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Industrial Controls & PLCs · Relay Logic

Ladder diagrams

Reading17mLesson 1 of 3 in this module

Read a ladder diagram, including a three-wire start/stop seal-in circuit with overload protection and an interlock.

Why it matters

Before PLCs, machines were controlled by panels full of relays wired together, and electricians drew those circuits as ladder diagrams. PLC programs still use the same picture, so the start/stop circuit you learn here appears on motor starters, conveyors and pumps everywhere. If you can read it, you can follow the logic of nearly any machine.

The ladder

A ladder diagram has two vertical rails and horizontal rungs.

  • The left rail is the control power supply (for example L1 of a 120 V AC control transformer, or +24 V DC).
  • The right rail is the return (L2, neutral or 0 V).
  • Each rung is one circuit: conditions on the left (switches and contacts) and one output on the right (a coil, lamp or solenoid).
  • Read each rung left to right: if there is a complete path of closed contacts, the output is energized.
  • Rungs are numbered down the left side, and contacts carry cross-references to the rung where their coil lives.

Contacts and coils

  • A normally open (NO) contact, drawn -| |-, is open when its device is at rest and closes when it is actuated.
  • A normally closed (NC) contact, drawn -|/|-, is closed at rest and opens when actuated.
  • "Normal" means the device's shelf state, not the machine's usual running state.
  • A coil -( )- is a relay or contactor. When it energizes, every contact with the same label changes state: its NO contacts close and its NC contacts open.

The three-wire start/stop circuit

This is the most important rung in industrial control:

   L1                                                     L2
    |                                                     |
1   |--[ STOP (NC) ]--+--[ START (NO) ]--+--[ OL (NC) ]--( M )--|
    |                 |                  |                    |
    |                 +----[ M (NO) ]----+                    |
    |                                                         |
2   |--[ M (NO) ]------------------------------------( RUN LAMP )--|

How it works:

  1. At rest, STOP is closed (NC), START is open, the M auxiliary contact is open. M is off.
  2. Press START: current flows through STOP, START and the overload contact, so coil M energizes. The contactor pulls in and the motor runs.
  3. M's auxiliary NO contact, wired in parallel with START, closes. This is the seal-in (or holding, or latching) contact.
  4. Release START: current now flows through the M contact instead, so M stays on.
  5. Press STOP: the path breaks, M drops out, and its seal-in contact opens. Releasing STOP does not restart the motor.
  6. If the motor overloads, the OL contact opens and drops M out the same way.

Rung 2 shows a run lamp driven by another M contact.

Why stop buttons are normally closed

A broken wire on a normally closed stop circuit has the same effect as pressing stop: the machine stops. If the stop button were normally open and its wire broke, pressing it would do nothing. This fail-safe principle is why stops, overloads and safety devices use NC contacts.

The three-wire circuit also gives low-voltage protection: after a power cut, M drops out and the motor will not restart by itself when power returns. A two-wire control (a maintained switch or float switch with no seal-in) restarts automatically, which is fine for a sump pump but dangerous for a saw.

Interlocks

An interlock stops two things from happening at once. A forward/reverse motor starter has two contactors, F and R. Energizing both would short two phases together, so each rung includes an NC contact of the other contactor:

3   |--[ STOP ]--+--[ FWD PB ]--+--[ R (NC) ]--[ OL ]--( F )--|
    |            +----[ F ]-----+                            |
4   |--[ STOP ]--+--[ REV PB ]--+--[ F (NC) ]--[ OL ]--( R )--|
    |            +----[ R ]-----+                            |

When F is on, its NC contact in rung 4 is open, so R cannot energize. Real reversing starters also use a mechanical interlock between the contactors.

Worked example: reading a fault

A conveyor starts when START is pressed but stops as soon as the button is released. Which part of rung 1 is suspect? The seal-in branch: the M auxiliary contact is not closing (a failed auxiliary block or a broken wire on that branch). The rest of the rung clearly works, because the motor runs while START is held.

Common mistakes

  • Confusing "normally closed" with "closed while running".
  • Forgetting that every contact with the same label as a coil changes when that coil energizes.
  • Drawing the seal-in contact in series with START instead of in parallel.
  • Putting more than one output coil in series on a rung.

Safety

Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

Recap

  • Rails are supply and return; each rung is conditions on the left, one output on the right.
  • Seal-in: an NO contact of the coil in parallel with START keeps it on.
  • Stops and overloads are NC so a broken wire fails safe.
  • Interlocks use NC contacts of the opposing output.

Try it

Open the Do tab and take the quiz on reading start/stop and interlock rungs.

Also in this lesson: