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I need to do some electrical work on a circuit in my home. One of the outlets on the circuit has a UPS battery backup plugged into it. The UPS is tucked away behind a lot of other equipment, so I'd rather not have to unplug it if its not necessary. Is there a risk of the UPS "leaking" (or whatever the term would be) back onto the circuit when the breaker is tripped? That is, for safety, should the UPS be unplugged before I work on the circuit?

Specifics in case they matter:

  • I am replacing some switches for lights on the same circuit as the UPS
  • the switches do not control the outlet the UPS is plugged into
  • the UPS is a bXterra BM1500AVRLCD (1500VA)
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    This is a reasonable question and, frankly, one I hadn't thought of! I had to do some rewiring on a circuit into which my server is plugged. I had the wife turn off the breaker while I disconnected the wiring at the furthest upstream point. I listened to the UPS scream while I broke the connections, then had her turn the breaker back on with the hot side wires all capped for safety. I did my downstream wiring changes, then had her turn the breaker back off, reconnected everything, and had her turn it back on. Power was off to the UPS < 1 minute each time because I planned ahead. – FreeMan Jul 2 at 17:55
  • @FreeMan you trust your wife a lot. Well, if I had to do the same, I would probably trust my wife, too. But: if the equipment powered by the UPS is that much important, an extension cord to the nearby working outlet is probably better. – fraxinus Jul 4 at 9:19
  • @fraxinus For right on 30 years - I trust her with my life! She can be an annoying "safety warden" sometimes, but she's probably kept me from killing/maiming myself more times than I'd like for her to count! ;) – FreeMan Jul 4 at 14:01
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While this should be acceptable, I have personally found a UPS which backfed 240AV out its input pins. I only found this out when brushing against the exposed pins on the wall plug - That UPS went straight in the junk pile!

In short, when dealing with mains power, make no assumptions. Use a non-contact voltage indicator or a plug-in lamp or even a voltmeter, to check your assumptions.


In your case, the switches are on a different circuit, and you will shut off the breaker to that circuit. When you open the switch plate, check for voltage immediately. And if its still live then stop and re-evaluate.

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    This: "when dealing with mains power, make no assumptions." +1 – Arvo Jul 3 at 7:19
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    Just to clarify, they are on the same circuit (a single circuit breaker switch shuts off power to both the UPS and the lights I am swapping out the switch on). While there are many good answers here, this one gets the nod for the check your assumptions bit. – asgallant Jul 3 at 15:51
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    I have heard horror stories of IT staff who chose to hit the emergency power-off button for the data center to shut it down for routine maintenance, only to find that the emergency power-off button wasn't actually connected properly! – Cort Ammon Jul 3 at 19:45
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    @CortAmmon I'm not sure what's more horrifying - that the e-stop was faulty or that some lazy git decided this was an appropriate way to gracefully shut down a data centre for regular maintenance. A bit like a fighter pilot just ejecting because landing is too much work. – J... Jul 4 at 12:03
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    @J... In the spirit of Chuck Yegar: pack your own parachute. (I can only assume he shut down the important computers properlry first, but the battery backups are live until they are told to go down) – Cort Ammon Jul 4 at 15:27
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The UPS battery won't be backfeeding into the house service. DC current won't be flowing back through the rectifier diode.

I'd be more concerned about doing something wrong with your switch replacements, turning on the power and frying your UPS board. We see this happening all the time here. Play it save and unplug the UPS and anything else on the circuit while you're working on it.

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  • It would appear that they CAN backfeed ... – Fattie Jul 4 at 3:59
  • I’ve had a USP back-feed 230 volts, discovered when I touched the plug after unplugged. – Tim Jul 4 at 9:10
  • @Tim could have been a cap discharge. I was referring to his UPS model. – JACK Jul 4 at 12:39
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    @JACK best to clarify that by saying “that model...” otherwise future visitors may be given dangerous information – Tim Jul 4 at 12:49
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    @Fattie in addition to your info, If the UPS's could backfeed into your service, then they could backfeed into the electrical grid during an outage and deliver 7000 + volts from a normal distribution transformer to a power line. There would be warning all over the UPS just like there are on generators. – JACK Jul 4 at 14:21
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It depends.

Cheaper ones have a single (or two) relays that separate the output from the input when there is no power at the input. Bad news are that the relays CAN stick and even if they are sane, a bug in the UPS firmware can engage them in a hard to predict moment.

If the UPS is one of those expensive full-conversion zero-switching time types that have a rectifier and an inverter at the input - the chance of it backfeeding something is minimal, comparable to the possibility of an unplugged laptop power supply to feed back something to the plug. Then again, one can never be sure, esp. if the UPS has "bypass mode" and associated circuity that connects output to the input and you are again at the mercy of the relay quality and the firmware sanity.

I would unplug. Everything.

p.s. Anyway, it is a bad practice to have an appliance that you cannot quickly unplug from the wall socket.

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You should not be too concerned, the UPS supplies power to the outlets on the UPS, not to the circuit it is plugged into. When the circuit is opened, the UPS will provide battery backup to the devices plugged into UPS.

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  • @Fattie he did say "should"... If things are done properly. However, we all know how easy it is to make mistakes. – FreeMan Jul 4 at 14:03
  • This sentence: "the UPS supplies power to the outlets on the UPS, not to the circuit it is plugged into" is incorrect, it seems. It seems in many cases of common faults/problems, UPS in fact DO backfeed. – Fattie Jul 4 at 16:58

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