WHAT IS EARTHING?
The main reason for doing earthing in
electrical network is for the safety. When all metallic parts in electrical
equipments are grounded then if the insulation inside the equipments fails
there are no dangerous voltages present in the equipment case. If the live wire
touches the grounded case then the circuit is effectively shorted and fuse will
immediately blow. When the fuse is blown then the dangerous voltages are away.
PURPOSE OF EARTHING
SAFETY FOR HUMAN/EQUIPMENTS/BUILDINGS:-
·
To save human life from danger of electrical shock or death
by blowing a fuse i.e.
·
To protect buildings, machinery & appliances under fault
conditions
·
To provide an alternative
path for the fault current to flow so that it will not endanger the user
·
To ensure that all exposed conductive parts do not reach a
dangerous potential.
·
To provide stable platform for operation of
sensitive electronic equipments i.e.
·
To provide safe path to dissipate lightning and short circuit
currents.
·
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.
VOLTAGE STABILIZATION:-
There are many sources of electricity. Every
transformer can be considered a separate source. If there were not a common
reference point for all these voltage sources it would be extremely difficult
to calculate their relationships to each other. The earth is the most
omnipresent conductive surface, and so it was adopted in the very beginnings of
electrical distribution systems as a nearly universal standard for all electric
systems.
OVER VOLTAGE PROTECTION:-
Lightning, line surges or unintentional
contact with higher voltage lines can cause dangerously high voltages to the
electrical distribution system. Earthing provides an alternative path around
the electrical system to minimize damages in the System.
DIFFERENT METHOD OF EARTHING
PLATE TYPE EARTHING:-
·
Generally for plate type earthing normal Practice is to use
·
Cast iron plate of size 600 mm x600 mm x12 mm. OR
·
Galvanized iron plate of size 600 mm x600 mm x6 mm. OR
·
Copper plate of size 600 mm * 600 mm * 3.15 mm
·
Plate burred at the depth of 8 feet in the vertical position
and GI strip of size 50 mmx6 mm bolted with the plate is brought up to the
ground level.
·
These types of earth pit are generally filled with alternate layer
of charcoal & salt up to 4 feet from the bottom of the pit.
PIPE TYPE EARTHING:-
·
For Pipe type earthing normal practice is to use
·
GI pipe [C-class] of 75 mm diameter, 10 feet long welded with 75
mm diameter GI flange having 6 numbers of holes for the connection of earth
wires and inserted in ground by auger method.
·
These types of earth pit are generally filled with alternate layer
of charcoal & salt or earth reactivation compound.
DESCRIBE THE METHOD OF CONSTRUCTION OF EARTH PIT
·
Excavation on earth for a normal earth Pit size is 1.5M X 1.5M X
3.0 M.
·
Use 500 mm X 500 mm X 10 mm GI Plate or Bigger Size for more
Contact of Earth and reduce Earth Resistance.
·
Make a mixture of Wood Coal Powder Salt & Sand all in
equal part
·
Wood Coal Powder use as good conductor of electricity, anti
corrosive, rust proves for GI Plate for long life.
·
The purpose of coal and salt is to keep wet the soil permanently.
·
The salt percolates and coal absorbs water keeping the soil wet.
·
Care should always be taken by watering the earth pits in summer
so that the pit soil will be wet.
·
Coal is made of carbon which is good conductor minimizing the
earth resistant.
·
Salt use as electrolyte to form conductivity between GI Plate Coal
and Earth with humidity.
·
Sand has used to form porosity to cycle water & humidity
around the mixture.
·
Put GI Plate (EARTH PLATE) of size 500 mm X 500 mm X 10 mm in the
mid of mixture.
·
Use Double GI Strip size 30 mm X 10 mm to connect GI Plate to System
Earthling.
·
It will be better to use GI Pipe of size 2.5″ diameter with a Flange on the top of GI Pipe
to cover GI Strip from EARTH PLATE to Top Flange.
·
Cover Top of GI pipe with a T joint to avoid jamming of pipe with
dust & mud and also use water time to time through this pipe to bottom of
earth plate.
·
Maintain less than one Ohm Resistance from EARTH PIT conductor to
a distance of 15 Meters around the EARTH PIT with another conductor dip on the
Earth at least 500 mm deep.
·
Check Voltage between Earth Pit conductors to Neutral of Mains
Supply 220V AC 50 Hz it should be less than 2.0 Volts.
THE FOLLOWING FACTORS ARE EFFECTING THE EARTH PIT
SOIL RESISTIVITY:-
·
It is the resistance of soil to the passage of electric current.
The earth resistance value (ohmic value) of an earth pit depends on soil
resistivity. It is the resistance of the soil to the passage of electric
current.
·
It varies from soil to soil. It depends on the physical
composition of the soil, moisture, dissolved salts, grain size and
distribution, seasonal variation, current magnitude etc.
·
In depends on the composition of soil, Moisture content, Dissolved
salts, grain size and its distribution, seasonal variation, current magnitude.
MOISTURE:-
·
Moisture has a great influence on resistivity value of soil. The
resistivity of a soil can be determined by the quantity of water held by the
soil and resistivity of the water itself. Conduction of electricity in soil is
through water.
