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I've got a computer that needs redundant power supplies. Unfortunately, it only has one PSU. So I want to plug my computer both into the wall and into my UPS (a battery) that's plugged into the wall. I'm thinking I could buy or splice a cable that puts the AC power supplies in parallel. As long as the frequency and phase shift of the AC supplies are equal, it should be fine, right? If so, are my UPS and house expected to supply equal frequency/phase, even if the power goes out (leaving just the UPS running) then comes back? I live in the United States.

I can't find this anywhere because it's too similar in keywords to a male-to-male AC power cable... and maybe because it's a horrible idea? Hope not.

sudo
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    That is a horrible, horrible idea. Why can't you just use wall power through the UPS? – Ignacio Vazquez-Abrams Aug 16 '16 at 03:56
  • I can, but I don't want my computer to shut down if my UPS fails. I've already had one UPS fail while the power was on, and it stopped supplying power even though the wall socket was active. So you're saying this isn't going to work? – sudo Aug 16 '16 at 04:18
  • ^ Secondly, it's useful to have two plugs in case I need to switch power supplies without taking down the computer. My server-grade computer has dual power supplies, and they have come in handy just for this reason. – sudo Aug 16 '16 at 04:26
  • Was my question off topic or something? I'm new to this exchange and don't want to be breaking rules unknowingly. Whoever downvoted, I'd appreciate a reason. – sudo Aug 16 '16 at 04:27
  • Downvoted are for a horrible idea. It's on topic, but just no. Dual power supplies have special circuitry to handle the loss of power and switch over. Redundant Power Supplies as well. – Passerby Aug 16 '16 at 04:55
  • Related: http://electronics.stackexchange.com/questions/231383/two-identical-dc-power-supplies-in-parallel-for-redundancy – MarkU Aug 16 '16 at 06:26

2 Answers2

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A simple sketch immediately shows why this is not going to work.

schematic

simulate this circuit – Schematic created using CircuitLab

  1. You can't assume that the UPS will be the same frequency as the mains. If they were even slightly off the UPS output circuitry would be destroyed.
  2. When the mains turns off the UPS will be feeding the rest of the house, the street and itself.

As well as overloading the UPS and you would be putting mains back on the grid which is not allowed.

Transistor
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Also if you have 2 male ends, that is a serious safety issue due to the potential to have exposed contacts at mains voltage.

With what you are trying to achieve, which is incorrect as others have stated the only possible explanation is to avoid UPS failure. However given I have seen several UPS faults trip the circuit breaker, you could potentially not have that as a backup anyway.

You can get modules that allow for several UPSes, concurrently or with changeover and monitoring, also need to consider such things as a using a seperate circuit or phase for each.

Your computer if it only has a single PSU is still a highly likely point of failure anyway.

D-on
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