A house wiring or a factory wiring may be earthed in three different ways:- 1. Earthing through a Water Main 2. G.I. Pipe Earthing 3. G.I. Plate or Copper Plate Earthing.
Type # 1. Earthing through a Water Main:
Before connecting an earthing system to a water main, it must be ascertained that all throughout galvanized iron pipes have been used- If any portion of the water main is made of cement concrete, earthing resistance will be highland the earthing arrangement will not be effective.
While connecting earth wire with the water main, care must be taken to see that the contact resistance is minimum. For this purpose specially made earthing clamps are used. If a stranded conductor is used as earth wire, all the strands should be made straight and placed parallel and soldered together to make it solid at that end which is to be connected with the water main.
A lead strip is bent round the pipe so that the earth wire can be properly placed over it and the contact between the wire and the pipe is perfect. Before placing the lead strip the surface of the pipe should be thoroughly cleaned, and all traces of dirt and grease are removed. Afterwards some more lead strip packings are placed in between earthing clamp and the wire, and the screws are tightened to complete the earth connection as shown in fig. 175.
Sometimes the earthing clamp is tightened over the water main and the end of the earth wire is soldered to a thimble or a lug fixed with the earthing clamp.
While connecting earth wire with a water main, the following Indian Electricity Rule is to be observed.
If the water main is the property of municipality or some other person, the consumer must not connect his earth wire with the water main without written consent of the owner and the Electrical Inspector.
Type # 2. G.I. Pipe Earthing:
For earthing house wiring, factory wiring (specially electrical installations of big factories), neutral wire of the supply line etc., galvanized iron pipe is generally used as earth electrode. The size of the pipe depends upon fault current and the condition of the soil. According to Indian Code of Practice if moist and soft soil is available in the earth pit, the length of the pipe should be at least 2 metres and its diameter 38.1 mm.
ADVERTISEMENTS:
If the soil is dry and rocky, the length of the pipe should be increased to 2.75 metres. In order that the contact between inner surface of the pipe and the soil is perfect, some holes of diameter 12 mm are made on the pipe at equal intervals along the length.
The pipe is placed upright in the pit as shown in fig. 176. It must be placed in permanently wet ground. The depth at which the pipe should be buried depends upon the condition of the ground moisture. According to Indian Code of Practice the pipe should be placed at a depth of 4.75 metres below the ground level. But this depth can be less if sufficient moisture of soil is available earlier.
The pipe at the bottom should be surrounded by charcoal or broken pieces of coke and salt for a distance up to 15 cm around the pipe. With this arrangement, on one side, charcoal increases the contact surface between the pipe and the earth, on the other side, salt decreases the earthing resistance.
Generally alternate layers of salt and charcoal are arranged in the earth pit for best results, in the first layer there is salt, in the second layer there is charcoal, in the third layer there is salt again, in the fourth layer there is charcoal again, and so on. During dry summer season moisture of the soil is considerably decreased and this increases the earth resistance.
ADVERTISEMENTS:
So in order to have an effective earth, arrangement must be provided so that three or four buckets of water may be poured into the earth pit for increasing the moisture of the soil.
This is achieved by connecting a galvanized iron pipe of diameter 19.05 mm at the top of the earth electrode with the help of a reducing socket. A funnel is placed at the top of this pipe. The mouth of the funnel remains covered with a wire mesh so that no piece of solid substance is dropped into the pipe and thereby the pipe becomes choked.
The practice is to use galvanized iron wire or strip as earth lead where galvanized iron pipe is used as earth electrode. The size of the earth leader earth wire depends upon the maximum current that will flow through it when a fault will occur anywhere in the installation. But generally the cross-section of this wire is 0.645 square centimetre.
ADVERTISEMENTS:
One end of the earth wire is connected to iron pipe of diameter 19.05 mm placed at the top of the earth electrode. It is then carried in a galvanized iron pipe of diameter 12.7 mm placed at a depth not less than 60 cm below the ground level. Further, when the earth wire is carried over from one machine or apparatus to another, it should be protected from external injury by carrying it through a pipe or in a recessed portion.
Type # 3. G.I. Plate or Copper Plate Earthing:
In case of plate earthing if galvanized iron plate is used as earth electrode, its size shall not be less than 60 cm x 60 cm x 6.35 mm (2 ft x 2 ft x ½ in) according to Indian Standard. With this plate galvanized iron wire is used as earth lead. If copper plate is used, its size must be at least 60 cm x 60 cm x 3.18 mm (2 ft x 2 ft. x ⅛ in), and copper wire should be used as earth lead with this plate. In both cases plate is kept vertically in the earth pit at a depth of at least 3 metres (10 ft) below ground level.
The depth at which the plate is kept in the earth pit depends upon the condition of the soil. It must be placed in a permanently wet ground. If the soil surrounding the plate becomes dry and hard, the earthing itself becomes a source of danger. For this reason periodical testing of earthing arrangement is necessary.
Copper earth plate should be embedded in alternate layer of charcoal and salt and G.I. plate should be embedded in charcoal for a minimum thickness of about 15 cm around the plate. The filling earth is thoroughly moistened with saline water. This reduces the earth resistance further. The details of the earthing arrangement have been shown in fig. 177.
At first a socket is soldered at the end of the earth wire. This socket is securely bolted to the earth plate with the help of two lock nuts, bolts and washers. It is to be remembered that the nuts and bolts must be made of copper for copper plate and galvanized iron for G.I. plate.
At the top of the earth plate a G.I. pipe of diameter 12.7 mm (1/2inch) is placed, and through this pipe earth wire is drawn up to earthing busbar or main switch. In the soil this pipe should remain at least 60 cm below ground level. The metallic frames of all instruments and switches mounted on the main switch board and other distribution boards should remain connected with the earth wire.
Another G.I. pipe of diameter 19.05 mm (3/4 inch) is also placed at the top of the earth plate. At the top of the pipe there is a funnel the mouth of which is covered with a wire mesh so that a piece of solid cannot be dropped into the pipe. The top of the pipe with funnel remains in reservoir of size 30 cm x 30 cm x 30 cm (1 ft x 1 ft x 1 ft) four sides of which have brick walls and the top remains covered with a cast iron plate. Periodically three or four buckets of water is poured into the earth pit through the funnel in order to keep the soil in the pit moist.