What Is Ultimate Breaking Capacity (Icu) & Service Breaking Capacity (Ics)

To all engineers…you must know what MCCB is! But have you notice the Icu and Ics percentage indicate at the MCCB?

For example; the above MCCB shows that the Ics = 50% Icu

Those values are actually the breaking capacity of the circuit breaker namely as per below;

Icu: Rated Ultimate Short-circuit breaking capacity (in kA) 

Ics: Rated Service Short-circuit breaking capacity (in kA) 

The purpose of those braking capacity is to ensure the Circuit Breaker fulfill and without fails its main purpose to protect the electrical installations against overcurrent.

Icu is the abbreviation for Rated Ultimate Short-circuit breaking capacity. This is the current for which the prescribe conditions according to a specific tests sequence do not include the capacity of the circuit breaker to carry its rated current continuously after the test.

Ics is the abbreviation for Rated Service Short-circuit breaking capacity. This is the current for which the prescribe conditions according to a specific tests sequence include the capacity of the circuit breaker to carry its normal rated current continuously after the test.

Ics is expressed as a percentage of Icu (value to be chosen by the manufacturer from 25%, 50%, 75% or 100%). This is the maximum current that the breaker can break!

To explain the differences between those two, we took the weight lifting event as an example.

We compare this two guy; Mr. A and Mr. B (consider both guy lifting the same weight..even though its obviously not in the picture)

Mr. A can lift the weight to its ultimate but Mr. B only can lift the weight up to half of Mr. A’s ultimate.

Now, we assume Mr. A and Mr. B is the same guy (same MCCB) but Mr. A is the Ultimate Current (Icu) and Mr. B is the service current (Ics).

By looking to the picture, we can conclude that both Mr. A and Mr. B can lift the weight to its ultimate but only during servicing both Mr. A and Mr. B can only operate at 50% of their ultimate current(Icu).

So, we go back to the standard percentage of Ics (normally 25%, 50%, 75% or 100%); as an engineer: What is the percentage that you prefer for your Ics compared to Icu?

It is you to decide…