All about MCB, its
Construction, Working, Types & Applications.
What is MCB (Miniature Circuit Breaker), its Construction, Working, Types & How to select proper
MCB for different loads rating?
Table of Contents
What is MCB (Miniature Circuit Breaker),
its Construction, Working, Types &
How to select proper MCB for different loads rating?
● Introduction
to Miniature Circuit Breakers (MCBs)
● What
is Miniature Circuit Breaker (MCB)?
● Construction
of MCB
● Working
& Operation of MCB
● Types
of Miniature Circuit Breakers (MCB’s)
● Type
B MCB
● Type
C MCB
● Type
D MCB
● How
to Select Proper MCB for different loads?
● Nominal
rating of the circuit breaker
● KA rating or breaking capacity ←很多時忽略了這參數
● Type
of MCB
Introduction to Miniature Circuit Breakers
(MCBs)
All fuses need
to be replaced when they have operated. Unlike a fuse, an MCB operates as automatic
switch that opens in the event of excessive current flowing through the circuit
and once the circuit returns to normal, it can be reclosed without any manual
replacement. MCBs are used primarily as an alternative to the fuse switch in
most of the circuits.
A wide variety of MCBs have been in use nowadays with
breaking capacity of 10KA
to 16KA, in all areas of domestic, commercial and industrial applications as a reliable
means of protection.
What is Miniature Circuit Breaker (MCB)?
An
MCB or miniature circuit breaker is an electromagnetic device that embodies complete
enclosure in a molded insulating material. The main function of an MCB is to
switch the circuit, i.e., to open the circuit (which has been connected to it)
automatically when the current passing through it (MCB) exceeds the value for
which it is set. It can be manually switched ON and OFF as similar to normal switch if necessary.
MCBs are of time
delay tripping devices, to which the magnitude
of overcurrent controls the operating time.
This means, these get operated whenever overload exist long enough to create a danger
to the circuit being protected.
Therefore, MCBs doesn’t
respond to transient loads such as switches surges and motor starting currents.
Generally, these are designed to operate at less than 2.5 milliseconds
during short circuit faults
and 2 seconds to 2 minutes in
case of overloads (depending on the level of current).
A
typical external appearance of an MCB is shown in figure. MCBs are manufactured
in different pole versions such as single, double, triple and four pole
structures with different fault current levels. Mostly, MCBs are linked to give two and three-pole
versions such that a fault in one line
will break the complete circuit and hence complete circuit isolation are
provided. This feature will be helpful in case of single phasing in three
phase motor protection.
These
are rated at 220V for DC
supply and 240/415 for AC
supply (single and three-phase) with different short circuit current capacity.
Typically,
single phase
devices have load current range of up to 100 A.
Some MCBs have facility to adjust its tripping current capacity while some
devices are fixed for some load current and short circuit rating.
MCBs
are used to perform many functions such as local
control switches, isolating switches
against faults and overload protection devices for installations or specific equipments or
appliances.
Construction of MCB
An
MCB embodies complete enclosure in a moulded
insulating material. This provides mechanically strong and insulated housing. The
switching system consists of a fixed and a moving contact to which incoming and
outgoing wires are connected. The metal or current carrying parts are made up
of electrolytic copper or silver alloy depending on the rating of the circuit
breaker.
As
the contacts are separated in the event of an overload or short circuit
situation, an electric arc is formed. All modern MCBs are designed to handle
arc interruption process where arc energy extraction and its cooling are
provided by metallic arc splitter plates. These plates are held in a proper
position by an insulating material. Also, arc runner is provided to force the
arc that is produced between the main
contacts. The
operating mechanism consists of both magnetic tripping and thermal tripping
arrangements.
The
magnetic tripping arrangement essentially consists of a composite magnetic system
that has a spring loaded dashpot with a magnetic slug in a silicon fluid, and a
normal magnetic trip. A current carrying coil in the trip arrangement moves the
slug against spring towards fixed pole piece. So the magnetic pull is developed
on the trip liver when there is a sufficient magnetic field produced by the
coil. In case of short circuits or heavy overloads, strong magnetic field produced by the coils (Solenoid) is sufficient to attract the armature of trip
liver irrespective of the position of slug in the dashpot.
The
thermal tripping arrangement consists of a bimetallic strip around which a
heater coil is wounded to create heat depending on the flow of current. The
heater design can be either direct where current is passed through bimetal
strip which effect part of electric circuit or indirect where a coil of current
carrying conductor is wound around the bimetallic strip. The deflection of
bimetallic strip activates the tripping mechanism in case of certain overload
conditions. The bimetal strips are made up of two different metals,
usually brass and steel. These metals are riveted and welded along their
length. These are so designed such that they will not heat the strip to the
tripping point for normal currents, but if the current is increased beyond
rated value, strip is warmed, bent and trips the latch. Bimetallic strips are
chosen to provide particular time delays under certain overloads.
Working & Operation of MCB
Under
normal working conditions, MCB operates as a switch (manual one) to make the
circuit ON or OFF. Under overload or short circuit condition, it automatically
operates or trips so that current interruption takes place in the load circuit.
