FUSE / MCB / ELCB

1. Why we need a fuse?

1. Fuses are used for the prevention of home appliances from the short circuit and damage
by overload, high current etc.
2. If it is not used, when an electrical faults occur in the wiring and it burns the wire
and electric appliances and may starts fire at home.
3. The lives of television, computers, radios and other home appliances may also put at
risk.
4. When the fuse blows, a sudden spark occurs and disconnects the power supply which
saves any further mis-happenings.
5. That’s why we need fuses to protect our home appliances from harm.
2. How does fuse work?
• The fuses work on the principle of the heating effect of the current.
• It’s made up of thin strip or strand of metallic wire with non-combustible material.
• This is connected between the ends of the terminals.
• Fuse is always connected in series with the electrical circuit.
• When the excessive current or heat is generated due to heavy current flows in the circuit,
the fuse melts down due to the low melting point of the element and it opens the circuit.
• The excessive flow may lead to the breakdown of wire and stops the flow of current.
• The fuse can be replaced or changed with the new one of suitable ratings.
3. Which material is used for Fuse?
The fuse can be made up of the element like
(i) zinc,
(ii) copper
(iii)silver and
(iv)aluminum.
4. What are different types of Fuses?
 5. Mention the applications of FUSE.

Re-wirable fuses are low voltage (LV) fuses, which are almost used in small applications like

 wiring in the house, to provide short circuit protection.

 small-scale industries to provide overload and short circuit protection
 other tiny current applications

HV fuses normally used for voltages over 1500V to 138000V and are used to

protect the transformers like instrument transformers,

 small power transformer and

 also used in power systems.
6. What Is Earthing?
 The process of connecting metallic bodies of all the electrical apparatus and
equipment to huge mass of earth by a wire having negligible resistance is called
Earthing.
 The term earthing means connecting the neutral point of supply system or the non
current carrying parts of the electrical apparatus to the general mass of earth in such a
manner that all times an immediate discharge of electrical energy takes place without
danger.
7. What is the importance / needs of Earthing?.
(i) To protect human lives as well as provide safety to electrical devices and
appliances from leakage current.
 (ii) To keep voltage as constant in the healthy phase (If fault occurs on any one
phase)
(iii) To Protect Electric system and buildings form lighting.
(iv) To serve as a return conductor in electric traction system and communication.
(v) To avoid the risk of fire in electrical installation systems.

8. Define the Terms used in Electrical Earthing

 Earth: The proper connection between electrical installation systems via conductor to the
buried plate in the earth is known as Earth.
 Earthed: When an electrical device, appliance or wiring systems connected to the earth
through earth electrode, it is known as earthed device or simple “Earthed”.
 Solidly Earthed: When an electric device, appliance or electrical installation is connected to
the earth electrode without a fuse, circuit breaker or resistance/Impedance, It is called “solidly
earthed”.
 Earth Electrode: When a conductor (or conductive plate) buried in the earth for electrical
earthing system, it is known to be Earth Electrode. Earth electrodes are in different shapes
like, conductive plate, conductive rod, metal water pipe or any other conductor with low
resistance.
 Earthing Lead: The conductor wire or conductive strip connected between Earth electrode
and Electrical installation system and devices in called Earthing lead.
 Earth Continuity Conductor: The conductor wire, which is connected among different
electrical devices and appliances like, distribution board, different plugs and appliances etc. in
other words, the wire between earthing lead and electrical device or appliance is called earth
continuity conductor. It may be in the shape of metal pipe (fully or partial), or cable metallic
sheath or flexible wire.
 Sub Main Earthing Conductor: A wire connected between switch board and distribution
board i.e. that conductor is related to sub main circuits.
 Earth Resistance: This is the total resistance between earth electrode and earth in Ω (Ohms).
Earth resistance is the algebraic sum of the resistances of earth continuity conductor, earthing
lead, earth electrode and earth.
9. What are the qualities of Good Earthing
1. has low electrical resistance
2. has good corrosion resistance
3. must be able to dissipate high fault current repeatedly
10. Purpose of Earthing
 To save human life from danger of electrical shock or death by blowing a fuse i.e.
 To provide protection and safety to the operator.
 To provide protection and safety to the equipment.
 To facilitate the balanced supply conditions.
 To provide safe path to discharge lightning and short circuit currents.
 To isolate the faulty section.
 To protect installation from sudden high voltages, switching surges with lightning
arrestor and surge suppressor
 To provide an alternative path for the fault current to flow so that it will not endanger the
user
 To protect buildings, machinery & appliances under fault conditions ie. To ensure that all
exposed conductive parts do not reach a dangerous potential.
 To provide safe path to dissipate lightning and short circuit currents.
 To provide stable platform for operation of sensitive electronic equipments i.e. To
maintain the voltage at any part of an electrical system at a known value so as to prevent
over current or excessive voltage on the appliances or equipment .
 To provide protection against static electricity from friction
11. What is Equipment Earthing?
Earthing can be done by electrically connecting the respective parts in the installation to
some electrodes placed near the soil or below the ground level.
When there is insulation failure, parts of apparatus connected to earth release the charge
accumulated on them immediately to earth, so that the person coming in contact may not
experience electric shock.

