Reading Ladder Diagrams for Control Circuits

A ladder diagram shows how a control circuit thinks. It does not show where parts are mounted or which way the wires run. It shows the logic: what has to be true for a motor, valve, light or fan to turn on. Electricians, HVAC techs and maintenance crews all use ladder diagrams because a drawing tells you what should be happening, and a meter tells you what is. A fault lives in the difference between the two.

What You Will Be Able to Do

• Recognize common control symbols and device labels.
• Explain why contacts are drawn in their de-energized state.
• Read a rung in the right order, and follow cross-reference numbers.
• Trace a seal-in circuit and an interlock.
• Plan a test using the drawing, before anyone opens a panel.

How a Ladder Is Laid Out

A ladder diagram has two vertical rails and a set of horizontal rungs.

• Left rail: the hot side of the control supply, often L1 or X1.
• Right rail: the return side, often L2, N or X2.
• Rungs: each rung is one circuit between the rails.
• Inputs on the left: switches, contacts and sensors.
• Outputs on the right: coils, lamps and solenoids.

Read each rung from left to right, and read the rungs from top to bottom. On each rung, ask one question: what must be true for this output to energize?

Rungs are numbered down the left margin. Wires often carry numbers too, and each number stands for one electrical point. Wire number 7 on the drawing is wire number 7 in the panel, wherever it goes.

Symbols and Labels

Symbols differ a little between standards, so read the legend. This is the common North American (NEMA) style.

Labels tie the drawing to the real device. A coil marked CR2 and a contact marked CR2 belong to the same relay, even if they sit on different pages.

Normally Open, Normally Closed

"Normally" means the state of the device with no power applied and nobody touching it. It does not mean how it usually sits while the machine runs.

Every contact on a ladder diagram is drawn in that de-energized state. A contact drawn open may spend all day closed, because its coil is on. Drawing them all at rest gives you a fixed starting point. From there, you can work out what happens one step at a time.

When a coil energizes, all of its contacts change state at once:

• NO contacts close.
• NC contacts open.

When the coil drops out, they all go back. Reversing NO and NC in your head is the most common beginner error.

Coils, Contacts and Cross-References

A single relay can have several contacts on different rungs. The drawing shows the coil on one rung, and its contacts wherever they do their jobs.

To help you find them, many drawings put numbers next to the coil at the right margin. Those are cross-references. They list the rungs where that coil's contacts appear. A mark such as an underline or bold type often shows which ones are NC.

 Rung 4 --[ PB1 ]----( CR1 )--   6, 9, 12

This says that CR1's contacts appear on rungs 6, 9 and 12.

Tracing a Seal-In Circuit

A seal-in is the most common circuit in motor control. A momentary Start button would stop the motor once released, so an auxiliary contact holds the circuit.

 L1                                                   L2
  |                                                    |
  +--[ STOP ]--+--[ START ]--+--( M )-----[ OL ]-------+
               |             |
               +--[ M aux ]--+

1. At rest: STOP is closed, START is open, M aux is open and OL is closed. The rung is broken at START and M aux.
2. Press START: the path is complete, and coil M energizes.
3. M changes state: the M aux contact closes in parallel with START.
4. Release START: current now flows through M aux, so the coil stays on.
5. Press STOP: the rung opens, M drops out and M aux opens. Releasing STOP does nothing, because both parallel paths are open.
6. Overload trip: the OL contact opens, and M drops out, as if STOP were pressed.

If you can draw that from memory, you can read a large share of what you will see.

Interlocks

An interlock uses a contact from one device to control another.

 Rung 1 --[ FWD ]--[ R NC ]--( F )--
 Rung 2 --[ REV ]--[ F NC ]--( R )--

Each coil has the other's NC contact in series with it. When F is on, its NC contact on rung 2 is open, so R cannot energize. This stops two devices from running at the same time.

An NO contact is used in the same way to set a sequence. A contact from a lube pump's coil, in series with a main motor's coil, means the main motor cannot run unless the lube pump is already running.

Timers

Timers add time to the logic. An on-delay timer starts timing when its coil energizes, and its contacts change after the set time. An off-delay timer changes its contacts right away, and changes them back after the coil drops out. The contacts are usually drawn with a small arrow or arc, and marked TR or TD.

