Why does a Transformer on the 20th Floor need a GEC?

[infinity]

I often wondered why you would need to run a GEC to building steel for a Delta/Wye transformer that's located inside of a building? I always assumed it had to do with 250.4(A)(1) but do any of the things mentioned in that section really apply to a transformer on the 20th floor? Any faults on the secondary side of the transformer will be carried back to the system via the system bonding jumper. Is this all about voltage stabilization or am I missing something?

(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that
are grounded shall be connected to earth in a manner that
will limit the voltage imposed by lightning, line surges, or
unintentional contact with higher-voltage lines and that will
stabilize the voltage to earth during normal operation.

 

[iwire]

I like the neutral of my SDS to be as close to 0 VAC to building steel as it can be. Relaying on the feeders EGC to do that could increase the impedance.

 

[don_resqcapt19]

I like the neutral of my SDS to be as close to 0 VAC to building steel as it can be. Relaying on the feeders EGC to do that could increase the impedance.

The only way the building steel and the neural of the SDS that has been bonded to the primary EGC will have a difference in potential is if either ( or both) the steel and EGC have current flowing to create a voltage drop.

I see no technical (only a code need) need for a GEC for a transformer that is installed in the same building as the service equipment and where the transformer feeder originates at the service equipment.