electric earthing and shock
TRANSCRIPT
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ELECTRIC EARTHING AND SHOCK
PRESENTED BY
P.RADHIKA
06C21A0241
TIRUMALA ENGINEERING COLLEGE
KEESARA
BOGARAM
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CONTENTS:
1. Introduction
2. Difference between neutral and earth wire
3. Why grounding is required
4. Methods of earthing
.Earthing through a water main
.Pipe earthing
.Plate earthing
5. Points to remember while providing earthing
.Distance
.Cross section of the earthing lead
.Crosse section of the earth continuity conductor
.Electrode
.Earth resistance
6. Conclusion
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INTRODUCTION:
The meaning of the earthing or grounding is to connect the
electrical equipment to the general Mass of the earth by wire
of negligible resistance. This brings the body of 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 Eale. [The earthed or connected
with earth means connected with the general mass of theearth In such manner as to sure at all times in immediate
discharge of energy without danger]
Difference between Neutral and Earth Wire:
1. The neutral wire is connected to the neutral point of thetransformer; which is earthed only at substation.The earth
wire originated from a solid earth at the substation and is
earthed at not less than four equally spaced points during
every 1.6km run of distribution line.
2. By reference to three phase four wire loads for which
student may consult any book dealing with poly phase
system it will be realized that the neutral wire carries the
return current in the case of single phase load and out of
balance current in the case of unbalanced three phase
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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 temted to connect a single phase load
between aphase wire and earth wire. Thise practice can be
dangerous because by dowing so the earth will assume
voltage above earth.
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 to earth
through his body this is very dangerous to human life in
this.
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 parallel paths through which
the circuit will be completed one path is through human
body and other earth wire. The earthed wire will offer an
easier path as it is having low resistance. The resistance
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device against the electric shock to human being human will
be saved against fatal accident.
METHODS OF EARTHING
There are mainly three methods of earthling
1.EARTHING THROUGH WATER MAIN: Before making an
earthing connection to the water main,it must be
ascertained that throughout GALVANIZED IRON pipe have
been used other wise ,if the cement convegeate pipe have
been used the eathing damp should be used . the standard
copper lead is fanned out is soldered to make it sold .thenthe lead strip is bent round the pipe so that may be seated
properly over the pipe .the surface of the pipe is dened
proply and all trace & grater are removed and then over it is
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placed the clamp in between the clamp and pipe is inserted
the lead and is tighted with the screws
2. pipe earthing:- If the water pipe cannot be used as
an earth the
GALVANIZED IRON pipe of approved length and diameter can
be used .the size of the pipe depends up on
A) The current to be carried.
B) The type of the soil.
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According to ISI std.NO.732-1963.the GALVANIZED IRON
pipe shall not be less than 38.1 mm. diameter & 2 m
.long for ordinary soil but if 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 permanetly 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 moister earlier. Pipe atthe 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 side of the coke bed .impregnating
the coke with salt decreases the 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 decreases 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 GALVANIZED IRON pipe
through 19 mm dia pipe.
The earth lead used must be
GALVANIZED IRON wire or GALVANIZED IRON stripe (not
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of copper) of sufficient cross sectional area 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 wire from the GALVANIZED IRON pipe
of 19 mm dia . should be carried in a GALVANIZED IRON
pipe of dia 12.7 mm at a depth of about 60 cm . bellow
the ground .further when the wire is carried over from
mechanical injury by,caring the earth wire bin a recessed
portion.
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3. PLATE EARTH ING: - The earth connection can again
cable provided with the help of a copper plate or IRON a
GALVANIZED. plate ,is used it should not be less then 00cm
x 60 cm x 6.36 m while for copper plate these dimension
may be 60 cm x 60 cm x 3.18mm . how ever the used of the
copper plate these days is limited.
The plate is kept with its phase vertical at a
depth of 3m.(10ft)and is so arranged that it is embedded in
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a alternate layer of cock and salt for a minimum thickness of
about 15cm.In case earthing is done by copper plate and in
cock layer of 15cm. if it done GALVANIZED IRON 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 GALVANIZED
IRON.For galvanized plate the other details of plate earthing
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are same as that of GALVANIZED IRON.pipe earthing.
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Points to remember while providing earthing:
1) Distance :-
The distance of the electrode from the installation at thepremises should be more than 1.5 meter.
2) Cross - Section of the earthing lead :-
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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 lessthan
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
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
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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.
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Conclusion:
As per the above theory of earthing. It is clear that earthing
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