What is a MCB?

Full form of MCB: MCB full form is Miniature Circuit Breakers. The MCB is an electromechanical device that automatically shuts down a circuit when an abnormality is detected. 

What is a MCB?

MCBs are Time-delay tripping devices. The operating time of these devices is determined by the magnitude of overcurrent passing through them. 

It means that they operate whenever an overload exists for long enough to endanger the circuit being protected. Transients such as motor starting currents or switch surges are not detected by MCBs

When there are short circuit faults, these devices are typically designed to operate in less than 2.5 milliseconds, and when there are overloads, they are typically designed to operate in 2 seconds to 2 minutes.

The Working Principle of the MCB 

Under normal operating conditions, the MCB acts as a switch (a manual switch) to turn on or off the circuit. The device automatically trips in the event of a short circuit or overload. This will result in a current interruption in the load circuit, resolving the issue. The trip is visually indicated as the operating knob automatically moves to the OFF position.

Magnetic tripping and thermal tripping are two methods for automatic operation/tripping of MCBs.

The current coursing through the bimetal causes the temperature of the bimetal to rise during overload. The heat generated by the increase in temperature within the bimetal causes the bimetal to deflect. 

This is because metals expand when they are heated. The trip latch is released as a result of the deflection, and the contacts are separated. 

The magnetic field created by the coil pulls on the bimetal, causing a deflection that activates the tripping mechanism in other MCBs.

Magnetic tripping is more commonly used in short circuit or heavy overload situations. The light spring holds the slug in place under normal working conditions

This is due to the coil’s magnetic field not being strong enough to attract the latch. When a fault occurs in the circuit and current flows through it, the magnetic field created by the coil is strong enough to pull the spring holding the slug in place, causing the slug to move and the tripping mechanism to activate.

The majority of MCBs employ a combination of thermal and magnetic tripping mechanisms. When the contacts begin to separate in both operations, an arc is formed. The arc is then forced into an arc splitter via an arc runner. 

Arc chutes are another name for arc splitter plates. When an arc is formed into a series of arcs, energy is extracted and the arc is cooled. As a result, the arrangement eliminates arcs.

As a result, the MCB is primarily used to protect the circuit (wiring, connected load, equipment, appliances, and so on) in the following situations:

  • Overload
  • Circuit Breaker
  • Over-current

What are the principal characteristics of a MCB?

Amperes, Kilo Amperes, and Tripping Curve are the three main features of an MCB.

Overload Current Rating – Amperes (A)

When too many appliances are connected to a single circuit, they use more electrical current than the circuit and cable were designed to handle. This may happen in the kitchen if the kettle, dishwasher, electric range, microwave, and blender are all in use at the same time. The MCB on this circuit cuts electricity, keeping the cable and terminals from overheating and catching fire.

Short Circuit Rating – Kilo Amperes (kA)

Short circuits are caused by a failure in an electrical circuit or appliance, and they can be even more harmful than overloads since the scale and speed of overcurrent are on a different scale. When the live and neutral conductors are connected directly, this happens. The electrical current rushes around the circuit in a loop without the resistance supplied by regular circuit integrity, multiplying the amperage by thousands of times in moments.

Tripping Curve

An MCB’s ‘Tripping Curve’ allows for real-world, and sometimes necessary, power surges. To overcome the inertia of large motors in commercial applications, large machines typically require an initial burst of power over their average working current. The MCB allows this small spike, which lasts only seconds because it is safe to do so for such a short period.

Use of MCB in household

While each installation is unique and should always be built by a fully qualified professional electrician on-site, most domestic wiring schemes follow the following guidelines:

6 Amp – standard lighting circuits

10 Amp – Large lighting circuits are becoming less common in household settings as technology and trends shift toward lower-energy lighting sources like LED.

16 Amp and 20 Amp – These are both often used for immersion heaters and boilers, depending on the power rating.

32 Amp – For example, a two-bedroom house might have two 32A power circuits to separate the upstairs and downstairs plugs. The number of 32 A circuits in a larger home is unlimited.

40 Amp – Cookers / electric hobs / small showers

50 Amp – 10kw Electric showers / Hot tubs.

Robin Singh

Robin Singh is education professional tutor of NCERT. I have good knowledge of CBSE all subjects. Expert in maths, physics and chemistry. if students have any doubt about NCERT Solutions so contact us info@cbsenotes.in.

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