• This moulded case circuit breaker (MCCB) is a type of electrical protection device that is used to protect the electrical circuit from excessive current, which can cause overload or short circuit.
• With a current rating of up to 2500A,
• MCCBs are used mostly in industrial areas on high voltage and heavy duty machinery.
• These breakers are used instead of miniature circuit breakers (MCBs) in large scale PV systems for system isolation and protection purposes.
• The MCCB uses a temperature sensitive device (the thermal element) with a current sensitive electromagnetic device (the magnetic element) to provide the trip mechanism for protection and isolation purposes.
• This enables the MCCB to provide:
  • Overload Protection,
  • Electrical Fault Protection against short circuit currents, and
  • Electrical Switch for disconnection.

Sizing

• MCCBs in an electrical circuit should be sized according to the circuits expected operating current and possible fault currents.
• The three main criteria while selecting MCCBs are:
  • 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.

Purpose of trip Button

• This test is conducted by testing the response of the MCCB under simulated over-current and fault conditions.
• Thermal protection of the MCCB is tested by running a large current through the MCCB (300% of rated value).
• If the breaker fails to trip, it is an indication of failure of thermal protection.
• The test for magnetic protection is conducted by running short pulses of very high current.
• Under normal conditions, magnetic protection is instant.
• This test should be conducted at the very end as high currents increase the temperature of contacts and insulation, and this may alter the results of other two tests.