• How Bypass Increases The Mission MTBF Of A Common UPS
• The Effect Of The Utility Source On Mission MTBF
• The Effect Of MTTR On Mission MTBF
• For Peace Of Mind, Buy A Spare Module
• Achieving An UPS MTBF Of Greater Than 400k Hours
• Conclusions

Automatic bypass has become an industry expectation for any well designed On-Line UPS. At the same time, considerable effort is spent in making the UPS so reliable that it never automatically transfers the critical load to the bypass source. In the early days the reliability of the UPS was not very good so adding an automatic bypass was an easy way to achieve a good increase in mission MTBF. These early UPS designs could not take a 100% load-step or maintain the output voltage when starting equipment so the bypass was used to make up for these deficiencies. 

New technologies have greatly increased the reliability of a UPS and have virtually eliminated the need for a transfer to bypass. Inverters can now take a 100% load step and have the overload capacity to start most loads. 

How Bypass Increases The Mission MTBF Of A Common UPS 

To understand how using bypass, as a backup for the UPS, increases the mission MTBF, we use a standard reliability model as shown in Figure 1. Because we can repair the UPS, to fully understand the effect of bypass, we must analyze failure interactions that are not independent. (This analysis is beyond the scope of this paper.) However, using the simple model in Figure 1 and a reliability model, we can gain a good understanding. 
 

The same formula can be used for a modular UPS. However, because the MTBF of the modular UPS can be greater than 400,000 hours, the effects of the bypass MTBF and the equipment MTTR on the overall mission reliability merit a closer look. 

The Effect Of The Utility Source On Mission MTBF 

Graph 1 shows how the mission MTBF varies for a range of utility MTBF values. For a UPS reliability of 400k hours or more, an interesting thing happens. The bypass circuitry (MTBF of 500k hours) reduces the overall mission reliability. This means that having a bypass makes the UPS less reliable. 

In countries or locations where the utility MTBF is not "high quality," (MTBF < 100 hours) it may actually be better not to include the bypass option in the UPS. 
 

 

Although the reliability of the utility source affects the mission reliability of the UPS, the MTTR of the UPS has a much bigger impact. Graph 2 assumes a high quality utility and shows how an MTTR of less than 20 hours greatly affects the mission reliability of the UPS. To achieve a very low MTTR, it is necessary to have a modular, hot swappable UPS and a spare module available on site. Until recently, UPS designs did not allow hot swapping a module while the UPS was powering the load. 

In a normal modular UPS, the UPS has to be put on maintenance bypass (putting the load at risk) to change a faulty module. If the repair module has to be shipped in, it may take 24 or 48 hours or even a week to repair the UPS. During this time, the load is at risk. 

In a UPS that allows the modules to be swapped while the UPS is powering the load, it is not necessary to use the maintenance bypass to change the faulty module and the UPS continues to protect the critical load. Unlike a normal modular UPS, the load is not at risk when a module fails because bypass is not used. If a spare module is on site, it can be changed easily in 10 minutes or so, retaining very high mission reliability. 
 

A spare module is an excellent investment that will cost about the same as the bypass 
option. Having a spare module will assure that the UPS can be repaired quickly. It is 
important to point out that unlike a normal UPS, which transfers to bypass when it fails, a modular redundant UPS will continue to power the critical load when a module fails. So even if a repair module has to be shipped in, the UPS will continue to protect the load, but having a spare on site is highly recommended. 

Achieving An UPS MTBF Of Greater Than 400k Hours 

New technologies, the use of fault tolerant techniques and keeping a module in reserve make it possible to achieve very high reliability through redundancy. The MTBF of a modular redundant UPS can be estimated using: 
 

u = 1/MTTR 
n = number of modules, 
s = number of modules in reserve

If 11 modules are used to power the same 10kVA load, the mission MTBF is 442k hours for an MTTR of 24 hours. The table below gives the mission MTBF for several MTTR values for a UPS using modules in reserve. 
 

Conclusions 

The recent development of UPS configurations that offer high reliability, fault-tolerance, hot- swappability and modularity means that previously unobtainable high reliability figures are now possible. This means peace of mind that the UPS will do its job, even if a module fails, easy module swapping while the UPS in powering the load and no worrying about the loss of critical data due to utility power problems. 

To get the most out of your UPS: 

• Specify a fault-tolerant, hot swappable, modular redundant UPS
• Size the UPS to keep at least one module in reserve.
• Purchase a spare module
• Don't buy the bypass option (Why waste the money unless you really need it?)

1. Suri, Mohammed and Perra, Andre UPS Reliability and System Configuration,  
    Power Quality, September 1992 Proceedings. 
2. Lewis, E.E. Introduction To Reliability Engineering. 1987, John Wiley& Sons , Inc.