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I have an ac circuit with 3 ground wires. This circuit switches between two ac power sources, accounting for two of the ground wires. The last ground wire comes from a switch. Is it safe to pigtail these ground wires together and connect them to a ground terminal on an electric receptacle?

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    Do you have a schematic? I'm that type of person that has to see what you're talking about in order to understand it :) – KingDuken May 17 '18 at 15:15
  • Are the "two ac power sources" from the same panel? – Dan D. May 17 '18 at 16:36
  • What country is this located in? (Electrical standards do vary by region of the world). Pictures of what’s going on here may help considerably, so far it sounds messy—especially if this involves two different services. – Tyson May 17 '18 at 16:47
  • My greatest concern is grounds from two different AC sources. If bonded upstream then you could mix them freely. If they are not bonded upstream you might unknowingly be creating danger without realizing it. Keep in mind that 99.99% of the time bridging two grounding systems probably won’t pose everyday problems, but you must always remember and consider the .01% of the time when excessive fault current might be present. I’ve seen an old satellite tv installation where a ground bridge was created and #10 ground actually burned and disintegrated for 50 feet when utility neutral dropped. – Tyson May 17 '18 at 17:39
  • When you use “ac” is that an abbreviation for “Air Conditioner”, or “Alternating Current”? – Tyson May 17 '18 at 18:20
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That would be usual and standard practice. Just make sure that it goes back to a quality ground in a panel. etc.

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    Actually after reading the question several times, I’m not sure how you arrive at this “would be usual and standard practice” because it’s nearly impossible to tell what’s going on based on what’s been presented so far. – Tyson May 17 '18 at 16:44
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Anytime a question comes over from electronics.se, we have to pause to get clear on terminology. In mains power, "Ground" is not GND/Vss/normal current return.

Mains power has "Ground" as the Equipment Safety Earthing. Only a horrible malfunction should cause current to flow, and then hopefully instead of through a human. That's important enough that it gets its own wire, and that wire is not used for anything else.

And then we follow through by actually spiking that into the earth, which also helps natural electricity (lightning, ESD) return to its natural source.

Your working conductors that are part of the circuit are obviously not ground. If one of them is intentionally designed to be near ground potential, it gets called "neutral".

When you are switching two separate power sources, you must switch all the working conductors, including neutral.

Now, there's one more thing. When neutral is intentionally rigged to be near ground potential, that's done by bonding in a very specific location, as close as possible to the source. The bonding is just a wire, nothing fancier than that. If you have two separate power sources, and both of them have a neutral-ground bond, and one of those neutral wires fails in the wrong location, then the current will return via a circuitous path through the other neutral-ground bond. Even if the wires don't have the capacity for that much current - traditionally neutrals are not fused, and grounds shouldn't be fused. This is Rather Bad, and it's why having two neutral-ground bonds in a system is super dangerous. So you need to select your sources with care.

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