The E-Stop That Restarted Automatically: Safety Circuit Reset Logic, Not the Relay

An e-stop button pressed. The machine stopped. Two seconds later, it restarted on its own. The customer thought the safety relay was defective. The relay was fine. The PLC logic was wrong — it automatically reset the safety circuit. This is about safety reset logic, not about the relay being broken.

The safety reset rules

Per ISO 13849 and ISO 12100, a safety stop must require a manual reset. The machine must not restart automatically after a safety event. The operator must press a reset button. This prevents a machine from restarting while someone is in the cell (e.g., after an e-stop, an operator clears a jam, and the machine restarts before they’re out).

What was wrong

The PLC program had a timer: after the e-stop was released (the button was pulled back out), the PLC waited 2 seconds and then energized the safety relay. The machine restarted. The operator didn’t press reset. The PLC did it automatically. This is a safety violation. The safety relay didn’t cause this — the PLC logic did.

What we changed

1. Removed the auto-reset timer. The PLC no longer resets the safety circuit on its own. The safety relay stays locked out until the operator presses a physical reset button.

2. Added a reset button. We installed a physical reset button (green, illuminated). The operator must press it. The PLC reads the reset signal and energizes the safety relay. The machine doesn’t restart until the button is pressed.

3. Added a cycle start interlock. Even after reset, the machine doesn’t start moving. The operator must press the start button (a separate, physical button). The reset just clears the safety lockout. The start button initiates the cycle. Two separate actions.

4. The safety relay’s manual monitor reset. Most Category 3 safety relays have a built-in reset circuit. The relay itself requires a reset signal (from the PLC or a physical button) before it closes the safety contacts. We wired the relay’s reset input to the physical reset button. The relay can’t auto-reset.

The e-stop rule: A safety stop must require a manual reset. No auto-restart timer. The operator presses reset, then start. Two separate actions. The e-stop that restarted automatically wasn’t a bad relay — it was bad PLC logic. Safety circuits don’t self-reset. That’s the whole point.