Dang, Scott - you sure get some Hellish things to shoot trouble on / Job walk fun!
But this is the "Glamorous Life" of the Project Manager, so we grin and bear it (ugggh!).
Anyhow, you have the right idea on tracing out the Ground Fault(s).
Hunt and Peck away on de-energized Branch Circuits until your Continuity Tester yells "Bingo", then search for the Fault, or Faults
(might need the Mounties to assist on this one, so call for Dudley Do-Right
You will definitely want to isolate the Circuitry from any PoCo Line Side stuff, or anything connected to Customer Owned Transformers (like the one in the second image), as these items may inaccurately show L-G connections.
(if this sounds odd, post a "Whaddaheck???" message for explanations).
Seeing how many older roof top conduit runs are found in an "Accordion Fashion", it's an easy bet that you will find a few Ground Faults up there.
Those Ground Faults on Grounded Systems are really fun - especially when the Equipment Bonding means back to the Panelboard has an "Unintentional Impedance" placed in series (poor connections), and when the OCPDs for Branch Circuitry has an Inverse-Time Trip Characteristic which allows blazing Faults to continue "much too long"
Got to see this a few times, and "Funny" was not the overall general feeling everyone had!
They were quite interesting, though!
So, to sum things up - Ring the Branch Circuitry out until no _Solid_ Ground connections exist (on Circuits which should not have a solid ground connection), then do a "Smoke Test".
BTW, are you sure the system is an Ungrounded Delta?
Although the Ungrounded flavors do exist in a 240 VAC version, I have seen a high majority of the 240 VAC Deltas being Corner Grounded, and nearly all the 480 & 600 Volt Deltas to be Ungrounded.
Just a thought.
edited tew git shed of baad speeellinn
[This message has been edited by Scott35 (edited 06-17-2006).]