the human body will remainwithin these tolerable limits.

22.9 Voltage gradients

The likely positions, in which a human body may comeinto contact with a phase or a ground conductor, and thecorresponding potential differences, that he may beexposed to, are illustrated in Figure 22.9.

22.9.1 Tolerable step voltage, Es

This is the difference in the surface potential to which ahuman body may be subject when bridging a distance of1 metre on the conducting ground through the feet withoutbeing in contact with any other conducting groundedsurface (position 1, Figure 22.9).

The safe step voltage, Es, should not be more than thetotal resistance to ground through the body, R2fsb (Section22.10.1) ¥ safe body current, Ib, as a function of time, where

R2fsb = 6 · Cs · rs + 1000 W (from Equation (22.10),discussed later)

and It

b = 0.116 for a 50 kg body, as in Equation (21.2)

= 0.157t

for a 70 kg body, as in Equation (21.3)

\ Es(50) £ (6 · Cs · rs + 1000) ¥ 0.116t

for a 50 kg body

(22.5)

and Es(70)£ (6 · Cs · rs + 1000) ¥ 0.157t

for a 70 kg body

(22.6)

22.9.2 Tolerable touch voltage (Et)

This is the potential difference between the groundpotential rise (GPR) and the surface potential at a pointwhere the person is standing on the conducting groundwith one hand in contact with a conducting roundedsurface (position 2, Figure 22.9). The safe touch voltage,


Et, should not be more than the total resistance to groundthrough the body, R2fps (Section 22.10.1) ¥ safe bodycurrent, Ib, as a function of time, where

R2fps = 1.5Cs · rs + 1000 W (from Equation (22.11)discussed later)

Et(50) £ (1.5Cs · rs + 1000) ¥ 0.116t

for a 50 kg body (22.7)

and Et(70) £ (1.5Cs · rs + 1000) ¥ 0.157t

for a 70 kg body (22.8)

Of the safe step and touch voltages, the requirement ofthe safe touch voltage Et is more stringent. These are

basic design parameters, which will decide the requiredsize of ground mat and the design of mesh. In all ourfuture assumptions, we consider the average weight of ahuman body to be 70 kg.

22.9.3 Mesh voltage or safe design voltage (Em)

This is the maximum touch voltage of a grounding stationthat may occur under the worst situation (position 3,Figure 22.9). The design of the grounding station mustensure that in actual service this voltage does not exceedthe permissible tolerable limits noted in Section 22.9.6.

22.9.4 Ground potential rise (GPR)

This is the maximum voltage, that a station groundinggrid (ground mat) may attain during a ground fault in the

Transmission line Outdoor Indoor

Vt

Wall

Bus duct

H.V. side

GL

1

2

3

Em

Et

Es

1mGenerator transformer

4

Etr

Generator

Indoor/outdoor Indoor

Wall

Bus duct

1 – Es – Step voltage

2

3

4

– Et – Touch voltage

– Em – Mesh voltage(max. touch voltage)

– Etr – Transferred voltage

4

EtrEm

3

2

1GL

Es1 m

Utility auxiliary transformer Switchgear

H.V. side

Et

Figure 22.9 Likely positions in which a human body may come in contact with phase and the ground conductors and the corresponding voltagegradients (illustrated through the layout of Figure 13.21)


presence of a remote grounding network. In normalconditions the grounded grid potential may be regardedas zero, except transferred voltages and surge pilferagesthat may be caused during a transient state (Section 18.5.2).During a ground fault the current will flow through thegrounding grid and cause its potential to rise with respectto a remote grounding station. This voltage rise is seento go up to 25 kV, but generally not beyond 10 kV (IEEE367) and can be expressed by

GPR µ Ig . Rg

whereIg = fault current through the grounding grid

and Rg = grid resistance at the station grounding grid, with respect to the remote ground.

The larger the grounded grid area, the lower will be thegrid resistance, and the lower the GPR and the mesh voltage.

22.9.5 Transferred voltage (Etr)

This may also be considered to be a type of touch voltagewhere the voltage may be transferred into a switchyardor a generating area as a result of a ground fault somewherein the power network in one of the supply sources and aperson standing in the local area of one grid station comesinto contact with a grounded conducting part, groundedat a remote grid station or vice versa (position 4, Figure22.9). In such a situation, if a ground fault occurs, thepotential to ground may exceed the full GPR of the localgrounding grid where the person is standing. Thetransferred voltage may exceed the sum of the two GPRsof the two grounding grids, due to the induced voltagesin the steel structures, neutral wires and metallic pipes inthe vicinity. It is not practical to make provisions forsuch an eventuality in the design of a station groundinggrid. To safeguard a human body from such voltages,IEEE 80 has recommended providing isolating devices,such as surge arresters or display danger boards at suitablelocations. For more details, refer to the Standard.

22.9.6 Design parameters

There are a few important parameters that must bedetermined before beginning the detailed engineering ofa grounding station.

Maximum ground grid current and its duration

This is the maximum grid ground fault current, IG, thatmay occur during the lifetime of the power plant. It mayincrease to the sum of two GPRs as noted above, i.e. upto 80 kA or even higher (IEEE 367). For system faultlevels refer to Table 13.10. It is advisable to carry outfault current studies every few years to assess the actualfault level compared to those considered at the time ofdesigning the grounding system. It is possible that thegenerating capacity of the power station and so also itsfault level has increased with time.

ts – duration of fault. Typical values may range between0.25 and 1.0 s

ts1 – shock duration. The value may be considered by

keeping a safety margin in the allowable body currentand its duration. In an automatic reclosure powersystem, reclosure after a ground fault is commonpractice in modern power systems. This may resultin repeat shocks in quick succession to a humanbody coming into contact with the ground conductor.Although this situation may last for less than 0.5 s,it may prove fatal. A reasonable allowance for suchan eventuality should be made when deciding onthe clearing time.

