Basics of Molded Case Circuit Breaker

Molded Case Circuit Breakers (MCCBs) refer to breakers where in the tripping mechanism is fitted in a molded frame and the breaker is used for rugged industrial use. These circuit breakers generally have higher breaking capacities as compared to MCBs and can handle larger amperes. Also, these breakers have features to provide extra protection from Short Circuits, Ground Faults and Overload protection.

The other low voltage circuit breaker types are : Miniature Circuit Breakers, Motor Protection Circuit Breakers, Earth Leakage Circuit Breaker, and Air Circuit Breaker.

MCCB Operation and characteristics

An MCCB is a thermo-magnetic device, having two methods of circuit interruption. An MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device. Both these devices act mechanically on the trip mechanism.

Depending upon the applications and the protection required, an MCCB may use the combination of trip elements to provide the below mentioned protections:

1) Thermal Overload

2) Short Circuits

3) Ground Faults

Thermal Overload

The overload current refers to the excessive current passing through a circuit. The heat generated from this current causes the temperature to build up between the insulation and the conductor.  At the threshold value, this thermal rise causes the tripping of the circuit breaker. The heat generated is a function of the square of the rms current (F), the resistance in the conductor (R), and the amount of time the current flows (t).

Short Circuit

Short-circuit condition. Usually, a short circuit occurs when abnormally high currents flow as a result of the failure of an insulation system. This high current flow, termed short-circuit current, is limited only by the capabilities of the distribution system. To stop this current flow quickly so that major damage can be prevented, the short circuit or instantaneous element of an MCCB is used.

Ground Fault

A ground fault is the most common type of short circuit happening in low voltage systems. Usually, the ground faults are not very high to be detected by the protective device of the MCCB. However, if left undetected, they can increase sufficiently to trip the protective system of the breaker. When this happens, the breaker trips but by this time, the equipment may already have been damaged. For addition protection in such cases, a ground fault protection device is attached the MCCB.With the advent of electronic trip units, this protection is now generally available inbuilt to the MCCB.

Types of Trip Units

1. Thermal Magnetic Tip Unit – As the name implies, a thermal magnetic trip unit combines the features of a thermal unit and a magnetic unit. The thermal trip action uses a piece of bimetal heated by the load current. This bimetal is actually two strips of metal bonded together, with each having a different thermal rate of heat expansion. They are factory-calibrated and not field-adjustable. The heat will cause the bimetal to bend. To trip the circuit breaker, this bimetal must deflect enough to physically push the trip bar and unlatch the contacts. The magnetic unit comprises of an electromagnet. The high current energises the electromagnet which in turn attracts the armature. The armature is connected to the trip unit and causes the breaker to trip.  In most breakers the magnetic trip units are adjustable.
2. Electronics Units – Electronic trip units typically consist of a current transformer (CT) for each phase, a printed circuit board, and a shunt trip. The CTs monitor current and reduce it to the required ratio for direct input into the printed circuit board, the brains of the electronic trip unit. The circuit board then interprets current flow information, makes trip decisions based on predetermined parameters, and tells the shunt trip unit to trip the breaker.

The basic principle of working of a circuit breaker remains the same irrespective of the type of circuit breaker.

The user can choose good quality electrical breaker components from L&T, Siemens, Schneider or other reputed brands.