Enhancing Understanding: Molded Case Circuit Breakers (MCCBs) in the PV Market



As the installed system capacity grows in the PV market, a grasp of molded case circuit breakers (MCCBs) becomes pivotal for system designers and installers. MCCBs act as electrical protection devices, safeguarding circuits from excessive current that can lead to overload or short circuit. With a current rating up to 1600A, MCCBs prove invaluable in large-scale PV systems, ensuring system isolation and protection.

Mechanisms of MCCBs:

Operation: The MCCB operates through a temperature-sensitive component (thermal element) and a current-sensitive electromagnetic component (magnetic element), providing a comprehensive trip mechanism for protection and isolation purposes.

  • Overload Protection,
  • Electrical Fault Protection against short circuit currents, and
  • Electrical Switch for disconnection.

  • Overload Protection: The temperature-sensitive bimetallic contact allows normal current flow. When the current surpasses the trip value, the bimetallic contact heats, bending away and triggering circuit interruption. The thermal protection includes a time delay for short-duration overcurrent situations.


  • Electrical Fault Protection: MCCBs respond instantaneously to short circuit faults via electromagnetic principles. The solenoid coil generates an electromagnetic field, attracting the trip bar and opening contacts during a short circuit.


  • Electrical Switch for Disconnection: MCCBs can serve as manual disconnection switches during emergencies or maintenance operations. To mitigate arc formation during contact opening, MCCBs incorporate internal arc dissipation mechanisms.

Deciphering MCCB Characteristics and Ratings:

Manufacturers provide crucial information on MCCB operating characteristics:

Rated Frame Current (Inm)

The maximum current that the MCCB is rated to handle. This rated frame current defines the upper limit of the adjustable trip current range. This value determines the breaker frame size.

Rated Current (In):

The rated current value determines when the MCCB trips due to overload protection. This value can be adjusted, to a maximum of the rated frame current.

Rated Insulation Voltage (Ui)

This value indicates the maximum voltage which the MCCB can resist in lab conditions. The rated voltage of MCCB is typically lower than this value to provide a safety margin.

Rated Working Voltage (Ue)

This value is the rated voltage for the continuous operation of MCCB. It is normally the same as or close to the system voltage.

Rated Impulse Withstand Voltage (Uimp)

This value is the transient peak voltage the circuit breaker can withstand from switching surges or lightning strikes. This value determines the ability of the MCCB to withstand transient over-voltages. The standard size for impulse testing is 1.2/50µs.

Operating Short Circuit Breaking Capacity (Ics)

This is the highest fault current that the MCCB can handle without being permanently damaged. MCCBs are generally reusable after fault interruption operation provided they do not exceed this value. The higher the Ics, the more reliable the circuit breaker.

Ultimate Short Circuit Breaking Capacity (Icu)

This is the highest fault current value that the MCCB can handle.

If the fault current exceeds this value, the MCCB will be unable to trip. In this event, another protection mechanism with a higher breaking capacity must operate. This indicates the operation reliability of the MCCB.

It is important to note that if the fault current exceeds Ics but does not exceed Icu, the MCCB can still remove the fault, but may be damaged and require replacement.

Mechanical Life

This is the maximum number of times the MCCB can be operated manually before it fails.

Electrical Life

This is the maximum number of times the MCCB can trip before it fails.

  • Sizing:
    • Match the MCCB's rated working voltage (Ue) with the system voltage.
    • Adjust the trip value based on the load's current draw.
    • Ensure the breaking capacity exceeds theoretical fault currents.
      • The rated working voltage (Ue) of the MCCB should be similar to the system voltage.
      • The trip value of the MCCB should be adjusted according to the current drawn by the load.

      • The breaking capacity of the MCCB must be higher than the theoretical possible fault currents.

Types of MCCB:

MCCB Maintenance:

Type of MCCB

Operating Current

Operating Time

Application

Suitability

Surge Current

Installation Location

Type B

Trips between 3 and 5 times rated current (In)

0.04-13 seconds

Domestic applications (lighting and resistive elements)

Resistive load application

Low

Sub feeder of Distribution board

Type C

Trips between 5 and 10 times rated current (In)

0.04-5 seconds

Commercial or industrial applications

Inductive load applications

Moderate

At incoming/outgoing of Distribution Board

Type D

Trips between 10 and 20 times rated current (In)

0.04-3 seconds

Commercial or industrial applications

Inductive – capacitive load applications (Pumps, motor, large winding motors etc.)

High

At incoming of Distribution Board/Panels

Type K

Trips between8 and 12 times rated current (In)

0.04-5 seconds

Industrial applications

Inductive and motor loads with high inrush currents.

High

At incoming of Distribution Board/Panels

Type Z

Trips between 2 and 3 times rated current (In)

0.04-5 seconds

Highly sensitive to short circuit and are used for protection of highly sensitive devices such as semiconductor devices

Medical instruments

Very low

At sub feeder of Distribution board for IT equipment.


Figure 1: Trip curve of type B, C, and D MCCBs

  1. Visual Inspection:

    • Check for deformed contacts, cracks, or burn marks on contacts or casing.

  2. Lubrication:

    • Some MCCBs require lubrication for smooth operation of manual disconnection switches and internal moving parts.

  3. Cleaning:

    • Remove dirt deposits that may deteriorate MCCB components.

  4. Testing:

    • Conduct insulation resistance, contact resistance, and tripping tests during maintenance.

Conclusion:

Selecting MCCBs correctly and implementing regular maintenance are crucial for ensuring the safety and reliability of high-power equipment in various applications, especially in the dynamic landscape of the PV market.