·
The resistance drops quickly to a more or less steady minimum
value of about 15% moisture. And further increase of moisture level in soil
will have little effect on soil resistivity. In many locations water table goes
down in dry weather conditions. Therefore, it is essential to pour water in and
around the earth pit to maintain moisture in dry weather conditions. Moisture
significantly influences soil resistivity
SOIL CONDITION:-
·
Different soil conditions give different soil resistivity. Most of
the soils are very poor conductors of electricity when they are completely dry.
Soil resistivity is measured in ohm-meters or ohm-cm.
·
Soil plays a significant role in determining the performance of
Electrode.
·
Soil with low resistivity is highly corrosive. If soil is dry then
soil resistivity value will be very high.
·
If soil resistivity is high, earth resistance of electrode will
also be high.
DISSOLVED SALT:-
·
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.
PHYSICAL COMPOSITION:-
·
Different soil composition gives different average resistivity.
Based on the type of soil, the resistivity of clay soil may be in the range of
4 – 150 ohm-meter, whereas for rocky or gravel soils, the same may be well
above 1000 ohm-meter.
WEATHER CONDITION:-
·
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.
LOCATION OF EARTH PIT:-
·
The location also contributes to resistivity to a great extent. In
a sloping landscape, or in a land with made up of soil, or areas which are
hilly, rocky or sandy, water runs off and in dry weather conditions water table
goes down very fast. In such situation Back fill Compound will not be able to
attract moisture, as the soil around the pit would be dry. The earth pits
located in such areas must be watered at frequent intervals, particularly
during dry weather conditions.
·
Though back fill compound retains moisture under normal
conditions, it gives off moisture during dry weather to the dry soil around the
electrode, and in the process loses moisture over a period of time. Therefore,
choose a site that is naturally not well drained.
EFFECT OF
CURRENT MAGNITUDE:-
·
Soil resistivity in the vicinity of ground electrode may be
affected by current flowing from the electrode into the surrounding soil.
·
The thermal characteristics and the moisture content of the soil
will determine if a current of a given magnitude and duration will cause
significant drying and thus increase the effect of soil resistivity
EFFECT OF GRAIN SIZE AND DISTRIBUTION:-
·
Grain size, its distribution and closeness of packing are also
contributory factors, since they control the manner in which the moisture is
held in the soil.
·
Effect of seasonal variation on soil resistivity: Increase or
decrease of moisture content in soil determines decrease or increase of soil
resistivity. Thus in dry weather resistivity will be very high and during rainy
season the resistivity will be low.
OBSTRUCTIONS:-
·
The soil may look good on the surface but there may be
obstructions below a few feet like virgin rock. In that event resistivity will
be affected. Obstructions like concrete structure near about the pits will
affect resistivity. If the earth pits are close by, the resistance value will
be high.
AREA AVAILABLE:-
·
Single electrode rod or strip or plate will not achieve the
desired resistance alone.
·
If a number of electrodes could be installed and
interconnected the desired resistance could be achieved. The distance between
the electrodes must be equal to the driven depth to avoid overlapping of area
of influence. Each electrode, therefore, must be outside the resistance area of
the other.
CURRENT MAGNITUDE:-
·
A current of significant magnitude and duration will cause
significant drying condition in soil and thus increase the soil resistivity.
MEASUREMENT
OF EARTH RESISTANCE BY USING EARTH
TESTER OR EARTH MEGGER:-
·
For measuring soil resistivity Earth Tester is used. It is also
called the “MEGGER”.
·
It has a voltage source, a meter to measure Resistance in ohms,
switches to change instrument range, Wires to connect terminal to Earth
Electrode and Spikes.
·
It is measured by using Four Terminal Earth Tester Instrument. The
terminals are connected by wires as in illustration.
·
P=Potential Spike and C=Current Spike. The distance between the
spikes may be 1M, 2M, 5M, 10M, 35M, and 50M.
·
All spikes are equidistant and in straight line to maintain
electrical continuity. Take measurement in different directions.
·
Soil
resistivity =2πLR.
·
R= Value of Earth resistance in ohm.
·
Distance between the spikes in cm.
·
π = 3.14
·
P = Earth resistivity ohm-cm.
·
Earth resistance value is directly proportional to Soil
resistivity value
THREE
POINT METHOD OF MEASURING EARTH RESISTANCE:-
·
In this method earth tester terminal C1 & P1 are shorted to
each other and connected to the earth electrode (pipe) under test.
·
Terminals P2 & C2 are connected to the two separate spikes
driven in earth. These two spikes are kept in same line at the distance
of 25 meters and 50 meters due to which there will not be mutual interference
in the field of individual spikes.
·
If we rotate generator handle with specific speed we get directly
earth resistance on scale.
·
Spike length in the earth should not be more than 1/20th distance
between two spikes.
·
Resistance must be verified by increasing or decreasing the
distance between the tester electrode and the spikes by 5 meter. Normally, the
length of wires should be 10 and 15 Meter or in proportion of 62% of ‘D’.
