Transcript Earthing

Earthing
PRESENTATION BY:
Er. Satnam Singh
Lecturer(Electrical engg.)
GPC Mohali (Khunimajra)
Concept of Earthing Systems
All the people living or working in residential,
commercial and industrial installations, particularly
the operators and personnel who are in close
operation and contact with electrical systems and
machineries, should essentially be protected against
possible electrification. To achieve this protection,
earthing system of an installation is defined,
designed and installed according to the standard
requirements..
What Is Earthing
The process of connecting metallic bodies of
all the electrical apparatus and equipment to
huge mass of earth by a wire having negligible
resistance is called Earthing.
What Is Earthing
The term earthing means connecting the neutral point of
supply system or the non current carrying parts of the
electrical apparatus to the general mass of earth in such a
manner that all times an immediate discharge of electrical
energy takes place without danger.
• Provide an alternative path for the fault current
to flow so that it will not endanger the user
• Ensure that all exposed conductive parts do not
reach a dangerous potential
• Maintain the voltage at any part of an electrical
system at a known value so as to prevent over
current or excessive voltage on the appliances
or equipment.
Good Earthing Means
Good Earthing must have low impedance
enough to ensure that sufficient current can
flow through the safety device so that it
disconnects the supply ( <0.4 sec ). Fault
current is much more than the full load current
of the circuit which melts the fuse. Hence, the
appliance is disconnected automatically from
the supply mains.
Qualities Of Good Earthing
• Must be of low electrical resistance
• Must be of good corrosion resistance
• Must be able to dissipate high fault current
repeatedly
Purpose of Earthing
• To save human life from danger of electrical shock or death by
blowing a fuse i.e. To provide an alternative path for the fault
current to flow so that it will not endanger the user
• To protect buildings, machinery & appliances under fault
conditions ie. To ensure that all exposed conductive parts do
not reach a dangerous potential.
• To provide safe path to dissipate lightning and short circuit
currents.
• To provide stable platform for operation of
sensitive electronic equipments i.e. To maintain the voltage
at any part of an electrical system at a known value so as to
prevent over current or excessive voltage on the appliances or
equipment .
• To provide protection against static electricity from friction
Electric shock
• An electric shock (electrocution)occurs when two portion of a
person’s body come in contact with electrical conductors of a
circuit which is at different potentials, thus producing a
potential difference across the body.
• The human body does have resistance and when the body is
connected between two conductors at different potential a
circuit is formed through the body and current will flow
• When the human body comes in contact with only one
conductor, a circuit is not formed and nothing happens. When
the human body comes in contact with circuit conductors, no
matter what the voltage is there is potential for harm.
Electric shock
• The higher the potential difference the more
the damage. The effect of an electric shock is a
function of what parts of body come in contact
with each conductor, the resistance of each
contact point the surface resistance of the body
at the contact as well as other factor.
• When the electrical contact is such that the
circuit path through the body is across the
heart, you have the greatest potential for death.
Electric shock
• As shown in fig the human body’s resistance varies
from as low as 500 ohms to as high as 600,000 ohms.
As the skin become moist the contact resistance drop.
If the skin is moist due to sweat that contain salt the
resistance drop further
• Fig 1 illustrates the amount of current that can flow
through human body at three different potential
differences across the body also shown is the effect of
different current level both AC and DC the ultimate
effect is fibrillation which cause the heart to stop and
result in death.
Electric shock
• When a high voltage such as 13,800V is
involved the body is literally cooked and at
times explodes
Short circuit
• To analyze how an electrical shock occurs and how
grounding is applied you need to look at the circuit
involved.
• Fig 2 illustrates the basic circuit that consist of a
source, a transformer or generator for all AC circuits,
circuit protection, conductors(R1s), and a load (RL).
Short circuit
• A short circuit is any unintended connection Rsc across the
circuit conductors between the power source and the load
• See the second circuit in fig short circuit are classified as
bolted shorts, momentary shorts,intermittent shorts, or high
impedance shorts. A bolted short which is rare is a very low
resistant connection such as two conductors being bolted
together.
Short circuit
• Most shorts are high resistant or they are
momentary or intermittent. The high resistant
short starts out as a high resistance or
impedance connection but usually progress to
a low impedance connection.
• In electrical system shorts are classified as
phase to phase, phase to neutral , or phase to
ground short.
Short circuit
• Most short are phase to ground and short which start as phase
to phase or phase to neutral progress to a phase to ground short
• When considering short circuit and protecting against the
damage they can cause one needs to know what is the
maximum amount of short circuit current that can flow in a
given circuit.
Equipment Earthing
• In case of insulation failure, the primary object
of connecting all the above points and
apparatus to earth is to release the charge
accumulated on them immediately to earth so
that the person coming in contact may not
experience electric shock.
