Replacement of Repairable and Non-Repairable Systems
However, the system is often considered for replacement based two important aspects.
This aspect is given preference if the cost of failure is not hazardous/disastrous and that may not lead to catastrophes. The average annual total cost to be spent is compared to its replacement cost for a new one and thus a decision is taken on replacement.
This aspect is considered if the failure is disastrous and may lead to a catastrophe in terms of heavy loss of money, human life or assets. For example, consider the failures in an aircraft. The cost is immaterial in such cases where its reliable operation is more important than its cost or replacement. In some cases, the reliability of the equipment is translated and measured in terms of cost of failures particularly with reference to that if it fails when it is under operation.
For the items, which are repaired while they fail, reliability is the probability that failure shall not occur in the period of interest. When more than one failure occurs, it can also be expressed as failure rate or rate of occurrence of failure (ROCOF). However, the failure rate expresses the instantaneous probability of failure per unit time, when various failures might occur in a time continuum.
Repairable system reliability can also be characterized by mean time between failures (MTBF), but only under the specific condition of constant failure rate. We are also concerned along the availability of repairable system, since repair takes time. Availability is affected by the rate of occurrence of failures and by maintenance time. Maintenance includes corrective (repair) or preventive (to reduce the likelihood of failures).
Sometimes an item may be considered as both repairable and non-repairable for example a missile is repairable system while it is in store and subjected to tests, but it becomes a non-repairable system when it is launched. Similarly, a vehicle is considered to be repairable as long as the cost of replacement is more than that of its repair, while may be considered to be replaceable after it reaches certain age. We must take into account of these separate states while reliability analysis of such systems.