PLC Electrical Noise: How EMI Causes Intermittent PLC Problems

Industrial PLC systems can operate reliably for years, but electrical noise and electromagnetic interference (EMI) can cause difficult-to-diagnose intermittent problems. A PLC may work normally for hours and then suddenly lose communication, read an incorrect sensor signal, trigger a false input, reset, or stop the machine.

These problems are particularly common in manufacturing environments with VFDs, servo drives, motors, welding equipment, contactors, solenoids, transformers, and high-current electrical equipment.

The challenge is that electrical noise problems often do not produce a permanent failure. The PLC may appear to be working correctly when a technician arrives.

This guide explains how to identify PLC electrical noise, common EMI sources, troubleshooting methods, and ways to prevent intermittent PLC problems.

What Is PLC Electrical Noise?

PLC electrical noise is unwanted electrical interference that affects PLC power, input/output signals, communication networks, or control circuits.

Common sources include:

• Variable frequency drives (VFDs)

• Servo drives

• AC and DC motors

• Motor contactors

• Solenoid valves

• Welding equipment

• Transformers

• Switching power supplies

• High-current cables

• Poor grounding

• Long signal cables

• Improper cable routing

• Nearby electrical equipment

Electrical noise can enter a PLC system through several paths:

1. PLC power supply

2. Digital inputs

3. Digital outputs

4. Analog signals

5. Communication cables

6. Shielding and grounding

7. Shared power circuits

Common Symptoms of PLC Electrical Noise

Electrical noise can produce symptoms that appear random.

Common symptoms include:

• PLC inputs turning ON unexpectedly

• PLC inputs turning OFF unexpectedly

• Analog values fluctuating

• HMI communication dropping

• Remote I/O going offline

• Ethernet communication errors

• PLC CPU faults or resets

• Sensor signals intermittently changing

• VFD communication problems

• Servo faults

• False alarms

• Machine stopping unexpectedly

• PLC restarting when motors start

• Outputs activating unexpectedly

• Intermittent safety-related faults

One of the most important clues is timing.

For example:

A PLC input becomes unstable every time a large motor starts.

That pattern strongly suggests that electrical interference should be investigated.

How EMI Enters a PLC System

There are three common coupling mechanisms.

1. Conducted Noise

Conducted noise travels through electrical wiring.

For example:

A VFD may generate high-frequency electrical disturbances that travel through a shared power system and affect sensitive control equipment.

Possible symptoms include:

• PLC resets

• Power supply instability

• Analog signal fluctuations

• Communication errors

2. Radiated Noise

Radiated EMI travels through the surrounding electromagnetic field.

This can happen when sensitive control cables run close to:

• VFD output cables

• Servo motor cables

• Welding cables

• High-current conductors

• Transformers

The interference can be picked up by nearby signal wiring.

3. Inductive or Capacitive Coupling

Noise can also transfer between nearby cables.

For example, a long sensor cable running parallel to a motor cable can pick up interference.

This is especially important for:

• Analog signals

• Encoder signals

• Thermocouples

• Load cells

• Pressure transmitters

• 4–20 mA signals

• 0–10 V signals

1. Check When the PLC Problem Occurs

Start by documenting exactly when the problem occurs.

Ask:

• Does it happen when a motor starts?

• Does it happen when a VFD changes speed?

• Does it happen when a servo moves?

• Does it happen when a solenoid activates?

• Does it happen when welding starts?

• Does it happen at a specific machine cycle?

• Does it happen only at high production speed?

• Does it happen only during certain shifts?

Create a timeline.

For example:

A strong correlation between an electrical event and a PLC fault is an important troubleshooting clue.

2. Check PLC Power Supply

Measure the PLC power supply under normal and abnormal operating conditions.

For a 24 VDC control system, check:

• DC voltage

• Voltage stability

• Power supply loading

• Grounding

• Wiring connections

• Fuse condition

• Power supply temperature

Do not only measure the voltage when the machine is operating normally.

