Reliability Indices

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  • Customer count: The number of customers.
  • Interruption: The loss of service to one or more customers.
  • Duration of interruption: The period measured in minutes from the initiation of an interruption to a customer or other facility until service has been restored to that customer or facility. It is essential to record the duration of each interruption
  • Forced interruption: An interruption caused by a forced outage.
  • Interruptions caused by events outside of distribution: An interruption due to the cause of outside influences such as transmission line, of generating plant failure or outages.
  • Loss of service: The loss of electrical power, a complete loss of voltage, to one or more customers. This does not include any of the power quality issues (sags, swells, impulses or harmonics).
  • Momentary event interruption: An interruption of duration limited to the period required to restore service by an interrupting device. Such switching operations must be completed in a specified time not to excess 5 min. This definition includes all reclosing operations that occur within 5 min of the first interruption. For example, if a recloser or breaker two, three, or four times and then holds, the event shall be considered one momentary interruption event.
  • Momentary interruption: Single operation of an interrupting device that results in a voltage zero.
  • Outage (electric power system): The state of a component when it is not available to perform its intended function due to some event directly associated with that component.
  • Scheduled interruption (electric power systems): A loss of electric power that results when a component is deliberately taken out on service at a selected time, usually for the purposes of construction, preventative maintenance, or repair.
  • Total number of customers served: The total number of customers served on the last day of the reporting period.
  • Reporting period: A period assumed to be one month unless otherwise stated.

Reliability Indices

SAIFI - System Average Interruption Frequency Index

This index is designed to give information about the average frequency of sustained interruptions per customer over a predefined area.

SAIFI = Total number of customer interruptions / Total number of customers served

[math] SAIFI = \frac{\sum_{i} N_i}{N_T} [/math]

where [math] N_i \, [/math] is the number of customers interrupted in the ith interruption

[math] N_T \, [/math] is the total number of customers

SAIDI - System Average Interruption Duration Index

This index is commonly referred to as customer minutes of interruption and is designed to provide information about the average time the customers are interrupted.

SAIDI = Customer interruption durations / Total number of customers served

[math] SAIDI = \frac{\sum_{i} r_i \times N_i }{N_T} [/math]

where [math] N_i \, [/math] is the number of customers interrupted in the ith interruption

[math] r_i \, [/math] is the customer restoration time (in minutes) for the ith interruption
[math] N_T \, [/math] is the total number of customers

CAIDI - Customer Average Interruption Duration Index

CAIDI represents the average time required to restore service to the average customer per sustained interruption.

CAIDI = Customer interruption durations / Total number of customer interruptions

[math] SAIDI = \frac{\sum_{i} r_i \times N_i}{\sum_{i} N_i} = \frac{SAIDI}{SAIFI} [/math]

MED - Major Event Days

A Major Event Day (MED) is any day that exceeds a daily SAIDI threshold called TMED. IEEE Std. 1366 proposes a methodology called 2.5 Beta for classifying major event days by quantifying the threshold TMED. The general process is as follows:

  • The natural log of daily SAIDI values for the last five years is calculated. The log-average ([math]\alpha \, [/math]) is then computed.
  • The standard deviation of the log SAIDI data is calculated ([math]\beta\, [/math])
  • Finally, TMED is calculated as follows:
[math] TMED = e^{\alpha + 2.5 \beta} \, [/math]


  • When calculating daily SAIDI, interruption durations that extend into subsequent days accrue to the day on which the interruption begins.
  • If any day in the data set has a value of zero for SAIDI, it shall not be included in the analysis.


When calculating the values of the above reliability indices i.e. SAIFI, SAIDI & CAIDI, the interruptions due to Load Shedding, EHV Openings (Distress load shedding), Forced interruption, Interruptions caused by events outside of distribution & Interruptions due to natural calamities are to be excluded.