Siemens SENTRON - Setting of Circuit Breakers

Methods of implementing selectivity using circuit breakers
3.5 Zone selective interlocking
Selectivity for 3VA molded case circuit breakers
36 Configuration Manual, 08/2016, A5E03603181010-01
3.5.2 Setting of circuit breakers
Simaris design, a software program published by SIEMENS, is recommended as a tool for
setting circuit breakers. One of the capabilities of this program is to calculate network short-
circuit values.
Two values – "I
kmax
" and "I
kmin
" – are required to select and set a circuit breaker at the
installation point.
● I
kmax
is the maximum potential short-circuit at the point of installation of a circuit breaker;
it is determined on the basis of the 3-pole, 2-pole and 1-pole short-circuit calculations.
● I
kmin
is the smallest possible short-circuit in the part of the network to be protected; it is
determined on the basis of the 3-pole, 2-pole and 1-pole short-circuit calculations.
This can refer, for example, to the zone to be protected extending from the circuit breaker
up to the end of the cable/busbar at the load, or up to the next protective device (circuit
breaker or fuse).
The following conditions apply to the selection and setting of circuit breakers:
1. A circuit breaker is selected in terms of its rated ultimate short-circuit breaking capacity
I
cu
according to the value of the highest potentially occurring short-circuit:
I
cu
is equal to or greater than I
kmax
.
2. The short-circuit release is set according to the minimum potentially occurring short-circuit
I
kmin
in the relevant network, whereby the tolerances of releases must be taken into
account. According to IEC 60947-2, 8.3.3.1.2, the maximum permissible tolerance of the
short-circuit release is ±20 %.
3. DIN VDE 0100, Part 410 and IEC 60364-4-41 differentiate between break-times
depending on whether the circuit is a distribution circuit or a final circuit.
– In distribution circuits (from the main distribution board to the subdistribution board, for
example), a break-time of 5 s applies in TN systems.
– In final circuits (from the distribution board to loads/socket outlets), the break-time is
as follows at a rated AC voltage U
o
= 230 V AC phase conductor to ground:
0.4 s at a rated current 32 A in a TN system
and
5 s at a rated current > 32 A in a TN system.