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I'm designing a system that uses a power supply to convert 120V AC current from the wall socket to 12V DC current. I'm looking at a Jameco SP-240-12.

enter image description here

Originally I thought pins 4 and 5 output negative DC current, meaning I would have to ground my circuit elsewhere. After seeing the datasheet though, I realized that DC- are grounding points.

But that got me thinking, from my understanding no current runs through the floating ground normally and it is just there as a safety feature. Is it possible that I could have used pin 3 and grounded DC current on an AC floating ground?

Edit: Originally I was unclear, I was asking if I can ground my DC devices to pin 3, even though it gets connected to the "frame ground" of an AC plug, like shown in the image below:

enter image description here

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    \$\begingroup\$ It's unclear what you are asking - can you draw how you would intend to wire it up? What do you mean by "AC floating ground". Also, although the floating ground is called floating there will indeed be currents flowing through this pin in order to reduce EMC problems. \$\endgroup\$ – Andy aka Jul 4 '17 at 19:03
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Terminal Pin No. Assignment (from OP's screenshot):

  1. AC Live
  2. AC Neutral
  3. Frame Ground (not "floating ground").
  4. DC output -V
  5. DC output -V
  6. DC output +V
  7. DC output +V

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Three output configurations are possible: floating, grounded negative or grounded positive.

... no current runs through the floating ground normally and it is just there as a safety feature.

It's not "floating ground" it's frame or chassis ground. This must be connected to mains earth as the power supply has a metal chassis. Failure to do this correctly could result in a live case. (e.g., If one of the heatsink insulation kits breaks down.)

You have the option of letting the output float with respect to ground or tie either positive or negative to ground. In most cases we tie negative to ground.

enter image description here

Figure 2. Datasheet entry for grounding continuity shows that it is specified at < 100 mΩ to chassis but that the test result was 8 mΩ. (PE stands for "protective earth".)

I was asking if I can ground my DC devices to pin 3, even though it gets connected to the "frame ground" of an AC plug, like shown in the image ...

No. There is no internal connection between FG and -V so there would be no current return path to your power supply. Connect your return path to -V and link the second -V to FG if required.

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