In order to register and classify reasons of electricity supply interruptions and to determine the numbers of interruptions in conformity with international standard, the ERC started to register the electricity supply interruptions by using the following 3 indices from 2005:

1. The System Average Interruption Duration Index (SAIDI) – the number of interruptions per consumer per year
   (number of interruptions/ number of all customers/year)
2. The System Average Interruption Frequency Index (SAIFI) – the average time of interruptions per consumer per year
   (duration of interruptions (min)/ number of all customers/year)
3. The Consumer Average Interruption Duration Index (CAIDI) - the average interruption time per consumer affected by the interruption per year
   (duration of interruptions (min)/ number of customers affected by the interruptions/year)