> There is NO hot plugging going on. Sure. People do not make mistakes and if they do - they always tell you. And all the wiring and connectors do not fail, ever. :D This could be the case of GND fail while GND of unpowered slave board collects last. This is only scenario I can imagine for your circuit at the moment. You shall test this scenario in your lab unless you have better idea of what's wrong. >Ok let say it is ESD. What class of ESD protection are you aiming at? As you name PLC then it's industrial application, shall be better than HBM, I would say TLP. Oh, and you cannot guess about ESD or verify it in the forum. You shall TEST it - using ESD tester. When your device and all it's open parts including contactors survives ESD tests - only then you can be sure that ESD is ok.
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