Guidelines for PLC wiring


The PLC – programmable logic controller – is an industrialised computer designed specifically for industrial control systems.To know more about PLC check the link given below.Now let us discuss about Guidelines for PLC wiring

There are two main styles:

  1. A small so-called all-in-one type, designed to be mounted directly into the panel on a DIN rail. These are wired in a similar way to contactors
  2. A modular type, which consists of a frame or rack and a number of separate, plug-in modules. The rack is first fixed on to the chassis – the smaller type fit on to a DIN rail, the larger type require bolts. The separate modules are then plugged into this rack

Cable connections may be to screw terminals mounted on the rack or use multi-way connectors that plug into each module. The rack should be connected to the protective bonding circuit.



  • They are generally robust but they contain electronic components and printed circuit boards so have to be fixed into a control cabinet away from heat, moisture, dust and corrosive atmospheres. Avoid mounting the PLC close to vibration sources, such as large-sized contactors and circuit breakers.

  • Ensure that the mounting surface is uniform to prevent strain. Excessive force applied to the printed circuit boards could result in incorrect operation. If equipment that generates electrical noise or heat is positioned in front of the PLC (as when such equipment is mounted on the back of a panel door), allow a clearance of 100 mm or more between the PLC and such equipment.

Power supply wiring

When wiring AC supplies:

  • Live – L
  • Neutral – N
  • Earth – E or symbol

With DC supplies:

  • Positive cable to +
  • Negative cable to –
  • DC power supply connections must never be reversed.


  • Use 2 mm2 wire or larger.
  • The PLC’s earth should not be directly connected to the same terminal as power devices, but should go to the operational earthing circuit. The PLC will work without an earth connection but it may be subject to malfunction due to electrical interference.
  • Where several PLCs or expansion units are used, link all the earth terminals together then connect to the operational earth terminal.

  • Where the PLC connections are by screw terminals, use an insulated crimp eyelet or spade terminal on the cable end.
  • Where the connections are to a terminal block type connecter, use an insulated ferrule on the cable end.
  • Make sure the terminal screws are tightened correctly.

Wiring of inputs and outputs

  • It is advisable to keep the input and output (I/O) lines separated from each other if possible.
  • When the I/O signal wires cannot be separated from the power wiring, use shielded multi-core cable earthed at the PLC end.
  • I/O signal wires should be kept at least 100 mm away from high-voltage and large-current, power circuit wiring.

To know more about

How to write a PLC ladder logic program?



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