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What are the best practices for wiring and grounding a PLC system to minimize interference and noise?

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PLC systems are widely used in control engineering to automate industrial processes and machines. However, they can also be affected by interference and noise from various sources, such as power lines, motors, switches, and sensors. Interference and noise can cause errors, malfunctions, and damage to the PLC and the connected devices. Therefore, it is essential to follow some best practices for wiring and grounding a PLC system to minimize these problems and ensure reliable and safe operation. In this article, we will discuss some of these best practices and explain why they are important.

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Greg Miller

Industrial Account Manager, Eaton Corporation
Mohammed Ghallab

Instrumentation Engineer @ GEA GRADE REFRIGERATION LLC Seeking advance for my company, my team and my self

1 Choose the right wires and cables

The first step to reduce interference and noise is to choose the right wires and cables for the PLC system. Depending on the type and length of the signal, you should use different types of wires and cables, such as twisted pair, shielded, coaxial, or fiber optic. Twisted pair wires consist of two wires twisted together to cancel out the electromagnetic fields generated by each wire. Shielded wires have a metallic layer around the wires to protect them from external electromagnetic fields. Coaxial cables have a central conductor surrounded by a shield and an insulator. Fiber optic cables use light pulses to transmit signals, which are immune to electromagnetic interference. You should also select the appropriate wire gauge, color code, and connector for each wire and cable.

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Greg Miller

Industrial Account Manager, Eaton Corporation
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the title question might be rephrased: how to properly construct an automation network, whether the controller is a traditional PLC, DCS system or soft PLC. The basic grounding and shielding concerns have been reduced by the adoption of copper ethernet field busses, so that the handiwork of grounding/shielding each node and termination is almost completely eliminated. The old RS485 media requirements regarding noise immunity and physically robust termination are mostly gone now... however, with the adoption of fiber optic media in these connective segments, a whole new field of expertise is required. The ability to properly pull and connect field FO runs between controllers and field devices is now the critical skill and knowledge set.

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Engr. Dr. Nwachukwu Aloy Iloanwusi DF, CMC, FIMC, FIMS(UK), FNIEEE, MNSE

Head, Transformer Service Workshop at Income Electrix Limited
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Interference and sensor failure occur when the wrong size of cable is not used or when it is not properly screened. Sensitivity is very important in Programmable Logic arrangements. So no mismatch or distortion should be allowed. Coaxial cable for individual signals is okay but will occupy a lot of space and make the wiring untidy so paired screened cable should be used in parts where the same size of cable is applicable

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Mohammed Ghallab

Instrumentation Engineer @ GEA GRADE REFRIGERATION LLC Seeking advance for my company, my team and my self
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Using what is called Clean Earthing where there is a grounding system for instruments and automation components and networks completely separate from the traditional grounding system for machinery and power sources and motors. This reduces the bad impact on instruments and modules that may caused by power system earthing. In addition, using shielded standard cables and sockets for communication helps protect the instruments.

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2 Separate the wires and cables

The second step to reduce interference and noise is to separate the wires and cables according to their function and voltage level. You should avoid running wires and cables that carry different types of signals, such as power, analog, digital, or communication, in parallel or close to each other. This can cause crosstalk, which is the unwanted transfer of signals between wires and cables. You should also avoid running wires and cables that carry different voltage levels, such as high-voltage AC, low-voltage DC, or signal-level, in parallel or close to each other. This can cause voltage drop, which is the loss of voltage due to the resistance of the wires and cables. You should use different conduits, trays, or ducts to separate the wires and cables, and label them clearly.

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3 Ground the system properly

The third step to reduce interference and noise is to ground the system properly. Grounding is the connection of the system to a common reference point, usually the earth, to provide a stable voltage level and a path for fault currents. Grounding can also help to dissipate static charges, reduce electromagnetic interference, and protect the system from surges and lightning. However, grounding can also introduce ground loops, which are unwanted currents that flow through the ground wires due to potential differences between different ground points. Ground loops can cause interference, noise, and damage to the system. To avoid ground loops, you should use a single-point grounding scheme, where all the ground wires are connected to a single ground point, such as a ground rod or a ground bus. You should also use shielded wires and cables, and connect the shield only at one end, preferably at the source.

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4 Filter the signals

The fourth step to reduce interference and noise is to filter the signals. Filtering is the process of removing unwanted frequencies or components from a signal, such as noise, harmonics, or spikes. Filtering can improve the signal quality, accuracy, and reliability. There are different types of filters, such as passive, active, analog, or digital, that can be used for different purposes and applications. For example, passive filters use resistors, capacitors, and inductors to filter signals without amplifying them. Active filters use op-amps, transistors, or ICs to filter and amplify signals. Analog filters use analog components and circuits to filter analog signals. Digital filters use software algorithms to filter digital signals. You should select the appropriate filter for each signal, and place it as close as possible to the source or the destination.

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Mohammed Ghallab

Instrumentation Engineer @ GEA GRADE REFRIGERATION LLC Seeking advance for my company, my team and my self
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Sometimes, Using signal isolators that convert from one type to the same type is essential to avoid issues related to instrument sensitivity and earthing effects. In addition, following manufacturer or standard recommendations regarding using diodes or RC filters or varistors or capacitors when required for relay or solenoid coils or similar or for earthing and EMC improvements

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5 Shield the system

The fifth step to reduce interference and noise is to shield the system. Shielding is the use of a metal enclosure or a mesh to surround the system or a part of it, to block or reflect external electromagnetic fields. Shielding can prevent interference and noise from entering or leaving the system, and protect the system from electromagnetic compatibility (EMC) issues. EMC is the ability of the system to function properly without causing or being affected by electromagnetic interference. There are different standards and regulations for EMC, depending on the industry and the application. You should comply with the relevant EMC requirements, and use proper shielding materials, techniques, and grounding for the system.

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6 Test and troubleshoot the system

The sixth step to reduce interference and noise is to test and troubleshoot the system. Testing and troubleshooting are the processes of checking and verifying the performance and functionality of the system, and identifying and resolving any problems or defects. Testing and troubleshooting can help to ensure that the system meets the specifications, requirements, and expectations, and operates safely and reliably. There are different tools and methods for testing and troubleshooting, such as multimeters, oscilloscopes, logic analyzers, signal generators, or software programs. You should use the appropriate tools and methods for each part of the system, and follow a systematic and logical procedure to test and troubleshoot the system.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are the best practices for wiring and grounding a PLC system to minimize interference and noise?

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1 Choose the right wires and cables

2 Separate the wires and cables

3 Ground the system properly

4 Filter the signals

5 Shield the system

6 Test and troubleshoot the system

7 Here’s what else to consider

Control Engineering

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