Reliability is a measure of the electric or natural gas system’s ability to provide supply to consumers when they desire to use the commodity. In the U.S., state or local regulators oversee reliability for distribution utilities. The Federal Energy Regulatory Commission (FERC) oversees reliability for the bulk electric system (generation plus transmission) and the interstate gas transmission system.

Due to the nature of the electric grid, reliability requirements for electric companies tend to be more stringent than for natural gas. Reliability requirements for the bulk electric system are overseen by the North American Electric Reliability Corporation (NERC), which is the Electric Reliability Organization authorized by FERC to develop and enforce compliance with mandatory reliability standards. NERC defines a reliable bulk electric system as one that meets the electricity needs of end-use customers even when unexpected equipment failures or other factors reduce the amount of available electricity. NERC divides reliability into two categories:

  • Adequacy, which is having sufficient resources to provide customers with a continuous supply of electricity at the proper voltage and frequency, virtually all the time.
  • Security, which is the ability of the system to withstand sudden, unexpected disturbances, such as short circuits or unanticipated loss of system elements due to natural causes or manmade physical or cyber-attacks.

Electric reliability is commonly measured using the metrics System Average Interruption Frequency Index (SAIFI), System Average Interruption Duration Index (SAIDI), Consumer Average Interruption Frequency Index (CAIFI), and Momentary Average Interruption Frequency Index (MAIFI).