The importance for which uninterruptible power supplies (UPS) were created means that their reliability requires some kind of measurement to give customers a means of comparing different manufacturers and UPSs. The purpose is to protect the loads that the UPS protects from vulnerability, therefore reliability is not to be guessed at.
Mean time between failure
MTBF or Mean Time Between Failures is one of those measures, an indicator of the reliability of an uninterruptible power supply. It is the average run time between system startup and shutdown due to a failure (not a power failure in this sense, but a UPS system failure). It is represented by a measure of hours.
Average failure rate is another measure of reliability. This is the total number of failures in a given period of time. The failure rate over the lifetime of any UPS system, therefore, is inversely proportional to its MTBF.
Uninterruptible power supplies are no different from any other electronic equipment in that the rate at which they fail is not constant. There are three distinct periods associated with UPS failure (often represented by a tub curve diagram showing a) infant mortality failures, b) random failures, and c) wear failures).
Infant mortality UPS failures
Infant mortality failures correspond to failures early in the life of the uninterruptible power supply. IT-sized uninterruptible power supplies can suffer what is called “death on arrival.” This could be due to a component manufacturing defect or damage during shipping. A sudden bump or jolt during transport can weaken a welded joint, for example. While UPS manufacturers strive to reduce these incidents as much as possible through strict quality controls and testing processes, they do happen. Various processes can be applied to minimize the chances of this happening. 10 kVA UPSs, for example, can operate for short periods of operation (up to 48 hours) at high ambient temperatures to reduce the potential for such failures.
Random UPS failures
Random crashes occur less frequently. During the normal life of a UPS, the rate of these is low and fairly constant.
Wear faults
End of life wear failures of an uninterruptible power supply are more common (and this is where the curves are steepest). Here, battery problems account for 98 percent of UPS wear failures. Particularly where the uninterruptible power supply has been subjected to high ambient temperatures for prolonged periods, the insulation of the internal wiring becomes brittle and breaks. There are other consumable items that should be part of a regular monitoring regimen, such as fans and capacitors, which will eventually wear out with use as well.
Just because a manufacturer shows you some favorable MTBF statistics does not necessarily mean that their products are the most reliable. Like most things, these can be massaged to make them look more relaxed than they actually are. The important question we must ask ourselves is: what was the basis for its calculation? There are two main approaches:
1) A record of the total number of failures for a particular size of UPS over a specified period of time.
Generally adopted by UPS manufacturers, this is a valuable approach if the field population is large and the time period is long enough (longer than the typical life expectancy of a UPS, which is five to ten years).
2) A system value calculated from known MTBF values of components and assemblies.
Obviously, this approach is more complex and is based on following standardized calculation formats.
Estimated or average time to repair
Mean Time to Repair (Mean Restore Time) is the time required to return an uninterruptible power supply to normal operation from shutdown.
Online UPSs are designed to safely fail to the grid; therefore, the calculation of the MTBF of the mains power supply is also an important consideration along with the mean time to repair (or mean time to repair).
Since it is highly unlikely that a service engineer will be on site at the same time a UPS fails, the MTTR must also include a travel time element. This also means that the service engineer brings the necessary parts to fix the problem in one visit, which is sometimes not the case. Manufacturers of UPSs can only provide a figure based on actual repair time. Although this can be a satisfactory comparison tool, it is not a true representation of reliability. Sometimes a degree of skepticism is necessary when comparing the marketing data of some manufacturers.