Earthing.ppt

EARTHING SYSTEM
BY
M.DURGA PRASAD.,B.E.,
RETD.CHIEF ENGINEER.

The primary objectives of a grounding system are to:
2
 Protects personnel and sensitive equipment.
 Dissipates lightning strikes.
 Discharges short circuit currents.
 Prevents damage from power and switching
surges.
 Provides stable reference.
 Safety to human life, appliances, machines,
equipments etc.)

What are the risks?
4
 20+ fires a day are caused by faulty
electrical installations
 Fires are 12 times more likely to be caused
by faulty electrics than, say, by gas
 2000+ people are injured through
electrocution every year
 As per National Crime Records Bureau
(NCRB) of India, around 15 people die every day
due to electrical accidents, which account for nearly
3% of total accidental deaths.

What are the risks?
5
 The majority of deaths, injuries and
damage would have been prevented by
an installation inspection.
 Older properties (50+ years old) are most
at risk.
 The danger is increased, as it is usually
out-of-sight, under floorboards, behind
walls.
 The function of the earthing system is
two- fold.

PURPOSE OF
PROTECTIVE EARTHING
6
 To ensure the safety of the
people and property within
the zone served by it.
 It requires a high current
capacity path with relatively
low impedence at the
fundamental frequency.
 To ensure that the voltages
developed under high fault
conditions are not
hazardous

Introduction
 1. Equipotential bonding of conductive objects
(eg.metallic equipment,building,piping etc.)to
the earthing system prevent the presence
of dangerous voltages between objects.
 2. The earthing system provides a low
resistance path for earth faults within the
plants which protects both personnel and
equipment.

contd
 3. The earthing system provides a low
resistance path for voltage terminals such as
lightning and surges/ over voltages.
 4. Equipotential bonding helps prevent Electro
static build up discharge,which can cause sparks
with enough energy to ignite flammable
atmosphere.
 5. The earthing system provides a reference
potential for electronic circuits and helps reduce
electrical noise for electronic instrumentation
and communication system.

The objective of a grounding system are:
1. To provide safety to personnel during normal and fault
conditions by limiting step and touch potential.
2. To assure correct operation of electrical/electronic
devices.
3. To prevent damage to electrical/electronic apparatus.
4. To dissipate lightning strokes.
5. To stabilize voltage during transient conditions and to
minimize the probability of flashover during transients.

Earth Resistivity
 Soil or Earth resistivity expressed in ohm-meter is the
resistance of cubic meter of earth measured.
 The resistivity of Copper is 1.6 micro ohm-cm, whereas
the normal value of Soil will be 10000 ohm-cm.The
resistivity of Soil is determined by the quantity of water
held in itself.In otherwords, it can be said that conduction
of electricity through soil due to water content present in
it.

 Poor grounding contributes to downtime and
increases the risk of equipment failure.
 Corrosive soils with high moisture and salt
content and high temperature can degrade
ground rods and their connections. So even
though the ground resistance is less when
installed, it will increase, if the ground rods are
corroded.
 So testing of ground rods are to be done once in
a year.If it increases by 20% the problem to be
investigated and rectified.

The PRIMARY goal of the grounding
system throughout any facilities is
SAFETY.
Why ground at all?
PERSONNEL SAFETY FIRST
EQUIPMENT PROTECTION SECOND

The three main types are:
 EQUIPMENT GROUNDING (SAFETY)
 SYSTEM GROUNDING
 LIGHTNING/SURGE GROUNDING
What are the three main types
of grounding?

. 15
Earthing can be broadly divided as :
 System Grounding ( System
Earthing)
 Equipment Grounding (Safety
Grounding).
 Discharge grounding.

 US National Fire Protection Agency & Institute of
Elec&Electronic Engineers(IEEE) recommended a
ground resistance value of 5ohms or less.
 What effects the grounding resistance?
 Four variables effect the ground resistance.
 1. Diameter of the electrode.
 2. No of ground electrodes.
 3. Length of electrode.
 4. Ground system design.
If double the dia, gr decreases by 10%.
Increasing no of electrodes and connecting them in
parallel gr can be decreased, the spacing of additional
rods must be atleast equal to the depth of the driven rod.
If Increasing the depth is not possible, (in case of rocky
soil)then increase the no of electrodes.

 The spacing of additional rods must be atleast equal to
the depth of the driven electrode.
 Ground rods be placed as deep as possible into the
Earth as soil and water are more stable at deeper strata
generally.
 Please note that when temperature falls,Resistivity
increases.
 Temp Resistivity
 20 7,200
 10 9,900
 0 13,800
 -5 79,000
 -15 3,30,000

19
MOISTURE:
• Moisture significantly influences soil resistivity
• Conduction of electricity in soil is through water.
• Soil resistivity drops significantly in soil with
moisture content.
• Moisture is the most important element in
conductivity.
• In many locations water table goes down in dry
weather conditions. Therefore it is essential to
pour water in and around earth pits to maintain
moisture in dry weather conditions.

. 20
DISSOLVED SALTS
• Pure water is poor conductor of
electricity.
• Resistivity of soil depends on resistivity of
water which in turn depends on the
amount and nature of salts dissolved in it.
• Small quantity of salts in water reduces
soil resistivity by 80%.
• Common salt is most effective in
improving conductivity of soil. But it
corrodes metal and hence discouraged.

. 21
GRAIN SIZE & DISTRIBUTION
 The grain size, distribution and
closeness of packing also contribute to
retention of moisture in the soil.
SEASONAL VARIATION
 Increase or decrease of moisture
content determines the increase or
decrease of soil resistivity.
 Thus in dry whether resistivity will be
very high and in monsoon months the
resistivity will be low.

REDUCTION OF EARTH RESISTIVITY:
METHODS.
22
 Chemicals traditionally used for changing
resistivity are:
-- sodium chloride, NaCl (salt)
-- magnesium sulphate MgSo4,
-- Copper Sulphate CuSo4,
-- Sodium Carbonate,NaCo3, (Washing
Soda),
-- Calcium Chloride, CaCl.
 Earth resistivity can be reduced to:
-- 0.2 ohm meter using NaCo3,or
-- 0.1 ohm meter using salt.

