What Is An MCB? How Does It Works? - SKengineers
WHAT IS AN MCB? HOW DOES IT WORK?
A Miniature Circuit Breaker is an electromechanical device
designed to protect an electric circuit from over-current - A term to describe
an electrical fault caused by either overload or short circuit.
Back in the day we protected against over-current by using
fuse wire (indeed, we used to sell it!). The principle was fairly simple - an
over-current would quite literally, 'blow' the fuse wire by rapidly heating and
melting it, thus breaking the electrical connection and in doing so, protected
the rest of the electrical circuit.
MCBs improve on this functionality as they are usually not
destroyed during over-current so are reusable. They are also much easier to
use, offering the convenience of 'on/off switching' for circuit isolation and
since the conductor is housed within a plastic casing, they are much safer to
use and operate.
It is important to note that MCBs do not protect humans
against electrical shock caused by 'earth leakage'. This service is provided by
RCDs and RCBOs.
An MCB has three principle characteristics, Amperes, Kilo Amperes and Tripping Curve.
Overload
Current Rating - Amperes (A) -
Overload occurs when too many appliances are put on one
circuit and draw more electrical current than that circuit and cable are
designed to take. This could occur in the kitchen, for example when the kettle,
dishwasher, electric hob, microwave and blender are all in use simultaneously.
The MCB on this circuit cuts power thus preventing overheating and fire in the
cable and terminals.
Short
Circuit Rating - Kilo Amperes (kA) -
Short Circuit is the result of a fault somewhere in the
electrical circuit or appliance and is potentially much more dangerous than
overload as the scale and speed of over current is in a different order of
magnitude. It occurs when there is a direct connection between the live and
neutral conductors. Without the resistance provided by the normal circuit
integrity, electrical current rushes around the circuit in a loop and
multiplies the amperage by many thousand
times in just milliseconds.
MCBs used in domestic installations are typically rated at
6kA or 6000 amps. The relationship between normal voltage (240v) and typical
domestic appliance power ratings mean that the over-current caused by short
circuit should not exceed 6000 amps. In commercial and industrial situations
however, using 415v and large machinery, it is necessary to use 10kA rated
MCBs. These ratings signify the maximum Prospective Fault Current (PFC) that
the MCB can withstand and still operate correctly by disconnecting the circuit.
Should the PFC be exceeded, the MCB could fail resulting in the contacts
welding shut.
How an
MCB Works -
1. Latch
2. Solenoid
3. Switch
4. Plunger
5. Incoming Terminal
6. Arc Chutes Holder
7. Arc Chutes
8. Dynamic Contact
9. Fixed Contact
10. Din Rail Holder
11. Outgoing Terminal
12. Bi-metallic Strip Carrier
13. Bi-metallic Strip
As you have now read, an MCB protects against both overload
and short circuit.
Since these two types of over-current manifest in very
different ways an MCB detects and then deals with them differently:
1) Thermal operation
and
2) Electromagnetic operation How
an MCB Works Inside
Overload protection is provided by the Bi-metallic strip
(number 13 in our diagram). During 'slow' continuous over-current,
characteristic to overload, the bimetallic strip is heated by the electrical
charge and thus bends, releasing the mechanical latch. The latch, which is
connected to the operating mechanism causes the contacts to open and breaks the
circuit.
During Short Circuit, the sudden and dramatic rise of
electric current causes electro-mechanical displacement of the plunger (4) from
the tripping coil or solenoid (2). The plunger strikes the trip lever causing
the immediate release of the latch mechanism, opens the contacts and thus
breaks the circuit.
Tripping Curve -
The 'Tripping Curve' of an MCB allows for real world and
sometimes entirely necessary, surges in power. In commercial environments for
example, large machines usually require an initial surge of power in excess of
their normal running current to overcome the inertia of large motors. This
brief surge lasting just seconds, is allowed by the MCB as it is safe to do so
for this very short amount of time.
There are three principle Curve Types which allow for the
surges in different electrical environments:
Type B MCBs are used in domestic circuit protection where
there is little need for surge permission. Any large surge in a domestic
environment is likely to be the result of a fault, so the amount of over
current allowed is relatively small. B Type MCBs trip between 3 and 5 times
full load current, so a 6amp MCB with a B curve will trip between 18a and 30a,
depending on how long the over-current lasts, but roughly between 10 and 8
seconds.
Type C MCBs trips between 5 and 10 times full load current
and are used in commercial and light industrial environments which may feature
large fluorescent lighting circuits, transformers and IT equipment such as
servers, PCs and printers.
