Overload Relays 3RB2 Solid-State Overload Relays 3RB20, 3RB21 for standard applications Function Basic functions The 3RB20/3RB21 solid-state overload relays are designed for: Inverse-time delayed protection of loads from overloading Inverse-time delayed protection of loads from phase unbalance Inverse-time delayed protection of loads from phase failure Protection of loads from high-resistance short-circuits (internal ground-fault detection only with 3RB21). Recovery time With the 3RB20/3RB21 solid-state overload relays the recovery time after inverse-time delayed tripping is 3 minutes when auto- matic RESET is set. This recovery time allows the load to cool down.
If the button is set to manual RESET and automatic RESET, the 3RB20/3RB21 devices can be reset immediately after tripping. Control circuit TEST function With motor current flowing, the TEST button can be used to check whether the relay is working correctly (device/solid-state test). Current measurement, motor model and trip unit are tested. If these components are OK, the device is tripped according to the table below. If there is an error, no tripping takes place. The 3RB20/3RB21 solid-state overload relays have an internal power supply, i. e. no additional supply voltage is required. Short-circuit protection Fuses or motor starter protectors must be used for short-circuit protection.
For assignments of the corresponding short-circuit protection de- vices to the 3RB20/3RB21 solid-state overload relays with/without contactor see "Technical specifications" and Catalog LV 1, "Selection and ordering data" . Trip classes The 3RB20 solid-state overload relays are available for normal starting conditions with trip CLASS 10 or for heavy starting condi- tions with trip CLASS 20 (fixed setting in each case). The 3RB21 solid-state overload relays are suitable for normal and heavy starting conditions. The required trip class (CLASS 5, 10, 20 or 30) can be adjusted by means of a rotary switch depending on the current start-up condition.
For details of the trip classes see "Characteristic Curves" . Note: The TEST button must be kept pressed throughout the test. In this case the motor current must be equal to more than 80 % of the current setting I e and have at least the value of the lower cur- rent setting. Testing of the auxiliary contacts and the control current wiring is possible with the switch position indicator slide. Actuating the slide simulates tripping of the relay. During this simulation the NC contact (95-96) is opened and the NO contact (97-98) is closed. This tests whether the auxiliary circuit has been correctly wired.
Phase failure protection The 3RB20/3RB21 solid-state overload relays are fitted with phase failure protection (see "Characteristic Curves") in order to mini- mize temperature rises of the load during single-phase operation. Phase failure protection is not effective for loads with star-connec- tion and a grounded neutral point or a neutral point which is con- nected to a neutral conductor. After a test trip the relay is reset by pressing the RESET button. Self-monitoring The 3RB20/3RB21 solid-state overload relays have a self-monito- ring feature, i. e. the devices constantly monitor their own basic functions and trip if an internal fault is detected.
Setting Display of operating state The 3RB20/3RB21 solid-state overload relays are set to the rated motor current by means of a rotary knob. The scale of the rotary knob is shown in ampere. With the 3RB21 solid-state overload relay it is also possible to se- lect the trip class (CLASS 5, 10, 20 or 30) using a second rotary knob and to switch the internal ground-fault detection on and off. The respective operating state of the 3RB20/3RB21 solid-state overload relays is displayed by means of the position of the mar- king on the switch position indicator slide.
After tripping due to overload, phase failure, phase unbalance or ground fault (ground- fault detection possible only with 3RB21) the marking on the slide is to left on the "O" mark, otherwise it is on the "I" mark. Manual and automatic reset Auxiliary contacts The 3RB20/3RB21 solid-state overload relays are fitted with an NO contact for the "tripped" signal, and an NC contact for swit- ching off the contactor. In the case of the 3RB20/3RB21 solid-state overload relays, a slide switch can be used to choose between automatic and ma- nual resetting. If manual reset is set, a reset can be carried out directly on the de- vice after a trip by pressing the blue RESET button.
Resetting is possible in combination with mechanical and mechanical reset options from the range of accessories (see Catalog LV 1, "Acces- sories") . As an alternative to the mechanical RESET options, the 3RB21 solid-state overload relays can be equipped with electrical remote RESET by applying a voltage of 24 V DC to the terminals A3 and A4. If the slide switch is set to automatic RESET, the relay is reset automatically. The time between tripping and resetting is determined by the recovery time.
20 Siemens · 2009 Trip classes Required loading with the rated current prior to pressing the TEST button Tripping within CLASS 5 3 min 30 s CLASS 10 5 min 1 min CLASS 20 10 min 2 min CLASS 30 15 min 3 min