What are the characteristics of phase unbalance protection of STR22ME and STR43ME motor trip units for NS100 to 630 circuit-breakers


30 September 2021

STR22ME and STR43ME electronic motor trip units incorporate protection against phase unbalance (current).

For the STR22ME trip unit: the unit compares the image of the current in the phases with phase 1 which is used as reference. If the unbalance is greater than 40% the trip will take place between 3.5 and 6 seconds.

For the STR43ME trip unit: the trip unit measures the maximum rms values of the currents on the three phases, calculates the average of these values and compares it with the minimum and maximum rms values. If the difference is greater than 30% (with a tolerance of +/- 10%) for more than 4 seconds (tolerance +/- 10%) the trip occurs.