SIRIUS 3RU2/3RB3 overload relays
5.4 Functions
SIRIUS Innovations
498 System Manual, 01/2011, A8E56203870002-03
Tripping characteristic curves for 3RB30/3RB31 solid-state 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 with the same
current at the same time. In the event of a phase failure, the 3RB30/3RB31 solid-state
overload relays disconnect the contactor more quickly to minimize the load's temperature
rise in accordance with the tripping characteristic curve for 2-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
temerature reserve. It is for this reason that the tripping time of 3RB30/3RB31 solid-state
overload relays is reduced to approximately 30 % following prolonged loading with the
current setting I
e
.
t [s]
I [A]
CLASS 30
CLASS 5
CLASS 10
CLASS 20
0,6
t [s]
I [A]
CLASS 30
CLASS 5
CLASS 10
CLASS 20
0,6
Figure 1 Figure 2
t [s] Tripping time
I [A] Current
Reference
The figures are schematic representations of the characteristic curves. The characteristic
curves for the individual overload relays can be downloaded from the Internet
(http://www.siemens.com/automation/service&support
).
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