1) A separately derived source of power is created with most transformers used in all commercial power systems. A Separately Derived Source, while having a specific voltage between the various derived legs, has no particular voltage relationship between its legs and anything else in the universe.
That means it can ultimately have any voltage potential between its legs and the earth. How? Static charge from blowing wind, inductive coupling of nearby circuits, lightning, capacitive coupling to nearby circuits, etc. A high potential difference raises the possibility of an unexpected and dangerous current flow. Typically this can be because the insulation system designed for the voltages of the transformer's output can be exceeded pretty easily. Grounding literally puts-a-stake-in-the-ground on the ultimate potential limits seen by the system's insulation and any the world around it. This is why ALL systems must have a ground reference of some sort.
The common Delta power system has no obvious potential centric aspect so to meet (1) above one phase can be grounded or often the middle of one phase is grounded. This constrains all the rest of the phases to be at a particular potential above earth and no higher.
2) Yes, earth resistance can change and is different everywhere. When the ground plane of a power distribution system is contemplated the earth grounding system is designed after earth resistivity in the area is studied. Wetter ground is more conductive than dry ground where a lot of troubles can haunt installing an effective ground system.
The greater world at-large has a very low overall resistance. This can be noted by the practice of places like South Africa where a hundred mile high power transmission system often uses the earth as one of the two conductors.
Keith Cress
kcress -
