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Definition of switching capacity
The switching capacity is the Rated Maximum Sinusoidal
(RMS) value of a current at a given power factor cos φ as well as
a given rated voltage at which a switchgear or a fuse can still shut-
off under specified conditions in an operationally safe way.
Both the short-circuit making capacity as well as the short-
circuit breaking capacity of circuit breakers must be larger than or
equal to the prospective short-circuit current at the place of
installation.
If this is not the case, then a suitable backup protection (for
example a fuse) should be provided to ensure the required
switching capacity of the device combination. Data regarding
devices for backup protection are given in the technical
documentation.

Rated short-circuit making capacity Icm
The rated short-circuit making capacity Icm is a quantity that
according to regulations must be in a certain ratio to the rated
ultimate short-circuit breaking capacity Icu and that has to be
guaranteed by the device manufacturer.
This is not a variable that must be considered by the user,
however it ensures that a circuit breaker is in the position to
connect onto a short-circuit – and to disconnect it subsequently.

Rated short-circuit breaking capacity ICU and ICS
IEC 60947-2 makes distinction between the rated ultimate
short-circuit breaking capacity (ICU) and the rated service short-
circuit breaking capacity (ICS):

Rated ultimate short-circuit breaking capacity ICU
ICU is the maximum breaking capacity of a circuit breaker at an
associated rated operational voltage and under specified
conditions.
ICU is expressed in kA and must be at least as large as the
prospective short-circuit current at the site of installation.
Circuit breakers that have switched-off at the level of the
ultimate short-circuit breaking capacity, are reduced serviceable
afterwards and should at least be checked regarding functionality.

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