. 23
 1.2 grams per liter of salt in dissolved water
has a resistivity of 5.0 ohm meter, while 6
grams per liter of salt in dissolved water has a
resistivity of 10 ohm meter.
GYPSUM:
 has water retention property.
 Low solubility.
 Resistivity of 5 – 10 ohm meter.
 Neither acidic nor alkaline with Ph value between 6.2
to 6.9.
BLAST FURNACE SLAG:
 Use of blast furnace slag on the granulated form is
on an experimental stage.

. 24
 BENTONITE:
-- Mixed in the ratio of 1 : 6 with black
cotton soil.
-- Is a volcanic product.
-- Is acidic by nature, with Ph value of
10.5.
-- Absorbs 5 times of water.
-- Swells upto 13 times its dry volume.
-- Non corrosive.
-- resistivity is 5.0 ohm meter.

25
• MARCONITE:
-- Developed by Marconi communications Ltd
in 1962.
-- Contains crystalline form of carbon with low
sulphur and Chloride content.
--
-- Resistivity of 2.0 ohm meter reduces to 0.1
ohm meter when mixed with concrete.
-- Retains moisture even in dry / hot climates.
-- Used for anti static flooring and electro
magnettic screening.
-- In slurry state, causes corrosion on metals.
-- Alumunium, tin coated, or galvanised steel
not to be used in marconite.
-- Used in Rocky locations.

26
• Connecting lead should have sufficient current
carrying capacity.
• L A s should have independent earth electrode
which should be inter connected to the station
grounding system.
• All paints, enamel, seals should be removed from
the point off contact of metal surfaces before earth
connections are made.
• The resistances of earth system should not exceed
2 ohms for 33/11 KV Sub Stations.
• But in the sub stations of Distribution companies
Earth resistance Maximum of 1 Ohm is maintained.
• Suitable grounding mat should be provided in the
sub station yard

27
In a Sub Station the following shall be earthed.
 The neutral point of the systems of different
voltages which have to be earthed.
 Apparatus, frame work and other non-current
carrying metal work associated with each
system, for example transformer tanks, switch
gear frame work etc.,
 Extraneous metal frame work not associated with
the power systems, for example, boundary,
fence, steel structures etc.,
The earthing Means connecting of Electrical
equipment, machinery or an electrical system
with the general mass of earth is termed as
earthing or grounding

. 28
 The earthing system must provide an
environment which is free from the possibility
of fatal electric shock.
 The earthing system must provide a low
impedance path for fault and earth leakage
currents to pass to earth.
 The earthing conductors must possess
sufficient thermal capacity to pass the highest
fault current for the required time
 The earthing conductors must have sufficient
mechanical strength and corrosion resistance.
A Sub Station earthing system has to satisfy four
requirements:

5. What are to be earthed
(a) Bodies of all the equipment's
(b) Neutral Point
(c) Battery mid point
(d) Tertiary winding
(e) Cable sheathe
(f) Structures
(g) Street Light Poles / LT / HT Poles
(h) Control Panel
(i) Lightning arrestor Dedicated earthing
with earth pits
(j) Computers / SCADA / Electronic
equip/ Servers Dedicated Earthing

A safe grounding design has two objectives:
1. To provide means to carry electric currents
into the earth under fault conditions
without exceeding any operating and
equipment limits or adversely affecting
continuity of service.
2. To assure that a person in the vicinity of
grounded facilities is not exposed to the
danger of critical electric shock.

Type of
soil
Soil
resistivity
Earthing Resistance -------- -------- ------- --------
Ground
electrode
depth
------------- --------- Earthin
strip
(meters)
--------- ---------- ----------
Ohm m 3 6 10 5 10 20
Very
moist
soil
30 10 5 3 12 6 3
Farming
soil
100 33 17 10 40 20 10
Sandy
clay soil
150 50 25 15 60 30 15
Moist
sandy
soil
300 66 33 20 80 40 20
Moist
gravel
500 160 80 48 200 100 50
ssDry Sandy 1000 330 165 100 400 200 100
soil

Step potential
 “Step potential” is the voltage
between the feet of a person standing
near an energized grounded object.
 It is equal to the difference in voltage,
given by the voltage distribution curve,
between two points at different
distances from the “electrode.”
 A person could be at risk of injury
during a fault simply by standing near
the grounding point.

Touch potential
 “Touch potential” is the voltage between
the energized object and the feet of a
person in contact with the object.
 It is equal to the difference in voltage
between the energized object and a point
some distance away.
 The touch potential could be nearly the
full voltage across the grounded object if
that object is grounded at a point remote
from the place where the person is in
contact with it.

 Mesh Potential :-- The mesh potential is defined
as the potential difference between centre of an
earthing grid mesh and a structure earthed to
the buried grid conductors.
 Transferred Potential:-- This is special case of a
touch potential in which a voltage is transferred
into or out of a sub-station for some distance by
means of an earth metallic conductor.

Basic Shock Situations in Substations

THE PERMISSIBLE LIMITS OF STEP POTENTIAL AND TOUCH
POTENTIAL SHALL BE
44
Maximum Acceptable step Voltage
Fault clearance times
Fault clearance times 0.2 Seconds 0.35 Seconds 0.7 Seconds
On soil 1050 V 600 V 195 V
On chippings 150mm) 1400 V 800 V 250 V
Maximum Acceptable Touch Voltage
Fault clearance times
Fault clearance times 0.2 Seconds 0.35 Seconds 0.7 Seconds
On soil 3200 V 1800 V 535 V
On chippings 150mm) 4600 V 2600 V 815 V

PERMISIBLE EARTH RESISTANCE
 As per IE Rules, one has to keep Touch Potential less than safe
value of 523V.
 I fault = Max current in fault conditions.
 Resistance= Touch Voltage/ If = Vf/If.
 Max fault current for a 100kva Transformer:-
 Full load current = 133Amps, Impedance =4.5%
 If= 100x100 = 2900 say
 1.732x4.5x.44
Therefore R = Vt/If = 523/2500=0.209.
As 0.209 ohms being quite low, quality work is to be done during
construction.
To obtain such a low value of Resistance of the earth system,the
expenditure will be very high.

