A molded case circuit breaker (MCCB) is a type of circuit breaker that protects the circuit from overloading, short circuit and current surges. It is an advanced version of miniature circuit breaker (MCB) since it operates like one. However, it offers extra features that make it a superior circuit breaker such as remote closing and adjustable trip settings i.e. its current settings and time settings can be adjusted according to our needs.
An MCCB works by using a thermal element and a magnetic element to detect and interrupt faulty currents. The thermal element is a bimetallic contact that bends away when the current exceeds the adjusted trip value due to heating. The magnetic element is a solenoid coil that produces a strong electromagnetic field that forces the trip bar to open the circuit when a short circuit occurs. The arc that is generated when the contacts open is extinguished by an arc chute that splits and lengthens the arc.
Applications of Molded case circuit breaker
MCCB has many applications in different environments, such as residential, industrial, or commercial systems. Some of the common applications of MCCB are:
Adjustable trip settings: MCCB is superior to MCB and has more application due to its higher current handling and adjustable trip setting capabilities. It can be used for both low and high breaking capacity requirements, mainly industrial.
Motor protection: MCCB is used in industries to protect large electrical motors from overloading. It can also provide earth fault protection and remote control for the motors.
Welding machine protection: Welding machines draw a very large amount of current that an MCB cannot handle. MCCB can protect the welding machines from short circuits and overcurrents.
Capacitor bank protection: Capacitor banks are used to improve the power factor and voltage stability of the power system. MCCB can protect the capacitor banks from overvoltage, overcurrent, and harmonic distortion.
Generator protection: Generators are used to supply power in case of grid failure or load shedding. MCCB can protect the generators from overload, short circuit, reverse power, and loss of excitation.
Main electric feeder distribution: MCCB can be used as the main switch for the electric feeder distribution system. It can isolate the feeder from the main supply and provide protection against faults and surges.
Wiring diagram
There are different types of wiring diagrams for MCCBs, depending on the type and model of the MCCB, the type and rating of the circuit, and the type and function of the auxiliary contacts. However, a general wiring diagram of an MCCB can be shown as follows:
This wiring diagram shows a simple MCCB with a trip unit and an operating mechanism. The trip unit monitors the current flow through the circuit and triggers the operating mechanism when a fault occurs. The operating mechanism opens or closes the contacts between the line and load terminals. The auxiliary contacts provide additional functions for the MCCB, such as remote control, indication, interlocking, or signaling.
The wiring diagram may vary depending on the specific features and functions of the MCCB. For example, some MCCBs may have more than one trip unit, such as thermal and magnetic trip units. Some MCCBs may have more than one set of contacts, such as main contacts and arc contacts. Some MCCBs may have more than one set of auxiliary contacts, such as normally open (NO) and normally closed (NC) contacts. Some MCCBs may have different types of terminals, such as screw terminals or plug-in terminals.
To wire an MCCB correctly, you should follow the wiring diagram provided by the manufacturer of the MCCB. You should also follow the safety precautions and installation instructions given by the manufacturer. You should use suitable wires and connectors for the MCCB and its related devices. You should check the polarity and phase sequence of the wires before connecting them. You should test and commission the MCCB after completing the wiring.
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