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Basically, I have a 120V device I would like to have powered if either/both of two outlets have power. So both could be on, or just one of them.

The device itself is typical in that it only has one power cord.

Two questions:

  1. How do I wire this correctly? Assuming I have to use these two 120V outlets and a single power cable into the device.

  2. How could this go horribly wrong? Although this may sound absurd, for the sake of my learning, say I "split" the power cable and connected it to each outlet. I assume this would end up providing much more voltage than I wanted and damage the device? I would get some varying voltage based on the phase of each outlet? Or am I horribly confused?

Of course I really need #1 answered, but I'd love some insight on #2.

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Could go horribly wrong in many ways:

  • if you chose two outlets on different phases
  • touch the pins on a disconnected plug whilst the other is plugged
  • bring up an isolated mains ring unexpectedly whilst someone if working on it

I've heard two-plug devices called "widow-makers" due to the obvious dangers. e.g. http://www.diynot.com/forums/electrics/widow-maker.309251/ where the example is for rigging generators. Also seen them used for running large PA systems from domestic sockets.

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You could use something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Of course, you could use whatever relay you want (even small voltage DC relay with adequate power supply).

How it works:

If powered from plug 2, power gets to the device via the NC switch of the relay. Relay is not powered. If powered from plug 1, relay is triggered and switches device's power to plug 1.

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    \$\begingroup\$ Using a relay fed directly by the power would seem to make sense to me! \$\endgroup\$ – Wouter van Ooijen Jul 30 '14 at 15:04
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    \$\begingroup\$ Looks good. Keep in mind that this is not a UPS- if you unplug Plug 1 there will be an interruption in power to the devices while the power transfers. Depending on the relay and how good the power supply filtering is (and other factors) that may or may not be enough of an interruption to affect the device. As an engineer, I'm compelled to mention the worst possible outcome which would be that the device gets enough interruption to screw up operation, but not enough to properly reset (a lot of products have poor reset circuits). \$\endgroup\$ – Spehro Pefhany Jul 30 '14 at 16:02

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