crocket wrote: ↑
Mon Mar 11, 2019 5:36 am
W. H. Heydt wrote: ↑
Mon Mar 11, 2019 5:03 am
one memorable evening, the UPS alarms went off and on checking, the input voltage was way over spec, so I called the electric company to fix it. (Nominal voltage here is around 177v to 120v. The UPSes were seeing 140v.)
Can a typical consumer UPS like APC BE-550 protect equipments against constantly high voltage levels? But, that doesn't seem like a surge.
Within limits, yes. Check the industry specifications for normal/acceptable voltage range for your line (aka "mains") voltage. In the US, the nominal voltage is 120v,+/-5%, so 114v to 126v. That's why my UPSes threw fits when the input was 140v. Mind you, the *output* voltage at that point was 120v, so the UPSes did, in fact, protect everything behind them from the overvoltage.
There are three levels of protection from power line problems. The first is surge protection. Depending on the quality of the surge protector, it will protect devices from voltage spikes or surges. If a surge protector stops a serious spike (say, a lightning strike on a near by section of power line), it should be replaced. The next level is a line conditioner. A line conditioner will correct for undervolt (brownout) conditions and overvolt conditions. Again, there are limits. The documentation on a given line conditioner will specify just how low it can boost and high high it can cut down. Line conditioners include surge protectors.
The top level is the uninteruptable power supply (UPS). When the power fails, batteries pick up the load and provide power. As noted in this thread, they may not output a "pure" sine wave when running on the batteries. This should not matter to the protected equipment. UPSes include line conditioners...which include surge protectors. A UPS is going to have limited run time that will depend on the supported load. It will also have a specified load limit, which I will address next because it is at the intersection of technical specifications and marketing exaggeration.
In general, "Small Office/Home" (aka "SOHO") UPSes list their output rating as "volt-amperes". Regular readers will have seen some of repeatedly try to drive home the point that power (Watts) is voltage (volts) times current (Amperes), of W=vA. That is true when dealing with direct current (DC). It is not true when dealing with alternating current (AC). The output of a UPS is AC, as is the line/mains input power.
For AC power, the power goes as W = (2*-2)/2 * v * A. That is, take the volt-ampere rating and multiply by one half of the square root of 2. In practice, just multiply by 0.7. Therefore a 1000vA (aka 1KvA) UPS output should be 700W. As a practical matter, the power rating will be somewhat lower than this. The 1.5KvA UPSes that I have should--theoretically--be rated at 1050W (1500vA * 0.7), but are actually rated at 900W.
This is a pet peeve of mine. The volt-ampere rating is real, but most people aren't sufficiently trained in electrical engineering to understand what it really means in practice. You have to go through the fine print to find the rating in Watts. So far as I can tell, this is done for marketing purposes--the volt-ampere rating is a bigger number. After all, how many people faced with one device labeled "700W" (or less) and one labeled "1000vA" where the price is the same, would buy the "700W" unit, even though they are identical?