I am going to try to give all the important info first. The jobsite is old factory building with a 480v ungrounded messenger supported overhead feeder assy consisting of 3 runs of 750 mcm copper travelling about 450 foot into a seperately tapped 200 amp 480v fused disconnect. the fuses are 200 amp 600v. Then, there is a 25' 100 amp tapped feeder going into a 100 amp rated fused 600v 3 phase disconnect. At this point, with no load, voltages are a-b 480, b-c 473, a-c 473. The 100 amp primary disconnect is fused at 90 amps and draws, with full load, 23.5 amps, 32.5 amps, and 29 amps 480v. The primary conductors then goes into a 45 kva 480/208y transformer with 1/0 secondary conductors to 200 amp 208 disconnect fused at 125 amp. The secondary volt readings(208 nominal) with no load are 208, 205, and 203. With 60 amp load the voltages decrease to 206, 203, and 199. My thinking was the loads are too much for the voltage drop but fixing the voltage drop by running new conductors back to the front panelboard service source was not economically feasible. My plan was to install a 30 kva delta delta mid point grounded 30 kva transformer to power all the 240v single phase loads off of to decrease the significance of the voltage drop for the 240v lights and a/c loads. This would then help the 45kva panel also due to significant decrease of loads.
The strange occurance happened at some point yesterday when all that had been completed is increasing the load side 45 kva transformer conductor to 1/0 copper and changing out the overheated initial 100 amp 240v fusible disconnect to a 200 amp 240v disconnect and increased the size fuses from 90 amp to 125 amp. Upon receiving a frantic phone call this morning stating that there was no power in his unit, both myself and my mentor went down to discover that phase 3 of the 480v primary disconnect had a blown fuse which was producing no power on leg 3. I examined the newly installed primary disconnect and saw no obvious signs of overheating or corrosion. Upon opening the 45 kva transformer, all connections were tight with no discoloration anywhere in the transformer terminal bar. There were no obvious signs of why the previously never blown primary 90 amp fuse had opened.
After 2 hours of continuous running, we could not replicate the failure and planned on continuing the initial plan of installing the new 30 kva delta delta transformer, Has anyone ever had a similar situation causing a primary overload device to fail with no obvious signs of any problems in the secondary side. In addition to turning off the secondary disconnect for testing, I removed all conductors from their respective terminals to verify the readings I was getting at the transformer termination lugs were the same that were at their respective terminals with no change.