Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 129)

Tripping delay
Fig. 4.1-21
Time selectivity of two circuit breakers in series
b = Overload release
s = Short-circuit release (switch 1 with short-time delay; utilization category B)
The cascading of trip times requires that Switch 1 is capable of carrying the short-circuit current
during the trip delay time. This is the case when using circuit breakers of utilization category B.
The critical variable is the rated short-time current I
cw
that determines the magnitude of the
permissible short-time current during a defined period. It is usually stated as the 1s-current and
can be converted for other times with I
2 t = const. (see also Section 2.3.4.3 ).
Selectivity between fuse and downstream circuit breaker
time
t
Current I (effective value)
Fig. 4.1-22
Selectivity between fuse and downstream circuit breaker
1 = Circuit breaker
2 = Fuse
In the overload range selectivity is given, if the trip characteristic of the overload release lies
under the characteristic curve of the fuse (considering the tolerance band). In the short-circuit
range selectivity is given to the extent that the total breaking time (incl. clearing time) of the
circuit breaker is below the melting characteristic of the fuse.
LVSAM-WP001A-EN-P - April 2009 4-21