The power system reliability (sometimes grid reliability) is the probability of a normal operation of the electrical grid at a given time. Reliability indices characterize the ability of the electrical system to supply customers with electricity as needed[1] by measuring the frequency, duration, and scale of supply interruptions.[2] Traditionally two interdependent components of the power system reliability are considered:[1]
Ability of the system to limit the scale and duration of a power interruption is called resiliency. The same term is also used to describe the reaction of the system to the truly catastrophic events.[4]