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Following the UPS issue at SO today I recall my experiences with UPS (uninteruptable power supply) systems in the work place.

Typically we would arrive at work in the morning and find all systems out. There was a power outage and the UPS failed to work correctly. In fact I can only remember one time when a UPS actually worked. That's a pretty poor track record - though of course my own experiences may be just unfortunate and UPS systems in reality are as reliable as having two slices of bread at opposite sides of a cheese sandwich.

EDIT The original question may invite opinion based answers. Apologies - my question is rephrased now as:

What factors can cause UPS systems to fail? What are the limitations of UPS?

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closed as too broad by Dave, LawrenceC, Ramhound, and31415, Ƭᴇcʜιᴇ007 May 16 '14 at 15:10

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.

Personally I think a lot of the issue with medium to small installations is once a UPS is installed it is completely forgotten about and the lack of maintenance leads to failure when it is actually put to use, I've seen that be the cause of UPS failure a lot anyway. Also I quite like your quantification of reliability. – Dan Nixon May 16 '14 at 14:34
It would help if you linked to the UPS issue that happened for historical purposes. I don't believe UPS is unreliable provided they are maintained like any mechanical equipment. This question honestly does not seem like a good fit at Superuser. You are clearly looking for an opinion. The simple fact is if UPS were not reliable to a certain specification those specific UPS would not be used and thus not be sold. – Ramhound May 16 '14 at 14:49
There was a power outage and the UPS failed to work correctly - What does that mean? What didn't work? Did you size it correctly? Were you expecting it to provide power indefinitely? What exactly happened that caused you to conclude that it didn't work correctly? – joeqwerty May 16 '14 at 14:50
On the recent occasion the UPS failed to provide any power after a mains supply outage. I was not responsible for the UPS. – suspectus May 16 '14 at 15:01
Ramhound - agreed. Question edited. Regarding commercial reliability there can be many years after purchase that UPS is actually required to function. Then it's too late to ask for your money back. – suspectus May 16 '14 at 15:06

To directly answer your question, not if they are good quality, used properly and within spec.

I have not had bad experiences with UPSes when they are used properly. Much of this depends on your specific situation. While IMHO the way you have worded your question invites extended discussion rather than precise answers and might be considered a poor fit for Superuser's Q&A format, there are some definite non-argueable points that one should consider when relying on UPSes.

  • Are you using name brand UPSes?

  • Are they new? Buying used UPSes is generally a bad idea.

  • Is everything connected under the wattage limit for the UPS?

  • Are you in an area with poor quality power or lots of storms? Those can shorten the life of any UPS.

  • Desktop UPSes are generally meant to give a system 15 or 30 minutes of uptime to allow for clean shutdown. Solutions that keep an entire datacenter running are typically diesel run generators and things like that. Are your computers configured to shutdown when the UPS is low on battery?

  • Batteries and UPSes wear out and must be refreshed periodically. So if you have a system on a 5 year old UPS it might not be as reliable as a new one.

  • If you are depending on UPSes as part of a business continuity plan, you really should buy a few, connect them, configure them properly, and then test them to be sure they will work according to your expectation. Rolling out something like this without a testing phase to confirm may lead to a bad time.

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+1 for testing, testing, testing. "Regarding commercial reliability there can be many years after purchase that UPS is actually required to function. Then it's too late to ask for your money back." Yes. But if you test it early on and re-test it on a reasonably regular schedule, then you don't have to wonder whether it'll work when it's really needed. – Michael Kjörling Aug 20 '14 at 8:37

I suspect it rather depends on a number of factors including:

  • What UPS? Is it perhaps under-specified for the task? Or were corners cut in procuring it?
  • How reliable is the incoming power source? It it perhaps very poor quality (brown outs?) causing the UPS to constantly kick in for a short period over night - I see that happen on my own home-office UPS sometimes. Could that be draining the battery?
  • What kind of load do you have on the UPS?

I've found good quality comms room and data centre UPS's to be very reliable indeed over the years. The most common failure being to forget to fill up the diesel fuel after an outage!!

Home and small office UPS's are also highly reliable, indeed I was able to work from home for a week or so after the Sheffield "Great Flood" a few years ago with the power going on and off like mad. However, I've found that the batteries can fail. I now only have one instead of two working (purchased at the same time) because one of the batteries failed after a couple of years.

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Line-interactive UPSes can handle moderate brownouts (and indeed moderate overvoltage) without resorting to fully battery-powered operation, through a buck/boost transformer. Such UPSes are likely a better choice than offline UPSes if you often experience brownouts. Honestly, I'm not sure how common pure offline UPSes even are these days; even my cheap 700VA unit, which retails here for around $150 US including taxes, is line-interactive topology. – Michael Kjörling Aug 20 '14 at 8:40

No, of course they need to have very good reliability. The problem is the old curse of the redundancy: to make things redundant, you need a system which integrates the childs. For example, in a hw raid array, you need a raid controller. If this controller dies, your whole array dies, although it was redundant.

Or if you try to make redundant servers, you need something which finds out which server is unavailable and redirects traffic to its clones. This thing can also die.

There are completely redundant systems possible, but they are normally too complex, and most sysadmin simply won't do that. Most of them simply don't even understand the concept. In most cases, they are using not redundant, but very reliable systems to connect and manage the redundant cluster of not so reliable sub-systems.

For example, there is a very costly raid controller, using a redundant disk array from the far east.

It is nearly always the case, but this is which won't work for UPSes. Why?

  1. UPSes are working with current. If the primary power source is lost, the secondary need to be powered on in 1/100 - 1/10000 seconds, or the outer voltage of the internal transformators of the servers will significantly drop.
  2. UPSes can't be tested. You can't plug out the power to find out if an UPS is okay, because your server will be shut down if it isn't.

For real ups redundancy you needed to use a redundant server cluster using servers with redundant power sources.

In most case, in big server farms, it is not done. Yes, google will also die if the actual server node dies which you are actually using. But they will find the bad nodes very fast, and eliminate that from their system.

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Why can't you pull the plug on the UPS? I've got a stack of real servers on my desk here, with redundant dual power supplies. Plug 'm in to two different UPSs, (so no SPOF). You can now safely test all redundant components including both UPS'es. – MSalters May 16 '14 at 14:55
@MSalters Then I congratulate to you. :-) – peterh May 16 '14 at 15:08
On commercial 24-7 systems not many sys admins would not dare test UPS on the live system. Because of the cost the test system often does not have UPS. – suspectus May 16 '14 at 15:14

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