 The earth electrode resistance value also causes
importance in view of protection against lightning by
lightning arrester.
 The earth electrode resistance value in that case is given
by
 R= Flashover voltage/ Lightning discharge current.
 Flash over voltage of 11kv= 75kv.
 LAS discharge = 40KA.
 Therefore R= 75/40=1.9ohms.
 Thus the earth electrode resistance for LAS of 11kv
system has to be < 1.9ohms.
 However taking the fault current and implementation
difficulties in view, the DTR structure the earth resistance
value is to be maintained below 5 ohms.

Types of Grounding Systems
Ground rod
Earth / Ground Basics
 Many different types
available
 Choice depends on local
conditions and required
function
 Simplest form is a single
stake
 Mostly used for:
 Lightning protection
 Stand alone structures
 Back-up for utility ground

Ground rod group
 ground rod group
 typically for lightning
protection on larger
structures or protection
around potential hotspots
such as substations.

Ground plate
 For areas where there is
rock (or other poor
conducting material) fairly
close to the surface ground
plates are preferred as
they are more effective

Ground mesh
 A ground mesh consists of
network of bars connected
together, this system is
often used at larger sites
such as electrical
substations.

Soil Characteristics
 Soil type. Soil resistivity varies widely
depending on soil type, from as low as 1
Ohm-meter for moist loamy topsoil to almost
10,000 Ohm-meters for surface limestone.
 Moisture content is one of the controlling
factors in earth resistance because electrical
conduction in soil is essentially electrolytic.

Test link
Cable(Earthing conductor)
Clamp
Rod(Earthing electrode)
Rod coupler

Recommended values of earth resistance
Recommended earth
resistance(ohm)
system
5
Low Voltage
2.0
33 / 11 kv SS
1.0
EHT SS
0.5
Gen.Station

Substation earthing system
•Step & Touch voltage
•Grounding grids

calculations
 Touch potential limit :-- The maximum potential
difference between the surface potential and the
potential of an earthed conducting structure during a
fault due to ground potential rise)
 50kg person : E touch50 = (1000+1.5 Cs ps)x0.116/ v
_
ts.
 70kg person : E touch70 = (1000+ 1.5cs ps)x0.157/v– ts
 Where E touch is the touch voltage limit.
 Cs is the surface layer derating factor.
 Ps is the soil resistivity(ohm.m)
 ts is the maximum fault clearing time
 Cs = 1- 0.09 ( 1- p/ps)
 2hs + 0.09

Where p
is soil resistivity, p s is the resistivity of the surface layer , hs is
the thickness of the surface layer.

contd
 Step potential limit:--
 E step 50 : -- [ 1000+ 6Cs- ps]0.116/v-t1
E step 7 5 :-- [ 1000 + 6 Cs – ps] 0.157/ v- t1
70 kg man will have more tolerable values than 50kg
man.

Safety Table according to IEEE std
Surface
Layer
resistivity
Fault
clearin
g time
Ohm-m 0.1sec 0.2sec 0.3sec
Step v Touch v St.v Tou v St v Tou v
None 985 469 763 363 646 308
500 1162 514 900 398 762 337
1000 1802 674 1396 522 1182 442
1500 2423 829 1877 642 1589 543
2000 3037 982 2352 761 1991 644
2500 3647 1135 2826 879 2392 744
3000 4257 1287 3298 997 2792 844
3500 4866 1440 3769 1115 3191 944
4000 5474 1591 4240 1233 3590 1043
4500 6082 1743 4711 1350 3988 1143

 Fault clearing Touch voltage
 Time
 0.04sec 800v
 0.08sec 700v
 0.14sec 600v
 0.2sec 500v
 0.29sec 400v
 0.39sec 300v
 0.49sec 250v
 0.64 sec 220v
 0.72sec 150v
 1.1sec 125v
 10sec 80v

Hence, it is very essential to measure the
resistance of individual electrode in both the
'connected' and 'disconnected' condition i.e. the
electrode connected to the earth mat and also
disconnected from the earth mat. Each electrode
individually as well as the earth mat in totality
shall be very efficient to give protection to all
equipments and to keep step and touch
potentials within limit for human safety.
Let us consider the effects of inefficient
earthing on sub-station equipment for
understanding :

Power transformers/ICTs :
a) Earth faults will not be cleared fast enough arid
the connected equipment will either be damaged
due to carrying the fault current for prolonged
period or its life will be reduced.
b) The voltage across the other two healthy phases
of a three phase transformer winding will be high
and insulation will be subjected to extra stresses
which will reduce the life of windings. Similarly,
the voltage will not be equal in all the phases, if
earthing is improper.

c) Because of over voltage caused due to
prolonged
fault clearing period, there is the possibility of
fire
to the transformer.
d) Step and touch voltages in the vicinity of
transformer will not be within limits, leading to
an accident to human beings.
e) Eddy currents in the transformer will not be
discharged promptly from core and transformer
body, causing heating of the transformer.
Transformer body is connected to station earth
mat.

f) The induced voltage on body of
transformer
due to nearby bus faults will not be
discharged and body will give shock.
g) Condenser bushing, if provided may
burst.
h) The arcing horns (wherever provided)
will
not give proper protection to
transformer.

Current Transformer (CT) :
a) If earthing is not efficient, in case of any open circuit
of secondary of C.T.. abnormal voltages will be
developed, and C.T. may burst.
b) Generally C.T. secondary is star connected. Neutral
is earthed. If neutral is not efficiently earthed, when
primary to secondary insulation fails, secondary will
be subjected to high voltage causing damages. In
such instances associated equipment will also be
damaged.

c) Generally for high voltage CT's insulation is graded
condenser type and this condenser is earthed to
body. Abnormal voltages due to poor earthing will
cause bursting of the CTs. This condenser will
cause bursting of the C.T, when earthing is
improper.
Potential Transformer (PT/CVT) :
a) In case of P.T. Primary lower end is taken to
terminal box and is earthed. All the secondary
phases are star connected there. The star point is
earthed. If not solidly earthed, under fault
condition, high voltage will be developed and the
weak insulation part will fail causing P.T. failure.
Secondary, terminals are also there. The fault could
spread.

