Electric Earthing And Shock

Electric Earthing and Shock

INTRODUCTION The meaning of the term earthing or grounding is to connect the electrical equipment to the general mass of earth by wire of negligible resistance. This brings the body of the electrical equipment to Zero potential and thus will avoid the shock to the operator. The neutral of the supply system is also solidly earthed to ensure its potential equal to Zero according to Indian Electricity Eale,[“The earthed or connected with earth means connected with the general mass of the earth in such manner as to sure at all times in immediate discharge of energy without danger”.]

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Electric Earthing and Shock

DIFFERENCE BETWEEN Neutral AND EARTH WIRE 1) The neutral wire is connected to the neutral point of the transformer, which is earthed only at Sub Station. The earth wire originated from a solid earth at the Sub-Station and is earthed at not less than four equally spaced points during every 1.6 km run of distribution line. 2) By reference to three phase four wire loads for which student may consult any book dealing with Polyphase System it will be realized that the neutral wire carries the return current in the case of single phase load and the out of balance current in the case of unbalanced three phase loads. The earth wire under normal condition does not carry any current. However, If an earth fault occurs the earth wire will carry large current, which will cause, the fuse to blow out thereby protecting life and equipment. 3) Some times when the neutral work is not available or “broken” one is tempted to connect a single-phase load between a phase wire and the earth wire. This practice can be dangerous because by dowing so the earth wire will assume voltage above earth.

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Electric Earthing and Shock

WHY Grounding Is Required The metal cover of every electrical equipment is earthed to get the protection against Metric Shock.

If the live wire accidentally makes

contact with metal frame the latter will be at the same potential as that of live wire if a person gets touched unknowingly with such a frame he will proved a path to the current from metal frame to earth through his body this is very dangerous to human life in this can be shown in fig. However a metal frame is connected to earth through an earth wire having low resistance path when live wire touches the metal frame and a person also comes in contact with metal frame, there will be two parallel paths through which the circuit will be completed one path is through human body and other through earth wire. The earth wire will offer an easier path as it is having low resistance. The resistance of human body is very high and will get protected. Thus earth wire works as a protective device against the electric shock to human being Human will be saved against fatal accident.

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Electric Earthing and Shock

Methods of Earthing There are mainly three methods of earthing. I)

Earthing through a water main :Before making an earthing connection to the water main, it

must be ascertained that throughout G.I.pipe have been used. Otherwise, if the cement convegeate pipe have been used the eathing will not be effective. When making an earthing connection, care must be taken to limit the contact resistance to the minimum.

For that purpose properly designed earthing damp should be

used. The standard copper lead is fanned out is soldered to make it solid. Then the lead strip is bent round the pipe so that it may be seated properly over the pipe.

The surface of the pipe is dened properly and all traces & greater are

removed and then over it is placed the clamp. In between the clamp and pipe is inserted the lead and is tightened with the screws as shown in fig. This method is however is not popular as water means are of concrete & cement.

2. Pipe earthing :If the water pipe cannot be used as an earth the G.I.pipe of approved length and diameter can be used. The size of the pipe depends upon A) The current to be carried. B) The type of the soil.

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Electric Earthing and Shock According to ISI std. No. 732-1963. The G.I. pipe shall not be less than 38.1 mm. Diameter & 2 m. long for ordinary soil but if the soil is dry and rocky, the length of pipe should be increased to 2.75 m. The pipe is placed up right as shown in fig. And must be placed in a permanently wet ground, the depth at which the pipe should be buried depends upon the condition of the ground moisture. According to I.S. the pipe should be placed at a depth of 4.75 m. it can be less if the soil provide sufficient moisture earlier. The pipe at the bottom should be surrounded by broken pieces of coke or charcoal for a distance of about 15 cm around the pipe. The coke increases the effective area of the earth practically to the out side of the coke bed.

Impregnating the coke with salt decreases the earth resistance generally

alternate system of layer of salt and coke are used for best result as represented in fig. In India in summer season the moisture in the soil will decrease to large extent which will increase the earth resistance. So in order to have effective earth resistance.

Whenever needed, 3 to 4 bucket of water should be put into the

funnel connected to the main G.I. pipe through 19 mm dia. Pipe. The earth lead used must be G.I. wire or G.I. strip (not of copper) of sufficient cross sectional area to carry fault current safely. (It should not be less then electrical equivalent of copper conductor of 12.97 sq.mm (80 swg), cross sectional area.