12. Max. Value of Earth Resistance to be achieved Equipment to be Earthed Max.

 Value of Earth Resistance to be achieved in Ohms Large Power Stations 0.5
 Major Substations 1.0
 Small Substations 2.0
 Factories Substations 1.0
 Lattice Steel Tower 3.0
 Industrial Machine and Equipment 0.5 *
 The Earth Resistance depends upon the moisture content in the soil.
13. Which Factors Affect Earth Resistance?
 Soil resistivity
 Soil condition
 Moisture
 Dissolved salts
 Climate conditions
 Physical composition
 Location of earth pit
 Effect of grain size
 Effect of current magnitude
 Area available
14. Which criterion is used for selecting Type of Earthing?
 Plate earthing : where digging is easy
 Pipe earthing : very common, used for domestic, small substations
 Strip earthing : where earth bed hard rocky soil
 Rod earthing : done sandy area/small amp post
15. Methods of Conventional Earthing
1. Plate Earthing
2. Pipe Earthing
3. Rod Earthing
4. Strip Earthing
5. Earthing through Water Mains
16. Explain Plate Earthing
 The earth connection is provided with the help of a copper/G.I. plate.
 If the G.I. plate is used it should not be of less than 60 cm x 60 cm x 6.35 mm. and for
copper plate it should not be of less than 60 cm x 60 cm x 3 mm.
 The plate is kept with its face vertical at a depth of 3 m 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 and in coke layer of 15 cm if it is done with GL
Plate.
 The earth wire is securely bolted to the earth plate with the help of bolt nut and washer.
1
 The earthing electrode should be situated at a place at least 1- 1 metre away from (i.e.
2
outside) the building whose installation system is being earthed.
 The earth wire should be of same material as that of earth electrode.
 The minimum sectional area of the earth lead wire should never be less than 0.02 sq. inch
and not more than 0.1 sq. inch.
 The size of earth conductor as a general rule should not be less than half of the section of
live line conductor.
 The size of the continuous earth wires used with cables should not be either less than
0.0045 sq. in or half of the installation conductor size in case of light wiring.
 17. Explain Pipe earthing
• Excavation on earth of size 2.7 M X 0.6 M X 3.0 M
• Use; GI pipe [C-class] of 75 mm diameter, and required length Having 6 numbers of
holes for the connection of earth wires
• Cover Top of GI pipe with a T joint to avoid jamming of pipe with dust & mud.
• The earth pit are generally filled with alternate layer of charcoal & salt up to 4 feet from
the bottom of the pit.
• The electrical installation which to be earthed, is connected to the top of the earth pipe by
means of copper or aluminum earth continuity conductor of sufficient cross-section.
• Use GI earthing wire of 10/8/6 SWG as per requirement
 18. Explain Rod type Earthing.
 In this system of earthing 12.5mm diameter solid rods of copper 16mm diameter solid
rod of GI or steel or hollow section of 25mm GI pipe of length not less than 3 meters are
driven vertically into the earth
 In order to increase the embeded length of electrode under the ground, which is some
time necessary to reduce the earth resistance to desired value more than one rod
section are hammered one above the other.
 This system of earthing is suitable for area which are sandy in character .
 This system of earthing is very cheap.
19. Enlist Applications of earthing
 Telecommunication
 Transmission
 Substations & Power Generations
 Transformer Neutral earthing
 Lightning Arrestor Earthing
 Equipment Body Earthing
 Water Treatment Plants
 Heavy Industries
 College, Hospitals, Banks
 Residential Building
20. Mention IE rules and IEE (Institute of Electrical Engineers) regulations.
(i) Earth pin of 3-pin lighting plug sockets and 4-pin power plug should be efficiently
and permanently earthed.
(ii) All metal casing or metallic coverings containing or protecting any electric supply
line or apparatus such as GI pipes and conduits enclosing VIR or PVC cables, iron
clad switches, iron clad distribution fuse boards etc should be earthed (connected to
earth).
(iii) The frame of every generator, stationary motors and metallic parts of all
transformers used for controlling energy should be earthed by two separate and yet
distinct connections with the earth.
 (iv)In a dc 3-wire system, the middle conductors should be earthed at the generating
station.
(v) Stay wires that are for overhead lines should be connected to earth by connecting at
least one strand to the earth wires.
21. Compare MCB and MMCB