Type D MCBs are used in heavy industrial facilities such as
factories using large winding motors, X-ray machines or compressors. Due to the
large potential surges they trip between 10 and 20 times full load current. A
10 amp D Type MCB therefore trips between 100amps and 200amps.
All three types of MCBs provide tripping protection within
one tenth of a second. That is to say, once the overload and period have been
exceeded, the MCB trips within 0.1 seconds.
MCB
Tripping curves B, C, D Type -
Typical
Uses for Domestic MCBs -
Whilst every installation is different and should always be
designed by the fully qualified professional electrician on site, there are
some standards which most domestic wiring schemes adhere to:
6 Amp - standard lighting circuits
10 Amp - large lighting circuits - though these are becoming
rarer in domestic settings as technology and trends move towards lower energy
lighting sources such as LED.
16 Amp and 20 Amp - Depending on the power rating these are
both commonly used for immersion heaters and boilers
32 Amp - Ring Final. The technical term for your power
circuit or sockets. A two bedroom house for example may have 2 x 32A power
circuits to separate upstairs and downstairs sockets. Larger dwellings can have
any number of 32 A circuits.
40 Amp - Cookers / electric hobs / small showers
50 Amp - 10kw Electric showers / Hot tubs.
Miniature Circuit Breaker [ MCB ]: What is it?
What is
MCB?
A Miniature Circuit Breaker (MCB) is an automatically
operated electrical switch used to protect low voltage electrical circuits from
damage caused by excess current from an overload or short circuit. MCBs are
typically rated up to a current up to 125 A, do not have adjustable trip
characteristics, and can be thermal or thermal-magnetic in operation.
Fuse vs
MCB -
Nowadays miniature circuit breakers (MCBs) are much more
commonly used in low voltage electrical networks instead of fuses. The MCB has
many advantages compared to a fuse:
It automatically switches off the electrical circuit during
the abnormal conditions of the network (both overload and fault conditions).
The MCB is much more reliable in the detection of such conditions, is it is
more sensitive to change in current.
As 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 a fuse, the fuse wire should be checked by opening
fuse grip or cut out from fuse base, for confirming the blow of fuse wire. Thus
is it much detect if an MCB has been operated compared to a fuse.
Quick restoration of supply can not be possible in case of
fuse, as fuses have to be re-wirable or replaced for restoring the supply. But
in the case of an MCB, quick restoration is possible by (literally) flipping a
switch.
The handling of an MCB is more electrically safe than a
fuse.
MCBs can be controlled remotely, whereas fuses can not.
Because of these many advantages of MCB over fuse units, in
modern low voltage electrical network, the miniature circuit breaker is almost
always used instead of a fuse,
The only one disadvantage of MCB over fuse is that this
system is costlier than a fuse unit system.
Working
Principle Miniature Circuit Breaker -
There is two arrangement of operation of a miniature circuit
breaker. One due to the thermal effect of over current and other due to
electromagnetic effect of over current. The thermal operation of the miniature
circuit breaker is achieved with a bimetallic strip whenever continuous
overcurrent flows through MCB, the bimetallic strip is heated and deflects by bending.
This deflection of the bimetallic strip releases a
mechanical latch. As this mechanical latch is attached to the operating
mechanism, it causes to open the miniature circuit breaker contacts.
But during short circuit conditions, the sudden rising of
current causes electromechanical displacement of plunger associated with
tripping coil or solenoid of MCB. The plunger strikes the trip lever causing
the immediate release of the latch mechanism consequently open the circuit
breaker contacts. This was a simple explanation of the miniature circuit
breaker working principle.
Miniature
Circuit Breaker Construction -
Miniature circuit breaker construction is very simple,
robust and maintenance-free. Generally, an MCB is not repaired or maintained,
it just replaced by a new one when required. A miniature circuit breaker has
normally three main constructional parts. These are -
Frame of
Miniature Circuit Breaker -
The frame of a miniature circuit breaker is a moulded case.
This is a rigid, strong, insulated housing in which the other components are
mounted.
Operating
Mechanism of Miniature Circuit Breaker -
The operating mechanism of a miniature circuit breaker
provides the means of manual opening and closing operation of a 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 the “OFF” position. In the closed condition of an 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.
Trip Unit
of Miniature Circuit Breaker -
The trip unit is the main part, responsible for the proper
working of the miniature circuit breaker. Two main types of trip mechanisms are
provided in MCB. A bimetal provides protection against overload current and an
electromagnet provides protection against short-circuit current.
