What is Electrical Earthing?
Electrical earthing means the connection of the neutral point of the supply system or connection of non-current carrying parts of electrical apparatus (examples; metal frameworks, earth terminal of socket outlet, etc.) to the general mass of the earth.
Types of Electrical Earthing
1.System or Neutral Earthing
This type of electrical earthing system deals with earthing of the system’s neutral to ensure system security and protection.
Examples of such an electrical earthing system are the connection of neutral points of star-connected three-phase winding of generator, transformer, or motors to the ground.
2.Equipment Earthing or Safety Earthing
This type of electrical earthing system deals with earthing of non-current carrying parts of the equipment to ensure the safety of personal and protection due to lightning.
Components to be Earthed by Electrical Earthing
The components that must be earthed to the point of electrical earthing are as follows:
- The frame, tank, and enclosure of electrical machines, transformers, and other electrical equipment.
- Frame board of switchboards and control panel.
- The metal structure of indoor, outdoor sub-station, metal conduits, etc.
Difference between Electrical Earthing and Neutral
Neutral is a return path for an AC circuit that is supposed to carry current in normal condition.
Electrical earthing or grounding is for safety concerns against leakage or residual current on the system through a low resistance path.
While phase and neutral are connected to the main wiring, electrical earthing may be connected to the body of the equipment or to any system which in normal condition does not carry current. But these parts of the equipment may carry current in the case of insulation failure or leakage.
Necessity of Electrical Earthing
Electrical earthing is necessary for an electrical installation because of the following reasons:
- To ensure that no current-carrying conductors rise to a potential concerning the groundmass of the earth than its designed insulation.
- In order to avoid electric shock to human beings.
- To avoid the risk of fire due to earth leakage current through the unwanted path.
Electrical Earthing Specifications
The various important specifications regarding electrical earthing are as follows:
- Distance of earth from the building:
Earthing electrode shall not be situated within the distance of 1.5 meters from the building whose installation system is being earthed.
- Size of earth continuity conductor:
The earth continuity conductor can be either metal conduit, metal sheeting of metal cheated, or special conductors. The cross-sectional area should not be less than 14 SWG (2.9 mm2).
- Resistance of earth:
The resistance of the earth should be low enough so that sufficient current flows to operate the protective relays or blow fuses in the event of the earth’s fault.
- The earth wire used for electrical earthing should be taken through a G.I. pipe or PVC conduit to protect it against mechanical damage.
- All the earth wires running along the various sub-circuits shall be terminated and loop firmly at the mainboard and from the mainboard or the main panelboard (MPB or MDB), finally, the earth wire shall be taken to the earth electrode.
- The earthing electrode shall always be placed in a vertical position inside the earth or pit so that it may be in contact with different layers inside the earth.
Rules and Regulation of Electrical Earthing
Following are the rules and regulations to be followed for electrical earthing.
- Earth pin of 3 pin lighting plug socket and similarly earthing pin of 3 or 5 pin power plug socket should be permanently and effectively earthed.
- All-metal casing such as iron clad switches, iron-clad distribution fuse boards, metallic reflectors, G.I. pipe, and conduits should be connected to the earth.
- The frame of every generator, station motor, portable motors, and the metallic parts of all transformer should be subsequently earthed.
Factors Influencing the Earth Resistance in Electrical Earthing
The resistance of the earth system depends upon the following mentioned points:
- Condition of the soil.
- The temperature of the soil.
- The moisture content of the soil.
- Size and spacing of earth electrode.
- Depth at which the electrode is embedded.
- Conductor material.
Methods of Electrical Earthing
For effective earthing purposes, there are mainly four methods of electrical earthing.
1.Strip or Wire Earthing
In this method of electrical earthing, strip electrodes of cross-section 25 mm x 1.6 mm of copper or 25 mm x 4 mm of galvanized iron (G.I.) are buried in horizontal trenches of a minimum of 0.5 meters depth.
The length of the buried conductors shall be sufficient to give the required earth resistance (approximately < 15 meters). Hence, this system of earthing is used at places that have rocky soil where excavation work is difficult to carry out.
In this method, 12.5 mm diameter solid rods of copper or 16 mm diameter solid rods of G.I. (galvanized iron) of around 2.5 meters in length are driven vertically into the earth.
More than one-rod sections are hammered one above the other to reduce the earth’s resistance to the desired value.
This method is suitable for the sandy area.
3.Electrical Pipe Earthing
In this method, a galvanized iron (G.I.) or copper pipe with a perforated topology of approved length and diameter is placed vertically in the soil.
The pipe is provided with a tapered casing at the lower end to facilitate the driving process. The pipe is inserted such that different layers of salt, coke, or charcoal and soil for a distance of 15 cm surrounds the pipe. Consequently, this reduces the earth’s resistance to the desired value.
4.Electrical Plate Earthing
In this method we burry an earthing plate either of copper ( 600 mm x 600 mm x 3.15 mm or 600 mm x 600 mm x 3.18 mm) or of G.I. ( 600 mm x 600 mm x 6.30 mm or 600 mm x 600 mm x 6.35 mm) into the ground with its face vertical to the depth of fewer than 3 meters (approx.. 2.5 meters) from ground levels.
We have to embed the earthing plate in alternate layers of coke (charcoal), salt, and soil for a minimum thickness of 15 cm. Then we securely bolt the earth wire to an earth plate with the help of a nut, bolt, and washer. The nut, bolts should be of the same material as that of the plate.
In order to provide water during dry seasons we have to place another pipe of 19 mm diameter and a minimum length of 1.25 meters at the top of the earth plate
Calculation of Earth’s Resistance for Electrical Earthing
For rod or pipe electrode
We can calculate the resistance of the earth electrode as
ρ = Soil resistivity
d= diameter of rod or pipe in cm.
l= length of rod or pipe in cm.
For plate electrode
Here, A is the area of both sides of the plate.
Approximate Values of Soil Resistivity
Types of Soil
|1. Alluvium (left by river or flood)||
6 to 40
|2. Alluvium dry sand||
60 to 200
|3. Clay black soil||
6 to 23
|4. Sandy loam||
6 to 14