[Q:-01]
What do you mean by 'Earth' in Electrical Engineering ?
Ans :
In electrical engineering 'earth' means
the conductive mass of the earth whose electric potential at any point is
conventionally taken as zero.
[Q:-02]
What is an earthing ?
Ans :
Earthing means connecting the non current
carrying metallic parts of electrical apparatus or the neutral point of the
supply system to the general mass of earth by means of an earth electrode in
such a manner that immediate discharge of electrical energy can take place at
all time without effecting any danger.
[Q:-03]
What is the difference between system earthing and equipment earthing ?
Ans :
System earthing means the intentional
connection of the neutral point i.e the star point of generator, transformer,
rotating machine and grounding transformer to the general mass of earth whereas
equipment earthing means the intentional connection of the non current carrying
metal parts of the motor body, generator body, switchgear structure,
transformer core and tank, sheaths of cable, body of portable apparatus etc. to
the general mass of earth.
[Q:-04]
What are the objects of equipment earthing ?
Ans :
The objects of equipment earthing are
(i) to eliminate the
danger of serious injury from electric shock to persons who may come in contact
with the metal frames of electrical equipment which may have become live due to
the insulation failure.
(ii) to flow the
earth fault current easily through earthing causing operation of over current
protective system or earth fault relay without creating a fire or explosive hazards
to building and contents and
(iii) To contribute
to better performance of the electrical system.
[Q:-05]
What is good earthing ?
Ans :
Good earthing is that earthing which gives
very low resistance to the flow of heavy leakage current of a circuit to earth.
[Q:-06]
What are the advantages of neutral point earthing ?
Ans :
The advantages of neutral point earthing
are as follows :—
(1) Under earth fault
condition the phase to earth voltages do not rise to high values.
(2) Under earth fault
condition it ensures efficient and fast operation of protective gear.
(3) Earth fault relays
can be provided to isolate the faulty portion.
(4) Arcing ground can
be avoided by using suitable protective gear.
(5) The magnitude of
the transient voltage can be limited to a very small value.
(6) The induced
static charges can be conducted to earth immediately before creating any
disturbance to the power system.
(7) The operation and
maintenance expenditure becomes less than that of isolated neutral system.
(8) From the safety
point of view earthed neutral system is much better.
[Q:-07]
What are the methods of neutral earthing ?
Ans :
The different methods of neutral earthing
are
(i) Solid earthing,
(ii) Resistance
earthing,
(iii) Reactance
earthing &
(iv) Arc suppression
coil earthing.
[Q:-08]
What Is called solid earthing ?
Ans :
When a direct metallic connection is made
between the system neutral and earth then it is called as solid neutral
earthing.
[Q:-09]
What Is called resistance earthing ?
Ans :
When a current limiting device resistance,
metallic or liquid, is introduced in between neutral and earth connection to
limit the earth fault current then it is called as resistance neutral earthing.
[Q:-10]
What Is called reactance earthing ?
Ans :
When an impedance, predominantly
inductive, is inserted in between the neutral point of generator or transformer
and earth connection then it is called as reactance earthing.
[Q:-11]
What are the advantages of reactance earthing over resistance earthing ?
Ans :
The advantages are as follows :
(i) For the same
maximum earth fault current the reactor device can be made smaller than the
corresponding resistance and
(ii) Dissipation of
electrical energy in the reactor device is very much less.
[Q:-12]
What is arc suppression coil earthing ?
Ans :
Arc suppression coil earthing is a logical
development of reactance earthing and is based on a value of reactance in the
system neutral such that the reactance current due to the coil exactly neutralizes
the network capacitance current at the fault. The resultant current being nil
is quite inadequate to maintain any arc at the earth fault point. Hence the
inductive coil is named as arc suppression coil which is usually provided with
tappings so that its value can be adjusted to suit the capacitances of the system.
It is also called in the name of its inventor as Peterson coil earthing.
