The primary function of un-interruptible power supplies (UPS) is to ensure continuous uptime of critical loads during power disturbances or interruptions. In order to do this, they themselves need to have some form of protection and preventative management. This comes in the form of warranties and maintenance contracts.
Standard UPS Warranties
Most un-interruptible power supplies and associated power protection equipment come with a warranty and the period of warranty will vary with UPS size. Typically, UPS up to 10kVA have a two-year warranty and larger systems a one-year. Many manufacturers offer warranty extension periods of up to five years (for a fee), which will often be achieved using ten-year design life battery sets in place of the standard five-year ones. Generators usually have a standard one-year warranty.
There are different types of warranty cover, which can affect the resilience of an un-interruptible power supply system. Below 3kVA, most UPS are covered by what is termed a Return to Base (RTB) warranty whereby the unit is returned to a service center or service agent, repaired or swapped out. This usually applies to un-interruptible power supplies that are not hardwired. For those that are, an onsite warranty provides a higher level of service. Generators are usually supplied with this type of cover too. A standard onsite warranty provides a next working day response time.
The Power Factor is a calculation used to account for the difference in power supplies used to convert AC power into DC for use in electrical appliances and computer equipment. There are two types of power supplies – the capacitor input supply and the power factor corrected supply.
Power factor corrected supplies are used in most high end computing and switching equipment, and have a ratio of 1VA:1W – allowing a very simple calculation for scaling electrical equipment and UPS. Thus, in theory, if your power factor is 1:1, and your UPS is an GE10kva UPS, then you would be able to load the UPS to 100% of its rating.
Typically when scaling a UPS for use in a data center you will use a 60% load factor on the UPS System. If you overload a UPS it is almost certain to fail during a power outage, as the draw on the batteries will exceed the capacity of the UPS. Most new UPSs will automatically go into battery bypass when an overload condition occurs. The 60% load factor accounts for the high probability that most of the equipment drawing power through the UPS will be of a category that has a power factor of between.55 and.75.
Comparisons between runtime performances of different un-interruptible power supply manufacturers can be misleading. Firstly, comparing battery runtime performance between them is difficult without a common basis. The best solution is to ensure all run-times are quoted using the same formula – load/Watt ratings and load power factors. An example could be: a 10kVA UPS supporting an 8kW load at 0.8pF, the runtime from a 12Ah internal battery set would be 14 minutes. However, at 75 percent load (6kW) and 0.6pF, the same UPS would provide almost 20 minutes of runtime.
It is possible to estimate battery set sizing, which may be useful for budgetary purposes at the specification stage of a power protection project, but as it is so critical to the overall performance and resilience of the un-interruptible power supply system, battery set sizing at the design stage should be carried out by industry professionals to achieve accurate and reliable ratings.