Miniature Circuit Breaker (MCB): How It Works, Rating & Types.

A miniature circuit breaker (MCB) is an automatic switch that protects an electrical circuit from overcurrents or power surge.

The term overcurrent refers to a faulty current that arises from short circuits, faulty wiring, and circuit overload.

Both fuse and miniature circuit breakers protect electrical equipment from damage. However, MCBs are more advanced.

Miniature circuit breaker (MCB) vs fuse

  • MCB last for a long time, while fuse gets damaged after a single trip.
  • Unlike a fuse, MCB has a plastic enclosure that covers the metallic conductor
  • MCB has a switch unlike fuse
  • MCBs are easy to restart after tripping while fuse must be changed

Note: MCB cannot protect you from electric shock, an RCD or GFCI does that.

Working principles of a miniature circuit breaker

Like other types of circuit breakers, MCBs protect electrical systems and appliances. They do that via two operational methods.

  • An electromagnetic operation for short circuit protection
  • Thermal operation for overload protection.

Under the working conditions of the circuit breaker, current flows from the incoming terminal, to the fixed and moving contacts, solenoid, a bimetallic strip, and then to the outgoing terminals.

Electromagnetic operation

Whenever a short circuit occurs, a heavy current flows through the solenoids. This causes the tripping plunger to shift, leading to the release of the latch mechanism that opens the contact.

Thermal operation

The thermal operation of an MCB works with the help of the bimetallic strip. Whenever there is a circuit overload, the switchgear trips.

Characteristics of miniature circuit breakers

  • Overload current rating – Amperes
  • Short circuit rating – Kilo Amperes (KA)
  • Operating types – B, C, D, Z and K curves.

Overload current rating – Amperes (A)

A circuit overload occurs when you connect too many appliances to one circuit, more than it can carry. For example, connecting your cooking unit, dishwasher, blender, and microwave together.

When such happens, the MCB cuts off power to prevent damages that may arise from the overload.

The rated current of a miniature circuit breaker ranges from 0.5 to 100 A.

Short circuit rating – Kilo Amperes (KA).

A short circuit occurs when a live wire touches the neutral. It is rated in Kilo Ampere.

The fault level of domestic MCBs is 6KA or 6000 A. While industrially used ones may need a 10KA rated breaker.

Types of MCBs according to their trip curves.

There are several types of miniature circuit breakers based on their trip curves. But before that, let us know the meaning of a trip curve.

A trip curve is the highest current an MCB can withstand before tripping off. Their various types are Type B, C, D, K, and Z.

Type B miniature circuit breaker.

This circuit breaker trips when the current is 3 to 5 times above the current rating. They are used in residential appliances and in some commercial applications where the surge current is low.

Type C MCB

This type of MCB trip when the current is 5 to 10 times higher than the current rating.

They are used in commercial industries where there may be greater chances of short circuits and overloads.

Items they protect include transformers, fluorescent servers, printers and pcs.

Type D MCB 

This circuit breaker trips when the current is 10 to 20 times higher than the current rating.

They are used in big industries where the inrush current is high. For example; X-ray machines, winding motors, big transformers, etc.

Type K MCB

This type of MCB trip when the current is 10 to 12 times above the current rating. They are suitable for inductive and motor loads with high inrush currents.

Type Z miniature circuit breaker

This MCB trips when the current is 2 to 3 times above the current rating. They are sensitive to short circuits and are suitable for protecting sensitive devices such as semiconductor devices.

Parts of a miniature circuit breaker.

1. Latch
2. Solenoid
3. Switch
4. Plunger,
5. Incoming Terminal
6. Arc Chute Holder
7. Arc Chutes
8. Dynamic Contact
9. Fixed Contact
10. Din Rail Holder
11. Outgoing Terminal
12. Bi-metallic Strip Carrier
13. Bi-metallic Strip

Types of MCBs based on the number of poles

Another way of classifying MCBs is on the number of poles they have. We have:

1. Single Pole MCB

A single pole circuit breaker has one switch and also protects a single phase of the circuit.

2. Double pole miniature circuit breaker

The double pole MCB has two switches and also protects two-phase and neutral.

3. Triple Pole MCB

A three-pole circuit breaker has 3 switches, and they also protect the three phases.

4. Three poles with a neutral

Three poles and a neutral circuit breaker protect the three phases of the circuit. It also has a neutral switch.

5. Four Poles

A four-pole MCB contains four switches, three phases and a neutral. But unlike the three poles with neutral, the four poles protect all the phases and the neutral.

They are used in places with an unbalanced circuit.

MCB rating

This is the value of current an MCB can contain without tripping. It is fixed and ranges from 1A to 100A.

Frequently Asked Questions about MCBs

Why is MCBs preferred to fuse?

MCB and fuse do the same work, which is overcurrent protection. However, MCBs are preferred because they last longer and don’t need a replacement after each trip.

What is the difference between MCBs and RCDs (GFCI)?

Residual circuit devices (RCDs) and miniature circuit breakers (MCBs) are both protective devices. However, they protect different things.

RCDs or ground fault circuit interrupters (GFCI) protect humans from electric shocks, which may arise from a ground fault. While MCBs protect appliances and electrical systems from high voltage.

What is the difference between MCBs and MCCBs?

MCBs and moulded case circuit breakers (MCCBs) are the same. Their only difference lies in their trip curve. The trip curve of MCBs is not adjustable and are all sub-100 amp devices designed for low-voltage circuits.

While MCCBs have an adjustable trip curve and can be used on high-voltage circuits.