The primary purpose of earthing is to avoid or minimize the danger of:
- Fire due to earth leakage of current
- Damage to people, equipment and assets
When the metallic part of electrical appliances comes in contact with a live wire, the metal becomes charged and static charge accumulates. If a person touches such a charged metal, the result could be catastrophic.
To avoid such instances, the power supply systems and parts of appliances have to be earthed so as to transfer the charge directly to ground.
Why have Earthing?
- Protect human lives
- Provide safety to electrical devices and appliances
- To keep voltage as constant in the healthy phase
- To protect electrical system and buildings from lightning.
- To avoid the risk of fire
Basic Terms used in Electrical Earthing
- Earth: The proper connection between electrical installation systems via conductor to the earth.
- Earthed: Connection to the earth through earth electrode.
- Solidly Earthed: When an electric device, appliance or electrical installation is connected to the earth electrode without a fuse, circuit breaker or resistance/Impedance.
- Earth Electrode: When a conductor buried in the earth for electrical earthing system. They are available in different shapes E.g. conductive plate, rod or mesh.
- Earthing Lead: The conductor wire, strip or tape connected between Earth electrode and Electrical installation system and devices.
- Earth Continuity Conductor: The conductor wire, which is connected among different electrical devices and appliances like, distribution board, different plugs and appliances.
- Sub Main Earthing Conductor: A wire connected the switch board and distribution board
- Earth Resistance: Is the total resistance between earth electrode and earth in Ω (Ohms).
POINTS TO BE EARTHED
Earthing is done according to AS, IE and IEE (Institute of Electrical Engineers) regulations:
- 3-pin lighting plug sockets should be permanently earthed.
- All metallic coverings containing or protecting any electric supply line or apparatus should be earthed.
- The frame of generators, stationary motors and metallic parts of all transformers used for controlling energy should be earthed by two separate connections with the earth.
- In a dc 3-wire system, the middle conductors should be earthed at the generating station.
- Overhead lines should be earthed by connecting at least one strand to the earth wires.
Components of Earthing System
A complete electrical earthing system consists on the following basic components.
- Earth Continuity Conductor
- Earthing Lead
- Earth Electrode
Earth Continuity Conductor or Earth Wire
That part of the earthing system which interconnects the overall metallic parts of electrical installation e.g. conduit, ducts, distribution boards, fuses etc… and components installed are known as earth wire or earth continuity conductor as shown in the above fig.
Size of Earth Continuity Conductor
The cross-sectional area of the earth continuity conductor should not be less than the half of the cross-sectional area of the thickest wire used in the electrical wiring installation.
Earthing Electrode or Earth Plate
A metallic electrode or plate which is buried in the earth and connected with earthing lead.
A metallic plate, pipe or rode can be used as an earth electrode which has very low resistance and carry the fault current safely towards ground (earth).
Earthing Ground Enhancement Material
It is recommended to bury the earth electrode in the moisture earth. If it is not possible, then it is recommended to use a ground enhancement material such as gypsum bentonite or even better GEM
Methods of Earthing | Types of Earthing
Earthing can be done in many ways. The various methods employed in earthing are:
- Plate Earthing: In plate earthing system, a plate made up of copper (preferable) or galvanized iron (GI) is buried vertical in the earth (earth pit) which should not be less than 3m (10ft) from the ground level.
- Rod Earthing: It is the same method as pipe earthing. A copper rod of min. 16mm diameter by 3m length are buried upright in the earth manually or with the help of a pneumatic hammer. The length of embedded electrodes in the soil reduces earth resistance to a desired value.
- Tape or Wire Earthing: Typically tape electrodes of 25mm x 3mm cross-section is buried in a horizontal trench of a minimum depth of 0.5m.
If 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.
Specification for Earthing
Various earthing recommendations are given below;
- An earthing electrode should not be situated (installed) close to the building whose installation system is being earthed at least more than 1.5m away.
- The earth resistance should be low enough to cause the flow of current sufficient to operate the protective relays or blow fuses. Its value is not constant as it varies with weather because it depends on moisture (but should not be less than 1 Ohm).
- The earth wire and earth electrode will be the same material.
- The earthing electrode should be placed in a vertical position inside the earth or pit so that it may be in contact with all the different earth layers.
Dangers of Not Earthing a Supply System
If excessive current is not earthed, appliances will be damaged without the help of a fuse. Excessive current is earthed at their generating stations which is why earth wires carry very little or no current at all.
In our electrical appliances, if excessive currents are not earthed, we would experience severe shock. Earthing takes place in electrical appliances only when there is a problem to save us from danger.
If you happen to touch a metallic part when charged or static charges you will be electrocuted. But 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.
Earthing is essential and mandatory. It’s an electrical connection between the exposed metallic parts of an electrical appliance or installation and the earth, regarded to have zero potential. Competent earthing provides an alternative and easy path for leakage or faulty current to flow that could save your life, equipment and asset.
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