What is the kA Rating on a Circuit Breaker? + Its Importance

What is the kA Rating on a Circuit Breaker?

kA rating on a circuit breaker is the maximum short-circuit current value that a circuit breaker can break at a given voltage and phase angle (cos ϕ). It can also be defined as Icu. If the current flows through the breaker exceeds this value, the circuit breaker could be damaged and useless.

The tests of Icu are executed according to the sequence O – t – CO. (According to IEC 60947-2) This test sequence includes the verification of the overload release of the circuit breaker.

O: Represents an automatic break operation

T: a time interval (3 min)

CO: A make operation followed by an automatic break operation.

Following the test, the circuit breaker must continue to provide a minimum level of safety (isolation, dielectric strength).

For example,

A value of 10kA on the circuit breaker means that this circuit breaker can withstand 10,000 amps of current under short-circuit conditions during the brief time it takes to trip the circuit breaker.

Why kA rating on a circuit breaker is important?

The circuit breaker must protect the installation from faults by switching off the faulty circuit while guaranteeing the continued operation of nonaffected circuits. The kA rating is very important because the maximum amount of current flow through a circuit breaker during a short circuit varies from one circuit to another. It is necessary to use the right circuit breaker to avoid dangers such as energy outages and fires.

IEC standard makes a distinction between the rated ultimate short-circuit breaking capacity Icu and the rated service short-circuit breaking capacity Ics. Let’s have a look at Ics…

What is service short circuit breaking capacity? (Ics)

Service short circuit breaking capacity (Ics) is the value expressed as a percentage of Icu. It will be one of the following values: 25% (category A only), 50%, 75%, or 100%. The circuit breaker must be capable of operating normally after breaking the Ics current several times using the sequence O – 3 min – CO – 3 min – CO

What is nominal breaking capacity? (Icn)

In standard IEC 60898-1, the breaking capacity of the device is tested in a similar way but is called Icn. After the test, the circuit breaker must retain its dielectric properties and be able to trip in accordance with the specifications in the standard.