Implementation Of Dynamic Selectivity - Siemens SENTRON Configuration Manual

With the dynamic selectivity concept, current-limiting circuit breakers are not only
coordinated according to their current-time response, but also on the basis of the energy
that they let through in the event of a short-circuit. If the let-through energy exceeds the
tripping energy assigned to a circuit breaker, the breaker will trip.
In order to implement selectivity between current-limiting circuit breakers, the let-through
energy of the downstream device must be lower than the tripping energy of the upstream
device. If the short-circuit current exceeds the selectivity limit current (I
circuit breakers will trip.
Advantages of dynamic selectivity
● Fast tripping times (a few milliseconds)
● Low thermal and mechanical loading of power distribution equipment
● Selective response up to high values in the short-circuit zone, even with category A circuit
breakers
● By contrast with time selectivity, molded case circuit breakers can be used to implement
total selectivity up to the rated ultimate short-circuit breaking capacity I
3.6.1

Implementation of dynamic selectivity

Dynamic selectivity cannot be achieved using the LI/LSI protective function, but requires
instead an additional tripping mechanism.
In order to implement dynamic selectivity with molded case circuit breakers, the short-circuit
zone is divided into two ranges:
● Low short-circuit current range
A tripping delay corresponding to a few milliseconds is permanently implemented in
circuit breakers for low short-circuit currents.
● High short-circuit current range
Accelerated instantaneous tripping is implemented as the breaker's response to high
short-circuit currents.
This ensures that the circuit breaker will not be irreparably damaged by the short-circuit
current.
The resulting "dual tripping principle" is a feature of the new Siemens 3VA2 molded case
circuit breakers for "Selectivity Applications". In the high short-circuit current range, an
additional mechanical tripping mechanism (energy release) within the switching pole trips the
circuit breaker.
This tripping mechanism exploits the fact that the forces caused by current flow at high short-
circuit currents cause the contacts of current-limiting molded case circuit breakers to open by
themselves. Arcs form at the contacts as they open. The arcing power generated in the pole
cassettes of the circuit breakers can be evaluated. This arcing power correlates with the let-
through energy and can therefore act as a criterion for the energy release.
A tripping mechanism is integrated in the switching poles of 3VA2 molded case circuit
breakers. At very high short-circuit currents, this mechanism uses the resultant arcing power
to move a mechanical lever. This lever trips the circuit breaker very quickly.
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
Methods of implementing selectivity using circuit breakers
3.6 Dynamic selectivity
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