SWITCH GEARS
Ø It is an assembly of
main and auxiliary switching apparatus for operation, regulation, protection or
other control of electrical installation.
Ø It is a means of
isolating the supply, protecting against excess current and cutting off the current
if a serious earth fault occurs.
Purpose of Switch Gears
i)
Isolation - Is to provide a mean of isolating the final
circuits and the mains supply
ii)
Protection against excess current – Is to provide means
of cutting off the current if a serious earth fault occurs.
iii)
Means cutting off electrical supply to a
circuit or item of equipment in order to ensure that the safety of those
working on the equipment by making dead those parts which are live in normal
service.
Types of Switchgears
a)
Fuse
b)
Circuit
breakers
c)
Change
over switch
d)
Isolator
e)
Main
Switch
(A) FUSES
A fuse
is defined as “A device for opening a circuit by means of a conductor designed
to melt when an excess current flows. There are three types of fuses:
1.
Rewirable fuse
2.
Cartridge fuse
3.
High breaking capacity (H.B.C.) fuse, formerly
known as High rupturing capacity (H.R.C.) fuse.
Rewirable
Fuse
The
rewirable fuse consists of a fuse holder, a fuse element and a fuse carrier.
The base and carrier are made of either Bakelite or porcelain.
The
fuse element is connected between the terminals of the bridge. An asbestos
tube, or pad, is generally fitted in the fuse to minimize the effect of arcing
when the fuse element melts. This type of fuse is termed “semi-enclosed fuse”
to distinguish it from the old type of fuse which consisted simply of a piece
of wire connected between two terminals.
The
following is the example of approximately sizes of tinned copper wire to be
used for element in semi-enclosed fuses.
|
Standard wire gauge |
Current rating |
|
0.15mm |
3A |
|
0.2mm |
5A |
|
0.35mm |
10A |
|
0.50mm |
15A |
|
0.60mm |
20A |
|
0.75mm |
25A |
|
0.85mm |
30a |
|
1.25mm |
45A |
|
1.53mm |
60A |
|
1.80mm |
80A |
|
2.00mm |
100A |
The current
rating: is the current which the fuse element will
carry continuously without deterioration or breaking.
The fusing
current: is the minimum current at which the fuse
element will melt. This is approximately twice the current rating of the fuse
element
The Fusing Factor: - In order to classify
protective devices, it is important to know their circuit breaking and fusing
performance. For fuses this is achieved by for by using the fusing factor.
Fusing factor is the ratio of fusing current to Current Rating.
Fusing
Factor = Fusing Current
Current Rating
(Fusing factor of Rewirable fuse is 2)
Advantages
of rewirable fuse
1. Cheap
2. Can
easily be seen when the fuse is blown
3. Easy
to replace fuse element
4. No
mechanical moving parts
Disadvantages
1. Fuse
element deteriorate in use
2. An
incompetent person can replace an incorrect size of the fuse element thus,
defeating the purpose of fuse.
Note: The
fuse must be capable of protecting the smallest conductor in the circuit.
3. Lacking
in discrimination. The rewirable is not capable of discriminating between a
momentary high starting current and a continuous fault current.
4. Easily
damaged, particularly with short circuit currents.
Cartridge
Fuse
This
type of fuse is commonly used with the 13A plug used on the domestic power
circuits. The fuse element is contained in porcelain or glass tube fitted with
two connecting metal caps, to which the fuse element is attached. The cartridge
fuse has a fusing factor of 1.6-1.8
The
following is the colour code table for cartridge fuses:
|
Current |
Colour |
|
5A |
White |
|
13A |
Brown |
|
15A |
Blue |
|
20A |
Yellow |
|
30A |
Red |
|
45A |
Green |
|
60A |
Purple |
Advantages
of Cartridge fuse
·
No mechanical moving parts
·
Declared rating is accurate
·
The element does not weaken with age
·
Small physical size and no external arcing.
·
Low fusing factor.
·
Easy to replace.
Disadvantages
of cartridge fuse
·
More expensive than rewirable fuses.
·
Can be replaced with an incorrect size fuse
·
Not possible to see if the fuse has blown.
·
Require a stock of spare fuses to be kept.
High
Breaking Capacity Fuse
This
type of fuse is designed to protect against heavy overloads and is capable of
opening a circuit under short circuit conditions without damaging the surrounding
equipment. It is a very fast acting fuse
and discriminates between a starting surge and an overload current.
A high
breaking capacity fuse consists of the followings:
1. Porcelain/Ceramic
tube
2. Silver
element
3. Indicating
element which ignites powder under the label to show when the fuse element has
opened.
4. End
caps
5. Silica
(fine sand) filling used to quench the arc.
