Chapter A General rules of electrical installation design

H - LV switchgear: functions & selection
Discrimination based on time-delayed tripping
uses CBs referred to as “selective” (in some
countries).
Implementation of these CBs is relatively simple
and consists in delaying the instant of tripping
of the several series-connected circuit-breakers
in a stepped time sequence
(1) Short-delay
4 Circuit-breaker
Fault
upstream
of B
Fault
downstream
of B
b The upstream circuit-breaker is a high speed device with a short-delay (SD) feature
These circuit-breakers are fitted with trip units which include a non-adjustable
mechanical short-time-delay feature. The delay is sufficient to ensure total
discrimination with any downstream high-speed CB at any value of short-circuit
current up to Ii A (see Fig. H54).
Schneider Electric - Electrical installation guide 2008
I peak
Fig. H53 : Downstream limiting circuit-breaker B
t
A
Is c
Fig. H54 : Use of a “selective” circuit-breaker upstream
B
A
Current limitation
curves
(see note)
B
Only B opens A and B open
Im A
delayed
Is c
prospective (rms)
Ii A
instantaneous
Example
Circuit-breaker A: Compact NS250 N fitted with a trip unit which includes a SD
feature.
Ir = 250 A, magnetic trip set at 2,000 A
Circuit-breaker B: Compact NS100N
Ir = 100 A
The Merlin Gerin distribution catalogue indicates a discrimination limit of 3,000 A (an
improvement over the limit of 2,500 A obtained when using a standard tripping unit).
Time-based discrimination
This technique requires:
b The introduction of time-delays into the tripping mechanisms of CBs
b CBs with adequate thermal and mechanical withstand capabilities at the high
current levels and time delays considered
Two circuit-breakers A and B in series (i.e. carrying the same current) are
discriminative if the current-breaking period of downstream circuit-breaker B is less
than the non-tripping time of circuit-breaker A.
I
H25
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