scope:

There is no method by which the short-circuit power factor can be determined with precision, due to variation of resistance with frequency and current and of inductance with time and current. However, for the purpose of this specification, the power factor of each phase of the test may be determined with sufficient accuracy by whichever of the three following methods is the more appropiate. Method I is recommended for the determination of power factors higher than 0.3 (for example test duty 3 for current-limiting fuses and test series 4 and 5 expulsion fuses) because it is generally not accurate for very low power factors due to difficulty in measurement of the resistance. Method II is recommended for the determination of power factors lower than 0.3 (for example test duties 1 and 2 for current-limiting fuses and test series 1,2 and 3 for expulsion fuses) because it is generally not accurate for power factors higher than 0.3 due to the short duration of the d.c. component and difficulty in determination of its time constant. Method III is recommended for the determination of power factors higher than 0.5 (for example test series 5 for expulsion fuses) because it is generally not accurate in the case of power factors below 0.5 due to the necessary degree of accuracy with which phase displacements need to be measured.