Siemens SENTRON 3WL10 Manual page 53

Selectivity with 3VA27 molded case circuit breakers
The 3VA27 circuit breaker is designed for selective (discriminating) tripping behavior in
conjunction with both high breaking capacity and high current limitation.
Energy selectivity and dynamic selectivity
For energy selectivity and dynamic selectivity, the tested devices are graded toward the end
load as follows:
The arising short-circuit energy and the current rise in case of a fault ensure that the contact
of the switching device directly upstream of the fault location open fastest, already within the
first half cycle, limiting and finally quenching the current. The other upstream protection
devices in the chain with the fault are set to a much slower response by the contact system.
For this reason, these devices, which may also be current-limiting, do not trip.
Summary: It is possible and quite common to combine these selectivity principles.
For protection requirements that apply more to the higher level and parts of the circuit near
to a transformer, devices must be selected that are better capable of time selectivity on a
short-circuit - i.e. use of a 3WL10.
For protection requirements applying to the outgoing side and parts of the circuit near to the
final load, devices are selected that are better at energy selectivity - i.e. use of a 3VA27.
In addition to short-circuit considerations, complete coordination of the current selectivity
throughout the chain is an important part of the configuration, especially in the overload
range.
This makes the power distribution system totally selective. Siemens protection devices are
designed for optimum coordination in order to achieve just that.
As an interface between characteristics of a molded case circuit breaker and an air circuit
breaker, the 3WL10 and 3VA27 platform product was designed.
For optimum configuration, the Simaris software tools Simaris curves, powerconfig and
selectivity tables (see Chapter Reference documents (Page 10)) can be used.
You will find more information in the manual on selectivity, see Chapter Reference
documents (Page 10).
See also
Characteristic curves (Page 66)
3VA27 molded case circuit breakers & 3WL10 air circuit breakers
Manual, 10/2018, L1V30499596002-01
Description
2.8 Electronic trip unit ETU
51