In IEC 60664-1:2020 table F.1 (Rated impulse withstand voltage for equipment energized directly from the mains supply) For 1250V, the calculated Rated Impulse with stand Voltage value is equal to 1000V (4000, 6000, 8000, 12000)
Ex) DC 1050V, Overvoltage Category I. which case is correct?
CASE1) Should the value of 1000V be ignored and calculated based on the value between 600V and 1250V? Result : 3538V
CASE2) Should the value of 1000V to 1250V be calculated without change at 1000V? Result : 4000V
