SIRIUS 3RU thermal overload relays / SIRIUS 3RB electronic overload relays
Manual, 09/2016, A5E03656507420A/RS-AB/003
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Introduction
3RU21 thermal overload relays
3RU21 thermal overload relays up to 100 A have been designed to provide current-
dependent protection for loads with normal starting against impermissibly high temperature
rises due to overload, phase asymmetry or phase failure.
An overload or phase failure results in an increase of the motor current beyond the set rated
motor current. Via heating elements, this current rise increasingly heats up the bimetal strips
located inside the device. The deflection of these bimetal strips eventually activates the
auxiliary contacts via a release mechanism. The contacts then disconnect the load via a
contactor. (The contactor function is not an integral component of the overload relay).
3RB20 / 3RB21 and 3RB30 / 3RB31 electronic overload relays
3RB20 / 3RB21 electronic overload relays up to 630 A and 3RB30 / 3RB31 electronic
overload relays up to 115 A with internal power supply have been designed to provide
current-dependent protection for loads with normal starting and heavy starting against
impermissibly high temperature rises due to overload, phase asymmetry or phase failure.
An overload, phase asymmetry or a phase failure results in an increase of the motor current
beyond the set rated motor current.
This rise in current is detected by the current transformers integrated into the devices and
evaluated by corresponding electronic circuits which then output a pulse to the auxiliary
contacts. The contacts then disconnect the load via a contactor. (The contactor function is
not an integral component of the overload relay).
In addition to current-dependent protection for loads against impermissibly high temperature
rise caused by overload, phase asymmetry, and phase failure, 3RB21 and 3RB31 electronic
overload relays feature internal ground-fault detection (not possible in conjunction with
contactor assemblies for star-delta (wye-delta) start). This provides protection of loads
against high-impedance faults to ground caused by damaged insulation, moisture,
condensation, etc.
The overload relays have been matched to the contactors in the 3RT series both electrically
and mechanically and can be integrated in the feeder by means of direct mounting. 3RU2
thermal overload relays are available in sizes S00 to S3 up to 100 A. 3RB30 and 3RB31
electronic overload relays are available in sizes S00 to S3 up to 115 A. 3RB20 and 3RB21
electronic overload relays are available in sizes S6 to S10 / S12 up to 630 A.
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