Not likely to help, but I had an interesting failure one time on an SCR Bus Supply feeding a group of PowerFlex 700s drives. Because my family and friends don't care for my boring stories, but some of you might, I'd like to share.
I walked into work one night, and this particular line had been down for several hours. The SCR Bus would not power up, and would only show a RED fault light, no fault code. I continued troubleshooting. Now, nobody ever really wants to change out these 150 lb. drives/inverters/power supplies, because it's actually a pain in the butt trying to strap, winch, cart, these units into a control cabinet, with little room, which ultimately ends up with the biggest 2 Bubba's just man-handling the thing into position while you squeeze between them and say "down and to the left," while trying to start the first screw.
Especially if it does not solve the problem. So, if you're going to change one, you better at least have a reason other than "I don't know what else to do." So, I can respect the previous shifts position.
So I'm trouble-shooting and following the manual verbatim, and the manual keeps stating to "VIEW THE FAULT CODE AND TAKE RECOMMENDED ACTION." The problem was, there was no fault code displayed. Only a RED fault LED.
Now, I wish I could say I solved this because I'm smart, but the reality is, after about two hours of chasing my tail, I powered up the cabinet, and just barely noticed a super tiny twinkle of a spark on the front of the power supply. Upon further inspection, I noticed a microprocessor board, about 4 inches by four inches, that had a super tiny transformer/reactor of sorts, with a hair thin wire burned open. I busted open the crate with the spare SCR Power Supply. Removed the four 2mm screws and the ribbon cable on the microprocessor, swapped it into the existing power supply and powered up without issue. Run The Line! That board apparently handled the fault codes and power up sequence.
Felt like a hero, right? Especially because I didn't need to swap out the whole power supply.
That night I got the work order to "Replace the entire power supply unit with the spare/ and swap the microprocessors back, because we're not convinced that was the root cause."
Point is, as rare as it is, don't forget there can be a problem with the fault handler as well.