Thus the earth – electrode resistance for the lightening
arrestor of 11 KV system has to be less than 1.90 ohm.
In this way justified, to keep the value of the earth
electrode resistance below 1.0 ohm from the point of
view of keeping the touch potential within limit and
below 2.0 ohm from the point of view of providing
protection to the system normally this range of the
values is maintained.

8. Earth at Domestic Premises (Houses)
9. Earth at DTR structure. All connection
through
earth strip but not through GI wire.
10. Tower dines.
11. 33 KV Substation. (Standard lay out)

EARTH GRID- MATERIAL
S.
No
Item Material to be used
1 Grounding Electrodes CI pipe 100 mm / 150mm
(inner dia) Meters long with a
flange at the top
2 Earth mat 75 X 8mm MS Flat
3 Connection to between
electrodes and earthmat
75 X 8mm MS Flat
4 Connection to between
earth mat and equipment
(Top Connections)
50 x 6mm MS Flat
68
The following are the minimum sizes of materials to used.

13. Earth Electrodes sizes
33 KV Substation - 75 x 8 / 50 x 6
132 KV Substation - 100 x 16 / 50 x 8
14. Bill of Materials (For a typical SS)
a) Earth Electrodes with Flange: 300x100Dx10Tmm
b) MS Flat 75x8 mm
50x6 mm
25x3 mm for neutral T/F
100x16 mm

c) Bolt and Nuts with Flat & Spring WASHERS
d) BENTONITE Powder
e) Copper Strip for T/F Neutral
300 x 50 x 60mm -1No. for one T/F
f) Copper Flexible Jumpers for fixing of MSFlat
50x12mm
Flexible Copper of 300mm-2Nos. for One T/F.
g) MS Channels for T/F Neutral – 2Nos.for One T/F.
h) Welding Rods
i) Paints to apply after welding and MS Flats.
1) Red Oxide 2) Bitumen

j) Metal GELLY of size 40mm
h) Cement COLOURS
for earth PIT 600mm dia x 300mm height.
k) Black Cotton Soil
l) Equipotential Bar: Copper Strip
750 x 75 x 8mm
15. How to reduce the earth resistance.
Methods to Improve Earth Electrode Resistance
value. Below given are various methods to
improve the earth-electrode resistance .

i) Chemical treatment.
ii) Multiple electrode system. Calculations required
for deciding number of multiple rods.
iii) Counter poise earthing.
Calculations for deciding the size and length of
conductor are required.
iv) Deep driven rod system.
Calculations for deciding the size and the depth
of the driven rods are required.

v) Use of Bentonite clay.
Importance of Bentonite Compound : Bentonite
compound reduces the earth mat resistance to ¼
level of its original. Bentonite consists of a clay
which, when mixed with water swells to many times
its own volume. It absorbs, moisture from the soil
and can retain it for a long time. Hence it doesn’t
require frequent watering to earth electrodes. It is
recommended that this back fill material be used to
surround vertical electrodes and used to bed
horizontal electrodes to improve the overall earth
resistance.

Importance of Bentonite Compound (Contd)
It is important to stop using coke/cinders and salt as
a back fill material as it corrosive, especially to
mild steel and cast iron.
The materials which are added to the soil to reduce the
soil resistivity are as follows :
a)Sodium chloride (NaCl), coke and sand are the most
common, popular and economical chemicals which
are used to bring down the earth resistivity of soil.
b) Aluminium sulphate is another chemical equivalent
to sodium chloride but it is slightly costlier.

c) Other effective chemicals like Magnesium Sulphate
(MgSO4, 7H2O), Calcium Chloride (CaCl2) and
Potassium Chloride (KC1) when mixed with soil brings
down the resistivity of soil. These chemicals are ten
times costlier than Aluminium sulphate and more than
ten times costlier than Sodium chloride. As such, choice
is preferably limited to Sodium chloride or Aluminium
sulphate but mostly the former one.
d) Bentonite clay, which has a property of retaining
moisture, is another chemical at our disposal but it is
used in high resistivity soil only.
e) Use of Bentonite clay and another effective material
‘electrocon' is discussed separately.

Points of earthing :
a) Earth mat of 75x8 MS flat should be laid as
outer of the switch yard compulsorily and see
that all the pole structures and all metallic
parts are enclosed in the outer mat.
b) Make vertical and horizontal sections for the
outer mat as shown in the fig. The internal
vertical and horizontal sections may be 75x8
or 50x6 MS flat.

c) The earth mat should be laid minimum 600 mm.
Below the ground level. Under the earth mat pure
bentonite powder is to be laid upto 2.5 mm and over
the earth mat, the same Bentonite compound with
Black cotton soil (a mixture of 1:6 ratio) is to be placed
upto 100 mm and the remaining earth trench is to be
back filled with the soil. It is therefore important to
ensure the earth conductor (MS flat) is laid atleast 0.6
M deep, but preferably deeper, say 1M as this will
reduce the surface potentials.

d) See that each and every pole structure is earthed
with 50x6 MS flat to the earth mat.
e) For every breaker there will be five
earth connections to the earth mat with 50 x 6 MS
flat (i) breaker body (ii) relay panel (iii) CTs of the
breaker (iv) and two side of the breaker structure.
f) Lighting arrester is to be connected one end
directly to the earth mat and the other end is to the
nearer earth pit or to the earth mat.
g) Line Isolators are to be connected directly to
the earth mat.
h) The Power Transformers body is to be connected
two sides to the earth mat.

Maintenance of earthing system
The following maintenance schedule is mandatory at each of the
substations.
Sl. No Item Periodicity
1. Watering of earth pits (Not required for
Bentonite treated
earth pits)
2. Measurement of earth resistance of
individual earth pits.
Half yearly @
3. Measurement of combined earth
resistance at all the earth pits.
Half yearly
4. Checking of interconnections
betweens earth pits and tightness of
bolts and nuts.
Quarterly.

