Reset circuit that cannot be defeated by a taped button

Make the reset act on an edge, not a level, and monitor the button for being stuck. The failure being defended against is simple: somebody tapes the reset button down, or its contact welds closed. If the reset is level sensitive, the machine then re-enables the instant the guard closes, with nobody deciding anything. That is precisely the hazard the interlock exists to prevent. Two requirements, and both are ordinary: - Trigger on the falling edge of the reset input, so a held button does nothing. The operator must press and release, which is a deliberate act. - Detect a held reset. If the input is true when the safety function first becomes ready, refuse the reset and raise a fault, because the button is stuck or taped. Any certified safety relay or safety block implements both already. The mistake is nearly always in application logic that adds its own reset on top and does it with a plain contact. While you are there, put the reset where the operator can see the hazard from it. A reset button around a corner from the machine defeats the point of a manual reset entirely.