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I've already faced this issue twice. There's a UPS and its mains cable is loose and all of a sudden the cable just falls out so the UPS gets disconnected from the mains. So it starts beeping which means it wants human attention. I come to the UPS and see that it has a yellow LED indicator with a "sine wave" sign next to it and the LED in permanently on.

Surely I don't have a manual with me and I don't know what "LED near the sine wave on" means, but I guess that the "sine wave" means "mains". I then have to decide whether the LED on is "good" or "bad".

Another design option would be to have an LED with two colors (or two LED of different colors close to each other) and green LED would mean "good" and red LED would mean "bad", but that could be suboptimal for people who don't reliably see colors.

I see the only clear option here - the LED should be permanently on when it's "good" and flash when it's "bad".

Is there a common best practice for such cases?

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Would appreciate a picture or the UPS and the sine wave. –  rk. Jun 13 '13 at 11:36
    
Please remember that there may be a wide variety of potential faults which should be signalled by a few LEDs. It is debatable how intuitive they shall be; after all, if you are using UPS, you must be a professional. Professionals read the docs. If there are existing conventions, you can follow them... –  Deer Hunter Jun 13 '13 at 14:55
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@DeerHunter: LOL. My local telephone operator is removing copper last mile wires and replacing it by a fiber. Yes, a dedicated fiber enters every apartment. Now the plain boring phone won't work with that fiber, it needs something like a router which in turn requires mains power and this means that either I'm a professional and use a UPS or I have no phone in case of power outage. –  sharptooth Jun 13 '13 at 15:02
    
sharptooth - you have my sympathy... –  Deer Hunter Jun 13 '13 at 15:07

1 Answer 1

There is no common best practice that I am aware of. However general UX principles would say that you should try to unambiguous indicators. The following options are clear and reasonable:

  1. If you only have one indicator, I would use a green LED indicating that the mains is connected and everything is fine, and change it to a flashing red LED to indicate that there is a problem.
  2. If you already have a power / working indicator, simply have an indicator for a problem with the mains. So when there is no light on the "mains problem" led, everything is fine. Then have a flashing red LED to show that there is a problem with the mains.

It's also common to have some sort of audible alarm (that can be turned off) when the mains fails, as most often people aren't looking at the UPS, and so wouldn't be aware of any indicators.


That said, a modern UPS should have far more indicators than a few LEDs are able to show. I would recommend using a small LCD anyway to give information on power quality, charge state, etc. In that case, it would be best to use the LCD for the warning that there is a problem with the mains.

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