Advantages
of High breaking capacity fuse
·
No mechanical moving parts
·
The element does not weaken with age
·
Operation is very rapid under fault conditions
·
It is difficult to interchange the cartridge,
since different ratings are made to different physical sizes.
Disadvantages
of High breaking capacity fuse
·
Very expensive than cartridge fuses and
rewirable fuses.
·
Stocks of these spares are costly and take up
space.
·
Care must be taken when replacing them in order
to ensure that the replacement fuse has the same rating and also the same
characteristics as the fuse being replaced.
Note: The
fuses must always be connected to the phase conductor and never be connected in
the neutral conductor.
(B)
CIRCUIT BREAKERS
A
circuit breaker is defined as “a mechanical device for making and breaking an
electric circuit both, under normal and abnormal conditions, such as short
circuit, over loading and earth leakage faults, the circuit being broken
automatically”.
Under
normal condition the circuit breaker is generally opened and closed by hand
(manually), but is automatically opened under fault conditions.
There
are two types of circuit breaker:
1. Miniature
circuit breaker (MCB)
2. Earth leakage circuit breaker (ELCB)
Miniature
Circuit Breaker (MCB)
Miniature circuit breakers are used as an
alternative to fuse. They are available in distribution board units for final
sub-circuits protection, and they have several advantages:
1.
It can be reset or closed easily
2.
It gives a close degree of small over current
protection (The tripping factor is 1.1)
3.
MCB
is much more sensitive to over current than fuse.
4.
Handling
MCB is more electrically safe than fuse.
5.
MCB
tends to give much better overall protection against fire and shock risks than
can be obtained with the use of fuses.
Working Principle: There are two
arrangement of operation of miniature
circuit breaker:
i)
Due to Thermal
effect of over current - Is achieved with a bimetallic strip when
continuous over current flows through MCB, the bimetallic strip is heated and
deflects by bending and releases mechanical latch which then opens the
miniature circuit breaker contacts.
ii)
Due
to Electromagnetic effect of over
current - Is achieved during short circuit condition. A 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 thus open the circuit breaker contacts. Therefore, a bimetal
provides protection against over load current and an electromagnet provides protection
against short-circuit current.
Electromagnetically operated over current release
Operating Mechanism of
Miniature Circuit Breaker
The
Operating Mechanism of Miniature Circuit Breaker provides the means of manual
opening and closing operation of miniature circuit breaker. It has
three-positions “ON,” “OFF,” and “TRIPPED”. The external switching latch can be
in the “TRIPPED” position, if the MCB is tripped due to over-current. When
manually switch off the MCB, the switching latch will be in “OFF” position. In
close condition of MCB, the switch is positioned at “ON”. By observing the
positions of the switching latch one can determine the condition of MCB whether
it is closed, tripped or manually switched off.
Disadvantages:
1. It is
expensive than fuse
2. There
is a tendency for the tripping mechanism to stick in operation after a long
period of operating.
Earth
Leakage Circuit Breakers (ELCB)
These
falls into two classes:
1. Voltage
operated Earth leakage circuit breakers
2. Current
operated Earth leakage circuit breakers
Voltage
operated type:
The
basic circuit consists of a coil connected between the metal frame work of the
installation and the general mass of earth as shown in the figure above.
Voltage
operated units are designed to be directly responsive to fault voltage which
appears on protected metal work, which happens when the earth path resistance
is high. (Ohm’s law applies. Where V = IR: I is the amount of fault current, R
is the resistance of the earth path)
Principle Operation
Voltage
operated type are actuated by rise of potential of the protected metal work
caused by earth leakage current. When an earth leakage current flows, the
potential of the protected metal work will rise because of the impedance of the
earth fault loop path. This potential causes a current to flow through the trip
coil of E.L.C.B. and so to earth via the reference earth electrode. The maximum
p.d. required to actuate the ELCB must not exceed 40V so that the danger of
shock is reduced to minimum.
Test
switch is required so that the unit can be tested at frequent intervals to make
sure that the continuity of the earth path is maintained and also to ensure
that the protection of the unit is satisfactory.
A high resistance resistor is also connected in the circuit to limit the test current.
Current
operated type:
Current
operated are actuated by excessive earth leakage current. As shown in the
diagram below, there is no direct connection between the earthing lead and the
ELCB. A ‘core balance’ transformer is being
used to detect the presence of earth leakage currents.
Under
normal condition the current flowing in the live conductor equals the current
returning from neutral conductor.
The two primary coils of the transformer are wound with opposite
polarity so, under normal conditions their magnetic effects balance each other
and no voltage is induced in the secondary coil. When an earth fault occurs,
the fault current returns via the earth loop path instead of the neutral
conductor, thus the current taken from the live conductor exceeds the current
returned to the neutral conductor. This disturbs the balance between the
magnetic effects of the two primary coils and the magnetic flux is set up in
the core thus inducing a voltage in the secondary coil which in turn operates
the tripping (operating) coil relay.
