Miniature Circuit Breakers (MCBs)

Miniature circuit breakers (MCBs) are electromechanical devices that protect an electrical circuit from over currents, in other words, MCBs are switches that automatically turns OFF when the current flowing them exceeds the maximum allowable limit. Over currents in an electrical circuit may be caused by short circuits overload or faulty designs. MCBs functions by interrupting the continuity of electrical flow through the circuits once a fault is detected. In general, MCBs are designed to protect against overcurrent and overheating faults.

MCBs are used for the control of small motors and domestic sub-circuits; they are considered primarily as a better alternative to the fused switches, since they don’t require replacement once an overload is detected.

MCBs Tripping Mechanisms

Methods of achieving the required operating characteristics can be classified as:

• Thermomagnetic
• Assisted thermal
• Magnetohydraulic

In the thermomagnetic method, the time delay is provided by a bimetal element, and the fast trip by a separate magnetically operated mechanism based on a trip coil.

In assisted thermal method, the bimetal is itself subjected to magnetic force. The thermal operation by bimetal elements implies that the effective current rating is a function of the ambient temperature. Hence, if complete ambient compensation is not fitted, MCBs are rated in such a way as to allow for the type of enclosure. The combination of thermal and magnetic functions is not easily controlled for low current ratings and for MCBs with such ratings the tolerances on operation must be wider than they are for larger currents.

The Magnetohydraulic mechanism incorporates a sealed dashpot with a fluid and a spring restraint, the dashpot plunger being of iron and subject to the magnetic pull of the trip coil. With Magnetohydraulic devices, the tripping is independent of the ambient temperature over a specified range, the small variations due to change of viscosity of the damping fluid being minimized by use of fluid with nearly flat viscosity-temperature characteristics.

The Operation of Miniature Circuit Breaker

Let’s consider the diagram below showing the cross section of MCB:

The MCB consists 3 main parts:

• One bi-metallic strip
• One trip coil
• One hand operated ON-OFF lever

In a MCB, they are two contacts – one is fixed and the other is moveable. When the current exceeds the set limit, a Solenoid forces the moveable contact to open, in other words, disconnect from the fixed contact, and the MCB turns OFF, stopping the current flowing in the circuit. The moving contact of the MCB is set by means of the spring, such that, with this latch point, a small displacement of latch causes release of spring and makes the moving contact to move to open the MCB.

Related: Switchgear

Overloading the circuit that contains MCBs for a long time can have a detrimental effect on them, since the bi-metallic strip becomes overheated and deformed. The deformation of bi-metallic strip causes the displacement of the latch point.

Also read:

• Toggle Switches
• Electromechanical Relays (EMR)
• Fuses & Circuit Breakers vs. Overload Heaters for Industrial Motor Protection
• Industrial Motor Control Using Contactors
• How to Wire an Industrial Control Motor
• Power System Protection