**8.2 System average interruption duration index (SAIDI)**

SAIDI is a measure of how many interruption hours an average customer will experience over the course of a year. For a fixed number of customers, SAIDI can be improved by reducing the number of interruptions or by reducing the duration of these interruptions. Since both of these reflect reliability improvements, a reduction in SAIDI indicates an improvement in reliability. This measure can be defined as

$$\text{SAIDI} = \frac{\text{Total customers interruptions durations}}{\text{Total number of customers served}} \ (\text{h/cust}) \tag{25}$$

#### **8.3 Customer average interruption duration index (CAIDI)**

CAIDI is a measure of how long an average interruption lasts and is used as a measure of utility response time to the system contingencies. CAIDI can be improved by reducing the length of interruptions but can also be reduced by increasing the number of short interruptions. Consequently, a reduction in CAIDI does not necessarily reflect an improvement in system reliability. This measure can be defined as

$$\text{CAIDI} = \frac{\text{Total customers interruptions durations}}{\text{Total number of customers interruptions}} \ (\text{h/cust}) \tag{26}$$
