Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 39)

breakers with I
n
≤ 2500 A, IEC 60947-2 requires l
CW
≥ 12 · I
n
, at least 5 kA. For I
n
> 2500 A
l
CW
≥ 30 kA is required.
2.3.4.4 Current limiting protective equipment
If the short-circuit withstand capacity of electrical equipment is lower than the prospective short-
circuit current at the installation site, its loading must be reduced in the case of a short-circuit by
upstream current limiting protective equipment to the permissible magnitude. For this purpose
fuses or current limiting circuit breakers may be chosen.
The I 2 t- and i
D
-values of this protective equipment are – usually in diagrams – stated as a
function of the prospective short-circuit current I
cp
(see example Fig. 2.3-2 ). It should be noted
that these quantities vary with the operating voltage. For fuses, limit-curves can be found in the
diagrams for the cut-off current for the largest and without direct current component (see
example
Fig. 2.3-3 ). As the time of occurrence of a short-circuit is coincidental, the cut-off
current for the largest direct current component is critical for engineering (i.e. most unfavourable
time point of occurrence of the short-circuit).
Fig. 2.3-2
i
D
-values and I
2
t-values as a function of the prospective short-circuit current I
cp
When the cut-off current is limited to ≤ 17 kA, in accordance with IEC 60439-1 no verification of
the short-circuit withstand capacity for the downstream circuits is required. This relates in
particular to the mechanical strength of the conductors. For the protection of electrical equip-
ment (for example of motor starters) smaller cut-off currents may also be required.
LVSAM-WP001A-EN-P - April 2009 2-11