Since a switchyard is normally connected to morethan one supply system, the ground fault current in apower station is contributed by the power plant as wellas by the switchyard and the transmission networks. Thefollowing possibilities may arise:

1 When there is a fault in the local generating area(Figure 22.10(a)) the return path will be through thegrounded neutral of the generator. The switchyard’s

Remotesource

Localpower station

G2 G1

Ig2*

GT

Ig2

GT

Ig1Ig1

The step and touchvoltages are not affected

in the generator area

Ig1Ig2

G

Ig1

G

O/GIg2

*Ig2 will exist only if the source is grounded star[If it is D or isolated neutral Ig2 = 0]

(a) Fault at the local area

Figure 22.10 Contribution to ground fault currrent by other supplysources when more than one system is operating in parallel

Remotesource

Localpower station

G2 G1

Ig1

Ig1Ig1

GTGT

Ig2

Ig1

Ig2

Ig1

Ig1 + Ig2O /G

Ig2

(b) Fault at a remote location

Generatorarea ¸

˝ÔÔ


other remote power sources may also contribute tothis fault as illustrated, provided that they are groundedstar. An isolated star or delta-connected source willremain unaffected by remote faults. The step and touchvoltages in the generator area will not be affected.The GT (generator transformer) area and the nearbysteel structures will develop high step and touchvoltages.

2 Similarly, when a fault occurs some distance fromthe generating area then this area will feed the remotefault as did the remote sources in the generator areain the previous case, thus, developing step and touchvoltages in the generator area (Figure 22.10(b)).

The flow of circulating currents in the groundingconductors or ground of region two caused betweentwo or more inter-connected grounding stations, for afault occurring in region one is termed the telluric effect.

3 When the generator and the switchyard grounding matsare inter-connected the ground fault current will dividebetween the two, depending upon their ground resistances,in inverse proportions (Figure 22.11) such that

Ig = Ig1 + Ig2

and II R

Rg1g g

1 =

◊

and II R

Rg2g g

2 =

◊

where RR RR Rg

1 2

1 2 =

+ ◊

For the grounding grid to remain effective over longyears of operation, in view of existing ground parallel pathsprovided by other grounding stations in the vicinity andexpansion of the power system in future, a moremeticulous design would also consider the followingfactors:

• Resistance of the grounding grid• Division of the ground fault current, Ig, between the

other parallel ground paths• The decrement factor to account for future expansion,

if any, and• The asymmetry (d.c. component, Section 13.4.1(8)).

\ IG = Ig · Df (22.9)

whereIG = maximum ground fault currentIg = symmetrical ground fault current

Df = decrement factor

Safe design voltage

Of all the grounding grid voltages derived above, the

Ig = Total symmetrical fault currentIg1 = Fault current shared by the power plant areaIg2 = Fault current shared by the switchyard areaR1 = Resistance of the power plant ground circuitR2 = Resistance of the switchyard ground circuit

Ig

R1 R2

Ig1 Ig2

Power plantarea

Switchyardarea

(b)

Figure 22.11 Sharing of fault current by the power plant and theswitchyard areas

Power plantground circuit

Switchyardground circuit

Grounding matinterconnected

G2 G1

Remotesource

Localpower station

Ig1Ig2

Ig1Ig2Ig2

O/G

Ig2Ig2

Ig1

Ig1

Ig2

GGGrid bus

(a)

Figure 22.12 Limits of touch voltages as a function of time

Touc

h vo

ltage

(E

t, E

m)

200

150

100

80

60

157

130

116

65

Et

t70 = 157

Et

t50 = 116(Upper voltage

limit)

(Lower voltagelimit)

0.50.8

11.46

2 33.2

4 5 6 7 8 9105.8

Allowable time (sec.)


mesh voltage or the maximum touch voltage, Em, mustfall within the safe limits and it forms the basic designparameter. The design of the grounding system mustensure that on a ground fault the actual touch voltage Emwill not exceed the maximum tolerable touch voltage Etmentioned above. An ideal design would mean a potentialdifference in the range of 65–130 V. For illustration, agraph is drawn of Et versus time, as shown in Figure22.12, assuming that there is no crushed rock and rs = 0.The graph reveals that a human body weighing 50 kgcan endure a shock voltage of 65 V for almost 3.2 s and130 V for almost 0.8 s. Similarly, a body weighing 70 kgcan endure a shock voltage of 65 V for almost 5.8 s and130 V for almost 1.46 s. A higher touch voltage than 130V would require a yet faster isolation of the fault. Usually50 V is considered as safe touch voltage.

22.10 Determining the leakagecurrent through a body

22.10.1 Body resistance

1 The proportion of the leakage current through a humanbody will depend upon the resistance of the bodycompared to the resistance through the ground. Todetermine the likely body current it is therefore essentialto determine the average body resistance. On thissubject many studies have been made and the followingdata established (Figure 22.13):

a = resistance hand to hand = 2300 Wb = resistance hand to feet = 1130 W(A leather shoe is considered as a part of the body)c = resistance between the two feet =1000 W

It is observed that the body’s resistance diminishes athigher voltages, above 1 kV or currents more than1 A, passing through the body, due to a puncture ofthe skin tissues. For all safety measures and grounddesign consideration, the average human body

resistance is considered universally, as 1000 W whichhas yielded satisfactory results.

2 To determine the total resistance of the ground circuitthrough the human body, the following may be adopted.

R2fs = resistance between the two feet in seriesR2fp = resistance between the two feet in parallel

There are many formulae to determine the above, allleading to almost the same results. The most adopted,assuming a layer of crushed rock (gravel) over theground surface, is expressed by

R2fs = 6 · Cs · rs

and total touch resistance R2fsb through the body

R2fsb = 6 · Cs · rs + Rb

= 6 · Cs · rs + 1000 (22.10)

and R2fp= 1.5 ¥ Cs · rs

and total step resistance, R2fps, through the body

R2fps = 1.5 ¥ Cs · rs + Rb

= 1.5 ¥ Cs · rs + 1000 (22.11)

where Rb = body resistance

� 1000 W

Cs = reduction factor for derating the nominal valueof surface layer resistivity, corresponding to acrushed rock layer of thickness hs and a reflectionfactor k

where

k =r rr r

– +

s

s

and r = ground resistivity in Wmrs = crushed rock (gravel) resistivity in Wm

NoteTo achieve a high contact resistance as a measure to provide highersafety to personnel working in the power plant and switchyardareas, common practice is to spread a layer of concrete or crushedrocks (gravel) over the finished ground surface. In the power plantarea, a layer of concrete (150–300 mm), depending upon the stationvoltage is spread to provide a resistivity of nearly 500 Wm or more.In the switchyard area, a layer of crushed rocks is spread (75–150mm) to provide a resistivity of nearly 2500–3000 Wm or more. Thevalue of Cs can be read from the hs versus k curves provided byIEEE 80, as in Figure 22.14.

