Earthing of Grounding

The process of connecting the metallic frame (i.e., non- current carrying part) of electrical equipment or some electrical part of the system (e.g., neutral point in a star-connected system, one conductor of the secondary of a transformer, etc.) to the earth (i.e., soil) is called grounding or Earthing. The potential of the earth is to be considered zero for all practical purposes. Earthing is to connect any electrical equipment to earth with a very low resistance wire, making it to attain earth’s potential, This ensures safe discharge of electrical energy due to failure of the insulation line coming in contact with the casing, etc. Earthing brings the potential of the body of the equipment to zero i.e., to the earth’s potential, thus ptotecting the operating personnel against electrical shock.

The earth resistance is affected by the following factors

(a) Material properties of the earth, wire and the electrode

(b) Temperature and moisture content of the soil

(d) Quantity of the charcoal used

Necessity of Earthing

The requirement for provision of earthing can be listed as follows :

(1) To protect the operating personnel from the danger of shock.

(2) To maintain the line voltage constant, under unbalanced load condition.

(3) To avoid risk of fire due to earth leakage current through unwanted path.

(4) Protection of the equipments.

(5) Protection of large buildings and all machines fed from overhead lines against lighting.

Methods of Earthing

The various methods of earthing in common use are

(i) Plate earthing

(ii) Pipe earthing

(iii) Rod earthing

(iv) Strip or wire earthing

(i) Plate earthing

In this method either a copper plate of 60cm × 60cm ×3.18 or GI plate of 60cm × 60cm × 6.35 is used for earthing. The plate is buried into the ground not less than 3m from the ground level. The earth plate is embedded in alternate layers of coal and salt for a thickness of 15cm as shown in figure. In addition, water is poured for keeping the earth’s electrode resistance value below a maximum of 5Ω. The earth wire is securely bolted to the earth plate.

A cement masonry chamber is built with a cast iron cover for easy regular maintenance

Plate earthing

(ii) Pipe earthing

Earth electrode made of a GI (galvanized iron) pipe of 38 mm in diameter and length of 2 m (depending on the current) with 12 mm holes on the surface is placed upright at a depth of 4.75 cm in a permanently wet ground. To keep the value of the earth resistance at the desired level, the area (15 cm ) surrounding the GI pipe is filled with a mixture of salt and coal. The efficiency of the earthing system is improved by pouring water through the funnel periodically. The GI earth wires of sufficient cross-sectional area are run through a 12.7mm diameter pipe (at 60cm below) from the 19mm diameter pipe and secured tightly at the top as shown in figure

Pipe earthing

When compared to the plate earth system the pipe earth system can carry larger leakage currents due to larger surface area is in contact with the soil for given electrode size. This system also enables easy maintenance as the earth wire connection is housed at the ground levels.

(iii) Rod earthing

It is the same method as pipe earthing, A copper rod of 12.5cm (1/2 inch) diameter or 16mm (0.6in) diameter of galvanized steel or hollow section 25mm (1 inch) of GI pipe of length above 2.5m (8.2 ft) 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.

(iv) Strip or wire earthing

In this method of earthing strip electrodes of cross- section not less than 25mm × 1.6mm (1 in × 0.06in) is buried in a horizontal trench of a minimum depth of 0.5m. If copper with a cross-section of 25mm × 4mm (1in × 0.15in) is used and a dimension of 3.0 mm2 if it’s a galvanized iron or steel.

If at all round conductors are used, their cross-section area should not be too small, say less than 6.0 mm2 if it’s a galvanized iron or steel. The length of the conductor buried in the ground would give a sufficient earth resistance and this length should not be less than 15m. The electrodes shall be as widely distributed as possible in a single straight or circular trench radiating from a point. This type of earthing is used where the earth bed has a rocky soil and excavation work is difficult.

Selection of Earthing

The type of earthing to be provided depends on many factors such as type of soil, type of installation, etc.

The following table helps in selecting a type of earthing for a particular application

Earth Resistance

The earth resistance should be kept as low as possible so that the neutral of any electrical system, which is earthed, is maintained almost at the earth potential. The earth resistance for copper wire is 1Ω and that of GI wire less than 3Ω. The typical value of the earth resistance at large power stations is 0.5Ω , major substations is 1Ω, small sub-stations is 2 Ω and in all other cases 5 Ω.

The resistance of the earth depends on the following factors

i. Condition of soil.

ii. Moisture content of soil.

iii. Temperature of soil.

iv. Depth of electrode at which it is embedded.

v. Size, material and spacing of earth electrode.

vi. Quality and quantity of coal and salt in the earth pit.

Difference Between Earth Wire and Neutral Wire

Neutral Wire

(i) In a 3-phase 4-wire system, the fourth wire is a neutral wire.

(ii) IT acts a return path for 3-phase currents when the load is not balanced.

(iii) IN domestic single phase AC circuit, the neutral wire acts as return path for the line current.

Earth Wire

(i) Earth wire is actually connected to the general mass of the earth and metallic body of the equipment

(ii) It is provided to transfer any leakage current from the metallic body to the earth.