How Does a PLC Seal-In Circuit Hold an Output?

The output now stays energized after the momentary start signal drops, while a separate release condition provides a controlled way to turn it off. Commission the holding branch one state at a time; the original two-branch circuit has no release path, so once B seals itself in, normal logic cannot clear it.

Reject the quick fixes first

Do not start by forcing the output, duplicating the output coil, replacing a relay, or cycling PLC power. A force bypasses the sequence you need to test. A duplicate coil lets another rung overwrite the state. Replacing hardware does nothing when the program is behaving exactly as written. A power cycle may drop the output temporarily, but it does not correct the missing release condition.

Prove this stage: remove all forces and find every instruction that writes to B. Continue only when one intended rung owns the output state.

Map the two current paths

The seal-in rung has two parallel paths feeding output B. One path contains input A; the other contains a contact labeled B. Current, or logical continuity in PLC ladder, can pass through either branch.

1. Confirm that operating the start device changes input A in the PLC input image.
2. Confirm that the output instruction controls B.
3. Confirm that the parallel holding contact also represents B.
4. Trace both branches to the same output instruction. A branch connected around only part of the rung can bypass a required permissive.

A contact instruction addressed to the output state proves that the PLC commanded B; it does not by itself prove that an external relay picked up or that the load energized. If the design requires physical feedback, use the wired auxiliary contact assigned to that purpose and verify its input separately.

Prove this stage: with B off, operate A and verify that the A branch becomes true and energizes B. Stop here if the PLC never sees A; repair the input signal before changing the seal-in logic.

Build and test the holding branch

The operating sequence is an OR function: B = A OR B contact while all series conditions remain true. Initially, both paths are false and B is off. Operating A makes the first path true, so the PLC energizes B. The B contact then becomes true and establishes the second path. Releasing A removes the start path, but the holding path keeps the rung true.

Series run conditions
        AND
(A contact OR B holding contact)
        --> B output

1. Start with A off and B off.
2. Operate A. Verify that B turns on.
3. While B is on, verify that the parallel B contact becomes true.
4. Release A. Verify that A becomes false while the B branch remains true.

If B drops at the last step, inspect the contact address, branch endpoints, and any series permissive that changes state when the output starts. Do not mask a changing permissive with another holding branch; find why it drops.

Prove this stage: repeat start-and-release several times from a known off state. The rung must hold only after B has actually been commanded on.

Add a deliberate release condition

The illustrated two-branch circuit has no way to turn B off after it seals in. Add a release condition in series with the entire parallel network, not inside only the A branch. When the release condition goes false, it must break both the start path and the B holding path.

The release may represent a normal stop command, a process trip, a lost run permit, or another machine-specific condition. Select it from the approved control narrative and electrical design. Do not treat ordinary PLC logic as a substitute for a safety-rated stopping function where the machine risk assessment requires one.

1. Place the selected run condition ahead of the parallel A/B network.
2. With the run condition healthy, operate A and confirm that B seals in.
3. Make the run condition false. Confirm that rung continuity breaks and B turns off.
4. Restore the run condition without operating A. Confirm that B remains off; otherwise another memory mechanism or write instruction is restoring it.

Prove this stage: a stop-and-restore cycle must leave B off until a new start command arrives.

Verify the complete operating sequence

Test the program state and the field result together. The PLC can show a true output instruction while an output channel, interposing relay, protection device, field supply, or load circuit prevents operation.

1. Begin with A false, the release condition healthy, and B off.
2. Operate A; verify the input indication, rung continuity, output indication, and load response.
3. Release A; verify that the B branch alone maintains the output and load.
4. Operate the release condition; verify that the rung, output indication, and load all turn off.
5. Restore the release condition; verify that no automatic restart occurs.
6. Repeat the sequence after returning the controller to its normal operating mode and removing temporary monitoring or forcing actions.

If the logic state and field response disagree, work outward from the output instruction: module indication, output terminal, interposing device, field power, then load. Follow the site electrical test procedure before measuring energized circuits.

Prove this stage: record the four decisive states—before start, during start, held after release, and stopped—and confirm that both PLC indications and physical operation agree in every state.

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