@ Earth resistance of individual earth pits can be
measured up by disconnecting the earth connections
to the electrode. This is possible if the connections are
made to a common clamp which is in turn is fixed
round the pipe.

Certain aspects regarding effectiveness of various
earthing material are discussed below.
Why for chemically charged Earth Electrode
diameter of filling material is kept as 300mm ?
If we refer to various drawings of earth electrode for
chemically charged earth electrodes like salt &
charcoal etc. it is seen that the diameter of filling
material is 300mm. Refer attached graph of voltage at
the center / of the electrode & its fall along ground
from the center, x/ in fig-5.0 It will be seen that
maximum voltage is at the center of the electrode & it
falls drastically within a radius of 150mm to some
value & then remains constant to some extent.

Hence the path upto 150mm radius from center should
be of very low resistivity material so that voltage does
not rise for long period and dies down as fast as
possible i.e. discharge is very fast & resistance offered
by ground is very low. Hence the diameter of charged
earth electrode filling material is generally kept
300mm. It can be kept more but it will consume more
material and space.
The distance between two electrodes shall be sum o1
their length to avoid overlapping of their effective area
and there by reducing its effect.

Why earth resistance depends on length and
diameter of electrode ?
Accompanying fig. 6.0 shows zone around an earth
electrode when a large current flows away from it tc
ground. This is generally the path and direction o] fault
current through electrode to ground. The surface area of
electrode depends on length & diameter of electrode.
More area is obtained by changing length than diameter.
Hence, generally variations are done in length of
electrode than diameter for better results

Advantages of separate earthing for PLCC equipment.
5 Essence of effective earthing :
Providing separate earthing is not sufficient if
earthing is not effective. Earth resistance of such
earthing should not be more than 0.5 Ohm. If
earthing is not efficient, it will have effect in
communication signaling (poor signaling).
Separate Earthing is mainly provided to avoid flow
of fault current through PLCC components and for
human safety. Hence ineffective earthing may
cause failure of components and also danger to
human beings.

Fig • 14.0 Earthing of PLCC System equipment

Why black metal used.
IMPORTAMCE OF BLACK METAL LAYER IN SUB-
STATION SWITCH-YARD.
It is common sight to see black metal spread in the
switch-yards of the HV and EHV sub-stations. There
are reasons to lay the black metal layers in the"
switch-yards of the sub-stations. Often, the black
metal spreading is found not upto the mark. Also, in
case of people not knowing the purpose of the black
metal in the switch-yards it is trifle matter. Actually,
much of attention and maintenance to the black metal
layers in the switch-yards is required at all levels. A
review is taken regarding the matter of black metal

Why the black metal is used in the switch-yards ?
Following are the reasons why the black metal is used
in the switch yards.
To provide high resistivity layer.
To avoid formation of pools of oil in case of leakages
from the equipments such as transformers, circuit
breakers, etc. and to eliminate spreading of fire.
To keep reptiles away.
To control the growth of grass and weeds.
To maintain moisture in the soil.
It discourages running of persons in the switch-yard
and saves them of the risk of being subjected to
possible high step voltage.

Point 1 : To provide high resistivity layer.
To understand this point one has to know the
concepts of the Touch voltage and Step voltage. As
per the Indian Electricity Rule no. 67 (1) in every
E.H.V./ H.V. installations :
(a) Touch voltage and step voltage shall be kept
within limits.
(b) The ground potential shall be limited to a
tolerable value.

The value of tolerable touch voltage in respect of human body is
less than the value of tolerable step voltage. Also, a person in the
switchyard may be exposed to touch voltage most often than to the
step voltage. The touch voltage, being predominant must be
considered for the purpose of analysis than the step voltage.
Following is the formula of permissible touch voltage.
E Touch = (116+0.174 ρ)
√t
where, ρ = The soil resistivity where the person is standing.
t = Fault clearing time.
The above formula clearly indicates that it is essential to provide
high resistivity layer under the feet of the person standing in the
switch-yard so as to keep the value of the Touch potential within
permissible limits

According to the I.S. 3043, the Touch voltage
should be less than 523 volts the step voltage should be
less than 1510 volts.
The black metal serves the purpose. The resistivity
of the black metal is taken as 3000 Ohm-m for
calculation of the tolerable touch voltages in most of the
designs of earth mat of sub-station. Crushed stone, i.e.
the black metal, of the size of 30 to 40 mm for a layer of
100 mm is recommended by the CBIP.

The values of resistivity of the different types of
rocks are given below .
Granite, Gneiss - 25000 Ohm-meter
Bolder Gravel - 15000 Ohm-meter
Lime Stone - 5000 Ohm-meter
Moran Gravel - 3000 Ohm-meter
Base Rock Hard - 1190 Ohm-meter
Rock, Hard - 1150 Ohm-meter
Boulders - 477 Ohm-meter
The range of the values of the resistivity is wide. It is,
therefore, essential to know the source of the rock from
which the black metal is obtained so that the idea of the
resistivity of the black metal can be had prior to laving
of the metal.

Resistivity Measurement
The purpose of resistivity measurements is to quantify the
effectiveness of the earth where a grounding system will be
installed.
Differing earth materials will affect the effectiveness of the
grounding system.
The capability of different earth materials to conduct current
can be quantified by the value E (resistivity in W.m).
Resistivity measurements should be made prior to installing a
grounding system, the values measured will have an effect on
the design of the grounding system.
Ground Testing Methods (1)

Resistivity Measurement ( Wenner method)
Resistivity measurements are performed by using a
four wire method.
Used to determine
which KIND of
earthing should be
used, so BEFORE
placing earth stakes
Ground Testing
Methods (1)

Resistivity Measurement
From the indicated resistance value RE, the soil
resistivity is calculated according to the equation :
E = 2  . a . RE
E ...... mean value of soil resistivity (W.m)
RE ...... measured resistance (W)
a ...... probe distance (m)
Ground Testing
Methods (1)

Resistance of driven rods:
 The Ground Resistance (R) of a single rod, of diameter (d) an
driven length (i) driven vertically into the soil of resistivity (ρ), can
be calculated as follows:
where: ρ Soil Resistivity in m
l Buried Length of the electrode in m
d Diameter of the electrode in m
The rod is assumed as carrying current uniformly along its rod.
 Examples
(a) 20mm rod of 3m length and Soil resistivity 50 Ω-m .....R=16.1 Ω
(b) 25mm rod of 2m length and Soil resistivity 30 Ω-m .....R=13.0 Ω













 1
8
ln
2 d
l
l
R



 The resistance of a single rod is not sufficiently
low.
 A number of rods are connected in parallel.
 They should be driven far apart as possible to
minimize the overlap among their areas of
influence.
 It is necessary to determine the net reduction in
the total resistance by connecting rods in
parallel.
 The rod is replaced by a hemispherical
electrode having the same resistance.

