Siemens SIRIUS 3RU - Page 53

Functions
5.1 Protection against overload, phase failure, and phase asymmetry
SIRIUS 3RU thermal overload relays / SIRIUS 3RB electronic overload relays
Manual, 09/2016, A5E03656507420A/RS-AB/003
53
Tripping characteristic curves of 3RB20 / 3RB21 and 3RB30 / 3RB31 electronic overload relays
The tripping characteristic curve for an overload relay loaded at 3 poles from cold (see
Figure 1) applies subject to the prerequisite that all three phases are loaded symmetrically.
In the event of a phase failure, the 3RB20 / 3RB21 and 3RB30 / 3RB31 electronic overload
relays disconnect the contactor more quickly to minimize the load's temperature rise in
accordance with the tripping characteristic curve for two-pole loading from the cold state (see
Figure 2). In the event of phase asymmetry, the devices disconnect dependent upon the
extent of the asymmetry between the two characteristic curves.
Compared with a cold load, a load at operating temperature obviously has a lower
temperature reserve. It is for this reason that the tripping time of 3RB20 / 3RB21 and
3RB30 / 3RB31 electronic overload relays is reduced to approximately 30 % following
prolonged loading with the current setting I
e
.
Figure 1
Figure 2
Tripping characteristic 1
Tripping characteristic 2
t [s]
Tripping time
t [s]
Tripping time
I [A]
Current
I [A]
Current
3
-pole load - 3RB20 / 3RB21 and 3RB30 /
3RB31
2
-pole load - 3RB20 / 3RB21 and 3RB30 / 3
RB31
Reference
The figures are schematic representations of the characteristic curves. The characteristic
curves for the individual overload relays can be downloaded from the Internet
(https://support.industry.siemens.com/cs/ww/en/ps/16269/char).