Latching in ladder logic plc programming

How Seal-In (Latching) Circuits Work in PLC Ladder Logic

Walk past almost any motor on your plant , a conveyor, a fan, a pump, a feeder and behind the panel you'll find some version of the exact circuit we're about to build together. A momentary Start button, a momentary Stop button, and one small trick that lets the motor keep running long after you've taken your finger off the button. It looks simple once you know it, but it trips up a lot of people the first time they see it. So let's take it apart piece by piece, the way I'd walk a new technician through it on the panel itself.

The Problem With a Simple Push Button

 we built the simplest rung there is  a push-button contact feeding a lamp coil.

 Press the button, the lamp lights up. Let go, it goes dark again. 

That's a perfectly honest little circuit, and it works fine for a lamp, because you generally want a light on only while you're actually pressing something.
But now think about what that same wiring would mean for a motor. If your Start button were connected exactly the same way — straight into the motor coil, nothing else, the motor would run only while your finger physically holds that button down. The moment you let go, it stops. 

Try to picture that on a real conveyor or a kiln ID fan: nobody wants to stand there with a thumb on a button for eight hours just to keep the belt moving.
So somewhere in the circuit, something needs to "remember" that Start was pressed, and keep the motor running even after the button springs back. That memory function is exactly what a seal-in contact gives you and once you understand it, you'll recognize it everywhere.

Building the Seal-In Circuit

 One Piece at a Time
Let's build it up slowly rather than jumping to the finished rung.
Start with the basics: a Start contact — normally open — wired in series with a motor coil, which we'll label M for motor. Press the Start button, the contact closes, power flows across the rung, and coil M energizes. The motor starts turning. So far, this is no different from the lamp circuit .
Here's the piece that changes everything: we add a second contact, also labeled M, wired in **parallel** with the Start contact — not in series, right alongside it, as an alternate path for power to take. This second contact is called an auxiliary contact. 

It isn't a separate device; it's physically tied to the same coil M, so the instant coil M energizes, this auxiliary contact closes too, at exactly the same moment, automatically.
Now read closely, because this is the part that usually makes it click. The moment you release the Start button, the Start contact springs back open — that path is gone, no power flows through it anymore. But by that same instant, the motor is already running, which means the auxiliary M contact is already closed. Power simply switches over and flows through that parallel path instead, and coil M stays energized without any help from the Start button at all.

That's the whole trick. The circuit is sealing itself in — locking itself on using a contact driven by its own output. That's exactly why it's called a seal-in, or sometimes a latching circuit: once it starts, it holds itself there.

 Adding the Stop Button

Of course, a motor that only ever seals itself in and never stops isn't much use, so now we need a way to break that seal. We add a Stop contact in series with everything else on the rung — but pay attention to how it's drawn, because this is the detail most people miss the first time: it's a normally closed contact, not normally open.
At rest, with nobody touching anything, this Stop contact is already sitting closed, quietly allowing power through. Press the Stop button, and it opens — and because it sits in series before the parallel Start/seal-in branch, it doesn't matter which path power was flowing through. Whether the motor was being held in by the seal-in contact or freshly started by the Start button, opening this one contact cuts the whole rung. Coil M de-energizes, and because the coil is now off, the auxiliary M contact drops open too. The seal-in releases itself automatically, and the circuit resets back to its starting condition.
There's a good reason Stop buttons are almost always wired normally closed rather than normally open, and it's worth pausing on. If the wire running to that Stop button were ever to break, chafe through, or come loose — which happens more often than anyone likes to admit in a busy plant — a normally closed contact fails open. In other words, a broken wire behaves exactly as if someone had pressed Stop. The motor shuts down, safely, on its own, rather than a broken wire silently disabling your ability to stop the equipment. That's not an accident of design; it's a deliberate safety principle, and you'll see the same normally-closed logic behind real emergency stop circuits throughout the plant, not just this simple example.

Running the Full Circuit Together

Let's put it all together and walk through the whole sequence in order, because seeing it end to end is what really makes it stick.
The Stop contact sits first in the rung, normally closed, quietly allowing power through by default. After it comes the Start contact, running in parallel with the seal-in auxiliary contact. Press Start — the motor runs. Release Start — the motor keeps right on running, held in by its own auxiliary contact, with the Start button doing nothing at all anymore. Press Stop — everything drops out immediately, the coil de-energizes, the auxiliary contact opens, and the rung returns to exactly where it started, ready for the next Start press.
Three components, really, is all it takes: two momentary push buttons and one coil with an auxiliary contact tied to it. That's the entire backbone of motor control logic behind an enormous share of the rotating equipment on your plant right now — feeders, fans, pumps, conveyors, all built on this same handful of parts.

Why This Is Worth Knowing Cold Out on a real plant, you'll rarely see this circuit sitting completely bare like this. Usually there are extra interlocks stacked in series ahead of the Start contact — no trip active, lubrication good, upstream equipment already running, permissives from other systems — before the operator is even allowed to press Start. But underneath all of that added complexity, the seal-in backbone is still there, unchanged. A feeder motor tag might read something like M-101, an ID fan might carry a completely different tag name, but the core three-component structure holding it in stays exactly the same.
Once you can trace this one rung with your eyes closed, you've effectively learned to trace the skeleton of nearly every motor control circuit you'll come across in your career — no matter how many extra permissives someone has piled on top of it.

Recap

Start contact

  normally open, wired in parallel with an auxiliary contact from the same coil. This parallel pairing is the seal-in.
Auxiliary M contact

 closes the instant the coil energizes, and takes over holding the rung on the moment the Start button is released.
Stop contact

 normally closed, wired in series ahead of everything else, breaking the whole rung the instant it's pressed. It also fails safe if the wiring itself ever breaks.
- Put together, two momentary push buttons and one auxiliary contact give you a motor that runs continuously from a single press, until you deliberately tell it to stop.