Correct use of varistors(I)

A: The calculation of varistor voltage can generally be calculated by the following formula:

U1mA = KUac Where: K is the coefficient related to the power quality, generally K = (2 ~ 3), the city with better power quality is desirable, and the rural area with poor power quality (especially in the mountains) may be larger. Uac is the AC supply voltage rms value. For 220V to 240V AC power surge protectors, varistors with a varistor voltage of 470V to 620V should be used. Selecting a varistor with a higher varistor voltage can reduce the failure rate and extend the service life, but the residual voltage increases slightly.

II: Calculation of nominal discharge current:

The nominal discharge current of the varistor should be greater than the required inrush current or the most likely to occur every year.

Large inrush current. The nominal discharge current shall be calculated based on the value of the varistor surge life rating curve over 10 times, which is about 30% of the maximum impact flow rate (ie, 0.3 IP).