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SIRIUS
Siemens Industry, Inc. Industrial Control Product Catalog 2017
3/19
3
OVERlOAd RElAYS
Design
Device concept
The 3RB2 / 3RB3 solid-state overload relays are compact
devices, i.e. current measurement (transformer) and the evalua-
tion unit are integrated in a single enclosure.
Mounting options
The 3RB2 / 3RB3 solid-state overload relays are suitable for
direct and space-saving mounting onto 3RT1 / 3RT2 contactors
and 3RW30/3RW31 soft starters as well as for stand-alone
installation. For more information on the mounting options,
please see Technical Specifications and Selection and
Ordering Data
.
Connection technique
Main circuit
All sizes of the 3RB2 / 3RB3 solid-state overload relays can be
connected with screw-type terminals. As an alternative for sizes
S3 to S10/S12, the main circuits can be connected via the Bus-
bar. Sizes S2 to S6 of the 3RB20/3RB21 relays are also available
with a straight-through transformer. In this case, the cables of the
main circuit are routed directly through the feed-through open-
ings of the relay to the contactor terminals.
Auxiliary circuit
Connection of the auxiliary circuit (removable terminal block) is
possible with either screw terminals or spring-loaded terminals.
For more information on the connection options,
see Technical Specifications and Selection and Ordering Data.
Overload relays in contactor assemblies for
Wye-Delta starting
When overload relays are used in combination with contactor
assemblies for Wye-Delta starting it must be noted that only
0.58 times the motor current flows through the line contactor.
An overload relay mounted onto the line contactor must be set
to 0.58 times the motor current.
When 3RB21 / 31 solid-state overload relays are used in combi-
nation with contactor assemblies for Wye-Delta starting, the
internal ground-fault detection must not be activated.
Operation with frequency converter
The 3RB2 / 3RB3 solid-state overload relays are suitable for
frequencies of 50/60 Hz and the associated harmonics. This
permits the 3RB2 / 3RB3 overload relays to be used on the
incoming side of the frequency converter.
If motor protection is required on the outgoing side of the
frequency converter, the 3RN thermistor motor protection
devices or the 3RU thermal overload relays are available for
this purpose.
Overload Relays
3RB2 / 3RB3 Solid-State Overload Relays
3RB20, 3RB21, 3RB30, 3RB31 up to 630A for standard applications