PLC Control Panel Electrical Conditions


The power supply, grounding, and noise make up the main elements of the electrical conditions. In the installation and wiring of the device, great care must be taken to ensure that there is no danger to the human body, and that electrical signals are not obstructed (noise).

■PLC Installation Site

For the sake of safety in maintenance and operation, installation should not be near high current (more than 600V) or high power devices. Where this cannot be avoided, installation should be as far away from such devices as possible.

●Arrangement of PLC and Each Unit

1.The CPU and other adjoining units should be special or input units where noise generation is limited.
2.The coils and contacts in electromagnetic contactors and relays in an external circuit are sources of noise. Locate them at least 100 mm away from the PLC.

■Power System Wiring

1.The power supply for the PLC and I/O devices should be separate, and a noise filter should be installed to the PLC power supply.
2.Noise between the PLC and the ground can be greatly reduced by adding an isolating transformer. In such an event, do not ground the secondary coil of the transformer.
3.Keep the wiring between the transformer and the PLC as short as possible, twist the wires well, and keep the wiring separate from high-voltage and power lines.

■Wiring External I/O Signal Lines

1.To absorb reverse electromotive force when an inductive load is connected to an output signal, connect a surge protector near the inductive load in an AC circuit, and connect a diode near the inductive load in a DC circuit.

2.The output signal line should not be near or parallel with high current or power lines.When nearby, they should be separated by a duct, and an alternative wiring pipe should be established. In this event, the duct and wiring pipe must be grounded.

3.Connect the shield to the ground terminal at the PLC, and leave it unconnected at the input device.
4.Wire the lines so that common impedance does not occur. Such wiring will increase the number of wires, so use common return circuits. Use thick wires with sufficient allowance for the return circuits, and bundle them with lines of the same signal level.
5.For long I/O lines, wire the input and output signal lines separately.
6.Use twisted-pair wires for pilot lamps (and particularly lamps with filaments).
7.Using a CR surge absorber or diode etc is effective in reducing noise sources from input devices or output load devices.

■External Wiring

The wiring method can have a large impact on creating a system that is strong against noise and a system that can operate with a high degree of reliability. Wiring, and noise countermeasures in particular, are based on experience, and it is necessary to closely manage wiring based on experience and information in the manuals.

●Preventing Faulty Wiring

1.By attaching marker bands to cables showing their direction, faulty wiring can be prevented, and wiring checks and maintenance can be facilitated.
2.Use different colors during wiring.
3.Place wires that transmit the same kind of signal into the same ducts, and separate them into groups.

●General Precautions

1.Do not join wires between terminals.

2.Support and fix the power line so that its end does not pull at the ends of multi-conductor cable.

3.Use flexible wires for the movable places such as doors etc. Install them one part on the panel body and the other on the door so that they may not be damaged by the movement of a door.

4.Connect crimp terminals to the end of the wire and use a torque driver for connection to the terminal, and screw in at an appropriate pressure. For terminal connection to an AC power unit, a round type crimp terminal should be used instead of a U shaped terminal, for safety purposes.

5.The dust protection cover attached to the PLC unit should be kept in place until the wiring is
completed to prevent dust from the wiring from entering. Further, as the internal temperature will increase greatly when the unit is in operation, the dust protection cover should be removed once wiring is completed to prevent loss in functionality.

6.All wiring for the power circuit should be twisted.

7.The primary or secondary noise filter should not be bundled, as this reduces its effectiveness.

●Wiring Method

1.Isolate signal cables with different characteristics when routed in the same duct.

2.Routing more than one power line in the same duct should be avoided as much as possible. Where this cannot be avoided, place a partition in the duct and ground it.

3.When using conduits in the course of wiring, the wires from one circuit should not place wires for a single circuit in separate conduits as the pipes heat up

4.Do not install power cables and signal cables in parallel as they will adversely affect each other.

5.When high-voltage devices are installed in the panel, due to noise, this should be separated as much as possible from high voltage and power lines when wiring.

6.Install the PLC at least 200 mm from any high voltage or power lines, or make sure that the high current and power lines are contained within metal tubing, and ground to a resistance of 100 W.


●Purpose of Grounding

There are two purposes for grounding.

1.Protective grounding which prevents electrocution to people by maintaining an grounding against leakage, induction and occasional accidents.

2.Grounding, including that to prevent external noise or noise which emenates from other devices or equipment, or noise that could disturb other devices or equipment, and that is necessary to protect
device and system functions. These grounding issues may need to be resolved through experience
and experimentation. It is important to take great care and consideration before performing the grounding.

●Grounding Method and Precautions

(1) Principles of One-point Grounding

It would be best to consider that it is grounding that “determines electrical potential”. Do not use common return circuits to return currents for grounding. Under normal conditions, do not send current through grounding lines (Do not use common return circuits to return currents for

(2) As much as possible use independent ground (keep grounding pole at least 10m away from other grounding poles).

1.Use Ground to 100 W or less, and use independent ground which is separated from the grounding of other devices

2.If using an independent ground is not possible, then use a common ground as shown in figure Connect to the ground pole of the other device.

3.Never connect to the same ground as a device that draws a large amount of power, such as a motor or inverter. Ground the devices separately to avoid mutually adverse influences.

4.To prevent electrical shock, do not connect to ground poles (especially steel frames) to which multiple devices are connected.

5.As much as possible keep the grounding pole close to the PLC, and keep the grounding line short.

Also read
Troubleshooting the plc system
What are the applications of PLC?
3 Commonly used Programming languages for PLC