Overload Relays 3RB2 Solid-State Overload Relays 3RB20, 3RB21 for standard applications Characteristic curves The tripping characteristics show the relationship between the tripping time and tripping current as multiples of the current setting I e and are given for symmetrical three-pole and two-pole loads from the cold state. Tripping characteristics for 3-pole loads 600 800 1000 The smallest current used for tripping is called the minimum trip- ping current. According to IEC 60947-4-1, this current must be wi- thin specified limits.
The limits of the total tripping current for the 3RB20/3RB21 solid-state overload relays for symmetrical three- pole loads are between 105 and 120 % of the current setting. Tripping time 400 200 s 60 80 100 The tripping characteristic starts with the minimum tripping cur- rent and continues with higher tripping currents based on the cha- racteristics of the so-called trip classes (CLASS 10, CLASS 20 etc.). The trip classes describe time intervals within which the overload relays have to trip with 7.2 times the current setting I e from the cold state for symmetrical three-pole loads. 40 20 CLASS 30 CLASS 20 The tripping times according to IEC 60947-4-1, tolerance band E, are as follows for:
6 8 10 CLASS 10 4 CLASS 5 2 1 0,6 1 2 4 6 8 10Ч e The tripping characteristic for a three-pole overload relay from the cold state (see illustration 1) only apply if all three phases are si- multaneously loaded with the same current. In the event of a phase failure the 3RB20/3RB21 solid-state overload relays switch off the contactor more quickly in order to minimize heating of the load in accordance with the tripping characteristic for two-pole loads from the cold state (see illustration 2) . With phase unba- lance the devices switch off depending on the degree of the un- balance between the two characteristic curves.
Illustration 1 Tripping current Tripping characteristics for 2-pole loads 600 800 1000 Tripping time 400 Compared with a cold load, a load at operating temperature obvi- ously has a lower temperature reserve. The tripping time of the 3RB20/3RB21 solid-state overload relays is reduced therefore to about 20 % when loaded with the current setting I e for an exten- ded period. 200 s 60 80 100 40 20 CLASS 30 6 8 10 CLASS 20 4 CLASS 10 2 CLASS 5 1 0,6 1 2 4 6 8 10Ч e Illustration 2 Tripping current The above illustrations are schematic representations of characte- ristic curves.
The characteristic curves of the individual 3RB20/3RB21 electronic overload relays can be ordered from "Technical Assistance": Either by e-mail to: technical-assistance@siemens.com Or on the Internet at: http://www.siemens.com/automation/service&support 26 Siemens · 2009 NSB0_01525 Trip class Tripping time CLASS 5 3 … 5 s CLASS 10 5 … 10 s CLASS 20 10 … 20 s CLASS 30 20 … 30 s NSB0_01526