Operation
of Miniature Circuit Breaker -
There are three mechanisms provided in a single miniature
circuit breaker to make it switched off. If we carefully observe the picture
beside, we will find there is mainly one bimetallic strip, one trip coil and
one-hand operated on-off lever.
The electric current-carrying path of a miniature circuit
breaker shown in the picture 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. All are
arranged in series.
miniature
circuit breaker -
If the circuit is overloaded for a long time, the bimetallic
strip becomes overheated and deformed. This deformation of bimetallic 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.
Again when the operating lever of the miniature circuit breaker
is operated by hand, that means when we make the MCB at off position manually,
the same latch point is displaced as a result moving contact separated from
fixed contact in the same manner.
Regardless of the operating mechanism – e.g. due to the deformation
of the bimetallic strip, or due
to the increased MMF of the trip coil, or due to manual
operation – the same latch point is displaced and same deformed spring is
released. This is ultimately responsible for the movement of the moving
contact. When the moving contact separated from fixed contact, there may be a
high chance of arc.
This arc then goes up through the arc runner and enters into
arc splitters and is finally quenched. When we switch on an MCB, we actually
reset the displaced operating latch to its previous on position and make the
MCB ready for another switch off or trip operation.
Difference between MCB, MCCB, RCCB, ELCB
MCB, MCCB, RCCB, and ELCB are circuit breakers but all of
them are designed for serving a specific purpose. Before knowing about the
difference among all, let’s get to know about them a little bit.
MCB
MCB stands for Miniature Circuit Breaker. It automatically
switches OFF electrical circuit during any abnormal condition in the electrical
network such as overload & short circuit conditions. However, fuse may sense
these conditions but it has to be replaced though MCB can be reset. The MCB is
an electromechanical device which guards the electric wires &electrical
load from overcurrent so as to avoid any kind of fire or electrical hazards.
Handling MCB is quite safer and it quickly restores the supply. When it comes to
house applications, MCB is the most preferred choice for overload and short
circuit protection. MCB can be reset very fast & don’t have any maintenance
cost. MCB works on bi-metal respective principle which provides protection
against overload current &solenoidshort circuit current.
MCCB
MCCB stands for Moulded Case Circuit Breaker. It is another
type of electrical protection device which is used when load current exceeds
the limit of a miniature circuit breaker. The MCCB provides protection against
overload, short circuit faults and is also used for switching the circuits. It
can be used for higher current rating and fault level even in domestic
applications. The wide current ratings and high breaking capacity in MCCB find
their use in industrial applications. MCCB can be used for protection of
capacitor bank, generator protection and main electric feeder distribution. It
offers adequate protection whenever an application requires discrimination,
adjustable overload setting or earth fault protection.
RCCB
RCCB stands for Residual Current Circuit Breaker. This
residual current device is basically an electrical wiring device that
disconnects the circuit whenever there is leakage of current flow through the
Human body or the current is not balanced between the phase conductor. It is
the safest device to detect and trip against electrical leakage currents, thus
ensure protection against electric shock caused by direct contacts. RCCB is
generally used in series with an MCB which protects them from over current and
short circuit current. Both phase and neutral wires are connected through a
RCCB device. These are an extremely effective form of shock protection &widely
used for protection from a leakage current of 30,100 & 300mA. It is
essential lifesaving equipment used to protect the human body from electrical
and is mandatory in many states for domestic installation.
ELCB
ELCB stands for Earth Leakage Circuit Breaker. They have the same function as RCCB but are voltage sensor devise. However, this is an old technology & is not in common use. RCCB being a current sensitive device have a better advantage over ELCB as under -
RCCB -
1. RCCB
refers to ear stands for Residual Current Circuit Breaker.
ELCB stands for Electric Leakage Circuit Breaker.
2. It is a
new name and refers to current operated devices.
ELCB refers to voltage operated earth leakage device.
3 It
ensures 100% detection of leakage current& is available to sense the AC as
well as DC leakage current.
It is not preferable as it can only detect current that flow
back through the main earth wire.
4 RCCB has
no connection with the earth wire and that’s why it can trip when both currents
(phase and neutral) are different and it withstands up to both the currents are
same.
ELCB -
ELCB is working based on Earth leakage current. These
devices measured the voltage on the earth conductor; if this voltage was not
zero this indicated a current leakage to earth.
ELCB stands for Electric Leakage Circuit Breaker.
ELCB refers to voltage operated earth leakage device.
It is not preferable as it can only detect current that flow
back through the main earth wire.
ELCB is working based on Earth leakage current. These
devices measured the voltage on the earth conductor; if this voltage was not
zero this indicated a current leakage to earth.
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