The visual indication of this trip can be observed by automatic movement of the
operating knob to OFF position. This automatic operation MCB can be obtained in
two ways as we have seen in MCB construction; those are magnetic tripping and
thermal tripping.
Under
overload condition, the current through the bimetal causes to raise the
temperature of it. The heat generated within the bimetal itself enough to cause
deflection due to thermal expansion of metals. This deflection further releases
the trip latch and hence contacts get separated. In some MCBs, magnetic field
generated by the coil causes develop pull on bimetal such that it deflection
activates the tripping mechanism.
Under
short circuit or heavy overload conditions, magnetic tripping arrangement comes
into the picture. Under normal working condition, the slug is held in a
position by light spring because magnetic field generated by the coil is not
sufficient to attract the latch. When a fault current flows, the magnetic field
generated by the coil is sufficient to overcome the spring force holding slug
in position. And hence slug moves and then actuate the tripping mechanism.
A
combination of both magnetic and thermal tripping mechanisms are
implemented in most of MCBs. In both magnetic and thermal tripping operations,
an arc is formed when the contacts start separating. This arc is then forced
into arc splitter plates via arc runner. These arc splitter plates are also
called arc chutes where arc is formed into a series of arcs and at the same
time energy extracted and cools it. Hence this arrangement achieves the arc
extinction.
Types of Miniature Circuit Breakers (MCB’s)
MCBs
are classified into three major types according to their instantaneous tripping
currents. They are Type B MCB Type
C MCB Type D MCB。
Type B MCB :敏感型,跳脫值 為 額定值 的 3 ~ 5 倍。
This
type of MCB will trip instantly at a rate of three
to five times its rated current. These are normally used for resistive or small inductive loads
where switching surges
are very small.
Therefore, these are suitable for residential
or light commercial installations.
Type C MCB :一般型,跳脫值 為 額定值 的 5 ~ 10 倍。
This
type of MCB will trip instantly at a rate of five
to ten times its rated current. These are normally used for high inductive loads
where switching surges are high
such as small motors and florescent lighting. In such cases, type C MCBs are
preferred to handle higher value of short
circuit currents. Therefore, these are suitable for highly inductive commercial and industrial installations.
Type D MCB:遲慢型,跳脫值 為 額定值 的 10 ~ 20 倍。
This
type of MCB will trip instantly at a rate of ten
to twenty five times its rated current. These are normally used
for very high inductive loads
where high inrush current are very frequent. These are suitable for specific industrial and commercial applications.
The common examples of such applications include x-ray
machines, UPS systems, industrial welding equipment,
large winding motors, etc.
The
above three types of MCBs provide protection within one tenth of a sec. The
minimum and maximum trip currents of these MCBs are given in a tablular form
below,
where
Ir ( I =
current 電流 , r = Rated 額定值) is the rated current of the MCB.
MCBs
can also be classified based on number of poles such as single pole, double
pole, triple pole and four pole MCBs.
How to Select Proper MCB for different
loads?
Choosing
a particular MCB for a specific application is a careful
task to ensure reliable protection
against overloads and short circuits. If it is not selected
according to the circuit requirements, there will be chances to lead frequent
unwanted trippings. before we going in details, We
must know difference between MCB & MCCB, How to Read MCB
Nameplate and difference between ELCB, RCB and RCD Circuit Breakers
If
it is undersized (MCB rating less than the nominal load current), MCB causes
frequent tripping and causes to interrupt the current to the load it is being
connected, because the MCB nominal current less than nominal current value of
the load. Similarly, if it is oversized (MCB rating more than the nominal load
current), the load to it is connected will not be protected efficiently. In
such case, the MCB will not trip even though load is drawing overcurrents.
The
following are the three factors to be considered for selecting an MCB for
specific application.
Nominal rating of the circuit breaker 斷路器的標稱額定值
This
is the rated ampere current rating of MCB. This value must be lower than the current carrying capacity
of wiring system 電線系統載流量and higher than or equal
to the maximum full load current in the
wiring system負載最大滿載電流. Generally, this rating should be such
that it can handle 125%
of continuous load連續長期負載plus
rating of noncontinuous
load 額定但非連續長期負載。Typically this can be
expressed as
Maximum full load current
in the system ≤ Current rating of MCB < Cable rating
負載最大滿載電流 ≤ MCB
額定電流 < 電線系統載流量
KA Rating 千安額定值 or Breaking Capacity 斷流能力
This rating refers to the ability
of MCB that can trip or interrupt the circuit under short circuit conditions. It is expressed in Kilo Amps (KA).
This rating must not
be less than the prospective
short-circuit current. (預期最大短路電流)
The prospective short-circuit current is the
maximum current that exist
in the circuit during short-circuit conditions occured.
In residential installations 6KA
MCB is sufficient while 10KA or above rating MCB is needed for
commercial and light industrial applications.
Type of MCB
The
type of MCB needed for a specific application decided by operating
characteristics such that various current ratings are required to operate the
loads instantaneously.
We
have already mentioned various types of MCBs for different applications above.