Rod Electrodes in Parallel
 If the desired ground resistance cannot be
achieved with one ground electrode, the overall
resistance can be reduced by connecting a
number of electrodes in parallel.
 These are called “arrays of rod electrodes”.
 The combined resistance is a function of the
number and configuration of electrodes, the
separation between them, their dimensions and
soil resistivity.
 Rods in parallel should be spaced at least twice
their length to utilize the full benefit of the
additional rods.

 If the separation of the electrodes is much
larger than their lengths and only a few
electrodes are in parallel, then the resultant
ground resistance can be calculated using the
ordinary equation for resistances in parallel.
 In practice, the effective ground resistance will
usually be higher than this.
 Typically, a 4 spike array may provide an
improvement of about 2.5 to 3 times.
 An 8 spike array will typically give an
improvement of may be 5 to 6 times.

Earth clamping 1
AT-090H AT-090H
Earth clamping 2
AT-087J AT-089J AT-093J

METHODS OF DECREASING GROUND
RESISTANCE
 Decreasing the ground resistance of a
grounding system in high resistivity soil is
often a formidable task.
 Recently, some new methods have been
proposed to decrease ground resistance.

1-Chemical Rods
 Chemical rods are electrodes with holes along
their length, filled with mineral salts.
 The specially formulated mineral salts are
evenly distributed along the entire length of the
electrode.
 The rod absorbs moisture from both air and soil.
 Continuous conditioning of a large area insures
an ultra-low-resistance ground which is more
effective than a conventional electrode.

Chemicals used in earthing
 Bentonite compound; sodium is
predominant.
 Marconite compound.
 Magnesium sulphate Mg so4, 7h2o
 Calcium chloride ca cl2
 Potassium chloride KCL,mixed with soil
brings down the earth resistance.
 Sodium based electrocon compound as
compared to calcium based have found
more suitable.

 If the conductive salts are running low, the
rod can be recharged with a refill kit.
 These rods are available in vertical and
horizontal configurations.
 They may be used in rocky soils, freezing
climates, dry deserts, or tropical rain
forests.
 They provide stable protection for many
years.

Disadvantages are:
 Chemicals concentrated around
electrodes will cause corrosion
 Chemicals reach through the soil and
dissipate
 Scheduled replacement may be required
 May be prohibited because they may
contaminate the water table

Soil Treatment Alternatives
 Ground enhancement material
Cement-like compound
 Non-corrosive
 Extremely conductive
 Installed around the electrode
 Easy installation
 Permanent

 Installing an EARTHLINK 101 earthling strip is
simple:
Dig a trench and lay in the wire.

Pour EARTHLINK 101 conductive cement, using the handy
applicator bag, and shovel in a thin protective layer of soil.

Backfill the remaining soil using a front-end loader
and restore the surface to grade.

 In an unbalanced system, if the neutral is disconnected
from the source,the neutral becomes floating neutral and
it is shifted to a position so that it is closer to the phase
with higher loads and away from the phase with smaller
loads.
 Let us assume the loads are as follows.
 R-phase 3kw, Y phase 2kw, B phase 1kw.
 If the neutral is disconnected from the main,the floating
neutral will be closer to the R phase and away from B
phase.
 So the loads with B phase will experience more voltage
than usual, while the loads in R phase will experience
less voltage, and loads in Y phase will experirnce almost
same voltage. So neutral disconnected unbalanced
system is dangerous.

Types of grounding
 1. Solid grounding.2. resistance grounding 3. Reactance
grounding. 4. Peterson coil.
 Solid grounding:-- The neutral is directly connected to
the earth with a wire of negligible resistance and
reactance.
 Since the neutral is directly connected to earth, the
neutral point is held at earth potential under all
conditions.
 Under fault conditions, the voltage of any conductor to
earth will not exceed the normal phase voltage of the
system.
 Adv:- 1. Neutral is at earth potential. 2. Fault current If and
 resultant cap current Ic are in phase oposition and
cancel each other. 3. No arcing ground.

Resistance grounding
 In order to limit the earth fault current,it is a common
practice to connect neutral through a Resistor.
 R is neither too small nor too high.
 If R is too low, it becomes solid grounding.
 If R is too high it becomes ungrounded system.
 So R is selected in such a way,that it limits the earth fault
to 2 times the usual full load current.
 Adv:- Avoids arcing grounds.
 Fault current is limited.
 Disadv:- costly than solid earthing.

Reactance grounding
 The purpose is to limit the earth fault current .
 It is not used nowadays.
 The disadvantage is, in this system,the fault current
required to operate protective system is higher than that
of Resistance grounding.
 High transient voltages appear during fault conditions.

Arc suppression coil
 Capacitive currents are responsible for producing arcing
grounds.
 So if we introduce some inductance and adjust L, If
flowing through L is in phase oposition to Ic.This
condition is known as Resonant grounding.also called
Peterson coil.
 It is an iron cored coil,having some steps for adjusting L
with the capacitance of the system.
 Adv:- Effecting in reducing arcing grounds.
 Disadv:- The capacitance of the system varies from time
to time.Therefore L is to be adjusted .
 The lines to be transposed.

Voltage transformer earthing
 Generator earthing:- Neutral is connected to the primary
of a transformer and secondary is connected to a low
resistance in series with a relay.
 When fault occurs on the system, voltage will be
developed across the relay and it operates.
 Adv;- The transient over voltages in the system are
reduced because, voltage transformer provided offer
high reactance.
 Application:- for Generator.

