What is kA rating (breaking capacity) of MCB and MCCB?

kA rating of an MCB or an MCCB is the maximum current it can safely interrupt in case of a short circuit. If the current goes beyond this value, the circuit breaker could be damaged. kA rating is known as the short circuit withstand capacity or ultimate breaking capacity of a circuit breaker.

For example, if the MCB is rated at 10kA, it means that the MCB can safely break the circuits during short circuit unless until the short-circuited current does not exceed 10kA.

MCB stands for ‘Miniature Circuit Breaker’ and MCCB stands for ‘Moulded Case Circuit Breaker’. These are circuit protection devices, capable of protecting circuits from greater damages caused during short circuit and overloads.

Under short-circuited conditions, the current flow increases exponentially. That means during short circuits a circuit designed for 10A can cause a current flow of over thousands of amperes. If the circuit breaker used has a very low rating or the ultimate short circuit withstand current rating, the circuit breaker, as well as the circuit, will be damaged. Therefore, the kA rating of a circuit breaker is significant.

Definition of Ultimate breaking capacity – Icu

The Ultimate breaking capacity of a circuit breaker can be defined as the maximum current it can safely interrupt in case of a short circuit. It is denoted by Icu. After clearing a fault at its Icu, the circuit breaker cannot be reused. It is also known as the ultimate current withstand capacity.

Definition of Service breaking capacity – Ics

The service breaking capacity of a circuit breaker can be defined as the maximum current it can safely interrupt and yet enter back to service after fault clearance. It is denoted by Ics. After clearing a fault at its Ics, the circuit breaker can be reused.

The difference between Icu and Ics is the re-usability of the circuit breaker after clearance of fault.

Importance of kA rating

The kA rating is a very important factor of any circuit breaker. The maximum amount of current flow through a circuit breaker during short circuit varies from one circuit to another. It is necessary to pre-calculate and use the right circuit breaker to avoid danger. Incorrect kA rating can be a potential hazard, causing fire and can be sometimes fatal.