PLC Latching vs Sealing Explained Simply
Why It Matters
Choosing seal‑in over latching for actuators eliminates dangerous unintended starts after power outages, directly impacting operational safety and downtime costs.
Key Takeaways
- •Seal‑in circuits keep outputs running after start button release.
- •Interlocks stop sealed outputs when stop button is pressed.
- •Latching uses OTL/OTU instructions and retains state through power loss.
- •Seal‑in is non‑retentive, preventing “ghost starts” after power cycles.
- •Use latching for data flags; sealing for motors and actuators.
Summary
The video walks through two fundamental PLC techniques—seal‑in (or sealing) and output latching—used to keep an output energized after its initiating condition disappears.
A seal‑in circuit combines a permissive (the start button) with an interlock (the stop button). When the start button is pressed, the motor output turns on; releasing the button leaves the output “sealed” by a branch that bypasses the permissive, while pressing stop unseals it. Latching, by contrast, employs the Output Latch (OTL) and Output Unlatch (OTU) instructions in Studio 5000; the latch sets the output true on start and remains true until an OTU instruction clears it.
The presenter highlights a safety nuance: latching is retentive, so the output survives a power cycle, potentially causing a “ghost start.” Seal‑in circuits are non‑retentive because the Examine On instruction forces a false pre‑scan, ensuring motors stop after power loss. The “golden rule” advised is to use seal‑in for any moving equipment and reserve latching for status flags or fault memory.
Understanding the distinction prevents hazardous unintended starts and guides engineers in selecting the appropriate method for control logic, ultimately improving plant safety and reliability while informing training needs.
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