Example 22.6Consider a large substation grounding system, having a layerof crushed rock, 150 mm thick at the surface, having a resistivityof 3000 Wm and the soil resistivity of 150 Wm:

\ k = 150 – 3000150 + 3000

= – 28503150

= – 0.90

\ Cs from Figure 22.14, corresponding to a rock surface of150 mm

b

c

= R2fs

h

Soil

Crushedrock

a

Figure 22.13 Resistances of different body parts


� 0.7

\ Ground resistance between the two feet in series

R2fs = 6 ¥ 0.7 ¥ 3000

= 12 600 W

and in parallel

R2fp = 1.5 ¥ 0.7 ¥ 3000

= 3150 W

Having determined the actual ground loop resistancethrough the body, one can find the ground leakage currentthat may flow through a human body during an actual groundfault under different body touch conditions with the groundinggrid.

For example, referring to Example 22.8 and Table 22.6,the safe touch voltage, Et, in the power plant area isestimated at 267 V. For this voltage, the leakage current, Ig�,through the two feet when in parallel which is a more severecase,

I

ERg

t

2fp = �

= 2673150

1000 mA¥

= 84.76 mA

22.10.2 Ground resistance

The ground resistance is a function of the area occupiedby the grounding station and the stratification of the soil.The stratification of the soil is usually of a non-uniformnature and may vary the resistivity of soil vertically aswell as horizontally, thus varying the resistance of soil.The minimum value of ground resistance (resistance of

the grounding station) at a certain depth h from the groundsurface may be expressed by

RA Lg =

4 +

r p r◊ (22.12)

where

r p4

= ground resistance at the surface of the soilA

and rL

= ground resistance of the total buried length(L) of the conductors

Rg = station ground resistance in Wr = average resistivity of soil in Wm

This will depend upon the condition of thesoil and its moisture content. This is why it isusually high where the moisture content is lessthan 15% of the weight of soil. The variationin soil resistivity is, however, low when themoisture content exceeds 22%.

A = area of the grounding grid

(i) in a rectangular grid

A = a · b m2 (Figure 22.15)

where

a = length of the grid in m andb = width of the grid in m

(ii) In a circular grid

A = p · r2 m2

wherer = radius of the grid in m

– 0.95– 0.9– 0.8– 0.7

Cs

in t

erm

s of

k

K=0

–0.1

–0.2

–0.3

–0.4

–0.5

–0.6

1.0

0.8

0.6

0.4

0.2

0

0.85

0.70.68

0 0.04 0.08 0.12 0.16 0.20 0.240.15 0.25

hs (metres)

Figure 22.14 Reduction factor Cs as a function of reflection factor kand thickness of crushed rock (gravel) hs

Figure 22.15 Area of a grounding grid and length of buried conductors

Number ofgrounding rods = ng Grounding rods

GL

h h

a b

Number ofconductors = na

Grounding grid

Number ofconductors = n b

h – Average depth of grid below the ground surface


L = total length of the buried conductors in m= Lc + Lr (see also Section 22.14.4)

Lc = total length of conductors, used in the grid in mand Lr = total length of the grounding rods in m

If there are na number of conductors lengthwise andnb widthwise and ng = number of grounding rods used ina grounding grid at a depth of h, then the total length ofthe buried conductors

L = na · b + nb · a + ng · h metres (22.13)

With an increase in the length of the buried groundconductors, the value of their ground resistance diminishes.It has been found that Equation (22.13) is more accuratefor a grid depth up to 250 mm. At greater depths ofstation grids, a more accurate representation is found inthe following equation:

RL A h A

g = 1 + 120

1 + 11 + 20/

rÊ

ËÁˆ

¯

Ê

ËÁ

ˆ

¯˜ (22.14)

22.11 Measuring the averageresistivity of soil

It is important to determine the average resistivity of soilat every site where a grounding station is to be located.To do this, a soil test is essential. For this, samples maybe collected from a number of nearby locations at thesite to arrive at an average value. As a result of soilstratification, samples must be collected at different depthsto ascertain variation in the resistivity to decide on asuitable depth for the grounding grid. For simplicity,Tables 22.1 and 22.2 suggest the likely average range ofresistivity for different kinds of soils and their moistureconditions.

A simple way to measure the resistivity of soil is afour-pin method in which four probes are drilled into theground along a straight line at equal distances a anddepth b. Then a voltage V is applied to the two innerprobes and a current, Ig, is measured in the two outerprobes (Figure 22.16). This test can also be conductedwith the help of a ground tester as discussed in Section22.3, which normally also has a provision for this test.The soil resistance

R VIg

g =

and rp

= 4

1 + 2

+ 4 –

+

g

2 2 2 2

◊ ◊a Ra

a b

a

a b

Since generally

a b b a >> for accurate results keep 20

£ÊË

ˆ¯

\ r � 2pa Rg (22.15)

The current tends to flow near the surface for smaller

probe spacing and deeply through the soil for largerspacing. As the soil resistivity may vary widely, it isrecommended that a wider assessment of the soil stratabe made by varying the probe spacing a and thusdetermining the variation in soil resistivity at the locationof the grid. The reflection factor, k, as noted below, formsan important parameter in the evaluation of a more accurateresistivity of soil where

k = – +

2 1

2 1

r rr r

and r1 = resistivity of the upper larger strata of soil r2 = resistivity of the lower larger strata of soil

22.12 Improving the performance ofsoil

22.12.1 Conductivity

As noted in Section 22.2, the use of salts such asmagnesium, copper sulphate or calcium chloride in thevicinity of the ground grid may improve the conductivityof the soil.

22.12.2 Soil moisture and contact resistance

Ground or crushed rock coverings, about 80–150 mmthick, are useful to slow the evaporation of soil moistureand hence retain the moisture of the topsoil layers. Itwill also diminish the intensity of shock currents due tohigher contact resistance between the feet and the soil.Typical values may vary from 1000 to 5000 Wm.

22.13 Determining the ground faultcurrent

Based on IEEE-80 recommendations, the following

Switch Battery

Ig

GL

p1

p2

VProbes

aaa

b

V

For accurate results keep b a 20

£

Figure 22.16 Measuring the resistivity of soil


simplified formula may be used to determine the groundfault current in the event of a phase to ground fault:

I

V

Z Z Zg

1 2 0=

3 [ + + ]�

◊ (see Section 20.4 or Table 13.5)

whereIg = ground fault currentV� = line voltageZ1 = positive sequence equivalent system impedance,

W/phase at the location of the faultZ2 = negative sequence equivalent system impedance,

W/phase at the location of the fault.Z0 = zero sequence equivalent system impedance,

W/phase at the location of the fault.

