Overload Relays 3RB2 Solid-State Overload Relays 3RB20, 3RB21 for standard applications Overview The devices are manufactured in accordance with environmen- tal guidelines and contain environmentally friendly and reusable materials. They comply with all important worldwide standards and approvals. 1 "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)". 2 3 5 6 2 4 7 Design Device concept The 3RB20/3RB21 solid-state overload relays are compact de- vices, i. e. current measurement (transformer) and the evalua- tion unit are integrated in a single enclosure. (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 possi- ble as an alternative (in some cases in conjunction with a stand-alone installation 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 ro- tary 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 auxiliary circuit can be connected with screw terminals and alterna- tively 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 (see "Function") against excessive temperature rises due to overload, phase unbalance or phase failure. An overload, phase unbal- ance 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 eval- uated 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 between the tripping current and current setting I e and is stored in the form of a long-term stable tripping characteristic (see "Characteristic Curves") .
In addition to inverse-time delayed protection of loads against excessive temperature rises due to overload, phase unbalance and phase failure, the 3RB21 solid-state overload relays also al- low internal ground-fault detection (not possible in conjunction with contactor assemblies for wye-delta starting). This provides protection of loads against high-resistance short-circuits due to damage 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 after the recovery time has elapsed (see "Function") .
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 specifications" and Catalog LV 1, "Selection and ordering data" . 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 openings 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 system (special device variants). For more information on the connection options see "Technical specifications" and Catalog LV 1, "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. 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 per- mits 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 motor protection devices or the 3RU11 thermal overload relays for this purpose. 19 Siemens · 2009