EARTHING SYSTEMS JUNIOR ENGINEERS.pptx

7/27/2019 EARTHING SYSTEMS JUNIOR ENGINEERS.pptx

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EARTHING SYSTEMS

ROHIDAS MASKE

Chief Engineer, MSETCL, Aurangabad

29-Oct-13


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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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EARTHING SYSTEMS JUNIOR ENGINEERS.pptx