The E-Stop That Restarted Automatically
We wired an e-stop circuit. When the operator hit the e-stop, the machine stopped. But when the e-stop was released (twisted to reset), the machine restarted on its own. The operator hadn’t cleared the fault or pressed “start.” The problem: the safety relay had an automatic reset (auto-monitored reset). When the e-stop released, the relay reset itself and the machine ran. We switched to a manual monitored reset: the operator must press a separate reset button (after the e-stop is released and the fault is clear) to re-enable the safety circuit. The machine doesn’t restart until the operator deliberately resets. The mistake was using auto-reset on a safety circuit.
E-stop reset and monitor design prevents unexpected restart. After an e-stop, the machine must not restart by itself. This article covers the circuit.
Why Auto-Reset Is Dangerous
When the e-stop is pressed, the machine stops. The operator might be reaching into the machine to clear a jam. If the e-stop is released (or the machine loses power and comes back), auto-reset restarts the machine. The operator’s hand is still in there. That’s a hazard.
The safety rules (ISO 13849-1) require a manual monitored reset: the machine must not restart until the operator deliberately presses a reset button (after confirming the hazard is gone).
The Monitored Reset Circuit
The safety relay (article 64) has two reset modes:
- Auto-reset (NOT allowed for e-stop): The relay resets automatically when the e-stop releases. The machine restarts. Dangerous.
- Manual monitored reset (required): The relay stays off after the e-stop releases. The operator must press a separate reset button (momentary) to re-enable the safety circuit. Then the machine can start.
The monitored reset requires the reset button to be pressed after the e-stop is released. If the operator presses reset while the e-stop is still pressed, nothing happens (the relay doesn’t reset). This confirms the e-stop is actually released.
The e-stop rule: Use a manual monitored reset, not auto-reset. The machine that restarted by itself had an auto-reset safety relay. After an e-stop, the operator must: (1) release the e-stop, (2) press a separate reset button, (3) press start. The machine doesn’t restart by itself.
Step 1: The Reset Button
The reset button is a separate (green or blue) momentary button. It’s labeled “RESET.” It’s not the same as the start button.
The operator sequence after an e-stop:
- The e-stop is pressed (machine stopped).
- The operator clears the hazard (jam, fault).
- The operator releases the e-stop (twists it).
- The operator presses RESET (the safety relay re-enables).
- The operator presses START (the machine runs).
Steps 3 and 4 are separate. The machine doesn’t start until the operator deliberately resets and starts.
Step 2: The Safety Relay Monitor
The safety relay (article 64) monitors the e-stop circuit. It checks:
- Channel 1 and Channel 2 (dual channel): The e-stop has two contacts (normally closed). The relay monitors both. If one welds (sticks closed), the relay detects it and faults (won’t reset).
- Reset circuit: The relay waits for the reset button (after the e-stop releases). It won’t auto-reset.
This is the “monitor” in “e-stop monitor.” The relay self-checks the safety circuit every time it resets. If a contact is welded or a wire is broken, the relay faults (and the machine won’t run).
Step 3: Reset After Power Loss
If the machine loses power (circuit breaker trips, power failure), the safety relay drops out. When power returns, the relay doesn’t auto-reset. The operator must press RESET again. This prevents the machine from restarting when the power comes back (which could happen while someone is servicing it).
This is the “power-loss reset” — a standard safety requirement. The machine is dead until someone deliberately resets it.
Reset After a Fault (Not Just E-Stop)
Other faults (overtemperature, overload, guard open) also require a reset. The fault must be cleared, then the operator resets. The machine doesn’t restart on its own.
The HMI shows the fault (article 97). The operator reads it, clears it, then presses reset. The reset button (or the HMI reset) acknowledges the fault.
| Condition | Reset Action |
|---|---|
| E-stop pressed | Release e-stop, press RESET, press START |
| Power loss | Restore power, press RESET (no auto-start) |
| Guard door opened | Close door, press RESET |
| Fault (overload, overtemp) | Clear cause, press RESET |
The Reset Button Location
The reset button is on the operator panel (next to the start/stop). It’s green (or blue), labeled “RESET.” It’s not hidden (the operator must find it after an e-stop).
Don’t use the e-stop’s release as the reset. The e-stop is a safety device (twist to release). The reset is a separate, deliberate action.
A Reset Circuit Checklist
- Is the e-stop circuit dual-channel? (Two contacts?)
- Does the safety relay use manual monitored reset? (Not auto?)
- Is the reset button separate? (Green/blue, labeled RESET?)
- Does the machine not restart when the e-stop releases? (Until reset?)
- Does the machine not restart after power loss? (Requires reset?)
- Does the safety relay detect a welded contact? (Dual channel monitor?)
- Does the HMI show the fault? (Operator knows what to clear?)
- Is the reset button accessible? (Not hidden?)
- Does reset work only after the e-stop is released? (Monitored?)
- Does the start button not bypass the reset? (Separate?)
- Is the reset sequence documented? (Operator trained?)
- Was this tested? (Press e-stop, release, no restart until reset?)
The Bottom Line
E-stop reset and monitor design prevents unexpected restart. The machine that restarted automatically had an auto-reset safety relay. Use a manual monitored reset: after the e-stop is released, the operator must press a separate RESET button (and then START). The safety relay monitors the dual-channel e-stop (detects welded contacts). After power loss, the machine stays dead until reset. The machine that’s safe after an e-stop wasn’t the one that restarted fastest — it required a deliberate reset.