22.14 Designing a grounding grid

22.14.1 Minimum size of grid conductors

This can be calculated from the formula derived by Sverakand recommended by IEEE-80:

AI

Tt

K tK t

= 10

log

+ +

g

cap–4

20 20e

0 max

0 amb

◊◊ µ ◊ ◊r

(22.16)

whereA = cross-sectional area of ground conductor

in mm2

Ig = ground fault current in kA (r.m.s.)This may be substituted with the estimatedmaximum ground grid current, IG (Section22.9.6), that may occur during the life ofthe grounding station

Tcap = thermal capacity factor from Table 22.5,in J/cm3/∞C. This is derived from formula(4.184) rh ·rs in Ws/cm3/∞C (for detailsrefer to IEEE-80)

rh = specific heat of ground conductor in Cal/gram/ ∞C

rs = specific weight in g/cm3

t = duration of fault in secondsµ20 = thermal coefficient of resistivity at a

reference temperature of 20∞Cr20 = resistivity of ground conductor at a

reference temperature of 20∞C in m W/cmK0 (at 0∞C)= reciprocal of µ0

= 1 – 2020µ

tmax = maximum allowable temperature in ∞Ctamb = ambient temperature in ∞C

Typical values of the above constants for the most widelyused metals are given in Table 22.5, based on IEEE-80.Example 22.7 in a tabular form gives a procedure todetermine the grounding grid conductor size.

22.14.2 Corrosion factor

Corrosion takes place in all metal conductors located ina humid environment, ground electrodes, being oneexample. It is therefore mandatory that certain corrosionmargins are considered when choosing the size of groundelectrodes to account for this in the long run, particularlyduring the considered life span of the generating station,switchyard or substations. Handbooks on corrosion suggestlikely corrosion depths. For steel this is considered to bearound 2.2% per year. For GI it will be much less.Considering economics of steel over GI it is a commonpractice to use only steel for such extensive and elaborategrounding stations. For steel grids, the depth of corrosionin 12 years is estimated to be around 3.48 mm in a soilhaving a pH value of 7.4. This figure can be used todetermine the depth of corrosion for any number of years.Considering the lifespan of a power generating station as40 years, the depth of corrosion during this period would be

= 3.48 1

1 + 1

2 + 1

3 + . . . 1

40

11

+ 12

+ 13

+ . . . 112

ÊË

ˆ¯

ÊË

ˆ¯

Table 22.5 Material constants

Description Conductivity K020

= 1 – 20µÊËÁ

ˆ¯ tmax, fusing r 20 Tcap

% a 20 temperature ∞C mW/cm J/cm3/∞C

Standard annealed soft copper wire 100.0 0.00393 234 1083 1.7241 3.422Commercial hard drawn copper wire 97.0 0.00381 242 1084 1.7774 3.422Copper-clad steel core wire 40.0 0.00378 245 1084/1300 4.397 3.846

Copper-clad steel core wire 30.0 0.00378 245 1084/1300 5.862 3.846Commercial EC aluminium wire 61.0 0.00403 228 657 2.862 2.556

Aluminium alloy wire 5005 53.5 0.00353 263 660 3.2226 2.598Aluminium alloy wire 6201 52.5 0.00347 268 660 3.2840 2.598Aluminium-clad steel core wire 20.3 0.00360 258 660/1300 8.4805 2.670Zinc-coated steel core wire 8.5 0.00320 293 419/1300 20.1 3.931Stainless steel No. 304 2.4 0.00130 749 1400 72.0 4.032

Refer to Figure 22.17 giving the nomograms for more widely used metals to determine the size of conductor A in terms of duration of fault.


= 3.48 4.27853.1032

¥

= 4.8 mm

This amount of corrosion will occur on each side of theelectrode,

\ total corrosion during a span of 40 years of operation

= 2 ¥ 4.8

= 9.6 mm

This is an average value of metal erosion during thelength of service and may vary with soil conditions. Incoastal areas, for instance, where the subsoil water issaline, erosion of metal would be much more rapid anda further safety factor must be considered. Field experiencewill be a better guide to assess this.

22.14.3 Maximum touch and step voltages of agrounding station

In Section 22.8 we discussed the tolerable step and touchvoltages that a human body can endure. In actual servicethese voltages of the grounding station should not exceedthe prescribed tolerable limits. The grounding stationdesign as carried out above must therefore be counter-

checked for these limits. If it exceeds these limits thestation must be redesigned, to contain the actual stepand touch voltages within the prescribed levels.

IEEE-80 has suggested the following formulae in termsof ground current and total length of buried conductorsto determine the actual step and touch voltages:

Max. step voltage Es(actual) = s i Gr ◊ ◊ ◊K K I

L(22.17)

and mesh or maximum touch voltage Em (actual)

= m i Gr ◊ ◊ ◊K K I

L(22.18)

wherer = resistivity of the soil

Ks, Km = geometrical factors, depending upon the moreimportant parameters such as area of thegrounding grid, its depth and conductor spacingand less important factors, such as diameter ofthe conductors and the thickness of the finishingsurface by concrete or gravel.

The touch voltage diminishes up to 1 m depthof the grid and then rises rapidly. The idealdepth for economic considerations may be takenas 0.5 m when the touch and step voltages arereasonably low.

Ki = corrective factor, accounting for the increasein current densities at the far ends of the gridsystem, the resistivity of the soil and the averagecurrent density per unit length, IG/L, of buriedconductors.

IG = maximum fault current contributed by thepower generating units.

L = total length of the buried conductors of thegrounding station (Equation (22.13)).

Estimating the step voltage Es (actual)

In this case

Kh D h D

ns

–2 = 1 12

+ 1 +

+ 1 (1 – 0.5 )pÊË

ˆ¯

and the maximum step voltage is assumed to occur at adistance equal to grid depth h, where h is more than 0.25m and less than 2.5 m. Depths less than 0.25 m may berare:

Kh D h D

Ws = 1 12

+ 1 +

+ 1 p ◊ÊË

ˆ¯

where

Wn

= 12

+ 13

+ 14

+ . . . 1 – 1

For n ≥ 6

W

nn 1

2( – 1) + log ( – 1) – 0.423e�

The step voltage falls sharply at higher depths.