·
Suppose, the distance of Current Spike from Earth Electrode D = 60
ft, Then, distance of Potential Spike would be 62 % of D = 0.62D i.e.
0.62 x 60 ft = 37 ft.
FOUR
POINT METHOD OF MEASURING EARTH RESISTANCE:-
·
In this method 4 spikes are driven in earth in same line at the
equal distance. Outer two spikes are connected to C1 & C2 terminals
of earth tester. Similarly inner two spikes are connected to P1 & P2
terminals. Now if we rotate generator handle with specific speed, we get
earth resistance value of that place.
·
In this method error due to polarization effect is eliminated and
earth tester can be operated directly on A.C.
PIPE EARTHING VS PLATE EARTHING:-
·
Suppose Copper Plate having of size 1.2m x 1.2m x 3.15mm thick.
soil resistivity of 100 ohm-m,
·
The resistance of Plate electrode to earth (R)=( r/A)X under
root(π/A) = (100/2.88)X(3.14/2.88)=36.27
ohm
·
Now, consider a GI Pipe Electrode of 50 mm Diameter and 3 m Long.
soil resistivity of 100 Ohm-m,
·
The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge
(4L/d) = (100X100/2X3.14X300) X loge (4X300/5) =29.09 Ohm.
·
From the above calculation the GI Pipe electrode offers a much
lesser resistance than even a copper plate electrode.
·
As
per IS 3043 Pipe, rod or strip has a much lower resistance than a plate of
equal surface area.
GI EARTHING VS COPPER
EARTHING:-
·
As per IS 3043, the resistance of Plate electrode to earth (R) = (r/A) X under root(P/A).
·
Where r = Resistivity of Soil Ohm-meter.
·
A=Area of Earthing Plate m3.
·
The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge
(4L/d).
·
Where L= Length of Pipe/Rod in cm
·
d=Diameter of Pipe/Rod in cm.
·
The resistivity of the soil and the physical dimensions of the
electrode play important role of resistance of Rod with earth.
·
The material resistivity is not considered important role in earth
resistivity.
·
Any material of given dimensions would offer the same resistance
to earth. Except the sizing and number of the earthing conductor or the
protective conductor.
LENGTH OF PIPE ELECTRODE AND EARTH PIT:-
·
The resistance to earth of a pipe or plate electrode
reduces rapidly within the first few feet from ground (mostly 2 to 3 meter) but
after that soil resistivity is mostly uniform.
·
After about 4 meter depth, there is no appreciable change in
resistance to earth of the electrode. Except a number of rods in parallel are
to be preferred to a single long rod.
AMOUNT OF SALT AND CHARCOAL (MORE THAN 8KG):-
·
To reduce soil
resistivity, it is necessary to dissolve in the moisture particle in the Soil.
·
Some substance like
Salt/Charcoal is highly conductive in water solution but the additive substance
would reduce the resistivity of the soil, only when it is dissolved in the
moisture in the soil after that additional quantity does not serve the Purpose.
·
5% moisture in Salt
reduces earth resistivity rapidly and further increase in salt content will
give a very little decrease in soil resistivity.
·
The salt content is
expressed in percent by weight of the moisture content in the soil. Considering
1M3 of Soil, the moisture content at 10 percent will be about 144 kg. (10
percent of 1440 kg). The salt content shall be 5% of this (i.e.) 5% of 144kg,
that is, about 7.2kg.
AMOUNT OF WATER POURING:-
·
Moisture content is
one of the controlling factors of earth resistivity.
·
Above 20 % of moisture content, the resistivity is very little
affected. But below 20% the resistivity increases rapidly with the decrease in
moisture content.
·
If the moisture content is already above 20% there is no point in
adding quantity of water into the earth pit, except perhaps wasting an
important and scarce national resource like water.
LENGTH VS DIAMETER OF EARTH ELECTRODE:-
·
Apart from considerations of mechanical strength, there is little
advantage to be gained from increasing the earth electrode diameter with the
object in mind of increasing surface area in contact with the soil.
·
The usual practice is to select a diameter of earth electrode,
which will have enough strength to enable it to be driven into the particular
soil conditions without bending or splitting. Large diameter electrode may be
more difficult to drive than smaller diameter electrode.
·
The depth to which an earth electrode is driven has much more
influence on its electrical resistance characteristics than has its diameter.
MAXIMUM
ALLOWABLE EARTH RESISTANCE:-
·
Major power station= 0.5 Ohm.
·
Major Sub-stations= 1.0 Ohm
·
Minor Sub-station = 2 Ohm
·
Neutral Bushing. =2 Ohm
·
Service connection = 4 Ohm
·
Medium Voltage Network =2 Ohm
·
L.T.Lightening Arrestor= 4 Ohm
·
L.T.Pole= 5 Ohm
·
H.T.Pole =10 Ohm
·
Tower =20-30 Ohm
METHOD OF MINIMIZING EARTH RESISTANCE:-
·
Remove Oxidation on joints and joints should be tightened.
·
Poured sufficient water in earth electrode.
·
Used bigger size of Earth Electrode.
·
Electrodes should be connected in parallel.
·
Earth pit of more depth & width- breadth should be made.
