The Machine That Restarted by Itself
We pressed the e-stop (article 98) to stop the machine. The machine stopped. When we released the e-stop (twisted to reset), the machine restarted on its own — it continued the cycle. The operator wasn’t ready. The part was still in the way. The problem: the e-stop reset was automatic (the machine restarted as soon as the e-stop was released). The safety circuit (article 64) didn’t require a deliberate reset. We changed the circuit: after the e-stop, the machine requires a deliberate reset (a button press, by the operator). The machine stays stopped until the operator presses reset. The mistake was auto-reset on e-stop release. The operator must deliberately restart.
E-stop reset sequence requires a deliberate reset (not auto-restart). This article covers the design.
The Reset Sequence
After an e-stop (or a guard opening, article 74), the machine stops. To restart, the operator must:
- Clear the hazard (the part is out of the way, the guard is closed).
- Reset the e-stop (twist the knob).
- Press a deliberate reset button (on the panel).
- Press start (to resume the cycle).
The machine doesn’t restart automatically when the e-stop is released. The deliberate reset (step 3) confirms the operator is ready.
Step 1: No Auto-Restart
The machine that restarted by itself had auto-reset (the cycle resumed when the e-stop was released). This is dangerous — the operator might not be ready. The safety relay (article 127) must require a deliberate reset (a button press) after the e-stop. The safety relay stays in the “stopped” state until the reset button is pressed.
The reset rule: After an e-stop, the machine requires a deliberate reset (a button press). Don’t auto-restart on e-stop release. The machine that restarted by itself had auto-reset. The operator must clear the hazard, reset the e-stop, press the reset button, then press start. This is the monitored reset (article 64).
Step 2: Monitored Reset (Article 64)
The safety relay (article 64, 127) has a monitored reset. The reset button is a momentary (not held) button. The operator presses it (and releases). The safety relay checks the reset (it’s a deliberate action, not a stuck signal). If the reset button is stuck (shorted), the relay faults (won’t reset). This prevents a hidden fault.
Step 3: The Guard Must Be Closed
If the e-stop was triggered by a guard opening (article 74), the guard must be closed (and the interlock switch confirms) before the reset. The operator can’t reset with the guard open. The safety circuit (article 64) checks the guard interlock (dual channel, article 98) before allowing the reset.
Step 4: The Start Button (After Reset)
After the reset, the machine is in a “ready” state (not running). The operator presses start (a separate button) to resume the cycle. The reset and start are two different buttons. The reset only clears the fault; the start resumes the motion. Don’t combine them (the operator might press start without checking the hazard).
Step 5: The Cycle Doesn’t Resume Mid-Motion
After an e-stop (and reset), the machine doesn’t resume the cycle mid-motion. It goes to a safe state (home, or a known position) before restarting. For a robot (article 95), after an e-stop, the robot doesn’t continue the path — it waits for the operator to confirm (and then starts from a safe point). This prevents the robot from moving into a hazard (the part that was in the way).
| Step | Action |
|---|---|
| 1 | Clear the hazard (part out, guard closed) |
| 2 | Reset the e-stop (twist the knob) |
| 3 | Press the reset button (deliberate, momentary) |
| 4 | Press start (resumes the cycle, from a safe state) |
An E-Stop Reset Checklist
- Does the machine auto-restart on e-stop release? (Bad?)
- Is a deliberate reset button required? (Article 64?)
- Is the reset monitored? (Momentary, not held?)
- Is the guard closed before reset? (Interlock?)
- Is reset separate from start? (Two buttons?)
- Does the machine go to a safe state? (Home?)
- Does the robot resume from a safe point? (Article 95?)
- Is the operator trained? (Clear hazard first?)
- Does the reset button fault if stuck? (Monitored?)
- Is the reset sequence tested? (Dry run?)
The Bottom Line
E-stop reset sequence requires a deliberate reset. The machine that restarted by itself had auto-reset. After an e-stop: clear the hazard, reset the e-stop, press the reset button (monitored), then press start. The machine goes to a safe state before resuming. The machine that restarted safely wasn’t the fastest one — it required a deliberate reset.
