System reliability is not the same as power quality. System reliability pertains to sustained interruptions and momentary interruptions.
Power quality involves voltage fluctuations, abnormal waveforms, and harmonic distortions. An interruption of greater than five minutes is generally considered a reliability issue, and interruptions of less than five minutes are a power quality concern.
This course explains the reliability indices used to measure distribution system reliability, how to calculate the indices, and discusses some of the factors that influence the indices.