Reliability describes the degree of performance of the elements of the bulk electric system that results in electricity being delivered to customers within accepted standards and in the amount desired. Reliability at the transmission level may be measured by the frequency, duration and magnitude (or the probability) of adverse effects on the electric supply/transport/generation. Electric system reliability can be addressed by considering two basic and functional aspects of the electric system:

1. adequacy: the ability of the electric system to supply the aggregate electrical demand and energy requirements of customers at all times, taking into account scheduled and reasonably expected unscheduled outages of system elements; and

2. security: the ability of the electric system to withstand sudden disturbances such as electric short circuits or unanticipated loss of system elements.