Description
2.4 Selectivity
3VA molded case circuit breakers with IEC certificate
Manual, 03/2019, A5E03603177010-03
55
Full selectivity
There is an increasing demand for full selectivity in order to safeguard continuity of service
by power distribution systems. A power system is said to be fully selective if only the
protective device located upstream of the fault location when viewed in the direction of
energy flow, i.e. from the infeed to the load, trips in the event of a fault.
Full selectivity always refers to the short-circuit current occurring at the installation point.
Partial selectivity
A system is said to be partially selective when selective tripping in response to a system fault
is not ensured up to the maximum ultimate short-circuit breaking capacity I
cu
of the switching
devices. Selectivity is then ensured only up to a certain I
s
current value (ultimate selectivity
value). If the calculated prospective short-circuit current at the installation point of the
downstream protective device is lower than the ultimate selectivity value specified for the
switchgear assembly, then it is still possible to describe the system as fully selective.
If the value determined by the short-circuit current calculation (e.g. according to
IEC / EN 60909, DIN VDE 0102) at the installation point of the downstream circuit breaker is
below the selectivity limit current listed in the respective table for the selected combination,
selectivity is guaranteed for all possible short circuits at the installation point.
If the calculated short-circuit current at the installation point is higher than the ultimate
selectivity value, selective tripping by the downstream circuit breaker is only assured up to
the value listed in the table. A judgment must be made as to whether the value can be
considered to be sufficient because the probability that the maximum short-circuit current will
occur is low, for example. Otherwise, a circuit breaker combination should be chosen whose
selectivity limit lies above the maximum short-circuit current.
Selectivity with 3VA2 molded case circuit breakers
The 3VA2 range is designed to deliver excellent selective tripping combined with optimum
current limiting and outstanding breaking capacity.
3VA2 molded case circuit breakers have been specifically designed to meet the following
requirements:
●
System-wide, high selectivity with a rated operational current interval of 1 : 2.5 up to the
miniature circuit breaker (for molded case circuit breakers with ELISA trip units, even 1
: 1.6 to each other or to LV HRC fuses)
●
Selectivity in combination with high current limiting and high breaking capacity
●
Cost-effective design / configuring of selective power distribution systems
These molded case circuit breaker requirements are achieved in engineering terms as
follows:
●
Rotary double-contact system for highly dynamic opening response
●
Coordinated electronic trip units
●
Dynamic selectivity
