Selectivity - terms and definitions
Selectivity for 3VA molded case circuit breakers
10 Configuration Manual, 08/2016, A5E03603181010-01
Over-current selectivity
Excerpt from EN 60947-1, 2.5.23:
Over-current selectivity (discrimination) is the "co-ordination of the operating characteristics
of two or more over-current protective devices such that, on the incidence of over-currents
within stated limits, the device intended to operate within these limits does so, while the
other(s) does (do) not".
See also DIN VDE 100-530: 2011-06 or IEC 60364-7-710 or DIN VDE 0100-710.
Partial selectivity
Partial selectivity exists in cases where the protective devices behave selectively only up to a
specific current, i.e. in addition to the protective device directly at the fault location, other
upstream protective devices also operate even though the maximum over-current at the fault
location is not yet reached.
Total selectivity is not achieved, i.e. sections of plant that are not immediately affected by the
over-current fault are also disconnected from the supply system.
If the actual short-circuit current is lower than the selectivity limit current, then total selectivity
is also afforded (see EN 60947-2, 2.17.4).
Excerpt from EN 60947-2, 2.17.3:
Partial selectivity is "over-current selectivity where, in the presence of two over-current
protective devices in series, the protective device on the load side effects the protection up
to a given level of over-current without causing the other protective device to operate".
Total selectivity
Total selectivity exists if only the protective device situated immediately upstream of the fault
location operates across the entire over-current zone (in the direction of current flow) that
can occur at the fault location.
Excerpt from EN 60947-2, 2.17.2:
Total selectivity is "over-current selectivity where, in the presence of two over-current
protective devices in series, the protective device on the load side effects the protection
without causing the other protective device to operate".
Total selectivity
Total selectivity is afforded if only the protective device situated immediately upstream of the
fault location operates across the entire over-current zone up to
● the rated ultimate short-circuit breaking capacity I
cu
of the downstream circuit breaker
according to IEC 60947-2, or
● the rated short-circuit breaking capacity I
cn
of the downstream miniature circuit breaker
according to IEC 60898, or
● up to the rated breaking capacity of the fuse according to IEC 60269-3.
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