EARTHING.
Earthing and it’s types.
It is very important to earth the line and electrical equipment. It will be electrically unsafe without earthing. The pole/ body of equipment connected solidly to earth is called earthing.
1)For Electrical supports and equipments.
It case of short circuit or leakage, current will pass with minimum resistance to earth so that maximum current will flow through effected circuit so that fuse will blow or circuit breaker to trip. This will isolate the faulty line or equipment from live circuit.
2)Transformer neutral earthing.
a.The leakage or unbalanced current will have path with minimum resistance.
b.Sensitive protecting equipment works properly. (Earth Fault Relay.)
c.It prevents the lines being charged to excessive high voltage due to lightening or switching surges.
d.By connecting resistance in the neutral earthing, fault current is controlled.
e.It helps for keeping neutral voltage always zero.
3)For Lightening Arrestor: -
The lightening arrestor earthing discharges the lightening charge with very low resistance, which prevents possible damages. For this, very low earth resistance is necessary. This quality can be achieved by piercing earth electrode deep in the ground up to wet soil. Earth resistance depends on-
1)Types of soil, (2) Temperature of soil, (3) Wetness of soil, (4) Minerals in soil, (5) Size of Electrode, (6) Cross section of electrode, (7) Deepness of electrode in ground, (8) Distance between two electrodes.
Earth tester measures Earth resistance and its unit is ohm.
LINE AND POLE EARTHING.
1)Every fifth metallic pole of L.T. line should be earthed.
2)Cross arms, top clip, insulator pins of P.S.C. pole should be earthed along with the pole.
3)Guarding at Railway crossing, Telephone crossing, Road crossings should be earthed along with pole on both side. If earth electrode is not available 8 S.W.G. G.I. wire coil of 25 mm dia and 120 to 150 mm long should be used and placed in a pit at a distance of 90 cms. from pole at a depth of 150 cms. It is very necessary to earth the guarding. All the metal fittings of L.T. pole and stay should be earthed and connected firmly to neutral because neutral is multiple earthed neutral.
4)A separate earth electrode to be used for L.A. earthing and the earth wire should be through an Alkathine pipe without touching the pole.
Methods of Earthing.
1)Plate Earthing :
A) In major power stations and major sub-stations 12 mm thick, 1200 m long, 1200 mm wide Cast Iron plates are used.
B) For minor sub-stations 18 mm broad, 50 x 50 cm. G.I. plates are used. These plates are dug vertically in the pit. Coal, sand and salt are filled in the pit each of 150 mm layer. The plate should be dug deep so that soil will be wet from all sides. The plate should be placed at a distance of 1200 cm. from each other.
2) Pipe Earthing.
A) For Power Stations, and major sub-stations 12 mm thick, 150 mm dia, 300 cms. Long G.I. pipes are used. A minimum distance of 1200 cm. should be kept between earth electrodes in major sub-stations and 180 cms in case of minor sub-stations. At least one electrodes to be used at every corner in sub-stations. Each electrical equipment structure and the entire metal fitting should be earthed. Three earth electrodes are used for pole-mounted transformer. The transformer neutral and body should be double earthed. A minimum 8 S.W.G. G.I. wire should be used for earthing. Separate earthing should be done for distribution box. 7/10 SWG G.I. wire should be used for tower earthing, and for S/s. gantries 3mm thick, 50 mm x 50 mm M.S. plate is used. G.I. pipe of 25 mm thick, 1910 mm long is used for H.T. line, and 20 mm thick, 1720 mm long for L.T. lines.
The purpose of coal and salt is to keep wet the soil permanently. The salt percolates and coal absorbs water keeping the soil wet. Care should always be taken by watering the earth pits in summer so that the pit soil will be wet. Coal is made of carbon which is good conductor minimizing the earth resistant. For sub-station earthing the provisions in I.S.I. 3043 (1966) Section 1,2 & 3 should be utilized.
Joints: - Rivets are used for joining the earth system, nut bolts or welding also used depending on the temperature rise of the system.
The joint to earth conductor in switchgear unit or cable sheaths required to be separated frequently, hence nut bolts are used at joint. In case of steel system they should inconsistently be by welding. Only the places where earth testing is carried, should be nut bolted.
All joints should be properly painted. Channel, supporting control boards and panels are used as earth electrodes (This is possible only when they are connected to earth system at both ends.)
EARTH TESTER & EARTH RESISTANCE.
Use of Earth Tester - Earth tester is used for measurement of Earth resistance. If earth resistance is more, proper treatments to be given.
Working of Earth Tester :- There is hand operated D.C.generator. While feeding current to spike. D.C.current is converted into A.C. current by the converter and A.C.current received from spike is again converted in D.C. current by the help of rectifier, while going to generator.
A.C. current is fed to the spike driven in earth because there should not be electrolytic effect.
Measurement of Earth Resistance (Three point method):-
In this method earth tester terminals C1 & P1 are shorted to each other and connected to the earth electrode (pipe) under test. Terminals P2 & C2 are connected to the two separate spikes driven in earth. These two spikes are kept in same line at the distance of 25 meters and 50 meters due to which there will not be mutual interference in the field of individual spikes. If we rotate generator handle with specific speed we get directly earth resistance on scale.
Note :- Spike length in the earth should not be more than 1/20th distance between two spikes.
Four Point Method :- In this method 4 spikes are driven in earth in same line at the equal distance. Outer two spikes are connected to C1 & C2 terminals of earth tester. Similarly inner two spikes are connected to P1 & P2 terminals. Now if we rotate generator handle with specific speed, we get earth resistance value of that place.
In this method error due to polarisation effect is eliminated and earth tester can be operated directly on A.C.
Earth Resistance :- (A) -Earth resistance is depended on following factors.
1) Type of earth soil.
2) Temperature of earth.
3) Humidity in earth.
4) Minerals in earth.
5) Length of electrode in the earth.
6) Electrode shape and size.
7) Distance between two electrodes.
8) Number of electrodes.
(B) Maximum earth resistance allowed is as below :-
Major power station 0.5 .
Major Sub-stations 1.0
Minor Sub-station 2
Neutral Bushing. 2
Service connection 4
L.T.Lightening Arrestor 4 L.T.Pole 5
H.T.Pole 10
Tower 20-30
If earth resistance is more than above values, following Treatments can be made for minimizing resistance.
1) Oxidation on joints should be removed and joints be tightened.
2) Sufficient water should be poured in earth electrode.
3) Earth Electrode of bigger size as far as possible be used.
4) Electrodes should be connected in parallel.
5) Earth pit of more depth & width- breadth be made.