Equipment Earthing(Cont.)
The other object is that a heavy current when
flows through the circuit that operates the
protective devices that is fuse or CB, which
open the circuit
Max. Value of Earth Resistance to
be achieved
Equipment to be Earthed
Max. Value of Earth Resistance to be
achieved in Ohms
Large Power Stations
0.5
Major Substations
1.0
Small Substations
2.0
Factories Substations
1.0
Lattice Steel Tower
3.0
Industrial Machine and Equipment
0.5
* The Earth Resistance depends upon the moisture content in the soil.
Methods of Earthing
• Conventional Earthing
• Maintenance Free Earthing
Conventional Earthing
• The Conventional system of Earthing calls for
digging of a large pit into which a GI pipe or a
copper plate is positioned in the middle layers
of charcoal and salt.
• It requires maintenance and pouring of water
at regular interval.
FIGURE:.
CONVENTIONAL EARTHING
Maintenance Free Earthing
• It is a new type of earthing system which is
Readymade, standardized and scientifically developed.
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.
Maintenance Free Earthing(Contd.)
• LOW EARTH RESISTANCE: Highly
conductive. Carries high peak current
repeatedly.
• NO CORROSION:
• LONG LIFE.
• EASY INSTALLATION.
Methods of Conventional Earthing
1.
2.
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5.
Plate Earthing
Pipe Earthing
Rod Earthing
Strip Earthing
Earthing through Water Mains
Earthing Electrode
The resistance of a ground
electrode has 3 basic
components:
A) The resistance of the ground
electrode itself and the connections
to the electrode.
B) The contact resistance of the
surrounding earth to the electrode.
C) The resistance of the surrounding
body of earth around the ground
Electrode. It consist of three basic components:
1. Earth Wire
2. Connector
3. Electrode
Plate Earthing
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In this type of earthing plate either of copper or of
G.I. is buried into the ground at a depth of not less
than 3 meter from the ground level.
The earth plate is embedded in alternative layer of
coke and salts for a minimum thickness of about
15cm.
The earth wire(copper wire for copper plate
earthing and G.I. wire for G.I. plate earthing) is
securely bolted to an earth plate with the help of
bolt nut and washer made of copper, in case of
copper plate earthing and of G.I. in case of G.I.
plate earthing.
PLATE EARTHING
Pipe earthing
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Pipe earthing is best form of earthing and it is cheap also in this system
of earthing a GI pipe of 38 mm dia and 2meters length is embedded
vertically in ground to work as earth electrod but the depth depend upon
the soil conditions, there is no hard and fast rule for this.
But the wire is embedded upto the wet soil.
The earth wire are fastened to the top section of the pipe with nut and
bolts.
The pit area arround the GI pipe filled with salt and coal mixture for
improving the soil conditions and efficiency of the earthing system.
It can take heavy leakage current for the same electrode size in
comparison to plate earthing.
The earth wire connection with GI pipes being above the ground level
can be checked for carrying out continuity test as and when desired,
while in plate earthing it is difficult.
In summmer season to have an effective earthing three or four bucket of
water is put through the funnel for better continuity of earthing.
PIPE EARTHING
ROD EARTHING
• In this system of earthing 12.5mm diameter solid rods of
copper 16mm diameter solid rod of GI or steel or hollow
section of 25mm GI pipe of length not less than 3 meters are
driven vertically into the earth
• In order to increase the embeded length of electrod under the
ground, which is some time necessary to reduce the earth
resistance to desired value more than one rod section are
hammered one above the other.
• This system of earthing is suitable for area which are sandy in
character .
• This system of earthing is very cheap
STRIP OR WIRE EARTHING
• In this system of earthing strip electrod of cross section not less
than 25mm into 1.6mm of copper or 25mm * 4mm of GI or steel
are burried in horizontal trenches of minimum depth of 0.5m
• If round conductor are used their cross sectional area shall not be
smaller than three if copper is used and 6mm2 if GI or steel is
used.
• The length of burried conductor shall be sufficient to give the
required earth resistance (about 0.5Ωto 1.5Ω)
• It shall however be not less than 15 m
• The electrod shall be as widely distributed as possible in a single
straight or circular trenches radiating from a point
• This type of earthing is used in rockey soil earth bed because at
such places excavation work for plate earthing is difficult
Procedure for filling up New
Earthing Pit
• Step A :
• Excavate the earthing pit size 2000 X 2000 X
2500 mm depth. Sprinkle sufficient quantity of
water in the bottom and surrounding walls to
become wetty only.
