Explanation of L.T. switch Gear, MCB , ELCB , MCCB , switch fuse unit , Types of wires , Types of cables.

2. -:Presentation:-
 Topic :----
1. Components of LT switch gear
2. Switch fuse unit (SFU)
3. MCB
4. ELCB
5. MCCB
6. Types of wires and cables

3. Components of LT switch gear
 The apparatus used for switching, controlling and protecting
the electrical circuits and equipment is known as
switchgear.
 A switchgear assembly has two types of components :
1. Power conducting components, such as switches, circuit
breakers, fuses and lightning arrestors , that conduct or
interrupt the flow of power.
2. Control system, such as control panels, current
transformers, potential transformers, protective relays, and
associated circuits, and the power conducting components.

4.  The term ‘switchgear’ is a generic term encompassing a
wide range of products like circuit breakers, switches,
switch fuse units, offload isolators, HRC fuses, contactors,
earth leakage circuit breakers (ELCBs), etc....
 A switchgear essentially consists of switching and
protecting devices such as switches, fuses, isolators, circuit
breakers, relays, control panels, lightning arrestors, current
transformers , and various associated equipments.

5. Switch fuse unit (SFU)
 The fuse is a device used in an electrical circuit for
protecting electrical equipment against over loads and short
circuits.
 A fuse is connected in series with the circuit to be protected
as shown in figure and carries the load current without
overheating under normal conditions.
 When abnormal condition occurs , an excessive current
flows through it. This raises the temperature which melts
the fuse element and opens the circuit.
 Thus, this protects the machines or apparatus from damage
which can be caused by the excessive currents.

6.  Fuse Wire Materials : The material used for fuse elements
must be low melting point, low ohmic loss and low cost.
Example: Tin, Lead, Zinc, Silver, Copper, Aluminium.
 Advantages : It is the cheapest form of protection available. It
requires no maintenance. It can break the electrical circuit
without noise or smoke.
 Disadvantages : It has to be replaced after each operation.
 Applications : Protection of cables. Protection of motors.
Protection of semiconductor devices.

7. Miniature Circuit Breaker (MCB)
 A miniature circuit breaker (MCB) automatically switches off
electrical circuit during abnormal condition of the network means
in over load condition as well as faulty condition. Now a days we
use an MCB in low voltage electrical network instead of fuse.
The fuse may not sense it but the miniature circuit breaker does
it in a more reliable way. MCB is much more sensitive to over
current than fuse.
 Handing a MCB is electrically safer than a fuse. Quick
restoration of supply is possible in case of fuse as because fuses
must be rewireable or replaced for restoring the supply.
Restoration is easily possible by just switching it ON. Let’s look
at the working of the miniature circuit breaker.

8.  Application of MCB :
1. MCBs are required in homes, offices, shops distribution
boards for protection of individual circuit and complete wiring
system in that building through main circuits.
2. MCBs are slightly more expensive than fuses and it is very
quick and simple process.
3. They are being used extensively these days and are virtually
replacing the traditional rewireable fuses.
 Advantages of MCB :
1. Easily identify the fault zone of a circuit.
2. Simple to resume supply.
3. Electrically safer and less maintenance.
 Disadvantages of MCB :
1. Only disadvantage over fuse is, this device is costlier.

10. Earth Leakage Circuit Breaker
(ELCB)
 An ELCB is a safety device used for installing an electrical
device with high earth impedance to avoid shock. The main
purpose of ELCB is to stop damage to humans and animals
due to electric shock.
 There are two types of Earth Leakage Circuit Breaker (ELCB)
:- - Voltage Operated ELCB
- Current Operated ELCB
 It is current operated device design to operate when a
leakage current exceeds a predetermined value. It is
essentially consists of an operating coil and a trip mechanism
which operates contacts controlling the supply to the circuit
concerned.

11.  Consider a single phase circuit. The two supply wires i.e. phase
and neutral are fed through separate winding of a current
transformer as shown in figure.
 Another winding control the tripping mechanism. The current
through the phase wire is equal to the current in the neutral
wire i.e.
Ip = In
 Here, phase and neutral conductor are equal in magnitude and
opposite in directions. Thus, no flux will produce.
 However, if more current flows in the phase conductor than in
the neutral conductor as a result of leakage or a fault between
phase and earth, an out of balance flux will result in an e.m.f
being in the trip coil wound on the transformer, which will open
the contacts of ELCB and isolate the circuit from the supply.

