In this section I am going to present the multitude of problems that can appear on a pretty nice but not- so-sunny Sunday while you are working on your computer. Of course all of them are related to electricity, as you probably expected.
The first one is the most obvious: a power failure-blackout. This translates to a sudden fall in the supply of electricity and results in an immediate stop of the devices connected to the network. For some devices, if this is for a shorter time (let's say about 0.5s) you won't always be able to observe it happening -- for example, an amplifier, light bulb, or an industrial motor might keep going.
The second problem is what we call the voltage sag. This provokes a short term under-voltage, which can be observed in the flickering of lights. Other devices, once the voltage drops under a specific level, might restart themselves. For example, this can happen with some modems on less than 210V AC. Here I should also mention that while in the US, the standard power is 110 volts of 60Hz, in Europe this is 240 volts of 50Hz.
The third potential issue is the voltage spike. This is the opposite of the previous problem; it is a short term over-voltage. It can cause acute damage to electronic devices, as the increase of tension increases the count of "Amperes" (i.e., electric current due to the count of charges) that run through them, and the wires/pieces all have an upper limit as to what they can support.
The fifth problem we may encounter is under voltage for an extended time period. This causes overheating in most devices, while some of them won't even make the effort to start or work. Of course the same can happen for an over voltage, and this can be even more dangerous; this is when the traditional light bulbs that Edison invented start to fail.
The sixth issue is line noise. As you probably learned in high school, AC power comes into your home in the form of a sine wave (and within a second it completes a full sine curve 50 times in Europe and 60 in the US). Line noise means that distortion is superimposed on the power waveform, causing additional magnetic fields to be carried in the wires, devices, and electromagnetic interfaces.
The seventh potential problem is frequency distortion, when there are more or less than 50/60 sinus curves under a second. For anything powered by motors (motion-producing devices), this is akin to a higher or lower speed. Many electric devices use this to measure time, so this is when a bad time measurement can occur.
The eighth issue is the switching transient. That's a sudden/instant under voltage (from milliseconds to seconds) that stresses the devices, because it can cause invalid data to be written and memory corruption. And finally, you may also meet with the harmonic distortion: multiples of power frequency superimposed on the power waveform. As with some other problems listed, this can cause overheating.
So many problems, and each of them is a threat. However, a UPS can fix most of them. As for each of them, it's a different solution; for price reasons, not all of them are resolved by all of the UPSes. Some producers like to categorize their UPS as to how many problems can handle on a scale from 1 to 9; obviously the higher, the better.
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