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Some words about UPS:
Q: What is a UPS?
A: An Uninterruptible Power Supply is a device that sits between a power supply (e.g. a wall outlet) and a device (e.g. a computer) to prevent undesired features of the power source (outages, sags, surges, bad harmonics, etc.) from the supply from adversely affecting the performance of the device.
The idea behind a UPS is pretty obvious from the name. In addition to filtering, enhancing or modifying the utility power, special circuitry and batteries are used to prevent the PC from losing power during a disruption (blackout) or voltage sag (brownout). These units are called different names depending on their exact design, but all fit into the general category of backup power .
Q: Vendor X says that (fill in description) is a UPS, but it's different than what you describe above. Who's right?
A: There really is no standard definition of what a UPS is. Anything ranging from a 9 volt battery backup in a clock radio to a building/compound wide backup generator has been called a UPS by someone. The majority of this document refers to objects larger than a beer can that help devices remain temporarily operational when changes to the power they receive would otherwise interrupt their function.
Maintaining power to a minicomputer (like a VAX 11) is beyond the scope of this document. This FAQ deals with UPS equipment that can be installed by a computer owner/administrator. If you have requirements that large, you need to talk to a qualified electrician. Basically, the focus of this document is on power protection devices that operate in the 3000 Volt-Amp (VA) range or lower. These are smaller machines that can be installed and managed without the assistance of licensed electricians.
There are basically three different types of devices, all of which are occasionally passed off as UPSes.
- Standby power supply (SPS). In this type of supply, power is usually derived directly from the power line, until power fails. After power failure, a battery powered inverter turns on to continue supplying power. Batteries are charged, as necessary, when line power is available. This type of supply is sometimes called an "offline" UPS.
The quality and effectiveness of this class of devices varies considerably; however, they are generally quite a bit cheaper than "true" UPSes. The time required for the inverter to come on line, typically called the switchover time, varies by unit. While some computers may be able to tolerate long switchover times, your mileage may vary. [ Some articles in the trade press have claimed that their testing shows that modern PCs can withstand transfer times of 100ms or more. Most UPS units claim a transfer time to battery of about 4ms. Note that even if a computer can stay up for 100ms, it doesn't mean that 100ms switchover is okay. Damage can still be done to a computer or data on it even if it stays up. ]
Other features to look for in this class of supplies is line filtering and/or other line conditioners. Since appliances connected to the supply are basically connected directly from the power line, SPSes provide relatively poor protection from line noise, frequency variations, line spikes, and brownouts.
[Some SPSes claim to have surge/spike suppression circuitry as well as transformers to "boost" voltage without switching to the battery if a modest voltage drop occurs. Often, as a "standby" UPS becomes more featureful it is called a "line interactive" UPS.
- Hybrid [ or ferroresonant ] UPS systems. I only know one vendor who sells them - Best Power, Inc. [ Now called Eaton Powerware. Note that Powerware also sells line interactive and online UPSes. ] The theory behind these devices is fairly simple. When normal operating line power is present, the supply conditions power using a ferroresonant transformer. This transformer maintains a constant output voltage even with a varying input voltage and provides good protection against line noise. The transformer also maintains output on its secondary briefly when a total outage occurs. Best claims that their inverter then goes on line so quickly that it is operating without any interruption in power. Other UPS vendors maintain that the transition is less than seamless, but then again it's not in their best interest to promote Best's products.
[ Note: According to some sources, ferroresonant transformers in an UPS system can interact with ferroresonant transformers in your equipment's power supply and produce unexpected results. On the other hand, ferroresonant UPS systems don't kick off a lot of heat, which is important in some environments. The Moral: Test equipment to make sure it meets your needs before you buy. -npc ]
- What I call "true" UPS systems, those supplies that continuously operate from an inverter. Obviously, there is no switchover time, and these supplies generally provide the best isolation from power line problems. The disadvantages to these devices are increased cost, increased power consumption, and increased heat generation. Despite the fact that the inverter in a "true" UPS is always on, the reliability of such units does not seem to be affected. In fact, we have seen more failures in cheaper SPS units. [ Note, though, that given the same quality inverter, you'd expect the one that runs least to last longest. These devices are often called "online" UPSes. ]
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