7/30/2019 Siemens Power Engineering Guide 7E 198

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Products and Devices

4.4 Low-Voltage Devices

198 Siemens Energy Sector t Power Engineering Guide t Edition 7.0

Selectivity and backup protection exemplified

for a data center

Computer centers place very high demands on the safety of

supply. This is particularly true for the consumers attached to

the uninterruptible power supply, and ensures a reliable data

backup in case of a fault and service interruption. Those solu-

tions providing selectivity and backup protection relying on the

previously mentioned SIMARIS design configuration tool should

be presented at this point. Fig. 4.4-13 shows a subdistribution

system in SIMARIS design. A SENTRON 3WL circuit-breaker as

outgoing feeder switch of the main distribution is upstream to

the subdistribution system shown here. The following figures

show the selectivity diagrams for the considered subdistribution

system automatically generated by SIMARIS design (fig. 4.4-14).

SIMARIS design specifies the characteristic curve band of the

considered circuit (red lines), the envelope curves of all

upstream devices (blue line) and all downstream devices (green

line). In addition to the specification of the minimum and max-

imum short-circuit currents, any selectivity limits for the indi-

vidual circuits are also specified.

Fig. 4.4-15 shows the selective grading of the 3WL circuit-

breaker from the main distribution system and the group backup

fuse (100 A LV HRC fuse) of the subdistribution system. The

consumers critical for functional endurance which are installed

in a redundant manner in the subdistribution system should not

be protected with the same backup fuse but rather be assigned

to different groups.

The selectivity diagram shows the circuit diagram of a single-

phase consumer in the subdistribution system. This circuit

diagram is protected with a 10 A miniature circuit-breaker with

characteristic B and for a maximum short-circuit current of

5,892 kA selective for the 100 A group backup fuse.

The same subdistribution system also contains an example for

backup protection. Fig. 4.4-16 shows the selectivity diagram for

the combination of the group backup fuse with a 13 A miniature

circuit-breaker of the characteristic B. Up to the breaking

capacity of the 6 kA miniature circuit-breaker, the two protective

devices are selective to each other. Above this value, the current

is limited by the fuse and the miniature circuit-breaker protected

by a fuse; both devices trip.

SIMARIS design automatically generates these characteristic

curves to provide exact information about the maximum and

minimum short-circuit currents of the associated circuit.

Fig. 4.4-16 also shows up to which current ( I sel-short-circuit 

) the

protective devices are selective to each other.

Fig. 4.4-16: Backup protection of the group backup fuse/miniature

circuit-breaker

Fig. 4.4-14: Selectivity of the group backup fuse to the upstream

protective devices

Fig. 4.4-15: Selectivity of the group backup fuse/miniature circuit-

diagram combination