The MCB is the most modern substitute for a conventional rewirable fuse and provides protection against overloads and short circuits.
The MCB operates at 125% of the rated current. e.g. a 10 amp. MCB will operate at 11.5 amp as compared to a rewirable fuse of the same rating which will operate at 20 amp.
Construction of an MCB
Fig. shows the cross-section of an MCB.
It consists of various components, whose functions have been discussed below:
(1) Silver Graphite Contacts:
While breaking very high short circuit currents, the breaker contacts separate. These contacts remain closed under normal conditions.
If ordinary silver alloy contacts are used, the contacts normally bounce, resulting in contact welding.
For this purpose silver graphite is used for the contacts since it is a non-melting material.
(2) Arc Chutes or Horns:
As the contacts separate, an arc is drawn between them. The arc is nothing but a self-sustained discharge of electricity that is produced when the contacts separate.
Under normal conditions, since the current carried by the contacts is less, the strength of the magnetic field produced is also less. However during short-circuits or overloads, since the current in the circuit
increases, the strength of the magnetic field produced also increases.
Under the influence of this field, the arc moves upward into the arc chutes. Here the arc is broken down into several arcs.
These arcs will require a considerably higher voltage than that required by a single arc. Since this is not possible, the arcs will be cooled and extinguished quickly.
(3) MCB Housing :
The body of the MCB is made of plastic material having a high melting point, high dielectric strength, and minimum water absorption.
(4) Inner Mechanism:
The moving contacts of the MCB are closed in a sheet metal casing and not inside the plastic. This is because plastic will tend to change its shape and accuracy under continuous stresses of load and consistent tripping.
(5) Angular Vents:
Angular Vents are provided to prevent dust from entering into the casing. Thus dust-free perfect electrical contacts are obtained.
(6) Simultaneous tripping of all phases in multiple MCB's:
The tripping design is so done that in case of any overload or short circuit in one phase, all the phases trip simultaneously. Thus, the damage done due to single phasing is reduced.
Working of MCB:
With respect to Fig., there are two main operations of the MCB.
(1) The first operation is thermal, resulting in the inverse time-current characteristics. This operation is obtained with the help of the bimetallic strip. Here, as the current flowing through the bimetallic strip increases during overloads, it deflects or bends and in the process causes the fixed and moving contacts to separate.
The inverse time-current characteristics mean, higher the current, the smaller will be the time in which the MCB operates and vice-versa.
(2) During short circuits, as the current rises to very high levels very rapidly, the rising current energizes the solenoid.
This in turn operates the plunger to strike the trip level. This causes the fixed and moving contacts to
separate.
Thus an MCB instantly switches off the supply automatically, when there is a short circuit or an overload. In this way, damage to the equipments and
wiring is prevented.
The supply is re-started after tripping by simply switching on the MCB switch. The MCB does not show wear and tear with time. It does not require any maintenance or repairs.
MCB's are used in residences, offices, shops, in the complete wiring of buildings, etc. Though they are expensive as compared to fuses, there is no cost involved in resetting them after the operation. We simply have to switch on the MCB.
In the case of the fuse, it will have to be re-wired once it has blown out. MCB's are being used extensively these days and have virtually replaced the re-wirable fuses.
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