Understanding MCCB (Molded Case Circuit Breaker): Functions, Working Principle, and Difference from MCB

Table of Contents

Image of MCCB (Molded Case Circuit Breaker)

Kelas Teknisi | MCCB is a useful component in the process of electrical power distribution, commonly found inside the LVMDP panel (Low Voltage Main Distribution Panel), which must be able to cover a load in both steady-state conditions and during disturbances.

The MCCB installed in the LVMDP panel must also be able to withstand short-circuit currents on the load for a certain period of time.

Definition of MCCB

MCCB stands for Molded Case Circuit Breaker, which is a protective device that operates automatically when it detects an overload or short circuit. The capacity of MCCBs ranges from 40 A up to 1000 A.

MCCBs commonly found in panels are of two types: fixed capacity and adjustable capacity. For fixed capacity MCCBs, operation is based on the nameplate specification, while for adjustable capacity MCCBs, the working capacity can be set as desired within the range indicated on the MCCB’s nameplate.

Image of MCCB Parts

Description:

1. BMC material for body and cover.
2. Arc extinguisher.
3. Terminal block for ST and UVT installation.
4. Operating mechanism (ON-OFF).
5. Movable contact.
6. Manufacturer data.
7. Magnetic trip unit.
8. Thermal trip unit.
9. Compact size.

Working Principle of MCCB

The working principle of an MCCB involves thermal protection against overloads and magnetic protection against short circuits. The thermal protection uses a bimetal, which consists of two bonded metal strips.

The heat generated by overload current (over current) causes the bimetal to bend, pushing the trip lever and making the MCCB trip. However, thermal protection responds much slower compared to magnetic protection.

Magnetic protection uses a coil. When a short circuit occurs, the coil detects it and generates a magnetic field. This magnetic field pulls the nearby shaft and moves the trip lever. Magnetic protection trips almost instantly.

Since magnetic protection works based on induction, it operates much faster than thermal protection. Therefore, magnetic protection has a very short response time to trip.

MCCB Characteristics

Here are 5 common characteristics of MCCBs:

• Operates on 3 phases.
• Ampere rating from 100A – 1000A.
• Uses Thermal Magnetic protection.
• Network frequency: 50/60 Hz.
• Can be set as Fixed or Adjustable.

How to Calculate MCCB Ampere Capacity

To calculate the Ampere capacity of an MCCB, we can use the formula shown in the following image:

Example:

A motor with a capacity of 37 kW. What size of MCCB should be used?

Here is the calculation result for the example above:

For safety factor, we multiply by 120%:

66.21 A x 120%

I = 79.452 ≈ 79.5 Ampere

Thus, the nearest available rating in the market is 80 Ampere.

Or you can use the calculator provided by Kelas Teknisi to quickly calculate MCCB requirements here: MCCB Ampere Calculator.

Comparison of MCCB and MCB

1. MCCB (Molded Case Circuit Breaker)

• MCCBs have higher Ampere ratings than MCB, ranging from 100 – 1000 A.
• kA rating ranges from 36 – 100 kA.
• MCCB Ampere value can be adjusted.
• Equipped with a reset button.
• Larger in size.

2. MCB (Miniature Circuit Breaker)

• Ampere rating ranges from 2 – 100 A.
• kA rating is limited to 4.5 kA.
• Ampere value cannot be adjusted.
• No reset button available.
• Smaller in size.

Hopefully this is useful!

References

1. Ampere Journal titled "Study of LVMDP Panel Capability Against Load" (M. Saleh Al Amin)

2. https://www.gses.com.au/molded-case-circuit-breakers/ (Accessed September 29, 2024)

3. https://www.fujielectric.com/products/mccb_elcb/ (Accessed September 29, 2024)

4. https://electrification.us.abb.com/products/circuit-breakers/molded-case-circuit-breakers-mccb (Accessed September 29, 2024)