Thank you for all the advice.

I was begining to doubt myself as fuses would run one rack and blow in the other.
I removed all the modules and discovered the transformer on the power supply had
overheated and melted.

My understanding is that this is pretty uncommon but can happen. It certainly explains
why I thought that I was not understanding fuses.

I ordered a PSUII from Analog Haven and all will be back to normal soon.

TD