Yes, the 48v power supply is connected to two stepper drives and two servo drives. If the PC PSU is disconnected from the servo drives, then the main 48v power supply has no connection to earth which is why I am thinking it is the PC PSU which is at fault.
The PC PSU powers a BOB with 5v, a couple of 12v fans and provides the 12v to the servo drives. Are there any other measurments I can take to verify the PCPSU is at fault before I strip it out?