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No, It’s NOT a Ground!
I was walking around at the AHR conference today, and I stopped at the booth of a popular hard start kit manufacturer (especially popular with white-shirt sales techs). I listened to the guy behind the booth “training” some municipal maintenance techs on the use of his hard start kit. (To be fair, I actually think it's a decent kit.)
However, I have an issue with what he KEPT SAYING over and over. He kept calling the C terminal on the compressor and the 5 terminal on the potential relay (hard start relay) a “ground.” He even said, and I quote here, “A 2-wire kit has no ground and should not be used. If you find one like that, just tell the customer it has no ground, and they will understand it's unsafe and should be replaced.”
Common and ground are not the same. They have no relation. Common is the common point opposite the run and start terminals; it has nothing to do with ground. Ground is a connection to ground and has everything to do with creating a safe path to ground in the case of a short. It has to do with preventing YOU from becoming that path—which would kill you.
This is one way that well-meaning people become techs that end up on the news for ripping off grandma.
Once again, I actually agree that 3-wire start kits are the way to go. Just don't call common “ground” a thousand times.