brief discussion about electrical installation earthing different type of fuse and cables

1. Electrical System Design
Suhail TP
MES Collage of Engineering

2. Design Considerations of
Electrical installations
ELECTRIC SUPPLY SYSTEM
250 V or less : LV
251 V to 650 V : MV
651 V to 33 kV : HV
Above 33 kV : EHV
DISTRIBUTION SYSTEM
Single phase ac supply using a 2 – wire system;
Three phase ac supply using 3 – wire system;
Supply of Three phase and neutral using a 4 wire system.
DC SUPPLY
A two wire system at 220 V
A three wire system with 440 V between the two outer conductors and 220V
between the outer conductors and the Centre wire

3. • SUPPLY VOLTAGE
– Single phase : 240 V, 50 Hz, 2 wire
– Three phase : 415V , 50Hz, 4 wire
Consumers with load requirement more than 250 kVA are provided with
supply at high voltage with a substation installed in the consumers
premises where voltage is stepped down to 415/240 V
– In India and many European countries the frequency of the supply is
50 Hz
– In USA 60 Hz
VOLTAGE TOLERANCES
Current carried by the electrical power distribution network varies at different times
of the day
It leads to varying voltage drops in the supply cables
Under IE Rules voltage fluctuations may not vary by more than 5 % above or below
the declared nominal voltage , frequency 1% from 50 Hz

4. CONDUCTORS AND CABLES
• All conductor must me surrounded by insulators
• In over head tr air act as insulation. Insulation requirement at supports
through which lines are suspended from the ground
• The conductor with insulation- A Cable
• Mostly used insulation in cables are
– paper (earliest material, works as good insulator as long as it is kept dry)
- Rubber ( from 1910 to 1950)
- Plastics ( most common PVC,)
For cables for withstanding high temperatures we use silicon- rubbers
STRANDED CONDUCTORS
Stranded conductor consist of no of Cu or Al wires place together like a rope
Stranding is employed to make the cable flexible
Stranded conductor is expressed as 3/.029, 7/.026 etc
first no represent no of strands and second no gives the diameter of each strand
in inch or mm

5. RATING OF CABLES
• Cables are always assigned a rating
• For rubber and plastic insulated cables rating is based on the
• temperature which the insulation can withstand.
• When several plastic insulated cables run together in the same duct
or conduit, rating may be reduced since each will tend to heat each
other
CRITERIA FOR SELECTING CABLE
• Minimum criteria for selecting a cable
It should be able to carry maximum expected current without the
insulation getting damaged
• Voltage drop inside the cable should be permissible
VOLTAGE DROP
• Depends on current , cross sectional area of the cable, and its length
• Voltage drop between supply point in any building and any outlet should not be
more than 2.5% of the nominal voltage

6. PROTECTION OF ELECTRIC INSTALLATIONS AGAINST OVERLOAD,
SHORT CIRCUIT AND EARTH FAULT
BASIC CONSIDERATIONS
1. There should be ready means of isolation of a circuit in the event of any
accident or for the purpose of servicing. This is provided by means of
switches
2. Conductors used should have adequate size. ie. Cross sectional area so that
the current they will carry will produce minimum heat and voltage drop.
3. Protection should be provided against excess current and electric shock
Excess Currents
Rated current for a wire is that value of current which does not
cause the temperature of the wire to reach a value which would damage the
insulation.
Any value of current which causes the insulation to be damaged is
called excess current or overload current.

7. • The wire size selected according to a certain expected load.
• If the load exceeds this value, system will draw the excess current.
• Value of current depends on the extend of overload
CAUSES OF OVERLOADS
• By switching on heating or light loads of higher power rating.
• By switching on a large no of heating or lights than what the wiring designed
for.
• Mechanical overloading of motors.
• Mechanical faults in the motor.
• Due to large friction at bearings & Jamming of rotor.
SHORT CIRCUITS
• When two phase wires or phase wire to neutral wire make direct contact.
• Fuses are used to prevent damage, fire

8. EARTH FAULT
• When the phase wire makes contact with earth or any conducting material
which is earthed, a short circuit exist between the wire and earth. Called
earth fault
• Fuses are used to interrupt large current caused by earth fault.
FUSES AND CIRCUIT BREAKERS

9. • When the excess current is very high the operating time is very short
• When the excess current is small the operating time is longer
• When short circuit occurs the resulting current will be extremely high and its
flow should be arrested in shortest possible time
• When extra load is switched on which may draw say 25% higher than the rated
current.
• This current will not cause any immediate damage, the protective device should
act after a time lag
FUSE
IS: 732-1983
A device that by fusion of one or more of its specially designed and proportional
components , opens the circuit in which it is inserted when the current through it
exceeds a given value for a sufficient time. The fuse comprises all the parts that
form the complete
• A fuse wire must have its rated current I 60 to 90 % of the critical current.
• Critical current is the value of current at which the fuse melts.
• A fuse wire of 5A rating will have a rating 5.6A to 8A

