Availability measures how much time per year a system is available to users. Power availability, the largest single component of system availability, is also the biggest cause of computer downtime. Therefore, increasing it is the most effective way for IT managers to increase their overall systems availability.
Availability is improved both by increasing mean time between failures (MTBF) and by reducing mean time to repair (MTTR). MTBF is the average number of hours until power system failure. It can be increased either by improving the reliability of every system component, or by building in redundancy, ensuring system availability even during an individual component failure. There is a finite limit to component reliability, even with increased cost. Today, typical power protection systems offering only high component reliability achieve an MTBF of between 50 000 and 200 000 hours.
By contrast, adding redundancy can yield a three – to six – fold improvement in MTBF. In practice, an optimum cost/MTBF performance is achieved by specifying highly reliable but not cost prohibitive components within a redundant configuration.
Decreased MTTR is best achieved by adding hot-swappability, where failed components can be replaced without downtime. Hot-swappability in a redundant design can drive MTTR close to zero, with failed components being repaired before the system fails.
IT managers can choose their own best compromise between cost and availability. This choice is sometimes represented by a Power Availability (PA) Index. A standalone UPS, rated PA-1, has the lowest initial purchase price, but its reliability is limited by that of its online components. A fault tolerant UPS is sometimes described as having affordable redundancy. Although its availability is high and rated PA-2, repair can be time-consuming and expensive, with serious downtime implications.
The same high availability level is offered by PA-3 rated modular UPS with multiple hot-swappable components and, often, redundant batteries. Planned downtime for servicing is eliminated.
The PA-4 rated PW modular UPS range offers the highest power protection level currently available. The power and processor electronics, and the batteries, are redundant and hot-swappable. This provides very high power availability and the highest level of protection for critical loads.