Measuring Reliability

7 min · difficulty 3/10

Reliability sounds vague until you measure it. Utilities use a few standard indices, and two cover most conversations.

SAIFI is about frequency. It is the average number of sustained interruptions a customer experiences over a year. A SAIFI of two means the typical customer lost power twice.

SAIDI is about duration. It is the total time the average customer was without power over the year. A utility can have few outages that each last a long time, or many short ones, and these two indices tell those stories apart.

Divide a customer’s total interruption time by the number of interruptions they experienced and you get the average restoration time per interruption. That is CAIDI, which equals SAIDI divided by SAIFI. Protective devices like reclosers improve these numbers by clearing temporary faults fast and isolating permanent ones to a small section, so fewer customers are out and for less time.

SAIFI, SAIDI, and CAIDI from one customer's year A one-year timeline for one average customer. Two surge-red outage blocks drop below the timeline axis: Interruption 1 lasting 50 minutes and Interruption 2 lasting 70 minutes, summing to 120 minutes total. A bracket above the timeline counts the two events and is labeled SAIFI equals 2 interruptions. A bracket below spans both outage widths and is labeled SAIDI equals 120 minutes. A gold derived box shows CAIDI equals SAIDI divided by SAIFI equals 60 minutes. Left-margin labels mark Frequency pointing at the SAIFI bracket and Duration pointing at the SAIDI bracket. One customer's year 1 2 SAIFI = 2 interruptions Frequency Jan Dec One year 50 min 70 min Interruption 1 Interruption 2 SAIDI = 120 minutes Duration CAIDI = SAIDI ÷ SAIFI CAIDI = 60 minutes Average restoration time per interruption SAIFI Frequency SAIDI Duration CAIDI Avg restoration time
From one customer's year: SAIFI counts the interruptions (2), SAIDI sums their total minutes (120), and CAIDI -- SAIDI divided by SAIFI -- gives the average restoration time per interruption (60 minutes).

How a utility builds and protects its distribution grid sets the baseline for every equipment order that follows. The Feeder tracks the infrastructure investment and modernization programs reshaping that baseline, free, every week.

Question 1 of 2

SAIFI counts how often the average customer loses power. What does SAIDI measure?

Educational material only. This is not engineering, safety, or procurement advice. Confirm any value against manufacturer documentation and a licensed professional before specifying equipment.