If possible, monitor the supply during the event that causes the PLC problem.

A brief voltage disturbance may not be visible with a basic multimeter.

3. Check the PLC Power Supply for Overloading

A control power supply may be overloaded by:

• Sensors

• Solenoids

• Relays

• Contactors

• HMI equipment

• Communication devices

• Remote I/O

• Other control components

If the power supply is operating close to its capacity, voltage disturbances may become more likely.

Consider separating sensitive PLC/control power from high-load devices when the system design allows it.

4. Check Grounding

Poor grounding is a common contributor to electrical noise problems.

Inspect:

• PLC panel grounding

• Cabinet grounding

• Power supply grounding

• Machine frame grounding

• Shield connections

• VFD grounding

• Motor grounding

• Ground straps

• Ground terminals

Look for:

• Loose connections

• Corrosion

• Broken ground straps

• Poor bonding

• Improper shield termination

A PLC control cabinet should have a properly designed grounding and bonding system appropriate for the equipment manufacturer's requirements and applicable electrical codes.

5. Inspect Cable Routing

Cable routing is one of the most important areas to investigate.

Avoid running sensitive signal cables directly alongside high-power cables for long distances.

Potential sources include:

• VFD motor cables

• Servo cables

• AC motor cables

• Welding cables

• Transformer wiring

Sensitive cables include:

• Analog signals

• Encoder cables

• Communication cables

• Sensor cables

• Thermocouple cables

When possible, maintain appropriate physical separation and follow the PLC, drive, sensor, and cable manufacturer's installation requirements.

6. Separate VFD Cables From PLC Signal Cables

VFDs are a particularly important source of high-frequency electrical interference.

A typical arrangement might contain:

VFD → Motor

with a high-power motor cable carrying switching waveforms.

If a sensitive sensor cable is routed next to this motor cable, the sensor signal may become unstable.

Possible symptoms include:

• False PLC inputs

• Analog signal fluctuations

• Encoder errors

• Communication errors

• Intermittent machine faults

Cable separation, appropriate shielding, grounding, and installation practices are important.

7. Check Shielded Cables

Shielding can reduce electromagnetic interference when correctly designed and installed.

Common shielded cables include:

• Encoder cables

• Analog signal cables

• Communication cables

• Sensor cables

Check for:

• Damaged shielding

• Incorrect termination

• Loose shield connections

• Improper grounding

• Excessively long unshielded sections

• Poor connector installation

Do not assume that simply having a shielded cable means the system is immune to EMI.

The shield termination method should follow the equipment manufacturer's requirements and the system's EMC design.

8. Check Analog Inputs

Analog signals are particularly sensitive to electrical interference.

Examples include:

• 4–20 mA

• 0–10 V

• Thermocouples

• RTDs

• Load cells

• Pressure sensors

Symptoms can include:

• Random value changes

• Unstable readings

• Sudden spikes

• Signal dropping to zero

• Incorrect engineering values

• PLC alarms

For example:

A pressure transmitter normally reads:

6.2 bar

but occasionally jumps to:

9.8 bar → 2.1 bar → 7.5 bar

while the physical process remains stable.

This can indicate a signal integrity problem, although the transmitter itself should also be checked.

9. Check Digital Inputs for False Signals

Electrical noise can cause a digital input to appear ON when the field device did not actually activate.

For example:

A proximity sensor should remain OFF.

But the PLC input monitor shows:

OFF → ON → OFF

without the target moving.

Possible causes include:

• EMI

• Poor sensor wiring

• Poor grounding

• Long cable runs

• Damaged cable

• Incorrect input configuration

• Sensor failure

Use a meter or appropriate diagnostic equipment to determine whether the signal is actually changing at the input terminal.

10. Check Relay and Solenoid Loads

Inductive loads can generate electrical transients when they are switched.

Examples include:

• Relays

• Contactors

• Solenoid valves

• Electromagnetic brakes

• Motor starters

When an inductive load is turned off, the collapsing magnetic field can generate a voltage transient.