 In TN system of earthing, the source is directly earthed
at one or more points. The conductive parts of the
installations are connected to the earth thro the earth
points of the source by means of conductor. So that any
earth fault current created in the installations will pass
thro the earth thro the earthing point of the source. Here
one conductor runs along with the supply line where the
earthing points of the installation are directly connected.
In overhead system, this conductor may be a separate
conductor but in underground system, the cable sheath
or armour is used for the purpose.

 In T T System of Earthing, the source is earthed. But the
conductive parts of the installations are connected to the
earth thro one or more local earth electrodes. These
local electrodes does not have any direct connection to
the earthing system of source.
 This T T System of earthing is applicable for both 3ph
and 1ph installations.

 I T system of earthing is generally used in un-earthed
3ph network, Here 3ph source is isolated from earth or
connected to earth thro a high impedance of suitable
value.The conductive parts including metal body of the
installations are connected to the earth thro one or more
local earth electrodes. These local electrodes does not
have any direct connection to the source.

T N S System of Earthing
 T N S system is similar to T N System of earthing. In both T N &
 T N S system, the neutral wire and earth wire run separately along
the network.The conductive parts of the installations are connected
to the earth wire to provide earthing. But in T N S system, in addition
to continuous earth line from source there are local earth pits
installed at consumer premises. The installations are also connected
to the local earth pits thro earth electrodes. The local earth
electrodes are inter connected to the earthing system of the source
by means of earth wire runs along the network. That means each
local earth electrode is individually connected to the earth wire.

 T N G Earthing system is similsr to T N S system of earthing. In
former, the neutral wire and earth wire do not run separately, rather
they are combined together to form a P E N wire which runs along
the network. The neutral and earth points of installation are
connected to the same P E N wire in this system.

The factors which influence
the design are:
140
• Duration of fault.
• Magnitude of the fault current.
• Resistivity of the underlying strata.
• Resistivity of the surface material
• Material of the earth electrode.
• Material of earthing mat conductor.
• Shock duration.
• Earth mat geometry.

Steps to be taken for design
purpose:
141
• Finalize lay out plan of the substation.
• Obtain the earth resistivity of the location where
substation is to be located.
• Determine the fault current likely to develop at the
SS through system studies. A correction factor of
1.2 to 1.5 may be used for the determined value.
• Assumption of duration of fault is taken as 1.0 sec
• For calculating safe step and mesh potentials, a
duration of 0.5 sec may be assumed.

142
 A uniform corrosion allowance of 0.12 mm
per year is considered for steel as ground
conductor.
 Life of a substation is taken as 40 years.
 Steel corrodes 6 times faster than copper.
 The electrodes spacing shall not be greater
than twice that of the length of the electrode.
Steps to be taken for design purpose:

. 143
 The spacing of the mesh earth conductors shall
be between 3 to 5 meters or as per calculated
value..
 Various specifications such as the area of the
earth mat, Number of electrodes, size of the
earth conductors shall calculated based on the
mathematical formulae and conductor constants.

144
• The number of electrodes is given by the following
thumb rule:
N = If / 250,
for a earth resistivity of 500 ohm meters.
N = If / 500,
for a earth resistivity of 5000 ohm meter.
• The current density of the unbalanced current in a
normal system shall not exceed 40A / Mtr2.
• Short time over load under fault condition is given
by
I = (7.57 x103) / t, where ‘t’ is the duration of
fault in secs.

. 145
• Safe step potential : (116 + 0.7 ) / t
• Safe touch potential : ( 116 + 0.17  ) / t.
where  refers to surface earth resistivity and ‘t’ ,
the duration of fault current in secs.
• The size of the earth bus and earth conductor are
given by
A = 0.0054 I x t,
for sweated and riveted joints.(250deg)
A = 0.0044 I x t,
for brazed joints.(450 deg.)
These values are applicable for copper only
and higher values are to be taken for steel.

Fault Current Carrying Capacity
146
Connection Bolted Brazed Welded
Maximum
temp.
250oC 450oC 700oC
Conductor Size 152 mm2 117 mm2 101 mm2
For a fault current of 25 kA and a duration of 1 second, the
conductor sizes required for each type of joint:

147
• 100 X 16 mm and 75 X 8mm size MS steel
flats form the earthing system for EHT Sub
station and 33/11 KV Sub Stations
respectively
• Earth mat shall be buried in the ground at a
depth of 500mm.
• Shall extend over the entire switchgear yard
and beyond the security fencing of structural
yard.
• The outer most peripheral earthing conductor
surrounding the earth mat shall be of 100 x 16
mm size MS flat.

148
• The intermediate earthing conductors forming
the earth mat shall be of 75 x 8 mm size flat.
• All the risers used shall be of 50 x6 mm size.
• L A s and transformer neutrals shall be of 100 x
16 mm or 75 x 8 mm.
• All crossing of the steel flats while forming the
earth mat and risers shall be properly welded.
• Proper earthing lugs shall be used for
connecting the earth terminals of equipments
to the earthing steel flat.

149
All the equipments, structures, conduits, cable
sheaths shall be solidly grounded at least at two
places.
• Neutral and body earthing shall be connected to
different earth pits
• Welded portion shall be given a coat of black
asphaltic varnish and then covered with jute to avoid
rusting.
• Provisions shall be made for thermal expansion of
the steel flats by giving suitable bends.
• 75x8 mm or 50x6 mm MS flat with a spacing of 5
meters duly welded at intersections.

. 150
• All paints, enamel and scale shall be removed from
point of contact in metal surfaces before giving
ground connections.
• The risers shall be clamped to the structures and
equipments at a height of not more than one meter
with ground connectors.
• Earth connections to cable trenches shall be given
at an interval of 5 meters.
• Power transformers neutral shall be provided with
double earthing.
.

151
 Soil resistivity ‘’ may be obtained from the
following formula :
 = 2  LR where,
R = Value of Earth resistance in ohm
L = Distance between the spike in cm
π = 3.14
= Earth resistivity ohm-cm or ohm-meter.

