What are the function and working principle of a molded case circuit breaker?

Molded case circuit breakers, also known as MCCBs, use a plastic insulator as the device's outer casing. They are suitable for use as protective switches in branch circuits and can automatically interrupt the current when it exceeds the trip setting. All parts used to isolate conductors and grounded metal parts are sealed within the plastic casing. Auxiliary contacts, undervoltage trip units, and shunt trip units are often modular. Due to their very compact structure, molded case circuit breakers are generally not repairable, so maintenance is usually not considered.

I. Main Functions

The core functions of a molded case circuit breaker (MCCB) can be summarized as "protection" and "control."

Overload Protection:

When the current in the circuit exceeds the circuit breaker's rated current (e.g., 1.13-1.45 times the rated current) for an extended period, but does not reach the short-circuit current, the circuit breaker will trip after a delay to disconnect the circuit. This is usually caused by abnormally increased load, equipment failure, etc.

Purpose: To prevent damage to lines and equipment due to overheating, and even to prevent fires. Its operating characteristics are matched to the withstand capacity of the lines and equipment.

Short-Circuit Protection:

When a fault such as a short circuit to ground or between phases occurs, generating a very large instantaneous current (usually several to tens of times the rated current), the circuit breaker will trip instantaneously or within a very short time (milliseconds), quickly disconnecting the circuit.

Purpose: To prevent the enormous electrodynamic and thermal effects from damaging the power distribution system and electrical equipment; it is the most critical protection against the escalation of electrical accidents.

Control and Isolation:

Normal Operation: Can be manually (or electrically) opened and closed to connect or disconnect the power supply circuit, enabling circuit control.

Isolation Function: When the circuit breaker is in the open state, its contacts have sufficient isolation distance, providing a visible and safe disconnection point for downstream lines and equipment, facilitating maintenance and repair.

Optional Additional Functions (achieved through accessory modules):

Undervoltage Protection: Automatically trips when the power supply voltage is too low or lost, preventing abnormal operation of equipment under low voltage and preventing self-starting of equipment when the voltage recovers.

Leakage Current Protection: By adding a leakage current protection module, trips when leakage current (which may cause electric shock or fire) is detected in the line.

Alarm Contacts: Used to indicate the open/closed status of the circuit breaker.

Electric Operating Mechanism: Enables remote or automatic control.

 

II. Working Principle

 

Molded case circuit breakers achieve the above-mentioned protection functions through the coordinated operation of a sophisticated internal mechanical and electromagnetic mechanism. Its core working principle is based on "thermal-magnetic tripping."

Core Structural Components:

Operating Mechanism: A linkage and spring mechanism that enables manual "closing-holding-opening" mechanical actions.

Contact System: Includes moving and stationary contacts, which are the components for connecting and disconnecting current. It is usually equipped with an arc-extinguishing grid to quickly extinguish the arc generated during disconnection.

Trip Mechanism: The "trigger" that triggers the operating mechanism to release and trip the circuit breaker.

Current Trip Unit (Core Protection Component):

Thermal Trip Unit (Bimetallic Strip) – Responsible for overload protection.

Magnetic Trip Unit (Electromagnetic Coil and Iron Core) – Responsible for short-circuit protection.

Housing: Made of high-strength engineering plastic, possessing good insulation and mechanical strength, encapsulating all components into a single unit.

 

Key Process Description

 

Overload Protection (Thermal Trip, Time-Delay Action):

Current flows through a bimetallic strip (thermal element) connected in series with the main circuit.

Overload current heats the bimetallic strip. Due to the different coefficients of thermal expansion of the two metals, the bimetallic strip slowly bends to one side.

When the overload persists for a certain period, the bending degree of the bimetallic strip reaches a predetermined value, which pushes a lever (trip lever), releasing the tripping mechanism. Under the action of a spring, the main contacts are forced to open, cutting off the circuit.

Time-Delay Characteristic: The bimetallic strip takes time to bend due to heat. The larger the overload current, the faster the bending and the shorter the action time. This "inverse-time" characteristic perfectly matches the thermal tolerance of the equipment and wires.

Short Circuit Protection (Magnetic Trip, Instantaneous Action):

Current flows through an electromagnetic coil.

When a short circuit occurs, the current instantly surges to a set threshold (e.g., 5-10 times the rated current). The strong magnetic field generated by the coil immediately attracts an armature (iron core).

The attracted armature strikes the trip lever like a hammer, instantly releasing the tripping mechanism and causing the circuit breaker to trip in milliseconds.

Arc Extinguishing Process:

When interrupting large currents (especially short-circuit currents), a strong electric arc is generated between the contacts. The arc-extinguishing grid inside the circuit breaker (composed of a series of mutually insulated metal grids) pulls in, divides, cools, and quickly extinguishes the arc, ensuring a safe and reliable circuit disconnection.

 

Summary

 

Moulded case circuit breakers serve as the guardians of low-voltage distribution systems. Employing thermomagnetic tripping principles, they intelligently distinguish between overload and short-circuit faults, interrupting faulty circuits at varying speeds (delayed or instantaneous). This reliably safeguards critical equipment—including lines, cables, motors, and transformers—from damage caused by overloads and short-circuit currents, thereby ensuring the secure and stable operation of power systems.

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