In this paper

the influence of voltage sag on power supply reliability from point of view of assuring to supply a certain quantity and quality of power to the power use and the necessity of setting up power supply reliability evaluation index system considering sag is set up. On the basis of existing power supply reliability indexes

a corresponding reliability index system for sag equivalent power supply is proposed

including equivalent interruption time due to lack of power supply

average power supply reliability rate of sag and equivalent user power supply shortage index due to sag. The index system can be used as a supplemental index for voltage sags or can be included in the calculation of existing indexes to consider the impact of voltage sags. At the same time

it is proposed to introduce the SARFI index as a supplementary index of the voltage sags for the system average interruption frequency index. Finally

the rationality of the proposed indexes is illustrated by standard examples and actual pilot cases.