Miniature Circuit Breaker

A miniature circuit breaker(MCB) is a safety switching mechanism used in electrical circuits to prevent short circuits or excessive current surges. It differs from a fuse in that the circuit breaker breaks the circuit reversibly, whereas the fuse breaks the circuit irreversibly in the event of an oversurge or short circuit. It is likewise an electromagnetic device, in contrast to the fuse.

Table of Contents

• What is a miniature circuit breaker?
• Miniature circuit breaker symbol
• Types of miniature circuit breaker
• Different types of MCB used in electrical protection system
• Components of miniature circuit breaker
• Working principle of miniature circuit breaker
• Conclusion

What is a Miniature Circuit Breaker(MCB)?

A miniature circuit breaker can be defined as an electromagnetically powered safety switching mechanism used in electrical circuits (primarily for home usage, where current is less than 100 amps) to prevent oversurge or short circuit is known as a small circuit breaker. A miniature circuit breaker diagram is displayed below:

Miniature Circuit Breaker

Image used courtesy of testbook

Miniature Circuit Breaker Symbol

The symbol for a miniature circuit breaker is displayed in the image below. Both single pole and triple pole circuit breakers are represented by the miniature circuit breaker symbols.

Miniature Circuit Breaker Symbol

Image used courtesy of testbook

Types of Miniature circuit Breaker

The types are:

1.Single Pole: The single-pole MCB is utilized for circuits operating on a single phase, as the name implies.

2.Double Pole: The circuit's phase and neutral are protected by a double-pole MCB.

3.Triple Pole: A triple-pole MCB offers switching protection for the circuit's three phases, which are RYB (standard wire color coding).

4.Triple Pole with Neutral: Similar to a three-pole MCB, this type of  MCB protects three circuit phases, but it additionally includes the neutral as a distinct pole.

5.Four Pole: The four pole MCB contains a protective release for the neutral pole and is constructed similarly to a three-pole MCB.

Different Types of MCB used in Electrical Protection Systems

The type of MCB that must be utilized for certain appliances or equipment is determined by the MCB trip curve. Types are:

A type MCB trips the circuit when the current surpasses 2-3 times the actual current rating. Because a type MCB, or miniature circuit breaker, is extremely sensitive to short circuits, it can be used in the production of semiconductors.

B type MCB are used in cable protection and trip off the circuit when the current surpasses 3-5 times the current flow.

C type MCB are used in both home and business appliances, including as transformers, fluorescent lighting circuits, and IT devices like PCs, servers, and printers. They trip off the circuit when the current reaches five to ten times the actual current flow.

D type MCB trips the circuit, when the current surpasses 10–20 times the current flow and provides significant resistance. It is used in motors.

G type MCB are made to guard against ground fault currents and overcurrent in electrical systems' residual current devices (RCDs).

In electrical systems that run on direct current (DC), Type H MCB are intended to guard against overcurrent and earth fault currents.

K Type MCB are used in heavy-duty load devices including compressors, winding motors, and X-ray machines because they can tolerate up to 8–12 times the current flow.

Components of Miniature Circuit Breaker

External Casing: An MCB's external casing, or outermost cover, is composed of ceramic. It shields the MCB from external factors that could corrode its components.

Contacts: An MCB contains two contacts. There are two types of contacts: one is fixed and the other is adjustable.

Knob: The switch-like extension that protrudes beyond the external casing's dimensions and is often black in color is referred to as the knob.

Mechanical Latch: In the ON position, the latch system, which resembles a collection of several metallic components, holds the two contacts together under spring strain.

Bimetallic strip: When heated by a high current, the bimetallic strip, which is composed of two distinct metals with varying thermal capabilities, experiences varying degrees of thermal expansion. They are often composed of steel and copper or steel and brass. Because it must be heated, the bimetallic strip really provides protection against extended high currents.

Solenoid:The spring-like element that can be seen in an MCB's cross-sections is referred to as a solenoid. Because it doesn't require heating like the bimetallic strip does, it provides immediate protection against short circuits by releasing the mechanical latch. When the circuit's current is almost three times higher than usual, the solenoid is triggered.

Arc Chutes: Arc chutes are used to split and quench arcs that are created when contacts separate. The air inside the miniature circuit breaker is ionized by the high current flowing through the contacts when they are disconnected, creating a plasma arc that, if left unchecked, might cause more issues because it is an electrically conductive environment.

Working Principle of Miniature Circuit Breaker

The bi-metallic strip overheats and deforms if the circuit is overloaded for an extended period of time. The latch point moves as a result of the bi-metallic strip's deformation. Spring pressure is used to set up the MCB's moving contact. With this latch point, a small movement of the latch releases the spring, causing the moving contact to move and open the MCB. The current coil, also known as the trip coil, is positioned so that, in the event of a short circuit fault, the magneto-motive force (MMF) of the coil causes its plunger to strike the same latch point, displacing the latch. 

Miniature Circuit Breaker working principle diagram

Image used courtesy of electronicsforu

Once more, when the miniature circuit breaker's working lever is manually operated, the same latch point is displaced because the moving contact separates from the fixed contact in the same way as the MCB is manually turned off. The movement of the moving contact is eventually caused by the displacement of the same latch point and the release of the same deformed spring, which may be caused by the deformation of a bi-metallic strip, an enhanced MMF of a trip coil, or perhaps a manual action.There may be a significant risk of arc when the moving contact is isolated from the fixed contact. After passing through the arc runner and arc splitters, this arc is ultimately quenched. The MCB is prepared for another switch-off or trip operation when we turn it on, which returns the displaced operational latch to its initial on position.

Conclusion

A miniature circuit breaker (MCB) is an electrical switch that, in the event of an abnormal network state, such as an overload or a malfunction, instantly cuts off the electrical circuit. These days, MCBs are used in low-voltage electrical networks rather than fuses. The little circuit breaker detects it in a more dependable manner than the fuse. Compared to a fuse, an MCB is far more sensitive to overcurrent.