How to Choose: Air Circuit Breaker or Moulded Case Circuit Breaker

The two main factors which are important for ACB or MCCB selection:

1. Load
2. Fault limitation.

Load:

The current-carrying capacity (In, A) of the breaker should be higher than the expected load in the circuit. MCCBs are less cost-effective for very large ratings (2000A and above). The advantage of MCCBs for very large ratings is their compact size. An ACB is physically larger, but more cost-effective for higher ratings.

Fault Limitation:

In a short circuit the contacts of  MCCB (Moulded Case Circuit Breaker)s open before the first peak of the current waveform (within five milliseconds in a 50 Hz system). The fault current flowing through the MCCB never reaches its peak, and the fault energy allowed downstream is limited. This fault limitation protects sensitive equipment which is not rated to withstand faults.

 ACB(Air circuit breaker)s are selected for their ability to withstand fault current rather than limit it -(see Discrimination - Selectivity). A typical ACB will open a short-circuit in between twenty-five and thirty milliseconds, allowing between one and two cycles of fault current through before opening. The load protected by an ACB (transformers, busbars for example) should be rated to withstand fault current for a short duration.