Basis for MCB MCCB

Comparison

Definition Type of switch which protects Protects the equipment from

the system from overloaded over temperature and fault
current. current.

Abbreviation Miniature Circuit Breaker Moulded case circuit breaker.

Tripping circuit Fixed Movable

Pole Available in single, two and Available in single, two, three

three versions. and four versions.

Interrupting 1800 A 10k A -200k A

Rating

Remote on / off Not Possible Possible

Applications In lightning circuit and for low In heavy current circuit

loads.

22. Explain with neat diagram, operation of ELCB and two applications.
 One terminal of the relay coil is connected to the metal body of the equipment to be
protected against earth leakage and other terminal is connected to the earth directly. If
any insulation failure occurs or live phase wire touches the metal body, of the
equipment, a voltage difference appears across the terminal of the coil of ELCB
(connected to the equipment body and earth). This voltage difference produces a current,
which flows through relay coil. If the voltage difference exceeds, a predetermined limit,
the current through the relay actuates the relay and trips the circuit breaker and
disconnects the power supply to the equipment. Thus it detects leakage current and
protect equipment and the personnel from damage.

Applications of ELCB:

(i) ELCB is used for protection against leakage current.

(ii) It prevents electrical shock and electrical fires that are caused by a short
circuit or overload in the control panel with an automatic shut off when
leakage is detected.
(iii) ELCB also detects faulty and inter-mixing of internal wiring – preventing
potential wastage of electricity.

23. State function of ELCB (Earth Leakage Circuit Breaker).

ELCB helps to break the circuit automatically whenever there is a current leak due to any reason.
Hence a person touching the electric circuit or a device does not get an electric shock. Thus it
prevent injury to humans and animals due to electric shock.

24. What are two types of ELCB

There are two types of Earth-leakage circuit breaker:

 voltage operated (referred as ELCB) and,

 current operated (referred to as RCCB).

25. Give the working of ELCB.

Under normal conditions, the current from the source flows into the load through the live
wire & flows out of the load through the neutral wire. Both currents are equal in magnitude.
If the current leaks through any unintended path, an imbalance between the live & neutral
wire occurs. The ELCB can sense the imbalance using current transformer & break the
contacts using an electromagnetic relay.

26. State any two methods of reducing earth resistance.

Methods to reduce earth resistance are

(i) Increase the diameter of earth rod.

(ii) Increase the length of earth rod.
(iii) Use multiple rods in parallel.
(iv) Perform Chemical treatment of soil.