10. REWIRABLE FUSE
A re- wirable fuse consists of a length of wire made either of tinned cu or other
metal of same size
It will melt and break the circuit if the current rises above the rated value
Cheap and simple in construction
Have the draw back of erratic in operation and of deterioration with time
Rated up to 200A

11. CATRIDGE FUSE
Fuse wire is enclosed in a sealed cartridge packed with a filler material which
will prevent deterioration
a) Diazed fuse (D type cartridge fuse)
b) Link type ( HRC fuse link)

12. D- TYPE CARTRIDGE FUSE
• Rating up to 2 to 63 amperes
• Quartz is used as the filler material
• Enclosed in a high compression ceramic material designed to withstand the
mechanical and thermal stress.
• Filler material is used for cooling the arc after the melting of fuse element
• A small coloured disc on top of the cartridge fuse is used as the indicator for the fusing
of a contact

13. HRC FUSE
• Available in rating from 2 to 1250 A
• High rupturing capacity
• Higher speed of operation
• Operation is silent and temperature rise is low

14. PROTECTION AGAINST ELECTRIC
SHOCK

15. EARTHING
Earthing means connecting earth terminals to electrodes installed solidly in
the mass of earth.
The objective of earthling is to ensure that a fault to earth produces the same
condition as a short circuit between the line and neutral cables.
The path of earth fault current includes the earth wire in the consumers
premises and the general mass earth between the earth point and the earth
electrode at the substation.
In under ground cables , the armour of the cable can be connected to earth
terminal.

16. EARTHING AND SOIL RESISTIVITY
• Resistance to earth of an electrode of given dimensions is dependent on
the electrical resistivity of the soil in which it is installed
• Earth conductivity is electrolytic in nature
• It is affected by moisture content, soil chemical composition, and
concentration of salts dissolved in the contained water.
• In places with high soil resistivity , the resistivity of soil immediately
surrounding the earth electrode should be reduced.
• This is done by using some artificial agents
• Sodium chloride, calsium chloride, copper sulphate, Common salt and soft
coke, common salt and charcoal in suitable proportion.

17. EARTH ELECTRODES
• Under ordinary conditions of soil use of copper, iron or mild steel
electrodes is recommended
• When soil conditions are such as are likely to cause excessive corrosion of
the electrode it is recommended to use either cu electrode or cu clad
electrode, or galvanised iron electrode
• There are two main earth electrodes in use
– Rod and pipe electrodes
– Plate electrodes

18. ROD AND PIPE ELECTRODES
• Made of metal rod or pipe having clean surface not covered by paint, enamel,
or poor conducting material
• Rod electrodes of steel or GI should be at least 16 mm dia, and that of cu
should be at least 12.5 mm in dia
• Pipe electrodes GI or steel should not be smaller than an internal dia of
38 mm, If cu – 10 mm.
• Min length of rod or pipe electrodes 2.5m
• Pipes and rod should be driven to depth of at least 2.5m.
• In rocky area if buried inclined to vertical depth should be 2.5m and inclination
not more than 30 degree from vertical
• If it is necessary to reduce the depth, it must be done without reducing the
resistance
• This can be achieved by connecting no of pipes or rods in parallel
• In such case distance between two electrodes should preferably be not less
than twice the length of the electrodes.

19. PLATE ELECTRODES
• GI or steel thickness not less than 6.30 mm
• If Cu thickness not less than 3.15 mm
• The size of plate electrode should be 60 cm X 60 cm
• Plate electrode should be burried such that the top edge is at a depth of
not less than 1.5m below the surface of the ground
• If plates are used in parallel two plates being separated from each other
by not less than 8.0m

20. Circuit Breaker
• A circuit breaker is a manually or automatically
operated electrical switch designed to protect an electrical
circuit from damage caused by overload or short circuit.
• Its basic function is to detect a fault condition and interrupt
current flow.
• Unlike a fuse, which operates once and then must be
replaced, a circuit breaker can be reset (either manually or
automatically) to resume normal operation.
• Circuit breakers are made in varying sizes, from small devices
that protect an individual household appliance up to
large switchgear designed to protect high voltage circuits
feeding an entire city.