A/mm2 mm2/kA

Siz

e of

con

duct

or ‘A

’

202530

4050

100

200

400

600800

1000

5040

30

20

10

5

2.5

1.0

0.1 0.2 0.3 0.5 1.0 2.0 3.0 5.0 10 20Duration of fault (sec)

For Zn coated steel wire

For 30% copper clad steel

For 97% cu– (250∞C)

For 40% copper clad steelFor 97% Cu-brazed (450∞C)2

For 97% Cu and 100% Cu

Note1. For critical installations, tmax is considered as 250∞C for annealed

copper conductors. At higher temperatures, the annealing mayerode.

2. For brazed joints, tmax is considered as 450∞C.3. A more prudent temperature rise may be considered to optimize

the use of metal, depending upon the type of jointing, such asby welding (preferably exothermic), bolting, brazing or crimpingetc. and their safe operating temperature, over long periods.Soldered joints must be avoided, which may fail under highfault currents because of excessive heat

Figure 22.17 Nomogram for conductor sizing at 40∞C ambienttemperature


L = Lc + Lr when there are only a few ground rodsat the periphery of the grid

or = Lc + 1.15Lr when there are proportionatelymore ground rods at the far end of thegrid

whereLc = total length of the grid conductors andLr = total length of the ground rods

The factor 1.15 represents the higher current densityin the ground rods that are placed at the far ends or peripheryof the grid. The ground current discharging through auniformly spaced ground grid is scanty at the centre, denseat the edges and a maximum at the corners. Accordingly,the worst step and touch voltages would occur at the outermeshes of the grid, especially at the corners. To make thecurrent density more uniform, a more non-uniform

conductor spacing would therefore be necessary with morenumber of meshes at the centre and fewer towards theperiphery. Increasing the number of meshes, i.e. reducingthe conductor spacing, would tend to reduce the step andtouch voltages until a saturation stage is reached, i.e. when,Lr approaches Lc. The factor 1.15 may now be increasedto 1.2 based on field experience and

Ki = 0.656 + 0.172 n

wheren = the number of conductors on each side of a square

grid. If the ground grid is not a square and thenumber of conductors lengthwise is na and width-wise nb then

n n n = a b◊

Example 22.7To determine the minimum size of ground conductor considering a station grid made of Zn coated steel, having the followingparameters:

Parameters As in IEEE-80 As in IS 3043

I (A) 1 or 0.001 kA 1 or 0.001 kA

(to calculate a generalized factor in mm2/A) (to calculate a generalized factor in mm2/A)

t (s) 1 1

µ20 0.0032 0.0045

r20(mW/cm) 20.1 13.8

Tcap(J/cm3/∞C) 3.931 3.8

tmax (∞C) 419 450

tamb(∞C) 40 40

K01

0.0032 – 20 1

0.0045 – 20

= 293 (Table 22.5) = 202

\ A = 0.001

3.931 100.0032 20.1

log 293 + 419293 + 40

–4

e¥

¥ÊËÁ

ˆ¯

ÊËÁ

ˆ¯

0.001

3.8 100.0045 13.8

log 202 + 450202 + 40

–4

e¥

¥ÊË

ˆ¯

ÊËÁ

ˆ¯

= 0.0010.0061 log 2.14e

= 0.0010.006 log 2.69e

= 0.0010.0061 0.76¥

= 0.0010.006 0.99¥

= 0.0147 mm2/A = 0.0123 mm2/A

or 10.0147

= 68 A/mm2 or 10.0123

= 81 A/mm2

Say 80 A/mm2

For an Ig of 30 kA,

size of ground = 0.0147 ¥ 30 000 = 0.0123 ¥ 30 000

conductor = 441* mm2 = 369* mm2

*If a future expansion in the generating capacity of the station is envisaged, the grounding grid conductor size so estimated maybe enhanced by a suitable decrement factor, D f (Section 22.9.6)


Estimating the maximum touch voltage, Em(actual)

This will largely depend upon the ratio of the currentdensities in the far-end conductors, i.e. conductors at theperiphery, and the innermost conductors and can beexpressed as follows, based on extensive research:

K Dhd

D hDd

hdm e

2 2 = 1

2log +

( + 2 )8

– 4p 16

ÊËÁ

ˆ¯

Ê

ËÁ

+ log 8(2 – 1)

ii

he

KK np

ˆ¯

where

Knii 2/n = 1

(2 )

For grids without or with only a few ground rods nearthe inner grid (not at the far ends):

K hhh

0 = 1 +

where

h0 is the reference depth of grid = 1 m

The values of Ki and L can be determined along similarlines to those for the step voltage.

NoteIn the above equations for maximum step and touch voltagesthe best results will be obtained when the following parametersare achieved:

n £ 25

0.25 £ h £ 2.5 m

d < 0.25 h and

D > 2.5 m

22.14.4 Estimating the value of groundconductor length (L)

Having determined the safe touch voltage Et (Equation(22.8)) the maximum mesh voltage Em (actual) (Equation(22.18)) should be equal to or less than this voltage.Thus by equating these two we can estimate the likelylength L of the ground conductors. Considering this foran average human body of 70 kg

Em (actual) £ Et(70)

orr

r

(1.5 + 1000) 0.157m i Gs s

◊ ◊ ◊£ ◊ ¥

K K IL

Ct

or LK K I t

C >

(1.5 + 1000) 0.157

m i G

s s

rr

◊ ◊ ◊ ◊◊ ¥ (22.19)

This is the parameter that will help to decide the size andtype of the grounding grid and design of the mesh.

Since there are too many variables and parametersrelated to a grounding station, the following practicalexample will illustrate a step-by-step procedure to designa grounding station.

Example 22.8To design a grounding grid, consider a power generatingstation as shown in Figure 22.18, transmitting power at400 kV through its own switchyard to another switchyardremotely located.

Size of grounding conductor

Consider GI for grounding, and the same parameters of Section22.14.1, leading to a minimum size of grounding conductoras 80 A/mm2, based on IS 3043. Fault level for a 400 kVpower station as in Table 13.10 is 40 kA (envisaging no furtherrise in the fault level, IG = Ig)

\ A = 40 1080

3¥

= 500 mm2

If we consider circular conductors then

Powerplant

3 ¥ 300MVAZp = 16 ± 10%

3 ¥ 315MVAZp = 14.5 ± 10%[Table 13.8]

G1 G2 G3

GT1 GT2 GT3

GL

Groundingrods

Stationgrounding

grid

Switchyardgrounding

grid

Figure 22.18 General layout of a power plant, station groundinggrid, and switchyard grounding grid etc.

400 kV trans-mission bus

Switchyardground bus

Stationground bus


pd 2

4 = 500

or d = 25.24 mm

For a power station, assuming a corrosion factor for a lifespanof 40 years as 0.12 mm/year per side then

Corrosion depth = 2 ¥ 4.8 = 9.6 mm.

