Reliability is a branch of power quality which investigates the impact of  “Electricity Interruptions” on customers. Interruption is an index of power quality which has two categories: short-term and long-term. In the “short-term interruptions”, indices of  momentary interruptions (with a duration between 30 cycles up to 3 seconds) and temporary interruptions (continued 3 seconds to 1 minute) and in the “long-term interruptions” category, the index of sustained interruptions (with continuity over than 1 minute) is defined. It must be mentioned that interruption refers to the reducing of voltage below the 10% of rated voltage.

In discussing the reliability of a power system, the dependent indices of interruption (eg. SAIFI, CAIDI, SAIDI, …), which evaluate the number of customers which is affected by the interruption and the duration of  interruption is studied.