Seal-in contact
A seal-in contact is a normally open auxiliary contact on a starter or relay, wired in parallel with the Start button, that keeps the coil energised after you let go of the button. It is also called a holding contact, a maintaining contact or an interlock; on ladder diagrams it usually carries the coil’s own name, like M.
How it works
Press Start and current flows through the button to the coil. The coil pulls in and closes all of its contacts — the big power contacts that run the motor, and the small auxiliary seal-in contact sitting across the Start button. Now there are two paths to the coil. Let go of Start and its path opens, but the seal-in contact is still holding the second path closed, so the coil stays in. The starter has “sealed in”.
To stop it, something in series with both paths must open — the Stop button, or the overload contact. The coil drops out, the seal-in contact opens with it, and the circuit is back where it started. Pressing Start again is the only way to bring it back.
Why the trade cares
This is three-wire control, and it has a safety payoff: after a power failure the coil drops out, the seal-in opens, and the motor does not restart by itself when the power comes back. Someone has to press Start. A two-wire control — a switch or a float that stays closed — would restart it the instant the power returned. When a motor will not stay running after you let go of Start, the seal-in path is the first thing to test.
One worked, so the idea sticks
Reading the rung
- Rung 1: Stop (NC) — then Start (NO) with M (NO) beside it in parallel — then coil M — then the overload contact (NC).
- Press Start: the coil path is Stop → Start → M → OL. The coil pulls in and the M contact beside Start closes.
- Release Start: the path is now Stop → M contact → M → OL. The coil holds until Stop or the overload opens.
Try it live on the start-stop lesson: press Start, let go, then fault the seal-in contact open and watch the motor quit the moment you release the button.
Questions people ask
What happens if the seal-in contact fails open?
The motor runs only while someone holds the Start button, and stops the instant it is released. That symptom — “it only runs while I hold it” — points straight at the seal-in contact or its wiring.
What if the seal-in contact is welded shut?
Then the contact bridges the Start button for good. Pressing Stop drops the coil only while you hold it; the moment you let go, the welded contact feeds the coil again and the motor restarts by itself. Treat that as a dangerous fault: isolate and lock out the supply, then find it — do not rely on the Stop button.
Is a seal-in contact the same as a latching relay?
No. A seal-in holds only while the coil is fed; lose the power and it lets go. A mechanically latching relay stays in its last position with no power at all, and needs a separate unlatch signal.
Related
- Start-stop with seal-in — the interactive lesson
- Two-wire vs three-wire control
- Start-stop station diagram
- Back to the glossary
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