\ minimum d = 25.24 + 9.6

= 34.84say, 35 mm

Sharing of ground fault current

Rated current of the power plant

Ir = 3 3003 400

¥¥

= 3 ¥ 0.433 kA

\ Ig1 =3 0.433

(16 + 14.5) 0.9 100¥

¥¥

(considering the lower side of Zp, to be on the safe side)

= 4.73 kA

and Ig2 =40 – 4.73

= 35.27 kA as illustrated in Figure 22.19

whereIg1 = fault current sharing by the power plant grounding

gridIg2 = fault current sharing by the switchyard grounding

grid (transmission system)Ig = total fault current, considered to be 40 kA for a 400

kV system as in Table 13.10.

For ease of understanding, the rest of the working is shownin the form of Table 22.6.

After the final designs are complete it is recommendedthat the actual touch Em (actual) and step voltage Es (actual)are rechecked for both power plant and switchyard areasseparately, to ensure that they are within the tolerable limitsas determined above. After the ground stations have beenfinally installed the actual step and touch voltages must bemeasured to verify the designs.

NoteThe above example illustrates a simple procedure to designa ground mat in a large power generating station, inter-connected to external supply sources through a power grid.The procedure would be the same with a large switchyard,receiving and transmitting large powers.

For small power houses, which may be captive and smallswitchyards or substations, receiving and distributing powerto industrial or domestic loads, such an elaborate design isnot required and simple grounding stations as discussed inSection 22.1 will be sufficient.

Remote powersystemG

Power plantground bus

Switchyardground bus

Ig = Ig1 + Ig2Ig2

Ig2

400 kV transmissionbus

Ig2

Ig2

Ig1

Ig1

G1 G2 G3

GT1 GT2 GT3

Figure 22.19 A simplified layout of Figure 22.18, illustrating thesharing of fault current by the station grounding grid and the switchyardgrounding grid on fault

22/818 Electrical Power Engineering Reference & Applications HandbookTa

ble

22.

6D

esig

ning

a g

roun

ding

gri

d

(1)

(2)

(3)

Step

no.

Par

amet

ers

Pow

er p

lant

are

aSw

itch

yard

are

a

1(i

)S

oil

resi

stiv

ity,

rW

m70

70(i

i)S

urfa

ce r

esis

tivi

ty f

or c

oncr

ete

of t

hick

ness

hs

as 2

50 m

m,

r s1

Wm

550

–(i

ii)

Sur

face

res

isti

vity

for

gra

vel

of t

hick

ness

hs

as15

0 m

m,

r s2

Wm

–25

00

2D

urat

ion

of f

ault

s1.

01.

0(m

axim

um c

lear

ing

tim

e of

the

int

erru

ptin

g de

vice

)

3A

vera

ge w

eigh

t of

a h

uman

bod

y (k

g)70

70(A

)\

Saf

e to

uch

volt

age,

Et =

(1.

5 C

s ·

r s +

100

0) ¥

0.15

7 t

Ref

lect

ion

fact

ork

= 70

– 5

5070

+ 5

50k

= 70

–

70 +

250

025

00

= –

480

620

= –

0.7

7=

– 24

3025

70 =

– 0

.95

\ C

s fr

om F

igur

e 22

.14

Cs

= 0

.85

Cs

= 0

.68

\ E

t=

(1.

5 0

.85

550

+ 1

000)

0.

157

1¥

¥¥

= (

1.5

0.6

8 2

500

+ 1

000)

0.

157

1¥

¥¥

= (

701.

25 +

100

0) ¥

0.1

57=

(25

50 +

100

0) ¥

0.1

57=

267

V=

557

V

(B)

Ste

p vo

ltag

e,E

Ct

ss

s =

(6

+ 1

000)

0.

157

◊◊

¥r

= (

6 0

.85

550

+ 1

000)

0.

157

1¥

¥¥

= (

6 0

.68

250

0 +

100

0)

0.15

71

¥¥

¥

= (

2805

+ 1

000)

¥ 0

.157

= (

10 2

00 +

100

0) ¥

0.1

57=

597

V=

175

8 V

90 0

00

Rgp

= 70 4

3.

1490

000

◊

= 0

.103

120

000

Rgs

= 70 4

3.

1412

0 00

0◊

= 0

.089

4G

roun

d re

sist

ance

(W

)

RA

g =

4

rp

(E

quat

ion

(22.

12))

L b

eing

lar

ge

ther

efor

er L

is

igno

red

for

ease

of

calc

ulat

ion

Ass

umpt

ions

by

expe

rien

ce: A

rea

m2

Gro

und

resi

stan

ceW

Tota

l gr

ound

res

ista

nce

of p

ower

pla

nt a

ndsw

itch

yard

are

as i

nter

conn

ecte

d in

para

llel

(re

com

men

ded

prac

tice

) w

here

W1

=

1 +

1

gtgp

gsR

RR

a

nd

Rgt

= 0.

103

0.0

890.

103

+ 0

.089

¥

= 0

.047

7


Rgp

= g

roun

d re

sist

ance

of

pow

er p

lant

are

aR

gs =

gro

und

resi

stan

ce o

f sw

itch

yard

are

aR

gt =

tot

al g

roun

d re

sist

ance

of

pow

er p

lant

and

sw

itch

yard

are

as i

n pa

rall

el

5F

ault

cur

rent

sha

ring

by

the

two

grou

nd m

ats

kA

(i)

Due

to

pow

er p

lant

,kA

(ii)

Due

to

tran

smis

sion

sys

tem

,kA

6To

est

imat

e ‘L

’, to

ach

ieve

saf

e po

tent

ial d

iffe

renc

es:

Equ

atio

n (2

2.19

) L

kk

It E

>

Safe

touc

h vo

ltage

()

mi

G

t

r◊

◊◊

◊

Ass

umin

g th

e fo

llow

ing:

∑A

rea

of p

ower

pla

ntm

2

∑C

onsi

der

a re

ctan

gula

r gr

ound

ing

mat

m2

∑S

paci

ng b

etw

een

cros

s co

nduc

tors

(m

esh)

,bo

th l

engt

hwis

e an

d w

idth

wis

e D

,m

\ N

o. o

f co

nduc

tors

len

gthw

ise

m

and

leng

thm

No.

of

cond

ucto

rs w

idth

wis

e

and

leng

th,

m

and

nF

orh

= 1

mK

id

= 0

.035

m

Kii

Kh

Km

I g1 =

40

0.0

477

0.10

3¥

= 1

8.54

4.73

0

.047

70.

