WHAT is MCB?
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
Types of MCB
(Miniature Circuit Breaker)
Several different MCB types are
available – types A, B, C, D, K, and Z. However, the three key versions are
type B, type C, and type D. Each is designed to be responsive to the likely
strength of electrical surges in different settings. These variations are
typically known as their ‘trip curve’ but can also be known as their tripping
characteristics or overcurrent characteristics.
B Type MCB
Tripping Current: 3-5 times the rated current
the electrical circuits of equipment that does not cause surge current
Application: Mainly used in residential applications or light
commercial applications where connected loads are primarily lighting fixtures
and domestic appliances with mainly resistive elements
C Type MCB
Tripping Current: 5-10 times the rated current
the electrical circuits of equipment that causes surge current
Application: Used in commercial or industrial facilities where
there is a risk of excessive short circuit currents. This includes small
transformers, lighting, pilot devices, control circuits, and coils
D Type MCB
Tripping Current: 10-20 times the rated current
the electrical circuits that cause high inrush current, typically 12-15 times
the thermal rated current
Application: Used in particular industrial/commercial facilities
where current inrush can be very high; concerned equipment include transformers
or X-ray machines, motors, power supplies, etc.
How to select an MCB?
When selecting an MCB (miniature
circuit breaker), the below parameters should be considered:
1) Rated current
Rated current is the maximum current
value the circuit breaker can withstand without tripping. MCBs have a rated
current from 0,5A to 125A. First, you should determine the total current of the
circuit. Then, you should select the proper MCB.
The current rating of MCBs should not be
greater than the cable current carrying capacity. It should also be greater
than or equal to the maximum current that the system is expected to carry.
Do not use an MCB lower than the total
current of the circuit. Otherwise, it trips unnecessarily.
2) Tripping characteristics
Tripping characteristics defines the
current vs tripping time relationship. They are differentiated based on the
instantaneous tripping range. You should determine the tripping class according
to the loads to be protected.
Typical loads by trip curve:
Designed for cable protection. Suitable
for resistive loads such as electrical heating, water heater and stoves, and
long cable runs (fault loop impedance)
Designed for medium magnetic startups.
Suitable for common loads such as lighting, socket outlets, small motors.
D and K Curves
Designed to allow for high inrush loads.
Suitable for high inrush loads and transformers.
Designed to protect circuits that need a
very low short circuit trip setting. Suitable for semiconductors.
3) Number of poles
You need to select the appropriate
number of poles to provide the necessary protection. Based on the number of
poles, the MCBs are classified as:
Single pole (1): Single pole MCBs are
used for protecting single-phase circuits, where neutral is taken directly from
the incoming supply.
Single pole and neutral (1+N): Single
pole and neutral types of MCBs are used when the phase has to be protected,
neutral has to be isolated. These are generally used for small loads.
Double pole (2): Double pole MCBs are
used when both phase & neutral have to be protected, and complete isolation
of supply is required for maintenance. These are generally used for small
loads, where only one MCB is used for switching/protecting the entire load.
Triple pole (3): Generally used for the
three-phase motor application, where neutral is not required. These are
generally used for motor protection.
Triple pole and neutral (3+N): Used for
incoming in distribution boards, where neutral isolation is required.
Four pole (4): Used for incoming in
distribution boards, where neutral protection is required.
capacity is the maximum current that an MCB can carry and break. High currents
above the breaking capacity could damage the MCB. 10kA or 6kA breaking
capacities are suitable for residential usage. MCBs with high breaking
capacities can be used in industrial applications.
an MCB Works?
There are two types of tripping mechanisms in MCB
(Miniature Circuit Breaker):
The thermal trip unit protects against
The thermal unit is based on a
bimetal element located behind the circuit breaker trip bar and is part of
the breaker’s current-carrying path.
When there is an overload, the
increased current flow heats the bimetal causing it to bend. As the
bimetal bends it pulls the trip bar which opens the breaker’s contacts.
The time required for the bimetal to
bend and trip the breaker varies inversely with the current.
The magnetic trip unit protects against
a short circuit. The magnetic trip unit is comprised of an electromagnet
and an armature.
When there is a short circuit, a high
magnitude of current passes through the coils creating a magnetic field that
attracts the movable armature towards the fixed armature.
The hammer trip is pushed against the
movable contact and the contacts are opened.