For an ungrounded system with a delta-wye secondary:
Is an EGC required?
Yup, sure is.
Is the EGC connected through the supply transformer? Such that the EGC in the building is connected to the ground(?) of the supply line.
Yes- only difference is there is no connection from the grounding block to an X0 on the trafo. Everything else is the same like on a grounded system.
If the EGC is required, whats the point of it? If there is a single phase line to ground fault then not enough current will flow to trip the breaker.
Picture it like this:
First, When one phase faults, the frame will be energized and will pass some level of current to earth, building steel, ect- that amount of current depends on the size and voltage of the system. Larger the systems the more capacitance between conductors and earth and as such more current flow. Large enough currents can shock or even kill.
Second- even if the currents are small enough (ie say 4ma), you still need an EGC. Reason being that if a phase faults down in a piece of equipment it will not open an OCPD and remain there until fixed. During this time if a phase faults down else where, you now have two objects in the same building at 240 or 480 volts potential relative to each other. Its possible for a person to touch the faulted equipment, have current flow through him, through the floor/earth/ect all the way to the other piece of faulted equipment. That current could be very large depending on the path resistance. Similarly there could be 2 pieces of faulted equipment in the same room, or a fault to the plumbing and something next to a sink- a person could contact one with one hand and another object with the other hand- that would make 240 or 480 across the chest. However, it doesnt stop there. If there is anything interconnecting the two pieces of faulted equipment- or even some high impedance path through building material- the current may be high enough to create enough heat to start a fire yet the current is low enough (from the resistance) to never trip the breakers associated with those circuits. If the the current path is through coaxial cable, a signaling circuit, ect it certainly is possible for the shields on those cables to burn. Even low levels of current passing through wood for years on end can lead to pyroforic carbonization.*
Summed up you do not want 2 or more objects in a building be it plumbing, conduit, switch boxes, motors, tools, ect to be at 240 or 480 volt potentials relative to each other.
Third- You need a good reference for your ground detector. The whole point of an ungrounded system is service continuity and as such a chance to repair a fault before another one occurs- thus the code mandates a ground detector. That detector needs a reliable circuit- and by having a low impedance circuit to ever piece of metal that could become contact by a phase conductor the detector will instantly and reliably signal a fault has occurred in the system.
