Wow, that's quite a welding job someone did there. I wonder what module was involved in that overload. Other than a power supply, is there something else that might pull so much current?
Clue #1 - it's the High Power compartment. It's the High Power compartment because, well, it can furnish more. It's hard to say what someone did - you know people can do some really dumb things (I know because I've done plenty myself). It also could have been a failed plug-in. Any plug in can go in the High Power slot, but of course it won't draw more power just because it's there.
The PS503 et. al. is designed to take advantage of extra pins on the back plane - I'd have to check the drawings to see what is furnished. There are fuses as well behind that backplane but all are OK ...
I do remember using these for years in industry that we had a lot of issues with the pads on the plug-ins getting dirty. We'd pull them out and push them back in (well I saw others slam them back in
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) till the connections were made. I do remember cleaning the pads on the units, but I do not remember that we ever cleaned the backplane connector. Unfortunately, for the components, when you can just go to the test equipment room and get another one, little care is given to them out there in the field. You had better things to do like performing the testing.
It's possible that a plug in had some dirty contacts and presented a high resistance connection, while at the same time drawing a lot of current through it. Some plug-in may have been in that slot for years, had some abuse, and when finally removed took out all those pins. We'll never know I suppose...
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