Importance of Earthing
Earthing or Grounding is to connect the metallic (conductive) Parts of an Electric
appliance or installations to the earth (ground).
In other words, to connect the
metallic parts of electric machinery and devices to the earth plate or earth
electrode (which is buried in the moisture earth) through a thick conductor
wire (which has low resistance).
To earth or earthing, means to
connect the part of electrical apparatus such as metallic covering of metals,
earth terminal of socket cables, stay wires that do not carry current to the
earth. Earthing can be said as the connection of the neutral point of a power
supply system to the earth so as to avoid or minimize danger
during discharge of electrical energy. Earthing and Grounding is the same terms used for earthing
. Need of
Earthing or Grounding
o Safety
of personnel
o Safety
of equipment. Prevent or at least minimize damage to equipment as a result of flow of heavy currents.
o Improvement
of the reliability of the power system.
The primary purpose of earthing is
to avoid or minimize the danger of electrocution, fire due to earth leakage of
current through undesired path and to ensure that the potential of a current
carrying conductor does not rise with respect to the earth than its designed
insulation.
In electrical gadgets, if excessive
currents are not earthed, we would experience severe shock by touching the same.
Earthing takes place in electrical appliances only when there is a problem and
it is to save us from electric shocks.
If the metallic part of the
appliance is earthed, the charge will be transferred to earth instead of
accumulating on the metallic part of the appliance. Current don’t flow through
earth wires in electrical appliances, it does so only when there is problem and
only to direct the unwanted current to earth in order to protect us from severe
shock.
Methods of Earthing | Types of Earthing
Earthing can be done in many ways.
The various methods employed for earthing are discussed as below:
1). Plate Earthing:
In this type of earthing system, a
plate made up of either copper with dimensions 60cm x 60cm x 3.18mm (i.e. 2feet
x 2feet x 1/8 inch) or galvanized iron (GI) of dimensions 60cm x 60cm x 6.35 mm
(2feet x 2feet x ¼ inch) is buried vertical in the earth (earth pit) which
should not be less than 3meter (10feett) from the ground level.
we need to maintain the moisture
condition around the earth electrode or earth for proper earthing.
2). Pipe Earthing:
It is the most common system of
earthing. The size of pipe to use depends on the magnitude of current and the
type of soil. The dimension of the pipe is usually 40mm (1.5inch) in diameter
and 2.75meter (9feett) in length for ordinary soil or greater for dry and rocky
soil. The moisture of the soil will determine the length of the pipe to be
buried but usually it should be 4.75meter (15.5feet).
3). Rod Earthing
It is the same method as pipe
earthing. A copper rod of 12.5mm (1/2 inch) diameter or 16mm (0.6inch) diameter
of galvanized steel or hollow section 25mm (1inch) of GI pipe of length above
2.5meter (8.2 feet) are buried upright in the earth manually. The length of
embedded electrodes in the soil reduces earth resistance to a desired value.
4). Earthing through the Waterman
In this method of earthing, the
waterman (Galvanized GI) pipes are used for earthing purpose. Make sure to
check the resistance of GI pipes and use earthing clamps to minimize the
resistance for proper earthing connection.
If stranded conductor is used as earth
wire, then clean the end of the strands of the wire and make sure it is in the
straight and parallel position which is possible then to connect tightly to the
waterman pipe.
5). Wire or Strip Earthing:
In this method of earthing, strip
electrodes of cross-section not less than 25mm x 1.6mm (1inch x 0.06inch) is
buried in a horizontal trenches of a minimum depth of 0.5meter. If copper with
a cross-section of 25mm x 4mm (1inch x 0.15inch) is used and a dimension of
3.0mm2 if it’s a galvanized iron or steel.
If at all round conductors are used,
their cross-section area should not be too small, say less than 6.0mm2 if it’s a galvanized iron or steel. The length of the
conductor buried in the ground would give a sufficient earth resistance and
this length should not be less than 15meter.
General method of Earthing / Proper Grounding Installation
(Step by Step)
The usual method of earthing of
electric equipments, devices and appliances are as follow:
1. First
of all, dig a 1.5m×1.5m pit about 20-30ft (6-9 meters) in the ground. (Note
that, depth and width depends on the nature and structure of the ground)
2. Bury
an appropriate (usually 2’ x 2’ x 1/8” (600x600x300 mm) copper plate in that
pit in vertical position.
3. Tight
earth lead through nut bolts from two different places on earth plate.
4. Use
two earth leads with each earth plate (in case of two earth plates) and tight
them.
5. To
protect the joints from corrosion, put grease around it.
6. Collect
all the wires in a metallic pipe from the earth electrode(s). Make sure the
pipe is 1ft (30cm) above the surface of the ground.
7. To
maintain the moisture condition around the earth plate, put a 1ft (30cm) layer
of powdered charcoal (powdered wood coal) and lime mixture around the earth
plate of around the earth plate.
8. Use
thimble and nut bolts to connect tightly wires to the bed plates of machines.
Each machine should be earthed from two different places. The minimum distance
between two earth electrodes should be 10 ft (3m).
9. Earth
continuity conductor which is connected to the body and metallic parts of all installation should be tightly connected to earth lead.
10. At
last, test the overall earthing system through earth tester. If everything is
going about the planning, then fill the pit with soil. The maximum allowable
resistance for earthing is 1Ω. If it is more than 1 ohm, then increase the size
(not length) of earth lead and earth continuity conductors. Keep the external
ends of the pipes open and put the water time to time to maintain the moisture
condition around the earth electrode which is important for the better earthing
system.
Factors
affecting on Earth resistivity
1. Soil
Resistivity
It is the resistance of soil to the passage of electric
current. The earth resistance value of an earth pit depends on soil resistivity. 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.
2. Soil
Condition
Different soil
conditions give different soil resistivity. Most of the soils are very poor
conductors of electricity when they are completely dry.
3. 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.
4. Dissolved
salts
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.
5. Climate
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 rainy season the resistivity
will be low.
6. 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.
7. 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.
8. Effect of
grain size and its 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.
9. 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.
10. 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.
11.
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.
12. Current
Magnitude
A current of
significant magnitude and duration will cause significant drying condition in
soil and thus increase the soil resistivity.
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
Treatments to
for 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.