PLC programs use the same layout. A TON instruction starts timing when its rung is true, and its done bit turns on when the accumulated time reaches the preset. If you can read a relay ladder, you already know the shape of a PLC program.

Power and Control Drawn Separately

A control drawing often splits the circuit in two. The power circuit shows the heavy conductors from the disconnect through the contacts to the motor. The control circuit is the ladder.

They are drawn apart to keep the logic clear. It is easy to forget that they connect in the real panel. Many control circuits use a control transformer that steps 480 volts down to 120 volts. That transformer may be fed from a different source from the motor.

Warning: Turning off the motor feeder does not always kill the control circuit. Before work starts, the feeder and the control power source must both be locked out and tested by a qualified person.

Schematic or Wiring Diagram?

A ladder diagram is a schematic. It shows logic, not location.

The two work together. The schematic tells you which contact you need to check. The wiring diagram tells you which terminal it lands on.

Using the Drawing to Find a Fault

Open the drawing before you open the panel.

1. Define the symptom. "Will not start" is different from "starts and drops out."
2. Find the coil for the part that is not working.
3. List every contact in series with it, left to right.
4. Predict the state of each contact right now.
5. Plan your test points, and mark them on the drawing.

A qualified person then measures one point at a time, against the prediction. The first reading that does not match your prediction puts the fault within one device of that point.

What a Reading Means

Measuring across a contact that has current flowing through it:

A closed contact has almost no resistance, so nearly no voltage drops across it. An open contact shows the supply voltage, because the meter reads through the rest of the circuit.

If a contact is open but should be closed, the fault may be upstream. Find the coil that controls it, read that rung and keep going back.

Worked Example: A Fan That Will Not Run

A rung reads: FU1, FS (flow switch, NO), TR1 (timer contact, NO), coil M2.

The symptom is that the fan never starts after the machine runs. On paper:

1. FU1 should be closed.
2. FS should be closed if air or water flow is proven.
3. TR1 should close after its delay.
4. M2 should pull in.

Tests show voltage after FS, but none after TR1. So TR1 has not closed. The cross-reference leads to the timer's coil on rung 11. On that rung, a contact is open, and the timer never starts. The fan is fine. The fault is on another rung, and you found it on paper.

Other Circuits You Will See

The same reading skills work on many circuits.

A three-way light switch. A schematic shows the common terminal of each switch and two travelers between them. At one switch, the common receives power from the line. At the other, the common sends switched power to the fixture. The light is on when both switches pick the same traveler.

A furnace. A thermostat closes R to W. The control board checks the safety switches, starts the inducer, proves draft with a pressure switch, lights the burner, senses flame and then starts the blower. Each step needs the one before it.

A lighting contactor. An occupancy sensor or a time clock closes a contact, which energizes the contactor coil. The contactor then switches the much larger lighting load.

A drive start circuit. Pushbuttons connect to a variable frequency drive's digital inputs rather than to a coil. A run output from the drive can drive a pilot light.

Common Mistakes

Check Your Understanding

1. A relay's coil is energized. Is its NC contact open or closed right now?

Open. It is drawn closed because the drawing shows the de-energized state.

2. Next to coil CR3, you see 8, 14. What does that tell you?

CR3 has contacts on rungs 8 and 14.

3. A motor runs only while the Start button is held. What part of the drawing do you check?

The seal-in branch, in parallel with Start.

4. With the circuit powered, a technician reads 120 volts across a contact drawn NC that should be closed. What does it mean?

The contact is open. Either it failed, or something has changed its state, so check the device or its coil.

5. Why do both motor feeder and control source need to be tested before work?

A control transformer may be fed from a different source, so the control side may be live after the motor feeder is off.

Key Takeaways

• Read left to right along a rung, and top to bottom down the ladder.
• Inputs sit on the left, and outputs sit on the right.
• Contacts are drawn de-energized, so that logic has a fixed starting point.
• Cross-reference numbers show where a coil's contacts are.
• 0 volts across a contact with current through it means closed, and full voltage means open.
• Trace the logic on paper before anyone opens the panel.