103

= 2

.19

¥

35.2

7 0

.047

70.

103

= 1

6.35

¥

I g2 =

40

0.0

477

0.08

9¥

= 2

1.46

4.73

0

.047

70.

089

= 2

.54

¥

35.2

7 0

.047

70.

089

= 1

8.92

¥

(1)

(2)

(3)

120

000

400

¥ 30

0

15 =

400

15 +

1 =

28

= 2

8 ¥

300

= 8

400

=

300

15 +

1 =

21

= 2

1 ¥

400

= 8

200

=

28

21

= 2

4¥

= 0

.656

+ 0

.172

¥ 2

4=

4.7

84

=

1(2

2

4)2

/24

¥

=

11.

91 =

0.5

2

=

1 +

1 =

1.4

14

To d

eter

min

e th

is, i

t is

nece

ssar

y th

at c

erta

in a

ssum

ptio

ns, b

ased

on

fiel

d ex

peri

ence

, are

mad

e fo

r a p

ossi

ble

grou

ndin

g sy

stem

and

if n

eces

sary

, fu

rthe

r m

odif

icat

ions

mad

e to

arr

ive

at t

he d

esir

ed r

esul

ts a

nd d

esig

n:

90 0

0036

0 ¥

250

12.5

=

360

12.5

+ 1

= 3

0

= 3

0 ¥

250

= 7

500

=

250

12.5

+ 1

= 2

1

= 2

1 ¥

360

= 7

560

=

30

21

= 2

5¥

= 0

.656

+ 0

.172

n=

0.6

56 +

0.1

72 ¥

25

= 4

.956

=

1(2

) =

1

(2

25)

2/n

2/2

5n

¥

=

11.

367

= 0

.73

1 +

0h h

=

1 +

1 =

1.4

14

=

1 2

log

1516

1

0

.035

+

(15

+ 2

1

)8

15

0.0

35 –

1

4 0

.035

+

0.52

1.41

4 lo

g

8(2

2

4 –

1)

e

22

e

pp

¥¥

¥¥

¥¥

Ê ËÁˆ ¯

¥

Ê ËÁ Á Á Á

ˆ ¯˜ ˜ ˜ ˜

=

1 2

log

12.5

16

1

0.0

35 +

(1

2.5

+ 2

1

)8

12.

5 0

.035

–

14

0.0

35

+

0.73

1.41

4 lo

g

8(2

2

5 –

1)

e

22

e

pp

¥¥

¥¥

¥¥

Ê ËÁˆ ¯

¥

Ê ËÁ Á Á Á

ˆ ¯˜ ˜ ˜ ˜

=

1 2lo

g16

.h.d

+ (

+ 2

)8

– 4

+

log

8

(2 –

1)

e

22

iie

pp

DD

hD

dh d

K kn

h

Ê ËÁˆ ¯

Ê ËÁˆ ¯


=

1 2lo

g(3

31.9

5) +

0.

731.

414

log

0.0

52e

ep

Ê ËÁˆ ¯

=

1 2

5.8

+

0.73

1.41

4(–

2.96

)p

Ê ËÁˆ ¯

= 0

.68

70

0.6

8 4

.956

1

8.54

1

000

126

7¥

¥¥

¥¥

= 1

6,39

2

= 7

500

+ 7

560

= 1

5 06

0

The

siz

e of

gro

und

mat

and

spa

cing

of

cros

s co

nduc

tors

,as

sum

ed a

bove

, se

em t

o be

acc

epta

ble,

exc

ept

for

smal

lad

just

men

ts. T

he d

efic

it i

n gr

ound

len

gth

of c

ondu

ctor

s m

aybe

mad

e up

thro

ugh

vert

ical

gro

und

rods

of t

he s

ame

cond

ucto

rsas

for

the

gro

undi

ng g

rid.

(T

he r

esis

tivit

y of

soi

l, r ,

at

the

dept

h of

the

grou

nd r

ods

may

als

o be

con

side

red

the

sam

e as

at th

e de

pth

of th

e gr

ound

gri

d co

nduc

tors

.) F

or m

ore

accu

rate

anal

ysis

ref

er to

IE

EE

-80.

In

this

cas

e co

nsid

er 1

50 s

uch

rods

10 m

in

leng

th

\L

r =10

¥ 1

50=

1500

man

dL

=15

060

+ 1

500

=16

560

m

=

1 2 (

log

(463

.46)

+ 0

.37

log

0.0

54)

ee

p

=

1 2

6.13

9 +

0.3

7 (

–2.9

2)p

¥(

)

= 0

.80

70

0.8

0 4

.784

2

1.46

1

000

155

7¥

¥¥

¥¥

= 1

032

1

= 8

400

+ 8

200

= 1

6 60

0

The

siz

e of

the

gro

und

mat

and

spa

cing

s of

cro

ss c

ondu

ctor

sm

ay b

e ec

onom

ized

. T

he s

ize

of t

he g

roun

d gr

id m

ay b

ere

duce

d an

d th

e sp

acin

g be

twee

n th

e cr

oss

cond

ucto

rs in

crea

sed.

It w

ould

requ

ire

a re

-exe

rcis

e fr

om s

tep

4 on

war

ds. F

or b

revi

ty,

this

is

not

bein

g do

ne.

\ M

inim

um l

engt

h of

con

duct

or r

equi

red,

Lm

>

((1.

5C.

+ 1

000)

0

.157

) (o

r E

t(70

))m

iG

ssr r◊

◊◊

◊¥

È ÎÍ˘ ˚˙

kK

It

Len

gth

avai

labl

e, L

cm

(1)

(2)

(3)


List of formulae used

Resistance to ground:

Plate grounding

RA

= 4

2

r p W (22.1)

r = resistivity of soil in WmA = area of each side of the plate in m2

Pipe or rod grounding

Rl

ld

= 100 2

log 8 – 1 e◊◊ ¥ ¥{ }r

p W (22.2)

l = length of pipe in cmd = internal diameter of pipe in cm

Strip or conductor grounding

Rl

lh w

Q = 1002

log 2

+ Ohmse

2rp ◊ ¥ ¥

◊ (22.3)

l = length of strip or rod in cmh = depth of strip or rod in cmw = width of strip or twice the diameter of the conductor

rod in cm

Q = –1 for strip and –1.3 for round conductor grounding

Size of the grounding conductor

SIk

t = g

(22.4)

S = cross-sectional area of a bare ground conductor inmm2

I = r.m.s. value of the ground fault current in amperesk = a factor that would depend upon the material of the

conductort = duration of fault in seconds

Tolerable step voltage

For a 50 kg body

Es(50) £ ◊ ◊ ¥ (6 + 1000) 0.116s sC

tr (22.5)

For a 70 kg body

Es(70) £ ◊ ◊ ¥ (6 + 1000) 0.157s sC

tr (22.6)

Tolerable touch voltage

For a 50 kg body

Et(50) £ ◊ ¥ (1.5 + 1000) 0.116s sC

tr (22.7)

Relevant Standards

IEC

60050-195/2001

60364-1 to 7

62271-200/2003

Title

Code of practice for earthing and protection against electricshocks.

