Molded Case Circuit Breakers: Advantages & Disadvantages

A circuit breaker is an automatically operated electrical switch used to protect electrical circuits from damage caused by overcurrent/overload. Overcurrent is an excessive current or current beyond the rating of protected equipment. Overload is a running overcurrent that causes overheating of protected equipment.

While there are numerous types of circuit breakers, this article will focus on the molded case circuit breaker or MCCB.

MCCBs are used to provide circuit protection in applications requiring adjustable tripping, high current, and manual tripping of the circuit. They are often used in industrial applications such as panelboards, switchboards, motor control centers, and control panels.

MCCBs have two working principles: an electromagnetic element, protecting against overcurrent (short circuit), and a thermal element, protecting from overload.

The electromagnetic element is a solenoid coil that generates a weak electromagnetic field during normal operation. If a short circuit occurs, large current flows through the solenoid creating a strong electromagnetic field that instantly pulls the contacts open, interrupting the circuit.

The thermal element is a bimetallic contact that expands and contracts with temperature changes. This allows current flow during normal operation but as current increases beyond the preset value, the contact expands until it opens, interrupting current flow from the main supply and protecting the equipment. This also helps prevent overheating of electrical equipment and wiring due to excessive current flow.

MCCBs, like all circuit breakers, have five main parts:

1. Frame: protects internal parts of the circuit breaker
2. Operating mechanism: opens and closes the circuit breaker, typically a handle or lever to manually operate the circuit breaker
3. Trip unit: detects and interrupts an overload or short circuit
4. Contacts: allows current flow when closed and prevents current flow when open
5. Arc extinguisher: extinguishes the arc formed when the contacts open thus preventing damage to the device and surrounding equipment

Specifications of MCCBs

1. Current rating: MCCBs are designed for higher currents, typically 10A-2500A. This is generally the most important factor to consider when choosing an MCCB for the application.
2. Breaking capacity: This is the ability of an MCCB to interrupt a circuit during a fault, specified in kA. This is the maximum amount of current the device can safely interrupt before damage occurs internally to the device.
3. Tripping characteristics: MCCBs feature adjustable tripping characteristics allowing for customization of the device’s response to overload conditions.

Advantages of MCCBs

• Higher current ratings
• Adjustable tripping characteristics
• High performance in a compact size
• Wide range of accessories for operating and automation requirements
• No possibility of phase failure – all poles trip simultaneously

Disadvantages of MCCBs

• Higher cost than miniature circuit breakers and fuses
• Susceptible to dust and corrosion
• Not suitable for high voltages

To explore the full selection of circuit breakers from MISUMI, visit our page here. You can also check out the selection of fuses at this link.