Molded Case Circuit Breakers (MCCB), Types and How They Work

A molded case circuit breaker (MCCB) is a type of breaker that protects electrical circuits from overcurrents arising from circuit overload, short circuits, or faulty wiring.

They are almost the same as the Miniature circuit breaker (MCB) as they do the same work. However, MCCB is more advanced.

It has a higher current rating of up to 2500, Amp, and adjustable trip settings.

These features make them useable on both low voltage, medium, and high voltage substations for circuit protection.

Parts of a molded case circuit breaker.

MCCBs have five major parts, they include:

  • Molded case or frame: This provides insulated housing to the breakers’ components.
  • Operating mechanism: It controls the opening and closing of the contact.
  • Arc extinguishers: This closes and opens the arc.
  • Trip units: It trips the operating mechanism.
  • Contact: Metallic conductors that carry currents.

Functions of molded case circuit breaker

MCCB has three major functions, they include

  • Protection against circuit overload: When certain parts of the circuit contain more loads than the rated value, the breaker cuts the power system to protect electrical devices.
  • Protection against short circuits: If there is a fault or a bridge between the hot wire and the neural wire such that it produces an overcurrent, the breaker interrupts the light without delay.
  • Switch ON and OFF the circuit, especially during maintenance.

Working principle of a molded case circuit breaker (MCCB).

Like miniature circuit breakers, the MCCBs also use thermal and electromagnetic operations to detect and interrupt faulty currents.

Under working conditions of an MCCB, current flows from the incoming terminal to the moving contacts, solenoid, bimetallic strip, and to the outgoing terminals.

But when a faulty current occurs, it trips to avert damage.

Overload protection.

It achieves overload circuit protection with the help of a thermal mechanism. MCCBs have bimetallic contacts that expand and also contract according to temperature change.

Under normal conditions, the current flows through the breaker. But once it exceeds the adjusted trip value, the contact heats up and expands, leading to circuit interruptions.

They build thermal protection of the MCCB against overload with a time delay to allow short duration overcurrents, which is common in many devices.

However, any over-current that exceeds the trip value represents an overload and should be extinguished by the breaker.

Short circuit protection

Besides overload protection, MCCB also protects against faulty currents using electromagnetic induction.

The MCCB produces a small electromagnetic force as the current flows through the solenoid coil.

But when a short circuit occurs, higher currents flow through the breaker. This makes the solenoid coil produce a strong electromagnetic field that forces the trip bar to open the circuit.

Besides the tripping mechanism of the breaker, MCCB also has a manual switch ON and OFF the circuit during emergencies or maintenance.

Note: MCCB doesn’t protect against electric shock, GFCI does that.

Types of MCCB.

Like miniature circuit breakers, MCCB also has types. We group them according to their trip curve, and they include the type B, C, D, K, and Z.

Type B MCCB.

This breaker trips when the current rate is 3 to 5 times higher than the current rating. They are used in domestic appliances and in light commercial applications with low surge current.

Type C MCCB

This circuit breaker trip when the current is 5 to 10 times above their trip curve.

They are used in commercial or industrial applications with moderate surge currents.

Examples include transformers, printers, fluorescent servers, and pcs.

Type D MCCB 

These MCCBs trip when the current is 10 to 20 times above the trip curve.

They are used in big industrial and commercial applications with high inrush current. For example; X-ray machines, pumps, large winding motors, big transformers, etc.

Type K MCCB

This MCCB trip when the current rate is 10 to 12 times higher than the current rating. They are suitable for inductive and motor loads with high inrush currents.

Type Z molded case circuit breaker

These MCCB trips when the current is 2 to 3 times higher than the trip curve. They are sensitive to short circuits and are suitable for protecting highly sensitive devices such as semiconductor devices.

MCCB Rating.

Before making your choice of a circuit breaker, be it MCB or MCCB, make sure you understand the rating or specification. They come in different sizes for unique purposes, so be sure your selection tallies with your intention.

Here are the things to watch out for:

  • Rated frame current (Inm): This shows the maximum current the breaker can withstand. It defines the upper limit of the adjustable trip, current range, and the frame size of the breaker.

There are several current ratings MCCBs for the same series of frame sizes.

For instance, DX100 Frame Size MCCB for rated current of 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A.

DX225 Frame Size MCCB for rated current of 100A, 125A, 160A, 180A, 200A, 225A.

From the above examples, DX100 and DX225 have two Types of frame Size for a rated current of 100A, but the shape and size of the breaking capacity of the breakers are different.

  • Rated current (In): This determines when the breaker trips because of overload. And unlike the MCB that has fixed rated current, MCCB is adjustable.
  • Rated insulation voltage (UI): This shows the maximum voltage the breaker can withstand in a lab test. It is usually higher than the rated working voltage, so as to provide a safety margin during field operation.
  • Rated working voltage (UE): This defines the rated voltage for the continuous operation of the breaker. It is usually the same or very close to the system voltage.
  • Rated impulse withstanding voltage (UIMP): This shows the value of transient peak voltage the breaker can withstand from switching surges or lighting strikes. This value shows the ability of the breaker to withstand transient overvoltages from lightning strikes. The standard size for impulse testing is 1.2/50 μs.
  • Operating short-circuit breaking capacity (LCS): It is the highest fault current the breaker can trip without damaging permanently. MCCBs are reusable after interrupting a faulty current, provided they do not exceed that value. The higher the LCS, the more reliable the MCCB.
  • Ultimate short-circuit breaking capacity (ICU): This is the highest fault current the MCCB can withstand. If the faulty current exceeds the value, the breaker will not trip again and will another protection mechanism with a higher breaking capacity.
  • Mechanical life: It shows the number of times we can manually operate the breaker before failure.
  • Electrical life: Shows the number of times the breaker can trip before failure.

MCCB testing and maintenance

According to  ANSI/NEMA AB 4, guidelines for Inspection and preventive maintenance of MCCBs, the following test can be useful in verifying the operational level of an MCCB.

  • Insulation resistance test: To determine the level of insulation between the phases, and across the supply and load terminals.
  • Inverse time overcurrent trip test: This checks the inverse time overcurrent tripping function of the breaker.
  • Individual pole resistance test (millivolt drop test): To evaluate the electrical integrity of the internal connections and contacts of the breaker.
  • Instantaneous overcurrent trip test: To check the operation of the instantaneous trip functions under field test conditions.
  • Mechanical operation test: To check if the operating mechanism is working fine.
  • Rated hold-in test. Check the capability of the breaker in carrying the rated current.

Maintenance

Since most of the MCCB casing are silled and cannot be opened, they rarely require maintenance.

The preventive maintenance you may give them depends on the operating conditions and includes regularly cleaning the breaker and manually turning it ON and OFF to make sure it is working fine.