Molded Case Circuit Breakers (MCCBs) are crucial components in electrical systems, designed to automatically cut off the flow of current when it exceeds a preset threshold. This functionality is essential in preventing electrical overloads and short circuits, which can cause damage to equipment and pose significant safety hazards. In this article, we will delve into the specifics of MCCBs, focusing particularly on their use in direct current (DC) applications.

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What is a Molded Case Circuit Breaker?

DC MCCB is a type of circuit breaker that uses a plastic insulator as its casing. This plastic casing serves to isolate the conductive parts of the breaker from each other and from any grounded metal parts. The term “molded case” reflects this integral plastic shell, which encapsulates all the breaker’s components. MCCBs typically include thermal-magnetic trip units, while larger models might be equipped with solid-state trip sensors.

Components and Functionality

MCCBs are also known as device-mounted circuit breakers because all their components are sealed within the plastic case. This design includes auxiliary contacts, undervoltage trip units, and shunt trip units, which are often modular in nature. Due to their compact structure, MCCBs are generally not designed for repair. They are typically operated manually, but larger capacity models may offer motorized operation options.

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Types of MCCBs

There are two main types of MCCBs based on their overcurrent trip units:

  1. Thermal-Magnetic MCCBs (Type A): These are more common due to their cost-effectiveness. They provide non-selective protection with overload long-delay and short-circuit instantaneous protection.
  2. Electronic MCCBs (Type B): These offer enhanced protection features, including overload long-delay, short-circuit short-delay, short-circuit instantaneous, and ground fault protection. Some newer models also feature zone-selective interlocking.

Operational Environment

MCCBs are designed to operate in various environmental conditions:

  1. Ambient Temperature: They can function within a temperature range of -5℃ to +40℃, with a 24-hour average not exceeding +35℃.
  2. Altitude: The installation location should not exceed an altitude of 2000 meters.
  3. Atmospheric Conditions: The relative humidity should not exceed 50% at +40℃, but higher relative humidity is permissible at lower temperatures, up to a maximum of 90% in the most humid month, with a minimum temperature of +25℃.

Working Principle

The main contacts of an MCCB are closed either manually or electrically. Once closed, a free trip mechanism locks the contacts in place. The overcurrent trip unit’s coil and the thermal trip unit’s element are connected in series with the main circuit, while the undervoltage trip unit’s coil is connected in parallel with the power supply.

  • Short Circuit Protection: When a short circuit or severe overload occurs, the overcurrent trip unit activates, causing the free trip mechanism to open the main contacts, thereby disconnecting the circuit.
  • Overload Protection: In the case of an overload, the thermal trip unit’s element heats up, bending a bimetal strip that triggers the free trip mechanism.
  • Undervoltage Protection: If the circuit voltage drops below a certain level, the undervoltage trip unit activates, releasing the free trip mechanism.
  • Shunt Trip: Pressing the shunt trip button activates the shunt trip unit, which in turn triggers the free trip mechanism.

Molded Case Circuit Breaker

Key Features and Benefits

MCCBs are widely used in low-voltage distribution systems due to their robust protection capabilities. They offer several key features:

  1. Overload and Short Circuit Protection: MCCBs can handle both overload long-delay and short-circuit instantaneous protection, safeguarding electrical circuits from damage.
  2. Modular Design: The modular nature of MCCBs allows for additional functionalities such as leakage protection and measurement.
  3. Isolation Function: MCCBs ensure that any residual current after tripping is minimized, protecting both equipment and personnel.
  4. Reliability: They are designed to perform reliably even after multiple short-circuit events.

Applications and Selection Criteria

When selecting an MCCB for DC applications, it is important to consider the following:

  1. Rated Current and Breaking Capacity: Ensure that the MCCB’s rated current matches the requirements of your system. The breaking capacity, especially the rated ultimate short-circuit breaking capacity (Icu) and the rated service short-circuit breaking capacity (Ics), should meet the needs of your system’s short-circuit currents.
  2. Type of Protection: Depending on the application, choose between thermal-magnetic and electronic MCCBs. While Type A MCCBs are cost-effective, Type B MCCBs provide comprehensive protection features.
  3. Environmental Suitability: Consider the operational environment, including temperature range, altitude, and humidity conditions.


Molded Case Circuit Breakers (MCCBs) are indispensable for ensuring the safety and reliability of electrical systems. By automatically disconnecting the circuit during overcurrent conditions, they protect both equipment and personnel from potential hazards. Understanding the different types and features of MCCBs can help in selecting the right breaker for specific applications, ensuring optimal performance and protection.