21. Operation
• All circuit breakers have common features in their
operation.
• Once a fault is detected, contacts within the
circuit breaker must open to interrupt the circuit.
• Mechanically-stored energy (using something
such as springs or compressed air) or energy from
fault current itself are used to separate the
contacts.
• When a current is interrupted, an arc is generated

22. Arc interruption
• Lengthening / deflection of the arc
• Intensive cooling (in jet chambers)
• Division into partial arcs
• Zero point quenching (Contacts open at the zero
current time crossing of the AC waveform,
effectively breaking no load current at the time of
opening. The zero crossing occurs at twice the
line frequency, i.e. 100 times per second for
50 Hz and 120 times per second for 60 Hz AC)

23. Types of CB
Based on medium in which arc is formed, CB can
be classified as
• Air circuit breaker
• Oil Circuit breaker
• Vacuum circuit breaker

24. Air circuit breaker
• Circuit breaker which operates in air at atmospheric pressure.
• It is used below 15 kV
• Arc voltage is defined as the minimum voltage required in maintaining the
arc
• ACB try to increase the arc voltage in three different ways
• By cooling arc plasma
• By lengthening the arc
• By splitting up the arc into a number of series arcs.
• ACB are two types
– Plain air CB
– Air blast CB

25. Operation of ACB
• Force arc into contact with as large an area as possible of insulating
material.
• air circuit breaker is fitted with a chamber surrounding the contact. This
chamber is called 'arc chute‘.
• High temperature plastics reinforced with glass fibre and ceramics are
preferable materials for making arc chute.
• lengthening the arc path, is achieved concurrently with fist objective.
• Inner walls of the arc chute is shaped in such a way that the arc is not only
forced into close proximity with it but also driven into a serpentine
channel projected on the arc chute wall.
• The main arc chute is divided into numbers of small compartments by
using metallic separation plates. These metallic separation plates are
actually Splitting up the arc into a number of series arcs

27. Air Blast Circuit Breaker
• These types of air circuit breaker were used for the system voltage of
245KV, 420KV and even more,
• The breaking speed of circuit breaker is much higher during operation of
air blast circuit breaker.
• Arc quenching is much faster during operation of air blast circuit breaker.
• The duration of arc is same for all values of small as well as high currents
interruptions.
• Requires much less maintenance compared to oil circuit breaker.

28. AXIAL BLAST ACB

31. What is MCB?
• Nowadays we use more commonly Miniature Circuit Breaker or MCB in low
voltage electrical network instead of fuse
• It automatically switches off the electrical circuit during abnormal condition
of the network means in over load condition as well as faulty condition. The
fuse does not sense but Miniature Circuit Breaker does it in more reliable
way. MCB is much more sensitive to over current than fuse.
• The switch operating knob comes at its off position during tripping, the
faulty zone of the electrical circuit can easily be identified. But in case of
fuse, fuse wire should be checked by opening fuse grip or cut out from fuse
base, for confirming the blow of fuse wire.
• Quick restoration of supply can not be possible in case of fuse as because
fuses have to be rewirable or replaced for restoring the supply. But in the
case of MCB, quick restoration is possible by just switching on operation.
• Handling MCB is more electrically safe than fuse.
• Only one disadvantage of MCB over fuse is that this system is more costlier
than fuse unit system.

32. Working Principle Miniature Circuit
Breaker
• There are two arrangement of operation of miniature circuit breaker.
• By thermal and electromagnetic effect of over current
• The thermal operation of miniature circuit breaker is achieved with a
bimetallic strip whenever continuous over current flows through MCB, the
bimetallic strip is heated and deflects by bending. This deflection of
bimetallic strip releases mechanical latch. As this mechanical latch is
attached with operating mechanism, it causes to open the miniature
circuit breaker contacts.
• During short circuit condition, sudden rising of electric current, causes
electromechanical displacement of plunger associated with tripping coil or
solenoid of MCB. The plunger strikes the trip lever causing immediate
release of latch mechanism consequently open the circuit breaker
contacts. This was a simple explanation of miniature circuit breaker
working principle.

33. Operation of MCB
• There are three mechanisms provided in a single miniature circuit breaker
to make it switched off.
• Main parts are one bi - metallic strip, one trip coil and one hand operated
on - off lever.
• Electric current carrying path of a miniature circuit breaker is as follows.
First left hand side power terminal - then bimetallic strip - then current
coil or trip coil - then moving contact - then fixed contact and - lastly right
had side power terminal.

34. Operation of MCB
• If circuit is overloaded for long time, the bi - metallic strip becomes over
heated and deformed.
• This deformation of bi metallic strip causes, displacement of latch point.
The moving contact of the MCB is so arranged by means of spring
pressure, with this latch point, that a little displacement of latch causes,
release of spring and makes the moving contact to move for opening the
MCB.
• The current coil or trip coil is placed such a manner, that during short
circuit fault the mmf of that coil causes its plunger to hit the same latch
point and make the latch to be displaced. Hence the MCB will open in
same manner.
• The current coil or trip coil is placed such a manner, that during short
circuit fault the mmf of that coil causes its plunger to hit the same latch
point and make the latch to be displaced. Hence the MCB will open in
same manner.
• In the manual operation, actually the same latch point is displaced and
same deformed spring is released