WIZ - Reliability indexes - page 3

RIs are additive only when the various systems are suitably far away from each other, or otherwise independent with regard to causes of failure.

For example, if a system with an RI of 3 fails because the building's power went down, then another system in the same building has a 100% certainty of failing as well. In this case the combined RI of two systems is not greater than the RI of just one of them.

Even if they are in the same city, there is some probability that a city-wide power failure or similar event would kill both systems simultaneously.

Thus, when computing the RI of a group of ZOZ objects, the ZOZ would take distance and several other factors into account in its estimation formula.