Availability of electrical power supply

From Electrical Installation Guide

Definition:

This is the probability that an electrical installation be capable of supplying quality power in conformity with the specifications of the equipment it is supplying. This is expressed by an availability level:
                                               Availability (%) = (1 - MTTR/ MTBF) x 100
MTTR (Mean Time To Repair): the average time to make the electrical system once again operational following a failure (this includes detection of the reason for failure, its repair and re-commissioning),
MTBF (Mean Time Between Failure): measurement of the average time for which the electrical system is operational and therefore enables correct operation of the application.
The different availability categories can only be defined for a given type of installation. E.g.: hospitals, data centers.

Example of classification used in data centers:

Tier 1: the power supply and air conditioning are provided by one single channel, without redundancy, which allows availability of 99.671%,
Tier 2: the power supply and air conditioning are provided by one single channel, with redundancy, which allows availability of 99.741%,
Tier 3: the power supply and air conditioning are provided by several channels, with one single redundant channel, which allows availability of 99.982%,
Tier 4: the power supply and air conditioning are provided by several channels, with redundancy, which allows availability of 99.995%.