Wiring and connections

Two worlds in one panel

Almost every PLC panel has two voltage levels:

• Power: the mains voltage that drives large loads. It depends on the country: for example, 230/400 V at 50 Hz in Europe, 220/380 V at 50 Hz in Chile, or 120/240/480 V at 60 Hz in North America.
• Control: usually 24 VDC, supplied by a power supply inside the panel. The PLC, sensors, push buttons and interface relay coils all work at this voltage.

The PLC lives in the control world. When it has to move something on the power side, such as a motor, it does so through a contactor: the PLC output energizes the contactor coil, and the contactor’s main contacts connect the motor to the mains.

The terminals of a PLC

Explore a generic compact PLC. Click each terminal, LED or port:

Click (or use Tab and Enter) on a terminal, LED or port to see what it is and how it is wired.

Select a part of the drawing.

Terminals are always grouped the same way:

• PLC power supply: L+ and M (or L1 and N).
• Inputs with their common: the common closes the circuit of the whole group. Wired to 0 V, the group accepts PNP sensors; wired to +24 V, it accepts NPN sensors.
• Outputs with their commons: on relay outputs, the common carries the voltage the contacts will deliver to the loads. Each group can work at a different voltage.

Terminal blocks: the bridge between PLC and field

Cables coming from the field (sensors, push-button stations, valves) are not wired straight to the PLC: they land on terminal blocks mounted on the DIN rail, and tidy internal wiring goes from there to the PLC.

This has very practical advantages:

• The PLC can be replaced without touching the field wiring.
• Every terminal has a number shown on the drawings, so signals are easy to trace.
• There are terminal blocks with a fuse, with a disconnect (to isolate a signal while measuring) and for earth.
• Distribution blocks for +24 V and 0 V avoid stacking many wires under one screw.

Typical panel layout

From top to bottom, a small control panel usually has:

1. Main switch and incoming supply protection.
2. Circuit breakers (or fuses) for each circuit: 24 VDC supply, PLC, outputs, service socket.
3. 24 VDC power supply.
4. PLC and its expansion modules.
5. Interface relays and contactors with their protection (for example, the motor overload relay).
6. Terminal blocks for inputs and outputs to the field.

Cable ducts run between rows of devices. A good practice is to keep power and signal cables in separate ducts, especially analog signals, to reduce electrical noise.

  Mains ── [Breaker] ── [24 VDC supply] ── +24 V ── [Fuse] ── L+ (PLC)
                              │                    └─ push buttons and sensors
                              └─ 0 V ────────────── M (PLC) and common 1M

  Output Q0.0 ── contactor K1 coil ── neutral
  K1 main contacts ── overload relay ── motor

Wire colours

The IEC 60204-1 standard (electrical equipment of machines) recommends, among others:

Colour	Recommended use
Green/yellow	Protective earth (PE). Exclusive, never anything else.
Light blue	Neutral
Black	AC and DC power circuits
Red	AC control circuits
Blue	DC control circuits
Orange	Interlock circuits supplied from an external source

Earthing and shielding

• Every panel has an earth bar. Metal parts, the rail and each device’s PE terminal connect to it with green/yellow wire.
• Analog signals and communications use shielded cable. The shield is earthed, usually at one end only (the panel side), to avoid ground loops; always follow the instrument manufacturer’s instructions.
• The negative of the 24 VDC supply (0 V) is usually earthed at a single point. Whether it floats or is earthed depends on the plant design: do not change it without checking the drawings.

Protecting the outputs

• Respect the maximum current per output and per common given by the manufacturer.
• Inductive loads (contactor coils, solenoids) produce voltage spikes when switched off. In DC a diode is placed across the coil; in AC, an RC snubber or a varistor. Many contactors offer these suppressors as accessories.
• For large loads or loads at another voltage, add an interface relay: if something fails, you replace a cheap relay instead of the PLC output.

Try it in the simulatorOpen a preloaded example and experiment yourself.

Summary

• The PLC works in the control world (usually 24 VDC) and drives power loads through contactors.
• Inputs and outputs are grouped with commons; the input common decides whether the group sinks or sources.
• Field wiring lands on terminal blocks, not directly on the PLC.
• Separate power and signal, use shielded cable for analog signals and respect earthing.
• Protect the outputs and use interface relays for large loads.