Everyone:
A few minutes ago, I was out in the yard pulling up the 1/2 emt pipes from my last test. While doing so, it dawned on me:
This is not a ground rod test.
It is a test of how much current can flow through the soil at my building under a 120 volt fault with earth as part of the path.
Somehow I let the discussion diverge to ground rod resistance. I purposely did not use a ground rod because I was trying to mimick some goofy possibility of a hot wire going to ground (meaning the earth). If I had used an 8 foot ground rod, most certainly one of you would have hit me with "Yeah, but when have you ever seen a 120 volt circuit get mashed against a ground rod? This isn't a real test of anything."
The reason the thread diverged to ground rod resistance is because it is this resistance that ohms laws will tell us how much current will flow in a given circuit, of a certain voltage, and when this is known you would have had your answer.
In your original post you asked "Can a ground fault through dirt trip the OCPD? "
While you got your answer of "yes if the resistance of the contact point is low enough" yes it can open an OCPD, but you also got the answer that because of the many types of soils and other circumstances that with these variables it can not be safely depended upon to always serve this function, so from a safety stand point the answer would be "no".
Now while at it, it does allow an easy method of finding the resistance of a ground rod, while not an accurate method, it serves to find how much current will pass through this point on its way back to the transformer, thus giving you an approximate of the resistance value of it's connection to Earth.
remember after you are 6' from the rod almost all the resistance of the connection point has diminished, and for the rest of the pathway back to the transformer will be at the given Earth over all resistance of 0 ohms, so it is only with in this close proximity of the rod that other metal pathways will even interfere with your over all measurement using the voltage injection method.
that close proximity of the rod is called "the sphere of influence" and there usually are two of them in each Earth circuit, one under the transformer pole, and one under the rod. If your rod is close (within the sphere) to other metallic paths also bonded to the X0 of the transformer then there can be more which can lower the resistance reading. but in general it will be toward the transformer pole.:wink: