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:
(1) Safety for Human life/ Building/Equipments:
To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the fault current to flow so that it will not endanger the user
To protect buildings, machinery & appliances under fault conditions.
To ensure that all exposed conductive parts do not reach a dangerous potential.
To provide safe path to dissipate lightning and short circuit currents.
To provide stable platform for operation of sensitive electronic equipments i.e. 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 .
(2) 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.
(3) 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.
Factors affecting on Earth resistivity:
(1) 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.
(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. 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.
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
(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.
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.
(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 monsoon months 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. 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.
(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.
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.
(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.
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.
Measurement of Earth Resistance by use of Earth Tester:
For measuring soil resistivity Earth Tester is used.
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
Measurement of Earth Resistance
1) Three point method :
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.
2) Four Point Method
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.
Maximum allowable Earth resistance:
Major power station= 0.5 Ohm.
Major Sub-station= 1.0 Ohm
Minor Sub-station = 2 Ohm
Neutral Bushing. =2 Ohm
Service connection = 4 Ohm
Medium Voltage Network =2 Ohm
L.T.Lightening Arrester= 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.