12.  A test button is included in the ELCB for testing the trip
mechanism. ELCB should be tested by pressing the ‘Test
Switch’ at least once in a month. If the ELCB does not trip even
after the ‘Test Switch’ it means that ELCB has gone faulty.
 Advantages of ELCB :
1. ELCB provides cheap and efficient earth leakage protection.
2. Supply is isolated when an earth fault occurs.
3. They are less sensitive to fault conditions, hence it has less
occurrences of nuisance tripping.
 Disadvantages of ELCB :
1. The complete supply is disconnected when a fault occurs in any
part of the installation.

14. Moulded Case Circuit Breakers
(MCCB)
 Moulded Case Circuit Breakers are electromechanical devices
which protect a circuit from over current and short circuit.
 They provide over current and short circuit protection for
circuits ranging from 63 amps up to 3000 amps.
 Types of MCCBs :-
1. Thermal Magnetic Release MCCB
2. Electronic Release MCCB
3. Microprocessor Release MCCB
 They are also used in applications of any current rating that
require adjustable trip settings, which are not available in
plug-in circuit breakers and MCBs.

15.  In thermal magnetic circuit breaker both thermal (strip)
element and magnetic element are connected in series with
load.
 In normal load, a bimetalic element is heated by the normal
load current, the bimetalic element does not bend and the
magnetic element does not attract the trip bar.
 If the temperature or current increases over a sustained
period of time, the bimetalic element will bend, push the trip
bar and release the latch. The switch will trip.
 If the current suddenly increases enough, the magnetic
element will attract the trip bar, release the latch and the
circuit breaker will trip.
 Advantages of MCCB :
1. Economical, tried and tested technology.

16.  Disadvantages of MCCB :
1. The operating characteristics of the breaker may very
depending on the ambient temperature.
2. It needs particular time to trip.
3. Provide less flexibility of adjustment than electronic releases.

17. Types Of Wires
 There are six types of wires :----
 Vulcanised India Rubber Wire :
 In this type of wires conductors are made up of aluminium or
copper. A vulcanised rubber is provided over it. These types
of wires are little used now a days.
 Tough Rubber Sheath Wire :
 In this type of wire layer of strong and durable rubber is
provided over the conductor. Its mechanical strength is more
than VIR wire. Effect of heat, water and moister is less on it.

18.  Weather Proof Wire :
 On this type of wire there is no effect of atmosphere. A layer of
rubber is provided on copper conductor. It is made waterproof by
dipping it into waterproof compound. It is useful in service
connections.

19.  Lead Covered Wire :
 In this type of wire coating of insulated rubber is provided on the
conductor. A tube made of lead kept over it. Easily affected by
mechanical stresses.
 Poly Vinyl Chloride Wire :
 In this type of wire insulation made of poly vinyl chloride is
provided over, copper or aluminium conductor.
 Its mechanical strength is more.
 There is good protection against moisture.
 Its life is long. There is no ill effect of vibrations.
 Its provides protection against oil, alkali, acid etc.
 Due to these advantages PVC wires are widely used.
 This type of wire is manufactured for voltage rating of 11kv.
 It is available in single PVC and double PVC types.
 Its dielectric strength is more.

20.  Flexible Wire :
 In this type of wire instead of using thick conductor many thin
copper conductors of 36 gauge are used. This is called stranding.
 Two wires of different colours of insulation are twisted together
and coil is prepared. As the flexible wire is used instead of thick
wire, the wire can be bent in any direction.

21. Types Of Cables
 Classification of cables is done on various basis :---
 {A} On the basis of number of core :
1. 1 core 2. 2 core 3. 3 core 4. 1.5 core 5. 4 core
 {B} On the basis of voltage :
1. Low tension cable operating voltage (1000v).
2. High tension cable operating voltage (11000v).
3. Super tension cable operating voltage (33000v).
4. Extra high tension cable operating voltage (66000v).
5. Extra super tension cable operating voltage (132000v).
 {C} On the basis of application :
1. Power cable used for the transmission of power.
2. Control cable used for the transmission of control system.

22.  {D} On the basis of insulation :
1. Rubber insulated.
2. PVC insulated.
3. Polythene insulated.
4. Paper insulated.
 {E} On the basis of sheath :
1. PVC sheathed.
2. Lead sheathed.
3. Aluminium sheathed.
 {F} On the basis of armouring :
1. Single layer steel wire armoured.
2. Double layer steel wire armoured.
3. Double layer steel tape armoured.

23.  {G} On the basis of construction :
1. Low tension cable.
2. Belted Cable.
3. Screened cable.
4. SL type cable.
5. HSL type cable.