Chapter A General rules of electrical installation design

H26
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H - LV switchgear: functions & selection
4 Circuit-breaker
Practical example of discrimination at several levels with Merlin Gerin
Masterpact circuit-breakers (with electronic trip units)
These CBs can be equipped with adjustable time-delays which allow 4 time-step
selections, such as:
b The delay corresponding to a given step is greater than the total current breaking
time of the next lower step
b The delay corresponding to the first step is greater than the total current-breaking
time of a high-speed CB (Compact NS for example) or of fuses (see Fig. H55)
Schneider Electric - Electrical installation guide 2008
t
Ir B
Fig. H55 : Discrimination by time delay
B
A
Only B opens
Non tripping
time of A
Current-breaking
time for B
Icc B
Energy discrimination with current limitation
I
Icc
Cascading between 2 devices is normally achieved by using the tripping of the
upstream circuit-breaker A to help the downstream circuit-breaker B to break the
current. The discrimination limit Is is consequently equal to the ultimate breaking
current Icu B of circuit-breaker B acting alone, as cascading requires the tripping of
both devices.
The energy discrimination technology implemented in Compact NS circuit-breakers
allows to improve the discrimination limit to a value higher than the ultimate breaking
current Icu B of the downstream circuit-breaker. The principle is as follows:
b The downstream limiting circuit-breaker B sees a very high short-circuit current.
The tripping is very fast ( 1.6
b The ratio of rated currents of the two circuit-breakers is > 2.5