I see where you're going here, a "Coil Blowover" situation!!! [is this correct?]
The situation would result with an Autotransformer connection - with the bonding connections being made to the "H2" lead of the primary coil, and the "X0" center tap of the secondary coil.
This is not something that could be thought of as "Rarely Happening", or "Not Very Likely To Happen", but more like "In Case It Happens", or "When It Happens".
This connection would function normally, as viewed on the primary side, but have catastrophic results on the secondary side.
As to your Q's regarding the output voltages from this type of coil blowover, here are some numbers for a 7.2 KV pri x 240/120 center tap:
- H1 to X1 = 7320 VAC,
- H1 to X2 = 7080 VAC,
- X1 to X2 = 240 VAC,
- X0 to X1 = 7320 VAC,
- X0 to X2 = 7080 VAC
If the system is grounded on the secondary side, then there's less of a hazard for persons to be shocked from one line to ground.
This is the primary reason for grounding the secondary at both the transformer and the service - it reduces the potential difference that can be created between one line and ground by physically driving a conductor to the local ground level.
This helps reduce the levels of voltage and current that could be imposed on a person from just unplugging something, or flicking on a switch, while the system is in an overvoltage state as described here.
It kind of creates "Series-Parallel" connections on a person that would grab a cord, flick a switch, or similar - so their is a much simpler path to the ground reference through the physical ground connection at the service, than thru some unsuspecting person.
Wish I could explain things better, but this is the primary motivation behind grounding the low voltage AC supply.
Lightning is the next reason, but not necessarily the primary one [kind of a residual effect of the first one!].
Maybe another member can explain more clearly than I can!
My messages, in a Nutshell, are too much
, resulting in
Just a little