TL;DR: Is there a cheap and simple tool for detection of very brief mains power outages?

Long story and explanation:

Yesterday and today we experienced three or four interruptions in mains electricity supply - some appliances with switched power supply units (PC screens, LED lights) flickered or failes for a very brief period of time, while appliances with good ol' transformer+rectifier setup (like the clock display on the oven panel) didn't seem to be bothered by them.

I firmly believe that we have had many of these in past two years, as these glitches manifest themselves by various strange effects that I can't find better explanations for (fridge-freezer box somehow stops refrigerating and sounds an alarm few hours later, the wifi access point seems to brown out and functions very erratically until next restart, one PC headless restarts itself).

I've had the utility guy over today, he measured our mains tap (voltage, impedance, inter-phase impedance difference etc.) and he said everything looked good on their part so he can't really do anything. He further said I need to check my own wiring, which I will do, but I suspect there is something amiss at the utility since the whole building of 5 units experinces similar weird occurrences as we do. Lastly, he said I should talk to our neighbors and ask if they have similar problems. If they do, he'll come back and do more checks (we are connected to an underground cable which may be failing somewhere but he has no means of detecting that before a full blown failure and as per his measurements right now everything looks good).

Now it took me some two years to actually witness this phenomena, as it usually happened when I was away or in broad daylight with no lights on, and one could only guess we had this brief outage hours later when the fridge/wifi problems manifested themselves. I'm a bit on the technical side of thing, whereas my neighbors are not. Which means even if I ask them, I doubt they won't be able to give me a definite answer whether they, too, get these glitches.

So I'm thinking if I could get them some simple and power efficient gadget for them to plug in to their own socket somewhere, I could just ask them whether the thing has gone dark/silent/pop/kaboom lately, and we all would get a nice answer to a complicated question.

All I could think of was a relay feedback loop which must be manually "activated" and the loop will open on power failure in the relay. But I would prefer a off-the-shelf solution one can buy chaply in a shop, as I don't really want to give my nice neighbors a strange gadget I engineered to put into their wall sockets (and I'm not certified to manufacture such things either).

I specifically don't ask about checking my own electrical installation, as I'll have a professional do it. Also our wiring is three years old and had to be checked officially before we were actually allowed to move in and use it.

Our location is the Czech Republic, Europe. We have three phase 240 V burried cable mains with 8 separate meters for various units, staircase etc.

  • 1
    I wonder if it’s a lost neutral. On your main circuit breaker,how many throws tied together are there? 1 or 3? Aug 4, 2020 at 19:50
  • 2
    Were these brief interruptions causing any problem with your a/c compressor? A brief interruption causes my scroll compressor (29 years in service) to run backwards under power and if I don't intervene, it runs backwards until it overheats and trips the thermal relay. Then it is an hour or two till the compressor cools unless I open the access panel and run water on the compressor. If I am home and I detect one of these (lights blink off, computer backup power supply clicks), then I manually switch the a/c off at the thermostat, wait 5 min, then it starts and runs with no problem. Aug 4, 2020 at 22:04
  • 2
    @JimStewart you should post that problem as a question. It might be the start-up capacitor and/or the start-up winding. If it's broken, you get stuttery start-up and possibly in reverse.
    – P2000
    Aug 4, 2020 at 22:14
  • 1
    @ThreePhaseEel I haven't checked yet, a log with timestamps would be a great help! It's an old entry-level APC with a serial port, so even if it doesn't, I guess I'll be able to get some sort of log over serial to some sort of raspberry/VM with USB serial. the important thing is to plug the logging computer into the UPS :-)
    – Pavel
    Aug 5, 2020 at 5:51
  • 2
    Well if your apartment gets 3 phases, then it's worth paying very close attention to the voltage on each phase as large loads are turned on and off. They should be equal (220? 230?) If they wander around as loads change, then look to see if some go above normal. That's a lost neutral. In particular, the heavily loaded phase will be pulled down, pushing the other phases up. Large motor starts will make this quite acute. Aug 5, 2020 at 5:51

2 Answers 2


Personally I would solder your self-latching relay solution. It's simple. If you don't like working with 240V, maybe an AC relay after a transformer or even a DC relay after a USB charger could work. Of course, check to make sure it triggers on brief interruptions.

You can even connect a clock to the switched side (e.g. a timer - but one with a mechanical motor) and determine when the outage happened based on when the clock stopped.

Have you looked at "power logger" "voltage logger" or "energy logger"? You'd have to check the specs if glitches are recorded. A "data logger" and the "voltage logger" are likely the most versatile to log brief/glitch events.

Alternatively, you can

  • record the DC voltage after a usb charger and a tone generator using a voice recorder app, and use a simple computer script or audio editor to find the time and duration. Or record the AC hum/cycle, double it to 100Hz using a rectifier bridge, in case 50Hz is filtered out before recording.
  • use a video triggered webcam directed at a light to get the time info. The ensuing "dark flash" would have to trigger a recording, and it can even notify you by WiFi
  • hook up a relay to an alarm circuit, a cheap one with WiFi/internet notification and logging.

If any of these get close to what you would like to try, we can discuss the details if needed, to help you further along.

  • 1
    Look for data loggers - Campbell is a name that comes to mind
    – Solar Mike
    Aug 4, 2020 at 20:38
  • @SolarMike, yes good point, I should mention that too. That would be my favourite engineering solution. Will edit answer.
    – P2000
    Aug 4, 2020 at 20:41
  • solid answer though +1...
    – Solar Mike
    Aug 4, 2020 at 20:52
  • Thank you for your answer, I'll look into the data loggers. The rest of what you propose isn't applicable as I can't attend to my "boxes" once installed. I need a simple plug-in gadget which with a green/red light or an on/off light, so that my neighbors can plug it themselves and tell me "hey your thing has gone dark yesterday" when it does. They are possibly very interesting, though, for others who may come across looking to solve their own problem!
    – Pavel
    Aug 4, 2020 at 21:16
  • 1
    Good luck @Pavel . It helps that you are clear about how you plan to use it and what you do and don't expect from it.
    – P2000
    Aug 4, 2020 at 22:09

I have a similar problem. I've used in my engineering life, many years ago, a printIing circular 7-day clock. This was a wind up clock with a printing needle on circular graph paper. When connected to the power line, it scribes a circular straight line. If power is momentarily or for longer term droppes, the ink trace will show it.

I haven't been able to find one of these 7 day power monitoring clocks, but I know that they did exist at one time.

  • You can use a wall wall wart that outputs AC, to power your relay circuit, then you are not working with the mains.
    – Gil
    Nov 5, 2022 at 3:29

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