Earthing is required for the following purposes:
To provide protection against lightning strikes & power surges.
To protect equipment from surge voltages & currents and human beings from electric shocks.
As one conductor in earth-return circuits where the earth is part of the circuit.
Discharging induced voltages, in communication systems.
To minimize RF radiation and EM interference.
Properties of Earth
Earth is not a good conductor.
But it is an ideal Equipotential surface.
It can SINK any amount of charge without any appreciable rise in its potential.
Effect of injected currents due to a fault or lightning is negligible, if not felt only locally.
What is system Earth and Equipment Earth?
System Earth:
It is the common conductive path in a circuit which is connected to one of the power supply terminals.
Equipment Earth:
It is the protective connection between equipment metallic body to an earth electrode of an earthing pit for the purpose of sinking fault currents or discharging induced/short circuit voltages
Characteristics of Good Earthing System (Equipment Earth)
Excellent Electrical Conductivity
High Corrosion Resistance
Mechanically Robust and Reliable.
The following locations are suitable for locating earth pit:
Low lying areas close to the building or equipment are good for locating Earth Electrodes.
The location can be close to any existing water bodies or water points but not naturally well-drained
The following locations are not suitable for locating earth pit:
Dry sand, gravel chalk, lime stone, granite and any stony ground locations should be avoided.
And also all locations where virgin rock is very close to the surface.
Earthing electrode should not be installed on high bank or made-up soil.
Soils suitable for making earth pit in the order of preference:
Wet marshy ground
Clayey soil, loamy soil, arable land, clay or loam mixed with small quantities of sand
Clay and loam mixed with varying proportions of sand, gravel and stones
Damp & wet sand or peat.
Earth Resistance
It is the resistance of soil offered to the passage of electric current.
Earth Resistance value of an earth pit depends solely on soil resistivity at the location.
Hence, it varies from soil to soil.
Soil Resistivity
Soil Resistivity mainly depends on:
Soil composition,
Its Moisture content,
Dissolved salts,
Grain size and its distribution,
Temperature and humidity.
Magnitude of current that flows.
Required Earth Resistance Value
Ideally, earth resistance has to be 0 Ω.
But practically it should be 1 or 2 ohms
Value of Earth Resistance depends on two factors
Soil Resistance ( Electrode to Soil Resistance) and
Electrode Resistance
Earth Resistance
The resistance offered by EARTH to the spread of electric current/field is called Earth/Soil Resistance.
It depends mainly on Soil Resistivity .
And also on Shape and size of earth electrode.
Electrode Resistance
Electrode Resistance is the resistance of the material of which the electrode is made of. It also depends on shape and size of the electrode.
Electrodes of GI, Copper and copper welded steel are used to keep electrode resistance negligible or less than 1 Ω.
Major Factors Deciding Earth Resistance
Geometry
Size of Electrode and
Soil resistivity
Earth Electrode Types
1. Rod Electrodes
2. Strip Electrodes
3. Pipe Electrodes
4. Plate Electrodes
A pipe, rod or strip has a much lower resistance than a plate of equal surface area.
The resistance is not inversely proportional to the surface area of the electrode.
Plate Electrode:
The approximate resistance to earth of a plate can be calculated from:
R = ρ x L/A
Where,
ρ = resistivity of the soil (in Ω.m)
A = cross-sectional area of plate (in m2)
L = the length of the conducting path
Strip Electrode:
In case of strip electrode earth resistance is given by :
R = ρ/2πL [ln (8L/T) + ln (L/h)-2+(2h/L)-(h/L)2]
L is length,
h is depth of laying,
T is thickness
L has major influence than T
Pipe Electrode:
In case of pipe electrode earth resistance is given by:
R = (ρ / 2πL) [ln {8L / 2.7183 D}]
Where
L is length of electrode,
D is diameter,
ρ is soil resistivity
Better earth Resistance can be achieved by:
Increasing the surface area of the electrode in contact with the earth,
Increasing the depth to which it is driven,
Using several connected ground rods,
Increasing the moisture of the soil,
Improving the conductive mineral content of the soil, and
Increasing the land area covered by the ground system
Types of earthing
1. Conventional earth
2. Maintenance free earth
Earth Electrode:
Procedure of Installation
Earth pit of 600 mm dia and 2.5 m depth is made by manual trenching or by using “Earth Auger”.