Choosing the cross section of
earth flat.
S=(I √t)/k
S= Cross sectional area in Sq mm.
I= Fault current which can flow in the earth mat.
t = Disconnection time not exceeding 5 sec.
k=Constant Factor dependent on protective
conductor

K for the
Material
Copper Aluminum Steel
1 Sec 131 86 47
3 Sec 76 50 27
For a 33/11 KV substation the main interconnecting flat
can be designed for 20 KA for 1 sec.

 S=(20000 √1)/47
 S= 425 sq. mm
Hence a interconnecting grid shall be 75 X 8
=600 Sq.mm as 50 X 6 is not sufficient as the
cross section area is only 300 Sq.mm

A 2.75m X 0.10 m cast iron pipe in 1000 ohms-
m soil will have approx. ? resistance.
σ =1000 ohms L =2.75 M d=0.1 M

Design EHT SS
 1. Grid Shape Rectangular.
 2. Depth of burial of grid 0.6 mts
 3. Length in X direction ( Lx) 80 mts
 4. Length in Y direction (Ly) 41 mts
 5. Spacing between conductors 4 mts
 6. No.of ground rods 50
 7. Length of ground rod 3 mts
 8. Fault current split factor 0.6
 9. Shock duration 0.5 sec
 10. Fault duration 1 sec
 11. Surface layer resistivity 1500 ohm-m
 12. Surface layer thickness 0.2 mts
 13. Soil resistivity 50 ohm-m
 14. Fault current 15 KA
 15. Material for grid conductor GI steel
 16. Material for ground rods GI steel

Specifications for 220 kv ss earthing
 Depending upon Soil resistivity, the earth conductor (flats) shall be
buried at the following depths;-
 Soil resistivity in ohm-m Economical depth of Burial in mts
 50-100 0.5
 100-400 1.0
 400-1000 1.5
 The following are the important features in Earthing :
 (1) The earth mat shall be as per the approved lay out.
 (2) The earth mat shall extend over the entire Switch yard as per the
lay out.
 (3) All the junctions of the steel flat with risers should be properly
welded.
 (4) The earth mat shall be formed by welding 50x8 mm steel flat to
the 100x16 mm peripheral earth conductor. The grounding grid shall
be placed about 5 meters ie,in longitude and 5 meters in the
traverse direction.After that earth resistance to be measured.

contd
 (5) All fence corner posts and gate posts shall be connected to the
ground by providing 32 mm dia ms rods of 3 mts length near the
posts and connected to the main grounding mat.
 (6) All paint enamel and scale shall be removed from surface of
contact on metal surface making ground connection.
 (7) The risers taken along the main Switch yard structures and
equipment structures (upto their top) shall be clamped to the
structures at an interval of not more than one meter.
 (8) 50x8 mm ground conductor shall be run in cable routes and shall
be connected to the ground mat at an interval of 10 mts.
 (9) Grounding electrodes of 32mm dia 3 mts long MS rods shall be
provided at the peripheral corners of the earth mat. The grounding
rods shall be driven into the ground and their top shall be welded to
the clamp and the clamp together with the grounding rods shall be
welded to the ground mat.

contd
 (10) LAS shall be provided with earth pits near them for earthing.
 (11) Cast iron pipes 125mm dia and 2.5mts long and 9.5mm thick
shall be buried vertically in the pits and a mixture of Bentonite
compound with black cotton soil a ratio of 1:6 is to be filled 300 mm
dia and the pipe for the entire depth.

MAINTENANCE FREE EARTHING
. 162
It is a new type of earthing
system which is readymade,
standardized, scientifically
developed.

MAINTENANCE FREE EARTHING (cont’d )
163
Its Benefits are:
• Maintenance Free:
No need to pour water at regular
interval- except in sandy soil.
• Consistency:
Maintain stable and consistent earth
resistance around the year.
• More Surface Area:
The conductive compound creates a
conductive zone, which provides the
increased surface area for peak current
dissipation. And also get stable reference
point.

164
• Low earth resistance. Highly conductive.
Carries high peak current repeatedly.
• No corrosion. Eco Friendly.
• Long Life.
• Easy Installation.
• As per IS: 3043- 1987.

165
TECHNICAL DETAILS :
Two ‘B’ class mild steel pipes, one
inside the other, are subjected to Hot
dip Galvanization : 80-100 micron on
the secondary electrode and 250 – 300
micron on the primary electrode.
Empty space inside the primary
electrode and the secondary electrode
is filled with CRYSTALLINE
CONDUCTIVE MIXTURE and then
sealed.

166
 Empty space inside the primary and the
secondary electrode is filled with
Conductor rich crystalline mixture which
contains metal alloys and natural
compounds which are :
 High conductive, Anticorrosive
 Does not disintegrate or collapse when
Outer electrode becomes inactive .

u. 167
• Back Fill Compound:-
• Empty space around the electrode is filled with a compound
which Contains eco-friendly materials.
• Maintains moisture and enhances conductivity around the
electrode.
• Does not mix with or leach in to the soil.
• Absorbs moisture 13 times its dry volume.
• No need to recharge pit. Except in sandy areas.
• Improves electrode performance and protects the system in
corrosive environment.
•

MAINTENANCE FREE EARTHING (cont’d)
u. 168
INSTALLATION OF MAINTENANCE FREE
EARTHING SYSTEM
Drill a 10 inch or 8 inch dia pit 2 m or 3 m deep
to install electrode of required length.
 Fill the space between soil and electrode with
specially developed BACKFILL COMPOUND
mixed with dug out soil in small quantities along
with water up to the neck of electrode.
 After installation pour a few buckets of water in
and around electrode for few days for the entire
system to set.
 About 4 electrodes can be installed in a day in
normal soil conditions.

MAINTENANCE FREE EARTHING (cont’d)
169
 When installing electrodes, there are three
conditions that must be satisfied: -
 The work must be carried out efficiently to
minimise installation costs,
 The backfill used must not have a pH value
which will cause corrosion to the electrode;
and Any joints or
 The connectors used below ground level
must be so constructed that corrosion of the
joint/connector will not take place.

Earthing.ppt

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