Application:
1. Voltage operated type: Used
in domestic and small industrial installations to give protection against shock
risk where a good earth cannot be obtained (because of rocky or dry sub soil)
2. Current operated type:
Useful in large installations where very heavy fault currents would flow before
the over current device is operated, thereby creating a serious fire risk.
Advantages:
1. The
ELCB provides a cheap and efficient form of earth leakage protection,
particularly in situations where it is difficult to obtain a good earth
connection.
2. All
poles of the supply are isolated when an earth fault occurs.
Disadvantages:
1. The
earth continuity conductor to the trip coil may become disconnected.
2. The
complete supply is completely disconnected when a fault occurs in any part of
the installation.
3. The
switch mechanism of ELCB can be wedged in the ON position thus allowing
dangerous earth fault condition to arise.
(C)
CHANGE
OVER SWITCH
A
change over switch - Is a switch, which has
got two ways of connections. When its
handle is switched to one of its side the other position will remain ‘off’. Furthermore, when is switched to the other
side (second) the previous position will be off making the second position ‘on’.
In other words, a changeover switch
is a two-way switch which is used connect two supplies to the same load.
The
use of a change over switch, for standby generator.
In
normal condition where the power supply from the mains is available that is
being connected to an external circuit the standby generators will be
‘off’. But as soon as there is power
failure from the mains a changeover switch is then used to connect the standby
generator to the external circuit, leaving the terminals of the main supply off
from the external circuit.
The
changeover switch must be constructed in such that as the electric power from
mains returns back there will be no collision between the phases of the mains
and the standby generators.
Job Safety
i.
Care must be taken when
installing a changeover switch.
ii.
A change over switch
must be installed in such that only an authorized person can reach
iii.
A change over switch
must be installed in a way that it will not operate in potential dangerous
situation
(D) AN ISOLATOR
It is a mechanical device which is
operated manually and used to open or close a circuit of load.
An Isolator switch must be provided
close to the supply point so that all equipment can be made safe for
maintenance. Isolators for motor
circuits must isolate the motor and the control equipment.
(E) MAIN SWITCH
Types of main switch.
i)
Fused
main switch
ii)
MCB’s
main switch
The consumer’s main
switch is normally interlocked by its cover so that it cannot operate when the
switch is open. This is for main switches containing fuses.
For three phase main
switch, each phase is connected on an independent fuse.
A main switch must be
capable of providing means of isolation,
means of supply as well as means of protection of the supply.
In other situations,
the main switch is also used as a circuit’s divider in place of a distribution
board.
A
main switch containing a fuse fulfils these conditions as the switch isolates
the supply and the fuse protects the circuit against excess current due to
overload or series earth fault.
SEQUENCE OF CONTROL EQUIPMENT
The two
common methods of controlling the incoming supply. The earth leakage circuit
breaker is used where it is difficult to get a good earth path.
(a)
The
supply authority’s sealing chamber for the termination of armoured cable. (Service Line)
(b) The supply authority’s
fuse and neutral block. (Cutout fuse)
(c) The supply authority’s
energy meter (kWh meter)
(d) The consumer’s Circuit
breaker (Earth leakage circuit breaker)
(e) Consumer’s control
unit. (Distribution main switch)
DOMESTIC CONSUMER’S UNIT.
This type
of unit is usually made up of the following:
(a) Main
switch (63Aor100A) which isolates both the phase and the neutral conductors.
(b) One 30A fuse for cooker circuit
(c) One
30A fuse for ring circuit (13A plug)
(d) 45
fuse for shower unit.
(e) 15A
fuse for immersion heater
(f) One
or two 5A fuse for lighting circuits.
Loading of final sub-circuits
INSTALL SWITCH GEARS.
Steps:
(a) Identify tools and materials
(b) Decide whether you want
fuses or MCB’S in the 1 phase in consumer unit.
MCB’S
are superior to fuses and should be the first choice.
If the
fuses are chosen, the first choice should be cartridge fuse, if not available
then rewirable fuse.
(c) Identify the position
for mounting switch gears
Hold the consumer against the wall and mark
the positions for the fixing holes hence drill and plug the holes for wood
screws.
(d) Before fixing the unit
it is better to connect the meter leads and the main earth conductor.
If the
fuses are chosen, remove fuse carries from the consumer unit and then mount the
consumer unit.
(e)
Draw
cable in the consumer’s unit main Switch
(f)
Remove
about 30mm of outer sheathing form the ends of the cable and about 25mm of
insulation
(g) Connect the circuit
cables to the consume unit terminals according to IEE regulations.
(h) Replace back the fuse
carries and covers to consumer’s unit.
(i)
Test
for function
(j)
Clean
the working area and tools.