1. What Does a Control Cabinet Wiring Diagram Have That a Distribution Box Wiring Diagram Doesn't? 3 Extra Blocks: PLC / Communication / Interlocks
A control cabinet drawing has3 more blocks than a distribution box drawing:
- PLC I/O assignment table: which input points (pushbuttons / sensors) go into the PLC, and which output points (contactors / indicator lights) come out of the PLC
- Communication topology diagram: how the PLC communicates with the HMI / VFD / host computer (Ethernet / Profinet / Modbus)
- Interlock circuits: safety interlocks between multiple motors (preventing simultaneous start / sequence control)
These 3 blocks make the information density of a control cabinet drawing 2–3 times higher than that of a distribution box drawing,Reading the diagram in the wrong order quickly gets confusing.
II. The 4 Main Blocks of a Control Cabinet Drawing 4 blocks + 1 cover sheet
A complete control cabinet drawing set, per GB/T 6988.1-2008, should include:
| No. | Drawing block | Reader | Stage of use |
|---|---|---|---|
| 1 | Cover sheet + main circuit single-line diagram | Purchasing / owner | Quotation / review |
| 2 | PLC I/O assignment table | Electrical engineer | Programming / commissioning |
| 3 | Secondary wiring diagram (control / signal) | Wireman / commissioning technician | Production / site |
| 4 | Terminal block diagram | Wireman | Production / site wiring |
| 5 | Layout drawing + bill of materials (BOM) | Assembler / purchasing | Production / acceptance |
Start with the main circuit single-line diagram (who the cabinet feeds) → then the PLC I/O table (the logic relationships) → then the secondary wiring diagram (how the wires connect) → finally the terminal blocks (both ends of every wire). Reverse that order and even 10 drawings will not make sense.
III. The 5-Step Method: From Supply to Load Standard engineer's routine
Given a control cabinet secondary wiring diagram, read it in 5 steps:
- Step 1: Check the power supply. Where the main supply enters (L1/L2/L3 busbars), where the control supply is (AC220V / DC24V), where the transformer / switching power supply sits
- Step 2: Check the main circuit. From the main breaker to the contactor to the thermal overload relay to the motor — this is the power section
- Step 3: Check the PLC inputs. Pushbuttons / sensors / limit switches → PLC input terminals (I0.0, I0.1...), to understand what signals enter the PLC
- Step 4: Check the PLC outputs. PLC output terminals (Q0.0, Q0.1...) → interposing relays → contactor coils, to understand what the PLC controls
- Step 5: Check the interlocks. Multi-motor interlocks, emergency stop circuits, safety doors — this is the safety logic; a mistake here causes an accident
After these 5 steps, the whole drawing takes shape in your head.
4. How to Read a PLC Wiring Diagram 3 Types: Input / Output / Communication
1. PLC Input Wiring
Input points (pushbuttons / proximity switches / photoelectric sensors / limit switches) → wired to the PLC I terminals. Commonly used PNP type (sinking) sensors connect to the PLC S/S terminal at 24V+; NPN type (sourcing) connects to 24V-.
2. PLC Output Wiring
Output points (Q terminals) → wired to relay coils or loads (note the PLC output point rating, typically 2A per point; small loads can be driven directly, large loads require an interposing relay).
3. PLC Communication Wiring
Ethernet / Profinet: use RJ45 cable; Modbus RTU: use shielded twisted pair connected to A+/B-; CPU to HMI is typically via Ethernet cable / serial port.
5. How to Draw Interlock Circuits in a Control Cabinet 3 Basic Interlocks
Control cabinet interlocks are the core of "accident prevention" — the 3 common types:
| Interlock Type | Wiring Method | Application |
|---|---|---|
| Contactor interlock | KM1 auxiliary normally-closed contact wired in series with KM2 coil circuit | Motor forward/reverse / duty-standby |
| Pushbutton interlock | SB1 normally-closed contact wired in series with SB2 circuit | Start-stop / start-stop-stop |
| PLC logic interlock | AND / OR conditions in the PLC program | Complex sequencing / remote control |
Emergency stop button (normally-closed contact)must be wired at the front end of the PLC inputor directly break the contactor coil circuit — a soft E-stop in the PLC program alone is not acceptable; it fails on loss of power.GB 50054-2011 Clause 5.4.3 mandates that the E-stop circuit physically disconnect the main circuit.
6. Factory Photos: 4 Examples of Secondary Wiring Workmanship (shop floor photos)
FAQ FAQ
Can AI drawing software produce control cabinet wiring diagrams?
Yes, but the output must be checked manually. AI software (e.g. EPLAN / AutoCAD Electrical) draws fast, but terminal numbers, wire numbers and PLC I/O are error-prone; at minimum, check three times: ① main circuit short-circuit withstand ② PLC I/O consistency ③ emergency stop / interlock logic. At Golden Unicorn, every drawing gets a 100% manual review before it leaves the plant.
What wire sizes are used inside a control cabinet?
Main circuit:copper-core BV wire, 1.5–95 mm²(selected by current); control circuit:0.75–1.5 mm² flexible stranded wire(easier to terminate on relays); signal circuit:shielded twisted pair(analog / communication). Color code: red = L phase, blue = N, yellow-green = PE, white = DC+, black = DC-.
How do you tell PLC inputs from outputs when wiring?
Siemens S7-200 SMART:inputs I0.0–I1.5, outputs Q0.0–Q1.1. Mitsubishi FX5U: inputs X0–X17, outputs Y0–Y17. Check the I / Q labels on the PLC faceplate, or the I/O assignment table in the drawing frame (usually a box with text labels).
How is an emergency stop button wired?
An emergency stop must use a normally closed contact, wired in series with the contactor coil circuit (or into a PLC safety input). Wiring options: ① emergency stop directly breaks the main contactor coil (safest) ② emergency stop feeds a PLC safety input, and the PLC program drops all outputs ③ dual circuit: both into the PLC and breaking the contactor (double protection). Golden Unicorn uses option ③ by default.
What voltage do indicator lights / pushbuttons in a control cabinet use?
By default DC 24V(fed by the cabinet's 24V switching power supply, capacity ≥ 60W); older designs use AC 220V. Golden Unicorn uses 24V by default — longer life, safer.If 220V indicator lights are specified, flame-retardant wire must be used.
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