Miniature Circuit Breaker (MCB)

Miniature Circuit Breaker (MCB)

The Miniature Circuit Breaker (MCB) becomes an integral part of domestic, commercial, and industrial applications.

What is a Miniature Circuit Breaker (MCB)?

It is a small trip switch operated in an overload condition and used to protect an electric circuit or home appliances in a domestic application. It is used as an alternative to a fuse.

What is the Function of MCB?

Miniature Circuit Breaker (MCB) is an automatically operated electric switch. It is designed to protect electrical appliances and human beings from electrical shocks and faults.

In an overload condition or a faulty condition, it is used to interrupt the electrical current flow through the circuit. These days MCB is very commonly used in low voltage electrical networks instead of a fuse.

Operation of MCB

The MCB is a tripping device that acts as a switch. Under the normal operating conditions, it acts as a closed switch and when the current goes above a predetermined level, it acts as an open switch, to break the circuit.

The MCB is available in the molded case which contains the tripping and switching mechanism. Based on the thermal and magnetic sensing devices, the switching mechanism works. Such a sensing mechanism is present inside the molded case of the MCB.

MCB is connected in series with each line of the AC supply going to the load just the way we connect fuse-links in series with each supply line.

If the load currents flowing through the MCBs are at safe levels, then the MCBs act as closed switches and connect the AC supply to the load.

But if the load current goes above a certain value (depends on the ampere rating of the MCB), then the corresponding rise in temperature is sensed by the temperature sensor inside the MCB. And the MCB will trip. It will act as an open switch to disconnect the load from the supply.

The MCB can replace the thermal overload relay and contactor successfully. So, we don’t have to use two separate devices for protection.

Miniature Circuit Breaker (MCB) can be manually switched ON and OFF similar to a normal switch if necessary. MCBs are time-delay tripping devices to which the magnitude of overcorrecting controls the operating time. That means, the MCB gets operated whenever overload exists for a sufficiently long time.

Therefore, MCBs don’t respond to transient loads such as motor starting surge current.

Construction and Working of MCB

The typical look of the MCB is as shown in the below figure. MCB is completely enclosed in a molded insulated structure.

MCB
MCB

The switching system consists of two contacts; a fixed and moving contact. By these contacts, incoming and outgoing wires are connected.

The operating mechanism consists of both magnetic tripping and thermal tripping arrangements. Under the normal operating condition, MCB operates as a switch (manual one) to make the circuit on-off.

Under the overload or fault condition, it automatically operates and trips to interrupt the load current. The automatic operation of MCB can be obtained in two different ways;

  • Thermal tripping
  • Magnetic tripping

Thermal Tripping

The below figure shows the operation of MCB under normal conditions and overload conditions. The contacts are closed and the load current is allowed to flow through the contacts. In the overload condition, the current flowing through the bimetal element raises its temperature.

The bimetal element bends due to thermal expansion, releases the trip latch, and opens the contacts.

Thermal tripping of MCB
Thermal tripping of MCB

Magnetic Tripping

In some MCBs, the magnetic field generated by the coil produces a pull on the bimetal element in such a way that the tripping mechanism is activated as shown in the below figure.

When a fault current flows the magnetic field produced by the coil is sufficient to overcome the spring force to open the contacts. Most MSBs implement a combination of both magnetic and thermal tripping.

Magnetic Tripping of MCB
Magnetic Tripping of MCB

Advantages of MCB over Fuse

MCB has several advantages over a fuse. Some of them are as given below:

  • MCB is more sensitive to current than a fuse.
  • In the case of MCB, the fault cone of the electric circuit can be easily identified.
  • MCB provides a better interface.
  • Handling MCB is electrically safer than handling a fuse.
  • MCB is reusable. It is not necessary to replace an MCB every time a fault occurs.
  • MCB has less maintenance and replacement cost.

Comparison of Fuse and MCB

The MCB has more advantages compared to the fuse. Let’s compare the MCB with the fuse.

Sr No Particulars MCB Fuse
1 Operation Requires equipment for automatic operation Inherently completely automatic
2 Operating time Comparative more time (approx. 0.1 to 0.2 sec) It takes a very small time to operate. (approx. 0.002 sec)
3 Breaking capacity Very large Small
4 Replacement No replacement after an operation. Requires replacement after every operation.
5 Safety High Low
6 Sensitivity to fault current High Low
7 Cost High Low

 

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