Earthing connects electrical equipment to the ground for safety, preventing excessive voltage and protecting both lives and equipment from faults and overvoltage. Various methods, such as pipe and plate earthing, utilize materials like galvanized iron and ensure low earth resistance through techniques involving coke and salt. Circuit breakers and fuses are safety devices that isolate faulty circuits, with different types available based on their operating conditions and current classifications.

29. Earthing
• Earthing refers to connecting the electrically
conductive part of an electrical equipment to the
ground through earth plate or electrode with
negligible resistance for safety.
• It helps in protecting the equipment and
provides a return path for the leakage or fault
current to pass to the ground through electrode.
• Galvanised iron is used as the material for
earthing purpose.

30. Functions of Earthing
(i) It ensures that electrical equipment through
which the current flows does not rise to a potential
greater than its designed value
(ii) It provide safety to human lives. It is noted that
the neutral of the supply system is also earthed.

31. Need for Earthing
(i) Protect the human lives and electrical
equipment from fault current.
(ii) Maintain the voltage at a constant level even
when a fault occurs in any system.
(iii) Protect the electrical equipment and buildings
from over voltages occurring due to lighting.
(iv) Provide a return path for the fault current
occurring in the system.
(v) Prevent fire in electrical systems

32. Terms Related to Earthing
(i) Earth: The connection between electrical systems to the buried plate in the ground is
called Earth.
(ii) Earthed: The electrical system that is connected to the ground through an electrode
is called earthed device or earthed.
(iii) Solidly Earthed: If there is no protection device like fuse, circuit breaker, etc.,
between an electrical system and the ground, then it is called solidly earthed.
(iv) Earth Electrode: The conductor that is buried inside the ground to provide safety for
the electrical system is called Earth Electrode and it is available in different shapes.
(v) Earthing Lead or Earth conductor: The connection between the electrical system and
the earth electrode is done with the help of a conductor wire or strip called Earthing
lead.
(vi) Earth Continuity Conductor: The conductor wire or strip that is used to connect
different electrical devices is called earth continuity conductor. It is also defined as
the conductor wire used in connecting earthing lead and electrical devices and it is
available in different shapes.
(vii) Sub Main Earthing Conductor: The conductor that helps in connecting switch board
and distribution board is called sub main earthing conductor.
(viii) Earth Resistance: It is defined as the total resistance between earth electrode and
earth. Also, it is defined as the algebraic sum of the resistances of earth continuity
conductor, earthing lead, earth electrode and earth.

33. components of earthing system
(i) Earthing continuity conductor
(ii) Earthing lead or earth conductor
(iii)Earth electrode

34. Methods of Earthing
(i) Pipe earthing and
(ii) Plate earthing.

35. Pipe Earthing
• In this method, a galvanised iron (GI) pipe is used as an earth
electrode. The length and diameter of pipe depend upon the
current to be carried and soil type to which earth electrode is
buried.
• According to I.S.I standard, the diameter of the pipe used for
earthing should be greater the 3.81 cm and length of the pipe
used should be 2 m, 2.75 m and 1.75 m for ordinary, dry and rocky
soils respectively.

36. Pipe Earthing
• The GI pipe is vertically placed and buried in the wet
ground. The depth at which the GI pipe is to be buried
depends on moisture level of the ground. According to I.S.I
standard, the depth should be 4.75 m and it can be less if
the moisture content of the ground is sufficient.
• In the underground, broken coke or charcoal piece is used
to surround the GI pipe for a distance of 15 cm. It is
necessary since coke along with salt helps in decreasing
the earth resistance.
• Generally, alternate layers of coke and salt are used. In
summer, to prevent the increase in earth resistance due to
decrease in moisture level, buckets of water is poured to
the funnel connected to GI pipe.

37. Plate Earthing
• In this type of earthing, a GI or copper plate is used as an earth electrode.
• The size of GI plate should be greater than 60 cm * 60 cm * 6.35 mm and the size
of copper plate should be 60 cm * 60 cm * 3.18 mm.
• These plates with their face vertical are buried inside the ground so that the
distance between the ground level and plate is greater than 2 m.
• If the moisture content of the place is high and it should be 0.6 m away from
building foundations. The plate shall be completely covered by 15 cm of coke and
salt.

38. Factors to be Remembered in Providing
Earthing
The factors to be considered while providing earthing are:
(i) Distance: The distance between earth electrode and electrical system should
be greater than 1.5 m.
(ii) Cross Section of the Earthing Lead: The cross section of the earthing lead
should be greater than half the cross section of the main wire or conductor. Its
minimum size should not be less than 12.97 sq. mm.
(iii) Cross Section of the Earth Continuity Conductor: The size of the earth
continuity conductor should be greater than 2.894 sq. mm.
(iv) Electrode: The material used for earth electrode and earth lead should be
same and it should always be placed in vertical position. The size of the electrode
varies with respect to load and insulation material.
(v) Earth Resistance: The maximum value of the earth resistance should be 5 W.
The earth resistance depends on the electrode area in contact with the ground,
coal, salt and quantity of earth.