Electrical installation of buildings-code of practice

A.C. metal enclosed switchgear and controlgear for ratedvoltages above 1 kV and up to and including 52 kV.

IS

3043/2001

732/2000

12729/2000

BS

BS 7430/1998, BS IEC60050-195/1998

BS7671/2001

BS EN 60298/1996

–

–

–

ANSI/IEEE-37.101/1993ANSI/IEEE-80/2000ANSI/IEEE-81/1983

ANSI/IEEE-141/1999ANSI/IEEE-142/1991ANSI/IEEE-241/1991ANSI/IEEE-242/2001

ANSI/IEEE-367/1996

ANSI/IEEE-C2/2002

Relevant US Standards ANSI/NEMA and IEEE

Guide for generator ground protection.Guide for safety in a.c. substation grounding.Guide for measuring earth resistivity, ground impedance and earth surface potentials of a ground system.Part I : Normal measurements.Recommended practice for electric power distribution for industrial plants. (IEEE Red Book)Grounding of industrial and commercial power systems. (IEEE Green Book)Recommended practice for electric power systems in commercial buildings. (IEEE Grey Book)Recommended practice for protection and coordination of industrial and commercial power systems. (IEEEBuff Book)Recommended practice for determining the electric power station ground potential rise and inductivevoltage from a power fault.National Electrical Safety Code (NESC).

Notes1 In the table of relevant Standards while the latest editions of the Standards are provided, it is possible that revised editions have become

available or some of them are even withdrawn. With the advances in technology and/or its application, the upgrading of Standards is acontinuous process by different Standards organizations. It is therefore advisable that for more authentic references, one may consult therelevant organizations for the latest version of a Standard.

2 Some of the BS or IS Standards mentioned against IEC may not be identical.3 The year noted against each Standard may also refer to the year it was last reaffirmed and not necessarily the year of publication.


For a 70 kg body

E Ct

t(70) s s (1.5 + 1000) 0.157£ ◊ ¥r (22.8)

Design parameters

IG = Ig · Df (22.9)IG = maximum ground fault currentIg = symmetrical ground fault currentDf = decrement factor

Body resistance

R2fsb = 6 · Cs · rs + Rb

= 6 · Cs · rs + 1000 (22.10)

R2fsb = total touch resistance through the bodyRb = body resistance

and,

R2fps = 1.5 ¥ Cs · rs + Rb

= 1.5 ¥ Cs · rs + 1000 (22.11)

R2fps = total step resistance through the bodyCs = reduction factor for derating the nominal value

of surface layer resistivity, corresponding to acrushed rock layer of thickness, hs and areflection factor k

where

k = – +

ands

s

r rr r

r = ground resistivity in Wmrs = crushed rock (gravel) resistivity in Wm

Ground resistance

(i) For grid depths up to 250 mm

RA Lg =

4 +

r p r◊ (22.12)

Rg = station ground resistance in Wr = average resistivity of soil in Wm

r p4

= ground resistance at the surface of the groundA

rL

L = ground resistance of the total buried

length ( ) of conductors

Total length of the buried conductors

L = na · b + nb · a + ng · h metres (22.13)na = no. of conductors lengthwiseb = width of the grid in m

nb = no. of conductors widthwisea = length of the grid in m and

ng = no. of grounding rodsh = depth of grounding rods in m

(ii) For grid depths > 250 mm

RL A h A

g = 1 + 120

1 + 11 + 20/

rÊ

ËÁˆ

¯

Ê

ËÁ

ˆ

¯˜ (22.14)

where A = a · b

Measuring average resistivity of soil

r � 2pa Rg (22.15)

Minimum size of grid conductors

AI

Tt

K tK t

= 10

log + +

g*

cap–4

20 20e

0 max

0 amb

◊◊ µ ◊ ◊r

(22.16)

A = cross-sectional area of ground conductorin mm2

Ig* = ground fault current in kA (r.m.s.)

= it may be substituted with IG (Equation(22.9))

Tcap = thermal capacity factor in J/cm3/∞Ct = duration of fault in seconds

µ20 = thermal coefficient of resistivity at areference temperature of 20∞C

r20 = resistivity of ground conductor at a referencetemperature of 20∞C in m W/cm

K0(at 0∞C) = reciprocal of µ0tmax. = maximum allowable temperature in ∞Ctamb. = ambient temperature in ∞C

Actual maximum touch and step voltages of agrounding station

Max. step voltage, Es (actual) = s i Gr ◊ ◊ ◊K K I

L(22.17)

and mesh or maximum touch voltage,

EK K I

Lmm i G(actual) =

r ◊ ◊ ◊(22.18)

For details refer to the text

Estimating the value of ground conductor lengthto design the grounding grid

LK K I t

C >

(1.5 + 1000) 0.157

m i G

s s

rr

◊ ◊ ◊ ◊◊ ¥ (22.19)

Further Reading

Central Board of Irrigation and Power, India, ‘Corrosion of earthingequipment’, Review No. 1, Jan (1973).

Central Board of Irrigation and Power, India, ‘Earthing systemparameters for HV, EHV and UHV substations’, Technical ReportNo. 49, Sept. (1985).

Central Board of Irrigation and Power, India, ‘Design of earthingmat for high voltage substation’, Publication No. 223, Jan(1992).

Central Board of Irrigation and Power, India, Workshop on Designand Earthing Systems, April (1994).

Reeves, E.A., Handbook of Electrical Installation Practices.

Documents

· PDF fileContents Section I 22/799 22.1 Grounding electrodes for buildings, industrial installations and substations 22/799 22.1.1 Plate grounding 22/799 22.1.2 Pipe or rod