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Miniature Circuit Breaker, (MCB), How does it work?
What is it? and How does it work?
A miniature circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current. Typically resulting from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. Unlike a fuse which operates once and then must be replaced. A circuit breaker can be reset either manually or automatically to resume normal operation. An MCB embodies complete enclosure in a mold of insulating material this provides mechanically strong and insulated housing (Picture 1.1).
Picture 1.1 miniature circuit breaker (MCB)
Parts of Miniature Circuit Breaker
Picture 1.2
- Incoming terminal:
Incoming terminal is made up of electrolytic copper or silver alloy and this is where the incoming phase is fixed. - Arc Chute:
A stack of mutually insulated parallel metal plates which divide and cool the arc. - Bimetallic strip:
It is part of thermal tripping arrangement, which a heater coil is wounded to create heat depending on the flow of current. - Magnetic coil:
Thermal tripping arrangement where in case of short circuit or heavy overloads strong magnetic field is produced in the coils. - Copper braid:
It Connects the moving element with a static element. - On off switch:
It is a operator with which we manually interrupt or turn on the electric circuit. - Outgoing terminal:
It connects outgoing phase from circuit breaker.
Picture 1.2 Parts of Miniature Circuit Breaker (Incoming terminal, Arc Chute,
Bimetallic strip, Magnetic coil, Copper braid, On off switch,
Outgoing terminal)
Working principle of Miniature Circuit Breaker?
The most important thing that circuit breaker is doing is to safely and reliably switches off electrical circuit during abnormal conditions of a network that means overload condition as well as faulty condition. Under normal working conditions circuit breaker work when we switch operator manually from position off to position on. Electric current carrying path shown in picture 1.3.
picture 1.3 Miniature Circuit Breaker, Electric current carrying path
First incoming terminal then contact point of moving and fixed contacts then magnetic coil then bimetallic strip then outgoing terminal all arranged in series. Under overload condition, current through the metallic strip causes to raise of the temperature of it the, heat generated within the bimetallic is enough to cause deflection due to thermal expansion of metals this deflection further releases the trip latch and contacts get separated. The visual indication of this trip can be observed by automatic movement of the operator to OFF position (picture 1.4). In tripping operations and arc is formed when the contacts start separating this arc is then forced into Arc chute plates where is formed into a series of arcs and at the same time energy extracted and cools it.
Picture 1.4 Tripping of Miniature Circuit Breaker caused by overload current.
Types of miniature circuit breakers
Miniature circuit breakers are classified into three major types according to their instantaneous tripping currents they are Type B, Type C, Type D.
Type B MCB
This type 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 (picture 1.5).
picture 1.5 tripping diagram for type B Miniature Circuit Breaker
Type C MCB
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 in fluorescent lighting in such cases type C are preferred to handle higher value of short circuit currents. therefore these are suitable for highly inductive commercial and industrial installations (picture 1.6).
picture 1.6 tripping diagram for type C Miniature Circuit Breaker
Type D MCB
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 or very frequent these are suitable for specific industrial and commercial applications in the common examples of such applications include X-Ray machines U.P.S. systems industrial welding equipment large winding Motors etc (picture 1.7).
picture 1.7 tripping diagram for type D Miniature Circuit Breaker
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Hrvoje Čočić
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