Voltage is produced across the transformer winding. ONLY one side of each of the windings is connected to the similar side of the other two windings. This junction of the common side of all three windings is called the neutral. The other sides of the three windings are the individual phases (A, B, & C). Usually the neutral is connected to the local ground.
If the non neutral side of the winding (phase A, B, or C) is connected to the same point as the neutral side(ground), THEN you have a ground fault. Depending on the total resistance of all the wires and connections and available power, you will probably have some fireworks until the overcurrent protection opens up the circuit.
I think the important thing to remember is a transformer is a separately derived source and does not have a ground reference at all until you establish one. That is not a fault. From them on any other contact with ground will be a ground fault. If the neutral faults to ground, it takes ground fault protection or inspection to detect it but an ungrounded conductor faults you operate an OC device, usually with some fireworks.
Article 100 Definitions. Ground-Fault Current Path.
Ground-fault current paths include equipment grounding conductors, metallic equipment, and any other electrically conductive material.
Ground-fault current paths include equipment grounding conductors, metallic equipment, and any other electrically conductive material. The term “ground-fault current path” is now included in Article 100 of the 2014 NEC. In previous editions of the NEC, this term was defined in Article 250.2. Because the term is used in two or more Articles, the definition and its accompanying Informational Note were relocated without change to Article 100.
A “ground-fault current path” is defined as an electrically conductive path from the place where a ground fault occurs in an electrical system to the electrical source.
A “ground-fault current path” is very different than an “effective ground-fault current path.” An effective ground-fault current path is intentionally constructed and low impedance. Its purpose is to carry ground-fault current back to the source so the circuit breaker on the faulted circuit will trip. A “ground-fault current path” may not be intentionally created, but it is the path of least resistance.
Current wants to go to back to the source, and it will get there along any path that leads back to the source but the majority of it will always flow along the path with the lowest resistance. Fault current will travel on building steel, raceways, or any metal surface. If lightning hits a tree, the ground-fault current path is through the tree to the earth. The human body can also become part of the ground-fault current path, sometimes with fatal results.
Fault current may return to the source through conductive materials that are not part of the electrical system such as equipment enclosures, metal gutters, drain pipes, metal roofing, and metal siding.
Although grounded electrical systems include a path for ground-fault current such as equipment grounding conductors, the fault current will take any electrically conductive low resistance path back to a grounded source of electrical energy.