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Section 12
Short-circuit current limiting
Technical advice
DB125768
DB125767
Definition
The limiting capacity of a circuit breaker is its ability to lessen the effects of a short
circuit on an electrical installation by reducing the current amplitude and the
dissipated power.
Benefits of limiting
Long installation service life
Thermal effects
Lower temperature rise at the conductor level, hence increased service life for cables
and all components that are not self-protected (e.g. switches, contactors, etc.)
Mechanical effects
Lower electrodynamic repulsion forces, hence less risk of deformation or breakage of
electrical contacts and busbars.
Electromagnetic effects
Less interference on sensitive equipment located in the vicinity of an electric circuit.
Savings through cascading
Cascading is a technique derived directly from current limiting: downstream of a
current-limiting circuit breaker it is possible to use circuit breakers of breaking
capacity lower than the prospective short-circuit current (in line with the cascading
tables). The breaking capacity is heightened thanks to current limiting by the
upstream device. Substantial savings can be achieved in this way on switchgear and
enclosures.
Discrimination of protection devices
The circuit breakers' current limiting capacity improves discrimination with the
protection devices located upstream: this is because the required energy passing
through the upstream protection device is greatly reduced and can be not enough
to cause it to trip. Discrimination can thus be natural without having to install a
time-delayed protection device upstream.
Acti 9 circuit breaker current limiting
Profiting from Schneider Electric's experience and expertise in the field of short-circuit
current breaking, the circuit breakers of the Acti 9 range have a top-level current
limiting characteristic for modular devices.
This assures them of optimal protection of the entire power distribution system.
I²sc
A²
Prospective
energy
100%
Limited
energy
100%
t
Isc
Prospective
peak Isc
Limited
peak Isc
Limited Isc
Prospective Isc
t
tc
Prospective current and real limit current.