System Average Interruption Duration Index (SAIDI) & System Average Interruption Frequency Index (SAIFI) are used to indicate the reliability of electric power utilities in terms of average interruption & average outage experienced by a consumer in a year.

System Average Interruption Duration Index (SAIDI)

The System Average Interruption Duration Index (SAIDI) is the average outage duration for each customer. SAIDI is measured in units of time, often minutes or hours. It is usually measured over the course of a year.

• SAIDI = (U₁N₁ + U₂N₂ + U₃N₃ + ......... + U_iN_i) / N_T
• Where, N_i is the number of customers and U_i is the annual outage time for feeder i, and N_T is the total number of customers served

System Average Interruption Frequency Index (SAIFI)

The System Average Interruption Frequency Index (SAIFI) is the average number of interruptions that a customer would experience. SAIFI is measured in units of interruptions per customer. It is usually measured over the course of a year.

• SAIFI = (I₁N₁ + I₂N₂ + I₃N₃ + ......... + I_iN_i) / (N₁ + N₂ + N₃ + ....... + N_i)
• Where, I_i is the number of failure and and N_i is the total number of customers served