Nick, you might take another look at 250.97 to see that you need a bond bushing for the primary side of the XFMR.
As for the 'transmission of vibration or noise' issue that requires one to use flex, that info is usually part of the transformer installation instructions.
Assuming you use an equipment grounding conductor on the primary side, which also connects through the bond bushing lug, fault current on the primary side will conduct through the low resistance grounding conductor and eventually back to the service neutral to close the primary side hot-to- neutral loop and trip the breaker feeding the transformer (hopefully).
On the secondary side, the equipment bonding conductor going from the transformer case to the equipment ground bus in the load side panel, if connected to the bond bushing at the panel end, will conduct fault current to the equipment ground bus, through the main bonding jumper of the separately derived system, on to the grounded conductor (neutral), back through the flex to Xo and complete the hot-to-neutral loop. This enables the fault current on the secondary to create an overcurrent situation on the primary and thus trip the supply breaker of the transformer. I put the bond bushing at the secondary panel end of the flex to shorten the fault current path and also to avoid having the choke effect. Don't let the info you undoubtedly have about 98% of fault current traveling on the raceway enclosing a grounding electrode conductor get mixed up in this scenario. If you connect the equipment bonding conductor at a bond bushing at the load panel end you won't have any net magnetic fields to create that choke effect.
[This message has been edited by Elzappr (edited 01-12-2002).]