[Q:-13]
What are the essential elements of an earthing system of an electrical
installation ?
Ans :
The essential elements of an earthing
system are earth electrode, earthing conductor, earthing terminal or earthing
bar, protective conductors, and earth continuity conductor.
[Q:-14]
What is an earth electrode ?
Ans :
Earth electrode is a metal plate, pipe or
other conductor or an array of conductors in intimate contact with and
providing an electrical connection to the general mass of earth.
[Q:-15]
What types of electrodes are generally used for earthing ?
Ans :
The following types of electrodes are
generally used :
(i) Pipe electrode,
(ii) Plate electrode,
(iii) Rod electrode
and
(iv) Strip electrode.
[Q:-16]
What should be the minimum size of (i) Pipe electrode, (ii) Plate electrode,
(iii) Rod electrode, (iv) Strip electrode ?
Ans :
(i) Pipe electrodes
shall not be smaller than 38 mm internal diameter if made of galvanised iron or
steel and 100 mm internal diameter if made of cast iron. The length of the pipe
electrode shall not be less than 2.5 metres.
(ii) Plate electrodes
when made of galvanised iron or steel the size shall not be less than 60 cm x
60 cm x 6.30 mm and the plate electrodes when made of copper the size shall not
be less than 60 cm x 60 cm x 3.15 mm.
(iii) Rod electrodes
of steel or galvanised iron shall be at least 16 mm in diameter and those of
copper shall be at least 12.5 mm in diameter. The length of rod electrode shall
not be less than 2.5 metres.
(iv) Strip electrodes
shall not be smaller than 25 mm x 1.60 mm if of copper and 25 mm x 4 mm if of
galvanised iron or steel. The length shall not be less than 15 metres.
[Q:-17]
What should be the distance of the earth electrode from the building ?
Ans :
Not less than 1.5
metres (5 ft).
[Q:-18]
What do you mean by the resistance area of an earth electrode ?
Ans :
The surface area of ground around an earth
electrode on which a significant voltage gradient may exist is known as the
resistance area of that electrode.
[Q:-19]
What do you mean by earth grid ?
Ans :
Earth grid means a system of grounding
electrodes consisting of interconnected connectors buried in the earth to
provide a common ground for electrical devices and metallic structures.
[Q:-20]
What do you mean by earth mat ?
Ans :
Earth mat means a grounding system formed
by a grid of horizontally buried conductors and which serves to dissipate the
earth fault current to earth and also as an equipotential bonding conductor
system.
[Q:-21]
What do you mean by protective conductor and how is it marked ?
Ans :
When a conductor is used as a measure of
protection against electric shock and intended for connecting exposed
conductive parts, extraneous conductive parts, main earthing terminal or any
earthed point of the source then it is called as protective conductor. The
protective conductor shall be marked with alphanumeric notation `PB' and
bicolour combination of green and yellow.
[Q:-22]
What is extraneous conductive part ?
Ans :
Extraneous conductive part is that conductive
part which is liable to transmit a potential including earth potential and not
forming part Of the electrical installation such as gas pipes, other service
pipes, and ducting, risers and pipes of fire protection equipment, exposed
metallic parts of the building structure, lightning conductors etc.
[Q:-23]
What do you mean by earthing conductor and how is it marked ?
Ans :
Earthing conductor is the final protective
conductor by which the earth electrode is connected to the main earthing
terminal (or the equipotential bonding conductor of an installation where there
is no earth bus). Earthing conductor shall be marked with alphanumeric notation
'E' .
[Q:-24]
What is main earthing terminal ?
Ans :
Main earthing terminal or earth busbar is
the intermediate connecting link between the protective conductor and the
earthing conductor used in large installation.
[Q:-25]
What do you mean by earth continuity conductor of wiring Installation ?
Ans :
Earth continuity conductor of wiring
installation means the protective conductor, including any clamp, running along
the wiring installation and connected to all metal parts which are required to
be earthed and finally to earthing terminal or earthing conductor.
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