EARTHING

 

                                        KENDRIYA VIDYALAYA NO -2 DARBHANGA                                                                                                                                                                                   

 

                                                                  WORK EDUCATION

TOPIC- EARTHING

. Earthing. A wire coming from the ground to 2.5 to 3  metres deep from an electrode (plate or so) is called earthing

The earth's potential is always taken as zero for all practical purposes. The electrical appliances or machines when connected  with earth attain zero potential and are said to be earthed

Object of earthing  -  To save human life  from danger or shock or death  on  blowing fuse of any apparatus which become leaky.                                                                                                                                                          2. To protect large buildings from atmospheric lighting

3. To protect all machines fed from O.H. lines from  atmospheric  lighting by taking all voltage of lighting through lightning arresters.

4. To maintain the line voltage constant ( neutr al of every alternator, transformer is earthed).

Good earthing --     It  is that earthing Which gives very low resistance to the flow of heavy Current of a circuit.

 Double earthing -  Double earth is used to give minimum resistance to the flow of whole current of that apparatus in case short circuit or leakage or any other such fault happens.                                                                             Second reason –If one earth is out of order, second will do the work

For double earthing.- Two earthing is done at a distance of about 5 metres, and the two earth wires should be fixed with the f thimble (lugs) of proper sizes with the apparatus at two different places. Precaution should be taken that the two wireshoud not each others.                                                         

What should be the resistance of a good earth ?                                                                                                                              The earth resistance for copper wire is 1 ohm  and for GI wire it should not be more than 3 ohm

                    Earth resistance is measured by earth tester. As the total current is to pass through the earth wire the earth resistance should be kept as low as possible. The resistance increases in hot days (summer days),

To Reduce Earth Resistance

1. By pouring water. The dampness of the soil and coal salt paste depends upon the atmospheric conditions. In summer season fresh salt water should be poured through the pipe over the coal bed.  The soil resistance can be lowered only to a certain limit by the above method, and in order to lower it further, following are the additional steps to be taken:

2 Increase in plate area, In case of increase in plate area, the decrease of resistance value is not in direct proportion to the area. It is found that to reduce the resistance value by one-sixth, the increase in area by 36 times more for the same soil condition and depth of Electrode. So this method is not used.                        3. Increase in depth. The increase of depth below the ground level  of the same plate reduces the resistivity of the  earthing system.It  is found that plate area reduces to fifty percent when the depth is doubled  for the soil resistance With this method also the soil resistance  value cannot be lowered as much as desired, on account of excavation  work; however, it helps to a great extent.                                                                                                                  4. Electrodes in parallel. In this system of lowering earth resistance the soil resistance fall considerably as the number of electrodes inter connected in parallel are increased for the same depth. This method  is suitable only where great area of a free soil is available for earthing The plates should be-so place d in parallel as not to overlap the earthing region covered by the individual electrode, If R and R, etc., are the earth resistance of each electrode and R The total earth resistance of the system,then

        1|R= 1\R1+1\ R2+……….

SPECIFICATION REQUIRED FOR EARTHING

 Following are recommended specifications as per 1sL for providing good earthing.                                     1. The earthing electrode should be situated at a place at least I.5  metres away from the building (outside) whose installation systemis being earthed.

2. The earth wire should be of same material as that ct electrode used.                                                                         3. The minimum sectional area of earth lead wire should be less than O'02 sq. inch (No. 8 S.W.G.) and not more than 0.1 inch.

4. The size of earth conductor as a general rule should notLess than half of the section of live line conductor.

5. The earth wire should be taken through G.I. pipe of 12 mm (1") dia. for at least 3'25 cm (1.5”) dia for at least 3.25cm (1’) length above and below ground surface to the earth electrode to safeguard against mecheni cal wear and tear

6. Loose earth and coal salt mixture should be filled around  the earth electrode for effective earthing                                                                                     7. The earth wire connected to the earth electrode should not  be necessarily run along the whole wiring system. All the earth  wire runs along the sub-circuits should be terminated and hook firmly at the main board and from where the main earth wire  should be run to the earth electrode. The loop earth wire show be of 14 SWG copper wire.

8. All the joints in the earth wire should be firmly done with  nut bolts of the same material as of earth wire.