almost all the customers at least at the "emergency level" (Range B of the ANSI C84.1), but a small section of the network that contains the original fault might require manual switching with a service interruption for about an hour. The popularity of contingency planning is based on its advantages: each of the N elements in the system is analyzed separately, limiting the amount of work to be done and simplifying the failure options (e.g., generator failure, short circuit); the process inherently provides a way to deal with the contingency if and when it will happen. The N-1 contingency planning is typically sufficient for the systems with the usual ratio of peak load to capacity (below 70%). For a system with a substantially higher ratio, the N-1 planning will not deliver satisfactory reliability, and even N-2 and N-3 criteria might not be sufficient; therefore the reliability-based planning is used that considers the probabilities of the individual contingencies. References Sources Power engineering