Recently a Major automotive parts manufacturer had their production line down after an unknown fault on a machine caused all four of their Siemens S7-300 PLCs to fail.
After repairing the PLCs, AFI Systems went our usual step further, and decided to see if we could not only understand what caused the initial fault and resulting damage that occurred, but to figure out how to stop it happening again. After a thorough on-site inspection, we realised that an overvoltage condition on an analog input module was causing the repetitive failures and risking an identical failure of the PLCs in the future.
Our solution was to design a new circuit board with an overvoltage protection diode combined with a fuse & fuseholder. The custom circuit would sit between the analog module and the PLCs. If the overvoltage condition ever occurred again, it would simply pop the fuse first and they could plug in a new fuse instead of it rendering any or all of the PLCs useless.