Overload Relays 3RU1 Thermal Overload Relays 3RU11 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. The tripping characteristic for a three-pole 3RU11 thermal over- load relay (see characteristic curve for symmetrical three-pole loads from the cold state) only applies if all three bimetal strips are simultaneously loaded with the same current.
If only two bi- metal strips are heated due to a phase failure, these two strips alone must generate the necessary force to trigger the tripping mechanism which would result in a longer tripping time or re- quire a higher current. If these higher currents are applied over a longer period, they usually cause damage to the load. To avoid damage, the 3RU11 thermal overload relays are fitted with phase failure sensitivity which ensures faster tripping in ac- cordance with the characteristic curve for 2-pole loads from the cold state by means of a suitable mechanical mechanism. The smallest current used for tripping is called the minimum tripping current.
According to IEC 60947-4-1, this current must be within specified limits. The limits of the minimum tripping current for the 3RU11 thermal overload relays for symmetrical three-pole loads are between 105 and 120 % of the current setting. The tripping characteristic starts with the minimum tripping cur- rent and continues with higher tripping currents based on the characteristics 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.
Compared with a cold load, a load at operating temperature ob- viously has a lower temperature reserve. This is taken into ac- count by the 3RU11 thermal overload relays by reducing the trip- ping time to about 25 % when loaded with the current setting I e for an extended period. The tripping times are as follows for: Tripping time min 100 10 000 5000 40 60 2000 s 10 1000 5 500 200 2 100 1 50 10 20 5 2 1 0,6 0,8 1 2 3 4 6 8 10 20 30 40 60 80 x n A 15 Current This is the schematic representation of a characteristic curve. The characteristic curves of the individual 3RU11 thermal over- load 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 15 Siemens · 2009 Trip class Tripping times CLASS 10A 2 s ... 10 s CLASS 10 4 s ... 10 s CLASS 20 6 s ... 20 s CLASS 30 9 s ... 30 s NSB0_00288 3-pole loading 2-pole loading