• Fill up the bottom layer of the pit up to 250 mm
height from the bottom by mixture black soft
soil + salt + wooden charcoal pieces. (Fig. I)
• Step B :
• Prepare the electrode assembly as per Sr. No. - 3
of the drawing and rest the entire Assembly in
the pit as shown in (Fig. II)
Procedure for filling up New
Earthing Pit
Step C :
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• Collect thin C.R.C. sheet approx 18 to 20 SWG having size
500 mm width 3.5 meter length approx. (Please make joints of
three to four pieces to achieve requirement of 3.5 meter length
(Fig. III-a)
• Prepare the Cylindrical Ring from the above sheet by bending
both ends & joining each other. The diameter "D" of the
cylindrical ring shall arrive approx. 1000 mm and height shall
be 500 mm. Collect two pieces of scrap G.I. wire of approx. 8SWG and prepare two lifting round handles (Hooks) on upper
side of the cylindrical ring to facilitate the lifting of the
Cylindrical Ring. (Fig. III-b)
• Now wear this cylindrical ring to the electrode pipe of the
electrode assembly such a way that the electrode pipe remains
in the centre of the cylindrical ring. (Fig III-c)
Procedure for filling up New
Earthing
Pit
Step D :
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• Fill up the inner part of the Cylindrical Ring with
Mixture - I ( Homogeneous mixture of Black Soft
Soil.
• The remaining part i.e. the Gap between walls of the
pit and outer periphery of the Cylindrical Ring by
Mixture - II. After completing filling work of both the
mixtures up to 500 mm height, proper ramming and
watering is to be done. (Fig. IV)
Procedure for filling up New
Earthing Pit
• Step E :
• There after lift the Cylindrical ring
by help of two lifting handles
(hooks) and again rest it on the
layer for carrying out filling of 2nd
layer cycle. Again fill up the inner
cylindrical part of the ring by
Mixture - I and outer gap between
walls of the pit & outer Cylindrical
periphery by Mixture - II up to
height of the Cylinder (i.e. 500
mm) (Fig. V)
Procedure for filling up New
Earthing
Pit
Step E :
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• There after lift the Cylindrical ring by help of two
lifting handles (hooks) and again rest it on the layer
for carrying out filling of 2nd layer cycle. Again fill
up the inner cylindrical part of the ring by Mixture - I
and outer gap between walls of the pit & outer
Cylindrical periphery by Mixture - II up to height of
the Cylinder (i.e. 500 mm) (Fig. V)
Procedure for filling up New
Earthing
Pit
Step F :
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• Lift the cylindrical ring by lifting handles (hooks) after proper
ramming and watering. Now again place the cylindrical ring
on upper layer and arrange 3rd cycle, subsequently complete
the filling of entire pit. Please see that water content is
minimum 20 %
• Fill up upper layer of the pit by crushed rock pieces (Gravel)
size 50 X 35 mm. 1 CMT. approx. to provide insulating layer
to person moving side by the pit, and to prevent reptile
movements subsequently causing hazards.
Standard Pipe & Plate Type Earthing Design for the
11 Kv. System Equipments, Distribution Transformer
Centers, L.T. Distribution System Equipments
• Design Details :
1. Earthing Pit : Size 1000 X 1000 X 1800 mm
Depth.M.S. / C.I. Plate : 500 X 500 X 8 mm
Thick.
2. Electrode Assembly : 40 mm Ø GI / CI
Perforated pipe duly fitted or welded with base
plate and 50 X 6 mm flat termination taken on
top for equipment earthing as shown in drawing.
Standard Pipe & Plate Type Earthing Design for the 11
Kv. System Equipments, Distribution Transformer
Centers, L.T. Distribution System Equipments
4. Mixture - I : Homogeneous mixture of black
soft soil 0.3 CMT. approx.
5. Mixture - II : Homogeneous mixture of
common salt 25 Kgs. + wood charcoal pieces
25 Kgs. + Black soft soil 1 CMT. Approx.
6. Crushed Rock pieces Gravel Size 50 X 35 mm
0.1 CMT. Approx.
7. Arrangement for earthing lead terminations
from equipment body, and connection for
main earthing Grid.
Typical arrangement for Pipe
electrode earthing pit (Bore Type)
• Design Details :
1. 75 mm thick RCC Cover.
2. 300 mm Ø 6000 mm deep (Approx. 20 ft.)
bore in the earth.
3. 65 mm Ø 6000 mm long (Approx 20 ft.) G.I.
pipe electrode. Forged at the top up to 75 mm
length and 12 mm hole provided for taking
earthing connection.
4. A homogeneous mixture of 50 kgs. wooden
coal pieces + 50 kgs. common salt
5. Water pouring purpose at the time of routine
maintenance
Applications
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Telecommunication
Transmission
Substations & Power Generations
Transformer Neutral earthing
Lightning Arrestor Earthing
Equipment Body Earthing
Water Treatment Plants
Heavy Industries
College, Hospitals, Banks
Residential Building