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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)
asgallant
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4 Answers4

13

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.

JACK
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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.

Criggie
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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.

fraxinus
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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.

ThreePhaseEel
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Programmer66
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