Uploaded July 2026 | Updated September 2026, 3 weeks ago
Why does STOP always beat START in a PLC seal-in circuit? Because of where the normally-closed stop contact sits — it can always break the rung.
⚡ QUICK ANSWER
In a start/stop (seal-in) circuit, STOP always wins over START by design. A momentary start button energizes the output, which then seals itself in through a parallel contact so it stays on after you release start. The stop button is wired normally-closed and placed in series with the whole rung — so whenever stop is pressed, it opens the rung and drops the output no matter what start is doing. Because stop is in series and normally-closed, holding start down can't override it, and even a broken stop wire drops the output (fail-safe). That series placement of the normally-closed stop is exactly why stop has priority: it can always break the circuit. This is the classic latching pattern behind relay logic, contactors, and PLC ladder logic alike. The full wiring and testing walkthrough is in the guide below.
📖 FULL WRITTEN TUTORIAL + DOWNLOADS (wiring, testing, ladder logic)
accautomation.ca/wiring-and-testing-our-p1-m622-16dr-practical-tutorial
📚 WHAT YOU'LL LEARN
✅ How a seal-in (latching) circuit keeps a motor running after a momentary start
✅ Why the normally-closed stop contact goes in series with the rung
✅ Why stop always has priority over start — it can always break the circuit
✅ Why a normally-closed stop is also fail-safe (a broken wire drops the output)
❓ FREQUENTLY ASKED
Q: Why does stop have priority over start in a PLC circuit?
A: The normally-closed stop contact is in series with the whole rung, so pressing stop opens the circuit and drops the output regardless of the start button. The full walkthrough is in the guide.
Q: What is a seal-in (latching) circuit?
A: A momentary start energizes the output, which then seals itself in through a parallel contact so it stays on after start is released — until stop breaks the rung.
Q: Why wire the stop button normally-closed?
A: So the circuit is normally complete and pressing stop — or a broken stop wire — opens it and drops the output. That makes the stop fail-safe.
👉 Full wiring & testing guide: accautomation.ca/wiring-and-testing-our-p1-m622-16dr-practical-tutorial
🔗 MORE RESOURCES
▶ Series: accautomation.ca/series/productivity-plc
▶ PLC Beginners Guide: accautomation.ca/programming/plc-beginners-guide
▶ Free eBooks: accautomation.ca (subscribe for download)
🛒 RECOMMENDED HARDWARE
▶ Shop: automationdirect.com
▶ Parts Guide: accautomation.ca/purchase-find-parts
💬 Questions? Drop a comment below.
🔔 New tutorials every Monday — Subscribe and hit the bell!
#PLC #LadderLogic #IndustrialAutomation #PLCProgramming #ACCAutomation #SealIn #StartStop #MotorControl #RelayLogic #Shorts
Regards,
Garry
ACC Automation
accautomation.ca
Why does STOP always beat START in a PLC seal-in circuit? Because of where the normally-closed stop contact sits — it can always break the rung.
⚡ QUICK ANSWER
In a start/stop (seal-in) circuit, STOP always wins over START by design. A momentary start button energizes the output, which then seals itself in through a parallel contact so it stays on after you release start. The stop button is wired normally-closed and placed in series with the whole rung — so whenever stop is pressed, it opens the rung and drops the output no matter what start is doing. Because stop is in series and normally-closed, holding start down can't override it, and even a broken stop wire drops the output (fail-safe). That series placement of the normally-closed stop is exactly why stop has priority: it can always break the circuit. This is the classic latching pattern behind relay logic, contactors, and PLC ladder logic alike. The full wiring and testing walkthrough is in the guide below.
📖 FULL WRITTEN TUTORIAL + DOWNLOADS (wiring, testing, ladder logic)
accautomation.ca/wiring-and-testing-our-p1-m622-16dr-practical-tutorial
📚 WHAT YOU'LL LEARN
✅ How a seal-in (latching) circuit keeps a motor running after a momentary start
✅ Why the normally-closed stop contact goes in series with the rung
✅ Why stop always has priority over start — it can always break the circuit
✅ Why a normally-closed stop is also fail-safe (a broken wire drops the output)
❓ FREQUENTLY ASKED
Q: Why does stop have priority over start in a PLC circuit?
A: The normally-closed stop contact is in series with the whole rung, so pressing stop opens the circuit and drops the output regardless of the start button. The full walkthrough is in the guide.
Q: What is a seal-in (latching) circuit?
A: A momentary start energizes the output, which then seals itself in through a parallel contact so it stays on after start is released — until stop breaks the rung.
Q: Why wire the stop button normally-closed?
A: So the circuit is normally complete and pressing stop — or a broken stop wire — opens it and drops the output. That makes the stop fail-safe.
👉 Full wiring & testing guide: accautomation.ca/wiring-and-testing-our-p1-m622-16dr-practical-tutorial
🔗 MORE RESOURCES
▶ Series: accautomation.ca/series/productivity-plc
▶ PLC Beginners Guide: accautomation.ca/programming/plc-beginners-guide
▶ Free eBooks: accautomation.ca (subscribe for download)
🛒 RECOMMENDED HARDWARE
▶ Shop: automationdirect.com
▶ Parts Guide: accautomation.ca/purchase-find-parts
💬 Questions? Drop a comment below.
🔔 New tutorials every Monday — Subscribe and hit the bell!
#PLC #LadderLogic #IndustrialAutomation #PLCProgramming #ACCAutomation #SealIn #StartStop #MotorControl #RelayLogic #Shorts
Regards,
Garry
ACC Automation
accautomation.ca
