1. The Two Sections of a Wiring Diagram (Main Circuit vs Control Circuit)
An electrical control cabinet wiring diagram is divided intotwo sections, top and bottom:
| section | Location | Contents | conductor size |
|---|---|---|---|
| Main circuit | upper half of the drawing | incoming line → breaker → contactor → thermal overload relay → motor | 1.5~35mm² |
| Control circuit | lower half of the drawing | pushbutton → relay logic → contactor coil → indicator light | 0.5~1.0mm² |
2. How to Read the Main Circuit (5 Steps, Left to Right)
Main circuit, left to right5 steps:
- Check the incoming line: L1/L2/L3 three-phase supply, incoming conductor cross-section (e.g., 3×16 mm² copper), whether a disconnect switch is fitted.
- Check the breaker: model (e.g., NSX100F), current setting In (e.g., 80A), breaking capacity Icu (e.g., 36kA).
- Check the contactor: model (e.g., LC1D50), main contact rated current (50A), whether auxiliary contacts are fitted.
- Check the thermal overload relay: model (e.g. LRD32), setting range (e.g. 23–32 A), and whether it includes phase-failure protection.
- Check outgoing circuits: motor connection (Y/Δ), outgoing cable cross-section (e.g. 3×10 mm²), and terminal block.
3. How to Read a Control Circuit Diagram (3 steps, top to bottom)
Control circuit, top to bottom3 steps to read it:
- Check the control power supply: control circuit voltage (AC 220 V, AC 380 V, or DC 24 V), whether it runs through a control transformer (e.g. 380/220 V, 500 VA), and fuse configuration.
- Check the logic chain: stop button (normally closed) → start button (normally open) → contactor coil → holding contact. The normally closed contact of the thermal overload relay is wired in series in the control circuit for overload protection.
- Check the indicator circuit: run indicator light (green, controlled by the contactor's normally open contact), stop indicator light (red, controlled by the contactor's normally closed contact).
4. Motor Forward/Reverse Wiring Diagram Example (interlock logic explained)
Motor forward/reverse is themost classic control circuit, and its core isInterlock— preventing the forward and reverse contactors from pulling in at the same time and causing a short circuit:
| Step | Forward start | Reverse start |
|---|---|---|
| 1 | Press forward button SB2 | Press reverse button SB3 |
| 2 | KM1 coil energized → KM1 main contacts close | KM3 coil energized → KM3 main contacts close |
| 3 | KM1 normally closed auxiliary contact opens →interlocks KM3 | KM3 normally closed auxiliary contact opens →interlocks KM1 |
| 4 | Motor runs forward | Motor runs in reverse |
The interlock isthe lifeline of a forward/reverse circuit. If an interlock contact fails, the forward and reverse contactors pull in at the same time, and the main circuitshort-circuits two phases, burning out the contactor main contacts or even starting a fire. Dual interlock: ① contactor normally closed auxiliary contact interlock; ② button interlock (the forward button's normally closed contact is wired in series in the reverse circuit).
5. Common Wiring Diagram Mistakes (4 red lines)
① Thermal relay normally-closed contactnot wired into the control circuit→ overload does not trip, motor burns out; ② interlock contactusing a normally-open contact instead of normally-closed→ interlock fails and causes a short circuit; ③ no fuse in the control circuit → short circuit burns out the coil; ④ contactor coil not paralleled with anRC snubber→ when the coil opens, it interferes with the PLC or burns the contacts.
FAQ FAQ
Can the main circuit and control circuit share one drawing?
Yes. On large drawings the main circuit is usually shown at the top and the control circuit below, separated by adashed box. For complex systems, however, issue separate drawings — one for the main circuit and one for the control circuit — to make assembly and maintenance reference easier.
Should the control circuit voltage be 220V or 24V?
DC24V is recommended(a safety voltage). AC220V is the traditional approach, but live wiring carries a shock hazard. In modern PLC control cabinets the control circuit is uniformly DC24V — safe and compatible with PLC I/O.
Must the thermal relay be wired into the control circuit?
Yes, it is required. The thermal relay normally-closed contact is wired in series with the contactor coil circuit; on overload the contact opens, cutting power to the coil, the contactor drops out and the motor stops. If a thermal relay is installed only in the main circuit and not wired into the control circuit, overload protection is ineffective.
What is the RC on a wiring diagram?
It is aRC snubber(varistor + capacitor), connected across the contactor coil terminals to absorb the surge voltage at the instant the coil opens and protect the control circuit contacts from erosion.
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