earthing systems junior engineers.pptx
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EARTHING SYSTEMS
ROHIDAS MASKE
Chief Engineer, MSETCL, Aurangabad
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What is Earthing/Grounding?
The term Earthing and grounding has same
meaning, which means an electrical
connection with a conductor of negligible
resistance to the general mass of earth to
provide safe passage to fault current to
enable the protective devices to operate and
provide safety to personnel and Equipments
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WHY EARTHING IS REQUIRED?
Safety to the humans & animals shocks & fire.
Protection of buildings, installations & power
system equipments against lightning.
Surge protection, (lightning/ switching)
Interference in Communication circuit.
System security / voltage stabilization.Correct operation of electricity supply network
and to ensure good power quality
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HUMAN HEART
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Effects of electric current with increasing magnitude
Dalziels classical experiment: (28-Womens,134-mens)
1mA Perception, slight tingling effect.10.5mA (women) Let go current, unpleasant to16 mA (men) sustain, releases the object away.
up to 25 mA Painful, impossible to releases theobject.
For higher currents Muscular contraction can makebreathing difficult, unconsciousness
such cases can respond toresuscitation
60-100 mA Ventricular fibrillation, stoppage ofheart, respiration stoppage mightoccur & causes injury or death.
More than 100 mA Burning29-Oct-13
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The effect of electric current passing through vital organsof the body depends on:-
magnitude, duration and
frequency of current.
The most dangerous consequence is a heart condition known as
ventricular fibrillation, which results in stoppage of blood
circulation.
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Consider a Tower having a DC
resistance of 5 and inductance of
50H subject to a typical lightning with
peak current and rise time of 50
kA/10S for 1st pulse and 25 kA / 0.5S
for repeat pulse (as is the case with
75% of lightning condition)
VOLTAGE RISE ON TOWERS
The Voltage rise is sum of Resistive and Inductive
term giving a peak voltage rise of 500 kV
For the repeat pulse, the results are even more
dramatic with a total voltage rise of being over 2600
kV, 2500 kV of which is due to inductive term alone.
Thus it is very important to measure impedance of the
tower rather than measuring resistance alone
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Normal Pipe type Earthing for trans. line
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Counter-poise Earthing
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Components of earthing system in Sub-stn.
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Earthing of LA
Straight risers
Separate earth rod per phase
Rods connected to each
other & to Earth-grid.
No G.I. pipe for riser.
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Touch potential:-
The potential difference between a point on the
ground and a point on an object likely to carry fault
current (e.g., frame of equipment) which can be
touched by a person29-Oct-13
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Step potential:
The potential difference shunted by a human bodybetween two accessible points on the ground
separated by a distance of one pace assumed to be
equal to one meter
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Step & Touch Voltages
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Transferred Potential
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MEASUREMENT OF EARTH
RESISTANCE AND GROUND
RESISTIVITY
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Objectives of Tests
Measurements of ground resistance or impedance andpotential gradients on the surface of the earth due toground currents are necessary to:
1) Verify the adequacy of a new grounding system
2) Detect changes in an existing grounding system 3) Determine hazardous step and touch voltages
4) Determine ground potential rise (GPR) in order todesign protection for power and communication circuits.
Scale-model tests are useful in studying or developing newdesigns for grounding systems which cannot be adequatelystudied by analytical methods (complex shape or complexsoil structure).
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Why Earth resistivity measurements ?
1) Estimating the ground resistance of aproposed substation or transmission tower
2) Estimating potential gradients including
step and touch voltages 3) Computing the inductive coupling
between neighboring power and
communication circuits 4) Designing cathodic protection systems
5) Geological surveys
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Methods of Measuring Earth Resistivity
TWO POINT METOD
THREE POINT METHOD
FOUR POINT METHOD
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Earth Resistivity Variations
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Earth Resistivity Variations
(a) Salt (b) Moisture (c) Temperature
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STATUTARY PROVISIONS
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Why 1m x 1m x 3m Earth pit
What should be the distance between two
electrodes.
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Any dam QUESTION PLEASE
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THANK U
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