Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 126)

4.1.3.3 Protection requirements
4.1.3.3.1 Switching capacity
The most important requirement for a short-circuit-protective device is sufficient switching
capacity so that it is able to reliably manage the fault current. The project engineers and users
have to ensure that the switching capacity I
cu
or I
cc
of the short-circuit protective devices or
device combinations (for example circuit breaker plus contactor) at the given operating voltage
is equal to or larger than the prospective short-circuit current occurring at the site of installation.
The reference quantity for rating is thereby the symmetrical value. While with fuses it is naturally
a question of the breaking capacity only, circuit breakers must also have a corresponding
making capacity, as they also may make a circuit in which a short-circuit is present. This is
ensured by means of test sequences.
With circuit breakers a distinction is made between the ultimate short-circuit switching capacity
and the service short-circuit switching capacity that relates to reusability after a short-circuit.
See Section
4.2.2.3.3 .
4.1.3.3.2 Current limitation
Due to the potential dangers of short-circuits it is desirable that they are quickly detected and to
break them already in the first phase of current rise as far as possible (
Fig. 4.1-18 ). This is
intended to reduce the destructive energy to a minimum and to keep the extent of damage low.
The smaller the damage due to a short-circuit, the lower will be the repair costs, the operational
interruption and the resulting production losses. Modern circuit breakers and fuses have
strongly current limiting properties.
IEC 60439 (low-voltage switchgear assemblies) takes these factors into account by dispensing
from the requirement of verification of the short-circuit withstand capacity, if the symmetrical
short-circuit current is
≤ 10 kA or the cut-off current I
D
≤ 17 kA.
t [ms]
10
20
30
40
50
60
70
80
9 8 7 6 5 4 3 2 1
Prospective short-circuit current at
50 kA symm. s-c-current and 50 Hz
Cut-off current of a
current-zero quenching
circuit breaker
Let-through current of a
fast acting current
limiting circuit breaker
I [kA]
I
D
0
t
k
Fig. 4.1-18
Current limiting circuit breakers or fuses reduce the fault current and hence the mechanical and thermal
stresses in the event of a fault
I
D
Cut-off current
t
k
Total breaking time
LVSAM-WP001A-EN-P - April 2009 4-18