51437246 Earthing System

7/27/2019 51437246 Earthing System

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SEMINAR PRESENTATION

ON

EARTHING SYSTEMS

Presented By:

SANDEEP KUMAR PARIDA

REGD NO:0501227577ELECTRICAL ENGG.

C.V.RAMAN COLLEGE OF ENGINEERING


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INTRODUCTION

EARTHING

The term Earthing means connecting the non current carrying parts of

the electrical equipment or the neutral point of the supply system to thegeneral mass of earth in such a manner that at all times an immediate

discharge of electrical energy takes place without danger.


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PURPOSE OF EARTH ING

The main objectives of the earthing are to :

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 .


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COMPONENTS TO BE EARTHED

In electrical installations the following components must be

earthed:

The frames, tanks and enclosures of electric machines, transformers,

lighting fittings and other items of equipment. The frame work of the switch boards, control boards, individual panel

boards.

The operating mechanism of the switch gear.

The structural steel work of indoor and outdoor substations, metal cable

joining boxes and metal sheaths of the cable.

The metal casing of all portable apparatus.


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TYPES OF EARTHING

System Earthing:It deals with earthing of the neutral to ensuresystem security and protection.

Equipment Earthing:It deals with earthing of non currentcarrying parts of the equipment to ensure safety of personnel.


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SOIL RESISTIVI TY

The chief requirement of goodearthing is low soil resistivity.

Soil resistivity varies greatly fromone location to another.

Some typical values are shown

in the following table.

Type

Sea water

Tap water

Clay

Sand claymixture

SandWet concrete

Dry concrete

Rock

Resistivity in -m

2.5

20

50

100

2000100

10,000

10,000


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PRINCIPAL FACTORS AFFECTING SOIL

RESISTIVITY

Type of soil :The soil composition can be clay, gravel, loam, rock,sand, stones, etc. Very often, the soil composition is in layers or strata,

and it is the resistance of the varying strata, especially at sub-soil level

where the moisture content is not subject to drying out, that is important

in securing a good electrical path. Climate:Arid and good rainfall climates are at opposite extremes for

conditions of soil resistivity.

Seasonal conditions:The effects of heat, moisture, drought andfrost can introduce wide variations in normal soil resistivity. Soil

resistivity usually decreases with depth and an increase in moisturecontent in a normally dry soil will decrease soil resistivity. Conversely

soil temp below freezing greatly increases soil resistivity.


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MEASURING SOI L RESISTIVI TY

Resistivity of the soil is usually measured by the four-spike method, in which fourspikes arranged in a straight line are driven into the soil at equal distances. Acurrent is passed between electrodes C1 and C2 and potential drop V is measuredacross electrodes P1 and P2.The current I develops an electric field which isproportion four terminal instrument is required for soil resistivity. The probes areinstalled in a straight line with an equal spacing of a metres and inserted to adepth of no more than a/20 metres, i.e.. for spacing of 2 metres, depth must be lessthan 100mm. Now keeping the centre position the same, resistance measurementsare taken at increasing spacing .Always ensure that the spacing between individualtest probes is identical .The soil resistivity can be obtained from the followingformula,

r = 2aR (Ohm metres)

where r = apparent soil resistivitya = spacing of probes in metres

R = resistance value in Ohms


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MEASURING SOI L RESISTIVI TY


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THE EARTH PATH

The resistance of the earth path is determined

By the resistance of the soil surrounding the earth rod,

By its contact resistance between the earth rod and the surrounding soil,

By the resistance of the earth rod and connecting conductors.

When an electrical current passes from a buried earth rod, it passes from alow resistance metal into an immediate area of high resistance soil. Theareas of resistance can be described as being that of a no. of sheaths ofever increasing diameter. The current path passes into the 1st sheathadjacent to the earth rod and then into the 2nd sheath which is of a largercross section with a greater area for current flow and, therefore, of lowerresistance than the 1st sheath, and so on into a succession of sheaths ofincreasing area and, because of this, of ever decreasing resistance. It is thisresistance at the interface where the current leaves the earth rod and flowsinto the main body of the earth.


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FACTORS INFLUENCING EARTH

RESISTANCE

1. Condition of soil.

2. Temperature of soil.

3. Moisture content of soil.

4. Size and spacing of earth electrode.

5. Depth at which the electrode is embedded.

6. Material of conductor.


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MEASUREMENT OF EARTH RESISTANCE

The resistance area of an earth electrode is the area

of soil around the electrode within which a voltage

gradient measurable with commercial instruments

exists.

This measurement is made by the

potential fall method.

E is the earth electrode under test. A is an auxiliary

earth electrode. B is a second auxiliary electrode. An a.c. of steady value is passed through the earth

path from E to A and the voltage drop between E and B

is measured.


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METHODS OF REDUCING EARTH

RESISTANCE OF THE SYSTEM

The earth resistance can be reduced by digging the earth electrode to a

depth of 1.5 or 2m,cleaning the surface of the earth plate of all rust and

then filling with charcoal soaked in salt solution. Pouring of fresh salt water

through pipe over all the coal bed will reduce the earth resistance. The earth resistance can also be reduced by :

increasing the plate area,

increasing of pit depth,

increasing number of electrodes in parallel.


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SIZE OF THE EARTHING CONDUCTOR

Minimum cross section of the earthing conductor which should have

thermal stability can be calculated from the relation,

A (min) =[ I *(t)^ ]/c

Where A (min) = minimum required section of the conductor in mm2I = fault current in ampere

t = time in sec

c = is a CONSTANT

For steel conductor at 400c,it is 70mm2

For copper conductor at 300c,it is 166mm2


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METHODS OF EARTHING

1. Strip or Wir e Earthing:Thistype of earthing is used at places whichhave rocky soil earth bed.

2. Rod Earthing:This system of earthing is suitable for areas which aresandy in character.

3. Pipe Earthing:It is the best form of earthing and is very cheap in cost.


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EARTHING MAT

A number of rods when joined together through copper conductors

constitute an Eathing Mat.

It reduces overall grounding resistance.

Such a mesh of conductors laid near the grounding surface helps in limitingthe potential gradient.

In a substation, it is bonded to the local supporting metal structure and to

the handle of the switchgear, so that the operator will not be exposed to a

high differential voltage due to a fault in the substation.

In systems where large fault currents are to be experienced, it is preferableto take care of the potential gradients by laying the earthing mat.


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CONCLUSION

In an electrical installation, if a metallic part of an electric

appliance comes in direct contact with a live wire, the metal being a

good conductor of electricity is charged. If any person comes in contactwith this charged metallic part, he will get a severe shock. But if the

metallic parts of the appliances are earthed, the charge will be transferred

to the earth. As discharge takes place to earth, the impedance of path of

the current is low, a large amount of current flows to earth, the current

exceeds the limiting value, the fuse provided in the circuit will blow offand cut off the appliance from supply. Thus, earthing of metallic parts of

electrical equipments and appliances provide safety.


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THANK YOU! ! !

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51437246 Earthing System