Earthing Resistance Doc
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Transcript of Earthing Resistance Doc
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Earthing Resistance
EARTH RESISTANCE
Impedance of Earth Electrode Systems
Measurement of the Ohmic value of a buried electrode is carried out for two reasons: -
To check the value, following installation and prior to connection to the equipment, against the
design specification.
As part of routine maintenance, to confirm that the value has not increased substantially from
its design or original measured value.
Important Facts
1. Electricity takes the least path of resistance
2. Earth Resistance Value (Ohmic value) of an earth pit depends solely on soil resistivity at the
location.
3. It is the resistance of soil to the passage of electric current
4. It varies from soil to soil.
5. It depends on the composition of soil, its Moisture content, & electrolyte content, Dissolved
salts, grain size and its distribution, seasonal variation, temperature and current magnitude.
EARTH RESISTANCE: DEPENDS ON SEVERAL FACTORS
SOIL RESISTIVITY
1. It is the resistance of soil to the passage of electric current
2. It varies from soil to soil
3. In depends on the composition of soil, Moisture content, Dissolved salts, grain size and its
distribution, seasonal variation, current magnitude.
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SOIL CONDITION
1. Different soil conditions give different soil resistivity.
2. Most of the soils are very poor conductors of electricity when they are completely dry.
3. Soil resistivity is measured in ohm-meters or ohm-cms
4. Soil plays a significant role in determining the performance of Electrode.
5. Soil with low resistivity is highly corrosive.
6. If soil is dry then soil resistivity value will be very high.
7. If soil resistivity is high, earth resistance of electrode will also be high.
MOISTURE
1. Moisture significantly influences soil resistivity
2. Conduction of electricity in soil is through water.
3. Soil resistivity drops significantly in soil with moisture content.
4. Moisture is the most important element in conductivity.
5. In many locations water table goes down in dry weather conditions. Therefore it is essential to
pour water in and around earth pits to maintain moisture in dry weather conditions.
DISSOLVED SALTS
1. Pure water is poor conductor of electricity.
2. Resistivity of soil depends on resistivity of water which in turn depends on the amount and
nature of salts dissolved in it.
3. Small quantity of salts in water reduces soil resistivity by 80%.
4. Common salt is most effective in improving conductivity of soil. But it corrodes metal and
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hence discouraged.
GRAIN SIZE & DISTRIBUTION
The grain size, distribution and closeness of packing also contribute to retention of moisture in the soil.
SEASONAL VARIATION
1. Increase or decrease of moisture content determines the increase or decrease of soil
resistivity.
2. Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be
low.
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- Through: Earth Tester
For measuring soil resistivity, and earthing device resistance, 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 C = Current spike
The distance between the spikes may be 1M, 2M, 5M, 10M, 35M, 50M.
All depending on space available.
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All spikes are equidistant and in straight line to maintain electrical continuity. Take measurement in
different directions.
Soil resistivity ‘P’ may be obtained from the following formula :
P = 2 Л LR where,
R = Value of Earth resistance in ohm
L = Distance between the spike in cm
π = 3.14
P = Earth resistivity ohm-cm
Earth resistance value is directly proportional to Soil resistivity value.
The following illustration describes how
resistance is measured Spikes must be in
straight line to maintain electrical continuity.
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 Mtr 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