You can't make a circuit immune to external events. Only adjust the amount of electromagnetic interference it takes to produce an error or kill the electronics.

Next thing is that some certifications don't care how well you protect your electronics. They still require proof that the device has working recovery mechanisms that can be trusted as long as you haven't actually fried the electronics. You would then be required to prove the existence of multiple independent clock sources, and that they can't affect each other when the unit is subjected to non-destroying interference.