Each major process line has run under PLC control for many years. A few years ago, you started seeing PLC module failures. These haven't come anywhere near the published mean-time-before-failure specs, so time in service isn't a factor.
After a couple of power supply modules failed, you used a power analyzer to identify anomalies but didn't find any. Someone suggested that maybe dirt was accumulating in the cabinets and causing overheating. However, all of the PMs have a line item "clean cabinet interior," and it's obvious that techs are wiping the bottom of the cabinets during PMs. Another person suggested grounding and bonding issues, but the systems comply with the NEC Art. 250 Part V and other standards.
Where do you go from here?
Because you've ruled out electrical anomalies, that leaves the other great enemy of electronics: heat. Excess heat often can go unnoticed because you vent the cabinet as soon as you open the door. Overheating can be localized within a cabinet (e.g., within a power supply), so cabinet temperature monitoring might not reveal it.
A commonly overlooked cause of overheating is dust. Yes, you found the cabinet floors were free of a significant layer of dust, dirt, and grime, but that doesn't mean dust isn't causing those failures.
If you look carefully around the door seal, you'll probably see a “spray" of dust. No seal is perfect. Even with positive pressure in the cabinet, dust can migrate into it (airflow past a seal can pull dust through it). Small dust particles get around and through air intake filters, and dust comes in through enclosure openings for raceway.
Over time, this dust accumulates in various components. It accumulates on power supply vents and fan blades, reducing airflow while thermally insulating — a double whammy. Conductive dust provides a triple whammy.
The solution is to expand the dust removal portion of your PMs to more than a swipe at the cabinet floor with a rag. You need a methodical cleaning with compressed air to remove dust (blow the dust in deliberate paths). Don't use plant air, as it contains oil. Canned air is acceptable, though in some facilities it makes more economic sense to have an oil-free compressor. Required PPE: dust mask and non-vented goggles. Clean cabinet exteriors as well.
PM frequency depends on the amount of dust in the environment. Start with quarterly, then adjust as conditions warrant.