How to read a control schematic: trace paths instead of memorizing

1. Read the requirement before the wires

Before studying the schematic, write what it should do. For Start–Stop: initially idle; Start picks up the coil; releasing Start keeps it energized; Stop releases it.

This gives you four states to check. If you read symbol names without knowing the machine’s job, you cannot easily distinguish an unnecessary branch from an intentional one.

Choose one coil as your target, here KM1. Understand its path before moving to indicators, interlocks, or timers.

2. Identify the control supply and return

This diagram uses 24 VDC: +24 V supplies power and 0 V is the return. They are not decorative symbols; the coil needs an appropriate supply and return path.

Real control supplies may be AC, DC, transformer-fed, or supplied by a power supply. Read voltage notes, rail names, and the project schematic before tracing. Do not assume every left rail is phase and every right rail neutral.

Separate control from power. Here you are explaining the command to the coil, not the complete three-phase circuit, protection, or motor wiring.

3. Recognize coils, contacts, and tags

KM1’s coil receives the electrical command. Compatible contacts tagged KM1 change state with that device. They may be drawn elsewhere; the name identifies their relationship, not an invisible wire joining every terminal.

Read conventional NO/NC contacts in the unactuated state: NO open, NC closed. Then ask which device changes their state.

Terminal numbers help cross-check real devices but do not replace manufacturer diagrams. TeSys D, for example, uses A1–A2 for the coil, 13–14 for auxiliary NO, and 21–22 for NC; do not assume every relay socket uses those numbers.

• Understand NO and NC with two lamps
• Distinguish relay/contactor coils and contacts

4. Trace a complete loop: supply, command, coil

Read connection points, not simply nearby lines. Here +24 V passes through Stop NC to a split. One branch passes through Start NO and the other through KM1 auxiliary NO; they rejoin before the coil.

With Start pressed: +24 V → closed Stop → closed Start → KM1 coil → 0 V. Energizing KM1 closes its auxiliary contact. Releasing Start leaves that auxiliary branch maintaining the coil path.

Pressing Stop opens the common supply, removing power from both branches. This is why the split and rejoining points matter: seeing a Stop symbol alone does not prove it can release the coil.

In this example, Stop and Start are spring-return buttons, with conventional coils and contacts. The illustration does not include complete protection or a power circuit for construction.

5. Do touching lines mean a connection?

Interpret crossings and junctions according to the drawing’s conventions. Do not infer a connection merely because lines cross or approach a terminal. Look for junction marks, wire names, and the relevant endpoints.

When recreating the circuit in EleRun, wires must attach to symbol terminals. A wire drawn near a coil without attaching does not create a valid supply path.

If the circuit will not run, trace the shortest supply-to-coil path first. Find the open segment before expanding to holding or indicator branches. This avoids redrawing a page because of one unattached endpoint.

6. Read a new circuit in five steps

Use this approach with timers or two contactors too, while considering each device’s extra conditions. Do not memorize the whole picture; explain why each contact is there.

1. Write the requirement and choose the coil/load to explain.
2. Read the branch’s supply type, voltage, and return.
3. Trace every possible coil supply path and note which contacts block it.
4. Change one command mentally: which device changes state, and which branch becomes complete?
5. Build a small example on one EleRun page, run it, and compare with your own state table.

• A series contact commonly adds a condition that must be satisfied along the path.
• Parallel branches can provide alternative supply paths; still check their common supply point.
• An interlock contact must block the required branch, including its holding path.

• Read the two branches of a reversing circuit
• Read when a timer starts counting and operates

7. Check whether you understand or only remember the picture

Cover the explanation and answer from the diagram: where does current flow after Start is released? Where does Stop interrupt it? What changes if the holding branch starts before Stop?

You understand when you can answer with current paths and device states. If the answer is only “because it holds,” trace again from after Stop to the coil.

After testing in EleRun, send a link and a question to a colleague. They can operate the circuit and point to the branch in question, which is clearer than a context-free image.

• Send a circuit link for others to test
• Troubleshoot a holding circuit