| Why MTTR Is Important
Reliability of a UPS is often specified in terms of its MTBF or Mean Time Between Failures. This figure will often be quoted in manufacturer's literature and is a measure of how reliable their UPS is. From the MTBF it is possible to predict how often the UPS will fail. Unfortunately this is only half the picture. When a UPS fails, the load is at the mercy of the incoming mains supply. If the mains supply were to fail while the UPS was down, then the load would lose power, and the UPS would have failed in its mission. How long the load is at the mercy of the mains supply depends upon how long it takes to repair the UPS. Clearly, if it takes 1 week to repair the UPS, and on average the mains supply fails once per week, then it is likely that the load will suffer a power failure while the UPS is down. Therefore, in order to evaluate a particular UPS solution, we need to know not only the UPS's MTBF, but also the mains' MTBF, and how long it takes to repair the UPS. This last quantity is given by a figure known as MTTR or Mean Time To Repair. Affect Of MTTR On Mission Success The affect of UPS's MTTR on mission
success also depends on the mains' MTBF. Clearly
if the mains never fails, then the UPS's MTTR is irrelevant (we will ignore
the point that if the mains never fails, there would be no need for a UPS).
The relationship between probability of loss of power, a UPS's MTTR, and
the mains' MTBF is given
by the equation :
Graph 1 shows this relationship for a range of UPS MTTR and mains MTBF values:  From the graph it can be seen that if the UPS MTTR is 1 hour, the mains MTBF does not have a great effect on the probability of a critical load failure How To Minimise The UPS's MTTR The best way to reduce a UPS's MTTR is to reduce the effort required to effect a repair. For example, if repairing the UPS involved complete disassembly, then this would obviously result in a long MTTR. If however, the UPS was of modular construction, then repairs would consist of simply removing the defective module and inserting a new module. This should take just a few minutes (assuming a spare module is available and close by). A further improvement can be made if the repair can be undertaken by the user as this eliminates the need for a technician to visit the site. This visit could easily take a few days depending upon how far the user was from their nearest supplier / service centre. |
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