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Basic steps for calculating the SCCR of a control panel
4.2 SB4.3 – Adaptation of the available short-circuit current rating based on the peak let-through currents when
using current-limiting devices in the feeder circuit
4.2
SB4.3 – Adaptation of the available short-circuit current rating based
on the peak let-through currents when using current-limiting devices
in the feeder circuit
When short-circuit-current-limiting components are used in the feeder circuit, all components
in the branch circuit must be rated for the peak let-through current of the short-circuit-current-
limiting components used in the feeder circuit.
4.2.1
Current-limiting transformers
For branch circuits powered through a load transformer with an isolated secondary winding,
the following applies to the short-circuit current rating on the secondary side:
● Transformer < 10 kVA ⇒ max. 5 kA on the secondary side, i.e. all devices on the
secondary side must be rated for 5 kA or higher
● Transformer < 5 kVA and max. 120 V ⇒ max. 2 kA on the secondary side, i.e. all devices
on the secondary side must be rated for 2 kA or higher
● All other transformers are not current-limiting, i.e. they have the same short-circuit current
on the secondary and the primary side
4.2.2
Current-limiting circuit breakers (according to UL489)
To be able to use a circuit breaker in accordance with UL489 as a current-limiting
component in the feeder circuit, it must be marked "current limiting".
In this case, the let-through currents of the circuit breaker must be considered. The let-
through current can be obtained from the circuit breaker characteristic.
Example
The SCCR of the assembly is the breaking capacity of the circuit breaker in the feeder circuit
under the following conditions:
● The SCCR of all individual components in the branch circuit is not lower than the peak let-
through current of the circuit breaker in the feeder circuit.
● The breaking capacity of all branch circuit protective devices or the SCCR of all
combination motor controllers is not lower than the breaking capacity of the circuit
breaker in the feeder circuit.
⇒ The SCCR of the assembly is the lowest breaking capacity of a branch circuit protective
device or combination motor controller connected on the load side of the circuit breaker in
the feeder circuit if this is smaller than the breaking capacity of the circuit breaker in the
feeder circuit.
⇒ The SCCR of the assembly is the lowest SCCR of each branch circuit on the load side of
the circuit breaker if the above conditions do not apply.
18
Short-circuit current rating (SCCR) of industrial control panels
Reference Manual, 06/2013, A5E31896849002A/RS-AA/001
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