Earthing

Earthing

The process of connecting the neutral point of a supply system on the non current carrying parts of electrical apparatus to the journal mass of earth in such a manner that at all times an immediate discharge of electrical energy takes place without danger is called earthing.

Or 

A wire coming from the the ground 2.5 to 3 dip from an electrode or plate is also known be earthing.

• Good earthing is dead earthing which gives very low resistance to the flow of heavy current for short circuit current of a circuit.

• Double Earth is used to give minimum resistance to the flow of whole current of the operators in case short circuit to leakage or any other such fault occurs.

• The earth resistance for copper wire is 1 ohm and for G.I. wire it should not be more than three ohm.

Objectives or purpose of earthing

- to save human life from danger by blowing fuse of an apparatus which becomes leaky.

- to protect large buildings from atmospheric lightning.

- to protect all machines fed from overhead lines from lightning arresters.

- to maintain the line voltage constant.

- to maintain the potential of any part of the system at a definite value with respect to earth.

             Factors affect the earth resistance

- size of electrode and Earth wire.

- material of the electrodes.

- depth to which the electrode is embedded.

- moisture of the soil.

- temperature of soil.

  Method of Earthing

1. Plate Earthing

In this method of earthing a plate of copper size 60cm×60cm×3.18mm or GI plate of size 60cm×60cm×6.35mm thick is kept with its faces vertical and stop not less than 3 metre below the ground level or to moisture level. The earthing plate is surrounded by 15 CM thick alternate layers of salt and charcoal. this keeps the earth electrode weight thereby reducing the earth resistance. Earth wire is bolted to the plate and brought out through  galvanised iron pipe. other pipe with funnel and match at top is used to pour water for dampness in the pit of earth plate. Funnel is enclosed in masonry and closer which is covered with hinged cover of cast iron. The material of plate, wire, bolts and washers should be same to avoid corrosion. The size of plate wire depends upon the rating of equipments.

The conductivity that is earthing efficiency depends upon the plate area and depth of embedding. If the plate area and depth of embedding increases.

- its disadvantage is that discontinuity of earth wire with plate below the earth wire cannot be observed physically. Hence misleading and sometimes results in heavy loss in case of any fault.

Figure as shown below-

2. Pipe Earthing

All dimensions as per standard specifications.

in this method of earthing galvanised iron pipe is used instead of plate. The lower end of the pipe is tapered to make its driving easy. The holes provided in a galvanised iron pipe helps in keeping this rounding of the pipe weight when water is poured through funnel.

To keep the surrounding of pipe always wet alternate layers of salt and charcoal are provided. The most surroundings the electrode is is necessary for very low earth resistance. The earth wire which is soldered and connected to pipe is covered through 12.7 mm pipe which is connected to another pipe earthing has preferred for domestic and other low power demand installation.

the contact surface area of galvanized iron pipe which soil is more in comparison to the plate because of its circular section and hands can take up heavy leakage current for the same electrode size. The earth wire connection with the galvanized iron pipe being above the ground level can be checked for carrying out continuously test as and when desired violin plate earthing it is difficult which is an advantage over the plate earthing the system of pipe earthing is as shown below.

3. Strip Earthing

This method of earthing is used where Earth bed have a rocky soil and excavation of earth is difficult. In this system 5 s w g copper wire or copper strip of size not less than 25 mm× 1.6mm is  buried in horizontal trenches. Under such circumstances earth resistance is high which is about five ohms. 

4. Rod Earthing 

this type of earthing is done in Sandy areas. In this method of earthing A copper rod is hammered directly into Earth. This is the cheapest method but earth resistance is also high like strip earthing.

Advantages of earthing

- over voltage due to restricting is minimised.

- the ground faults can be located and isolated fastly.

- steady state voltage stress to Earth is reduced.

- sensitive protective apparatus can be used.

- battery safety is ensured.

• What equipment is to be connected to earth?

- metal frame of generators, motors and other metallic parts of the equipment used for regulating the energy should be earthed by two separate and distinct connections with earth.

- all the metal parts of electrical installation such as metal conducts, light fitting ironclad main switches etc. Should be properly Earthed.

- Earth terminals of the three pin power plug socket should be permanently and efficiently Earthed.

- the metal casing of portable apparatus should be earthed. This covers electric drill, heaters etc.

• Maximum permissible resistance of earth system

- there is no hard and fast rule that the following representative value uses of the combined overall resistance of earth system.

Large power station 0.5 ohm

Major substation 1.0 ohm

Small substation 2.0 ohm

In all other cases 8.0 ohm

Earth continuity inside and installation date is from the Earth plate to any other point - 1.0 ohm in the Installation.

The main principle is that the resistance should be low enough to allow the passage  of sufficient current to operate the overload relays for blow fuses. Hindi event of an Earth fault, it follows that the larger the installation, lower should be the total earth resistance. The lower the value the better it is from the point of view of the protection.

Maintenance of earthing system

- Earth resistivity should be checked half yearly during dry season and results should be compared with the installation records.

- in case of small substations, water should be poured at regular interval during dry season.

- when chemical has been used to reduce conductivity, condition of electrode should be checked to and sure whether these have been corroded or not.

- tight the earth connections and should be properly welded with earth electrodes.

- examine and replace broken Earth leads conductors with proper size.

- in case of earth is found corrosive replace immediately.

- cross sectional area of earth conductor should be checked.

- the value of earth resistance measured should not exceed 1 ohm for better performance.

Methods of reducing earth resistance

The maximum value beyond which the earth electrode resistance should not be increased is 1.5 ohms. Thus for better performance at power house coma at substation its value should be 0.5 ohms and 1.0 ohms respectively. It can be achieved by applying following methods

- Earth resistance can be reduced by increasing number of earth electrodes inter connected in parallel.

- by increasing large size of earth electrodes.

- by increasing the depth of pit for earthing system.

- the methods mention above have their own limitations, but the common method is that the area around the earthing pipe for earth electrode is increased and charcoal soaked in salt water is filled in the pit point in dry seasons, the fresh salt water is poured into the earth through the pipe. This is done to increase moisture level within the earth pit and in this way earth resistance LV considerably reduced.

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