Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 128)

At high short-circuit currents the melting I 2 ·t value of the upstream fuse must be larger than the
breaking I
2
·t value (melting and clearing time) of the smaller downstream fuse. This is usually
the case if their rated currents differ by a factor of 1.6 or more.
Selectivity of circuit breakers connected in series
Current selectivity
In distribution networks, the rated currents of the switches decrease constantly from the
transformer to the load. As the short-circuit releases normally operate at a multiple of the rated
current, their release levels decrease in the same way with distance from the supply. As the
prospective short-circuit currents also become smaller with increasing distance from the supply
point due to line damping, a so-called natural selectivity can be created via the current magni-
tude. This means that the maximum short-circuit current with a short-circuit on the load-side of
the switch 2 (
Fig. 4.1-20 ) is below the trip value of the magnetic release of switch 1. The short-
circuit currents must be known at the installation sites of the switches.
Selectivity is usually not assured with short-circuit currents above the response value of the
magnetic release of the upstream circuit breaker.
Area of
current-selectivity
Fig. 4.1-20
Current selectivity of two circuit breakers in series is given, if the prospective short-circuit current
downstream of Circuit breaker 2 is smaller than the trip value of the magnetic release of Switch 1
b = Overload release
s = Short-circuit release
When assessing the current selectivity the tolerance of the short-circuit trigger (+/-20 % in
accordance with IEC 60947-2) should be taken into account.
Time selectivity
If current selectivity between circuit breakers is not possible, selectivity must be achieved by
cascading of the trip times, i.e. the upstream circuit breaker operates with a short delay to give
the downstream circuit breaker time to clear the short-circuit. If the short-circuit occurs between
the two switches, then it will continue during the short trip delay time of the switch 1 and after
lapse of this time it will be switched off by the latter (
Fig. 4.1-21 ).
LVSAM-WP001A-EN-P - April 2009 4-20