210622-0956 EDT
Zalkon:
I really do not understand what you are really asking in post #1.
Virtually all commercial power generated in the US is 3 phase at the point of origin. This is stepped up to a higher voltage for distribution. Then back down to a lower user level. At the user level it will be either a wye or delta. How this is broken up at the user level can be of several forms.
At my home the primary distribution is a three wire delta source. There is no neutral on the poles. Back at the substation is it really a wye source, or some other way to reference it to earth. Doesn't matter to me because all that is on my poles is 3 phase delta.
To power my home there is a single phase input 50 kVA transformer tied to two of the primary delta wires. So two insulated primary posts on my transformer. On the secondary is a center tapped winding with the neutral wire connected to a ground rod at the transformer pole. That secondary is described as a single phase source. I will accept that general description, but when you look at it using the neutral point as a reference, then it is really a two phase source. But don't let that confuse you as most electricians will call it single phase. There are three wires from the transformer to my main panel, two hots, and one neutral.
If no additional grounding (earthing) was done at my main panel, then the system is still tied to ground at the transformer pole. But it is required that a second grounding be done at my main panel. In my case this is a 150 ft long 1" copper water line.
As you progress from one neighbor to another on my block their single phase power will be supplied from different three phase wire pairs so as to approximately equalize power loading on the 3 phase system.
For a very few neighbors that require 3 phase power a second transformer is added to the pole. This is connected between the third primary phase wire, and one of the two phase wires supplying the single phase transformer. The output side of these two transformers now provides 3 phase power to the customer from what is called an open delta. A third transformer could be added to make a closed delta. But that is not economical. If three phase loading increased a lot, then the power company would add a third transformer. In our area we have a lot of open delta supplies.
There are areas in our city where primary power is distributed as a wye system. This means that in various areas there are just two primary distribution wires, and single phase pole transformers with one insulated primary bushing. One primary wire is earthed, and this is also connected to the ground rod at the pole, and to the neutral of the secondary. In these areas you can not get three phase power unless another primary side wire is added.
A power company is not going to provide you with a wild leg supply unless you pay for it. A wild leg requires a second transformer vs just one transformer for regular single phase service. Some wild leg services will use three transformers. At my son's shop he has a wild leg service. The single phase service transformer is probably close to 200 kVA, and the wild leg transformer somewhat smaller. If three phase loading went up, then a third transformer would be added, but it would still remain a wild leg structure, and become a closed delta.
What did you mean by this --- "No one here has ever seen such a thing."
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