The electrode is placed at the center.
Top of the electrode is kept 30 cm above the ground.
After inserting the electrode, the hole is filled with earth properly and water is spread to ensure good contact between electrode and filling.
In the soil is of high resistivity, it is treated with salt and charcoal in appropriate proportion.
The pit is filled alternately with layers of common salt and charcoal each layer of about 2.5 cm thick up to a depth of about 150 cm from the bottom of the electrode.
A brick wall of 400 mm height below the ground level is constructed in rectangular fashion and walls are plastered and then filled with sand.
The surroundings of the earth electrode are kept moist by periodically pouring water through the pipe in order to keep the resistance below specified value.
Coke treated electrodes shall not be situated within 6 meters of other metal structure. (This results in rapid corrosion)
Bolt nut mechanism is not a permanent solution
It is corrosive
It can not offer consistent resistance
It is not Maintenance Free
Maintenance of conventional Earth
1) Check earth and its connections periodically at interval of not more than one month, to ensure that all connections are intact and soldered joints are in proper condition.
2) Measure the earth resistance once in a year. Enter the value, date of last test on the inspection pit cover and in a register.
3)Water to be added every day to the earth electrode in summer and once in two days in other seasons.
4)If earth resistance is more than the nominal value either renew the old earth or provide a new earth.
Maintenance free Earth
Earth Electrode:
Steel circular rods, bonded with copper on outer surface to meet the requirements of Underwriters Laboratories (UL) latest specifications.
(strength, corrosion resistance, low resistance path to earth and cost effectiveness).
Copper Bonded Rods:
Steel rods (minimum 17 mm dia., 3 m long)
Copper bonded
Coating thickness - 250 microns min.
UL marking
Service Life - 40 years in most soil types
Earth Enhancement material
A superior conductive material that improves conductivity of the electrode and ground contact area. It improves earth’s absorbing power and humidity retention capability.
Non-corrosive, low water solubility, highly hygroscopic.
Resistivity of less than 0.2 ohms-meter.
Suitable for installation in dry/slurry form.
Does not depend on continuous presence of water to maintain its conductivity.
Permanent & maintenance free and maintain constant earth resistance.
Construction of Earth Pit
A hole of 100mm to 125mm dia is augured/dug to a depth of about 2.8 meters.
Earth electrode is placed into this hole.
It is penetrated into the soil by gently driving on the top of the rod. Here natural soil is assumed to be available at the bottom of the electrode so that min. 150 mm of the electrode shall be inserted in the natural soil.
Earth enhancement material (30-35 kg) is filled into the hole in slurry form and allowed to set.
Remaining portion of the hole is covered by backfill soil, which is taken out during digging.
A copper strip of 150mmX25mmX6mm is welded to earth electrode for taking connection to the main equipotential earth bus-bar in the equipment room and to other earth pits, if any.
Exothermic weld material is UL listed.
The main earth pit is located as near to the MEEB in the equipment room as possible.
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Comparison with Conventional System
1. Space Savings
2. Corrosion Proof
3. Maintenance Free
4. Steady Performance
5. Very Long Life
6. All over Fit and forget Technology
Construction of Loop/ring Earth by Providing Multiple Earth Pits
At certain locations, it may not be possible to achieve earth resistance of ≤1 ohm with one earth electrode/pit due to higher soil resistivity. In such cases, provision of loop/ring earth consisting of more than one earth pit is done.
The distance between two successive earth electrodes should be between 3 to 6 m.
These earth pits are inter-linked using 25mmX2mm copper tape to form a loop using exothermic welding.
The interconnecting tape is buried at depth of 500mm below ground level. This tape is also covered with earth enhancing compound.
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