The earth wire from the G.I. pipe of 19 mm dia.

should be carried in a G.I. pipe of dia.12.7 mm at a depth of about 60 cm. bellow the ground. Further when the earth wire is carried over from one machine to the other it should be well protected from mechanical injury by, caring the earth wire in a recessed portion.

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Electric Earthing and Shock 3) Plate Earthing :The earth connection can again be provided with the help of a copper plate or a G.I. plate, when G.I. plate, is used it should not be less than 00 cm x 60 cm. x 6.36 m while for copper plate these dimension may be 60 cm x 60 cm x 3.18 mm. However the used of the copper plate these days is limited. The plate is kept with its face vertical at a depth of 3 m. (10 ft.) and is so arranged that it is embedded in an alternate layer of coke and salt for a minimum thickness of about 15 cm. in case earthing is done by copper plate & in coke layer of 15 cm. if it is done with G.I. plate. The earth wire is securely bolted of which are shown in fig. It should be remembered that the nuts and bolts must be of copper for copper plate and should be of G.I. for galvanized plate. The other details of plate earthing are same as that of G.I. pipe earthing.

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Electric Earthing and Shock

Points to remember while providing earthing. 1) Distance :The distance of the electrode from the installation at the premises should be more than 1.5 meter.

2) Cross - Section of the earthing lead :It should never be less than half the cross - section of the mains wire or conductor. The selection should be made in such a way that even in worst condition it shall not fuse out. The minimum size of the earthing lead should not be less than electrical equivalent of copper conductor of 8 SWG (12.97 sq.mm x -section). The size of earthing lead for various loads and installation are given below.

3) Cross section of earth continuity conductor :The size of the earth continuity conductor should not be less than 14 SWG (2.894 Sq.mm.)

4) Electrode :As already discussed, the earthing can be done either with plat or pipe electrode. It should be remember that the material for earth electrode & earth lead should be same and the electrode should always be placed in vertical portion. The various size of earth electrode are given in table.

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Electric Earthing and Shock

5) Earth resistance :The earth resistance of any earthing should not be more than 5 cms. As already soil that for better performance at power hour and Sub-Station or for high horse power motor beyond 100 H.P. The earth resistance should be up to 0.5 cm. & 1 cm. respectively. The resistance of earth dependent upon area of electrode in contact with the general mass of earth.

The quantity of earth, chock & salt.

The

resistance of an existing earth can be increased by purring water for G.I. plate earthing salted water for copper plate earthing.

The low resistance can be

obtained by increasing the X-section area of electrode.

S

Types of Load &

Size of the earth

Size of the earth

r.N

Installation

Lead copper : G.I.Pipe.

electrode

o. SWG

Area in

SWG

Sq.mm

1.

Consumers

8

Govt. Polytechnic, Washim

12.97

Area in

Copper

G.I.

60 x 60 cm2 x

60 cm x 60 cm x

Sq.mm

8

12.97

8

Electric Earthing and Shock residential 2.

3.18 mm

premises Pole earthing

of

transmission

&

6.35 mm.

--//--

--//--

--//--

--//--

--//--

--//--

3.

distribution line Industrial load

--//--

--//--

--//--

--//--

--//--

--//--

4.

upto 10 HP Industrial

--//--

--//--

6

18.68

--//--

--//--

load

between 10 to 15 5.

HP Industrial

load 6

18.68

2

38.60

--//--

90 cm x 90 cm x 6.35 mm.

between 15 to 30 6.

7.

HP Industrial

load 4

Used

90 cm x 90 cm

50 HP Industrial

mm

load i) 2

HP

Industrial

load

above 100 HP

9.

-not-

between 30 HP to

between 50 to 100 ii)Co

8.

27.27

Power Houses &

x

Not used

6.35

38.60 12.7

pper

mm

strip

2.54

Copp

mm 25.4

er

mm

strip

2.54

-do-

mm -do-

-not-

Used

-//-

Not used

-not-

Used

-//-

Not used

Sub-Stations

Conclusion As per the above theory of earthing. It is clear that earthing is most important in installation system & without earthing all the power system is incomplete & risky.

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Electric Earthing and Shock Reference :Electrical winning, estimating & costing --by UPPAL

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Electric Earthing and Shock

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Electric Earthing and Shock

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Electric Earthing and Shock

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