39. Circuit breaker
• A switching device, which can be used to make or break a circuit
manually, automatically or with the help of remote control, under
different conditions i.e., normal and under faulty conditions, is
known as a circuit breaker.
• Special attention must be given while designing a circuit breaker,
to safely interrupt the arc produced during its operation.

40. • The fixed and moving contacts, which are called
electrodes, exist in a normal circuit breaker.
• The medium in which these contacts are placed could
either be oil or air.
• When the power system is operating normally, these
contacts will remain closed and will not open
automatically until a faulty condition occurs in the system.
• Whenever a fault occurs in the system, these contacts can
be opened either manually or automatically or by using a
remote control.
• During a faulty condition, due to energization of trip coils
of the circuit breaker, the moving contact is pulled apart,
which opens the circuit and an arc is formed between
these contacts.

41. • An arc develops between the fixed and moving
contacts, when a fault occurs in the power system.
• The faulty current in the power system will continue
to flow until this arc is extinguished or stopped.
• Therefore, the formation of the arc not only delays
the interruption of faulty current, but also generates
huge amounts of heat, which might cause damage
to the power system or the circuit breaker itself.
• Hence, it is necessary to extinguish the arc
developed in a short interval of time, so that the
magnitude of heat generated will not exceed a
maximum value. (High-resistance method and Low-
resistance method or current zero method)

42. Classification of Circuit Breakers
Circuit breakers are classified based on:
(i) medium in which the circuit breaker operates
● Oil circuit breaker
● Air Blast circuit breaker
● SF6 circuit breaker
● Vacuum circuit breaker
(ii) actuating signal in which it works
● Spring-operated circuit breaker
● Pneumatic circuit breaker
● Hydraulic circuit breaker

43. (i) Construction type
● Low-voltage circuit breakers (< 1 kV)
● Medium-voltage circuit breakers (1-72 kV)
● High-voltage circuit breakers (> 72 kV
(ii) Based on voltage level:
● Outdoor circuit breaker
● Indoor circuit breaker

44. Fuse
• A fuse is a short piece of wire or a thin strip of metal, which is inserted
in series to the circuit.
• When the fault current flows through the fuse for a sufficient time, it
melts the fuse, thus isolating the circuit.
• Under normal operation, the fuse is kept at a temperature below the
melting point of the material used, which helps in carrying the normal
current without any rise in temperature.
• But when fault occurs in the power system due to a short circuit or
when an overload current, which is greater than the normal current,
flows through the fuse, this fault current will increase the temperature
above the melting point of the material used for the fuse.
• Hence, the material melts or blows, thereby isolating the healthy part
and protects the circuit.
• The magnitude of excessive current flowing in the circuit is an
important factor in deciding the time taken for melting or blowing out
the fuse. Greater the fault current, lesser the time required to melt or
blow out the fuse.

45. Advantages and Disadvantages
Advantages :
1. Cheapest form of protection device.
2. Requires no maintenance.
3. Operation of fuse is completely automatic.
4. Easily breaks a large amount of fault current.
5. Pollution-free protection device i.e., does not create any smoke or noise.
6. Suitable for over-current conditions due to its inverse current–time
characteristics.
7. Requires less time for isolating the faulty part of the circuit.
Disadvantages
1. Rewiring or replacing a fuse takes a considerable time.
2. Discrimination between fuses connected in series is not possible.
3. Correlation of the characteristics of fuse with the protected device is not always
possible.

46. Characteristics of Fuse Element Materials
Low melting point, e.g., tin, lead
High conductivity, e.g., silver, copper
Least reactive to oxidation, e.g., silver
Affordable, e.g., lead, tin, copper

47. Materials used as fuse
Lead, tin, copper, zinc and silver are the most
commonly used fuse materials.
Tin, or an alloy of lead and tin (0.37 and 0.63
respectively) is used as a fuse element material,
where the rating of current is up to 10 A.
For larger currents, copper or silver is used as
fuse element material. Usually, the copper is
tinned to prevent oxidation effect.
Zinc, in strip form, is used where a considerable
time-delay is required

48. Classification of Fuses
1. Low-voltage fuses
– Semi-enclosed re-wireable fuse
– High rupturing capacity (HRC) cartridge fuse with
and without tripping device
2. High-voltage fuses
– Cartridge type
– Liquid type
– Metal-clad fuses

49. Comparison Between Circuit Breaker and
Fuse