**8.1 System average interruption frequency index (SAIFI)**

SAIFI is a measure of how many sustained interruptions an average customer will experience over the course of a year. This measure can be defined as

$$\text{SAIFI} = \frac{\text{Total number of customers interruptions}}{\text{Total number of customers served}} \ (\text{inter}/\text{cost}) \tag{24}$$

For a fixed number of customers, the only way to improve SAIFI is to reduce the number of sustained interruptions experienced by customers.
