By reliability, I am referring to longevity of MCU - not being damaged by surges/spike & other common events in industrial automation systems. Which is why I opted to use photocouplers for my MCU IOs, but didn't' know common GND still could cause an issue.
My 3.3V circuit is powered through an LDO which is fed from main 24V supply. I am looking for ways to protect my 3.3V GND at minimum cost, without external 3.3V power source.