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Overload Relays
3RB2 Solid-State Overload Relays
3RB20, 3RB21 for standard applications
■
Overview
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2
5
2
(1)Connection for mounting onto contactors:
Optimally adapted in electrical, mechanical and design terms to the
contactors and soft starters, these connecting pins can be used for di-
rect mounting of the overload relays. Stand-alone installation is possible
as an alternative (in some cases in conjunction with a stand-alone instal-
lation module).
(2)Selector switch for manual/automatic RESET and RESET button:
With the slide switch you can choose between manual and automatic
RESET. A device set to manual RESET can be reset locally by pressing
the RESET button. On the 3RB21 a solid-state remote RESET is
integrated.
(3)Switch position indicator and TEST function of the wiring:
Indicates a trip and enables the wiring test.
(4)Solid-state test (device test):
Enables a test of all important device components and functions.
(5)Motor current setting:
Setting the device to the rated motor current is easy with the large rotary
knob.
(6)Trip class setting/internal ground-fault detection (only 3RB21):
Using the rotary switch you can set the required trip class and activate
the internal ground-fault detection dependent on the start-up
conditions.
(7)Connecting terminals (removable joint block for auxiliary circuits):
The generously sized terminals permit connection of two conductors
with different cross-sections for the main and auxiliary circuits. The aux-
iliary circuit can be connected with screw terminals and alternatively
with spring-type terminals.
The 3RB20 and 3RB21 solid-state overload relays up to 630 A
with internal power supply have been designed for inverse-time
delayed protection of loads with normal and heavy starting
"Function")
against excessive temperature rises due to overload,
phase unbalance or phase failure. An overload, phase unbalance
or phase failure result in an increase of the motor current beyond
the set rated motor current. This current rise is detected by the
current transformers integrated into the devices and evaluated by
corresponding solid-state circuits which then output a pulse to the
auxiliary contacts. The auxiliary contacts then switch off the load
by means of a contactor. The break time depends on the ratio be-
tween the tripping current and current setting I
the form of a long-term stable tripping characteristic
acteristic
Curves").
In addition to inverse-time delayed protection of loads against ex-
cessive temperature rises due to overload, phase unbalance and
phase failure, the 3RB21 solid-state overload relays also allow in-
ternal ground-fault detection (not possible in conjunction with
contactor assemblies for wye-delta starting). This provides pro-
tection of loads against high-resistance short-circuits due to dam-
age to the insulation material, moisture, condensed water etc.
The "tripped" status is signaled by means of a switch position in-
dicator. Resetting takes place either manually or automatically af-
ter the recovery time has elapsed
The devices are manufactured in accordance with environmental
guidelines and contain environmentally friendly and reusable ma-
18
Siemens · 2011
3
6
4
7
(see
and is stored in
e
(see "Char-
(see
"Function").
terials. They comply with all important worldwide standards and
approvals.
"Increased safety" type of protection EEx e acc. to ATEX di-
rective 94/9/EC
The 3RB20/3RB21 solid-state overload relays are suitable for the
overload protection of explosion-proof motors with "increased
safety" type of protection EEx e;
see Catalog LV 1, Chapter 20 "Appendix" --> "Standards and ap-
provals" --> "Type overview of approved devices for explosion-
protected areas (ATEX Explosion Protection)".
■
Design
Device concept
The 3RB20/3RB21 solid-state overload relays are compact de-
vices, i. e. current measurement (transformer) and the evaluation
unit are integrated in a single enclosure.
Mounting options
The 3RB20/3RB21 solid-state overload relays are suitable for di-
rect and space-saving mounting onto 3RT1 contactors and
3RW30/3RW31 soft starters as well as for stand-alone
installation.
For more information on the mounting options see "Technical spe-
cifications" and Catalog LV 1, "Selection and ordering
Connection methods
Main circuit
All sizes of the 3RB20/3RB21 solid-state overload relays can be
connected with screw terminals. As an alternative for sizes S3 to
S10/S12, the main current paths can be connected with the help
of rails. Sizes S2 to S6 of the 3RB20/3RB21 relays are also avail-
able 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 or spring-type terminal connection sys-
tem (special device variants).
For more information on the connection options see "Technical
specifications" and Catalog LV 1, "Selection and ordering
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.
An assignment of the 3RB20 solid-state overload relays to the line
contactors of our 3RA contactor assemblies for wye-delta starting
can be found in Chapter 3 "Controls: Contactors and Contactor
Assemblies".
When 3RB21 solid-state overload relays are used in combination
with contactor assemblies for wye-delta starting, the internal
ground-fault detection must not be activated.
Operation with frequency converter
The 3RB20/3RB21 solid-state overload relays are suitable for fre-
quencies of 50/60 Hz and the associated harmonics. This permits
the 3RB20/3RB21 overload relays to be used on the input side of
the frequency converter.
If motor protection is required on the outgoing side of the fre-
quency converter, Siemens recommends the 3RN thermistor mo-
tor protection devices or the 3RU11 thermal overload relays for
this purpose.
data".
data".
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