Depending on the circuit design, this can contribute to interference.

Potential mitigation methods include appropriately rated:

• Flyback diodes

• RC suppression

• Surge suppressors

• Snubbers

The correct suppression method depends on whether the load is AC or DC and on the manufacturer's requirements.

11. Check VFD and Servo Drive Installation

If intermittent PLC faults started after installing a VFD or servo drive, investigate the drive installation.

Check:

• Motor cable routing

• Shielding

• Grounding

• Drive grounding

• Control cable routing

• Encoder cable routing

• Communication cables

• Filter installation

• Drive parameters

• Motor cable length

• Cabinet layout

A new VFD can change the electrical environment inside a control cabinet significantly.

12. Check Industrial Ethernet Problems

EMI can also contribute to intermittent communication problems.

Symptoms include:

• HMI disconnects

• PLC communication timeout

• Remote I/O offline

• EtherNet/IP errors

• PROFINET errors

• Device reconnects

• Packet errors

• Network instability

Before replacing a PLC communication module, check:

• Ethernet cable

• Connectors

• Switch

• Shielding

• Cable routing

• Grounding

• Network topology

• Device power

See also:

PLC Communication Failure: Troubleshooting Ethernet, I/O and HMI Problems

13. Check PLC Diagnostic Information

When the PLC faults, capture the diagnostic information before resetting the system.

Record:

• Fault code

• Timestamp

• CPU status

• I/O status

• Communication status

• Network diagnostics

• Module diagnostics

• Program error information

If the system is restarted before collecting the information, valuable troubleshooting evidence may be lost.

14. Check PLC Scan Time and Watchdog Faults

Electrical noise does not always cause a direct communication fault.

It can sometimes trigger abnormal conditions that contribute to:

• Excessive scan time

• Communication retries

• Program errors

• Watchdog faults

• CPU faults

If a PLC reports a watchdog or scan-time problem, investigate the actual diagnostic information rather than assuming that the CPU itself is defective.

15. Check the Control Cabinet Layout

Cabinet layout can significantly affect noise performance.

Look for situations where:

• VFDs are mounted directly beside sensitive PLC equipment

• High-current wiring crosses signal cables

• Communication cables run beside motor cables

• Power and control wiring are mixed unnecessarily

• Heat-producing equipment is concentrated around the PLC

• Shield termination is poorly designed

A well-designed cabinet should consider both electrical noise and thermal management.

16. Check for Recent Electrical Changes

Ask:

What changed before the problem started?

Potential changes include:

• New VFD

• New motor

• New servo

• New PLC module

• New sensor

• New HMI

• New Ethernet switch

• New power supply

• New conveyor

• New welding equipment

• Cabinet modification

• Cable rerouting

• Software change

• Production expansion

A new electrical device may introduce a new source of EMI.

PLC Electrical Noise Troubleshooting Flowchart

Use this basic troubleshooting sequence:

Intermittent PLC problem

↓

Does the problem correlate with a machine/electrical event?

→ No → Continue general PLC troubleshooting

→ Yes → Identify the electrical event

↓

Motor / VFD / Servo / Solenoid / Welding equipment?

↓

Check:

• Power supply

• Grounding

• Cable routing

• Shielding

• Cable condition

• Suppression

• Drive installation

↓

Is the PLC signal itself changing?

→ Yes → Investigate input/output/signal path

→ No → Investigate PLC program, communication, or hardware

↓

Does the problem disappear after isolating the suspected noise source?

→ Yes → Correct EMC/installation issue

→ No → Continue troubleshooting

PLC Electrical Noise Troubleshooting Checklist

How to Confirm an EMI Problem

Finding EMI problems can require more than a simple multimeter.

Depending on the application, technicians may use:

• Oscilloscope

• Power quality analyzer

• EMI/EMC measurement equipment

• Network diagnostics

• PLC diagnostic software

• Data logging

• High-speed signal monitoring

The objective is to correlate the electrical disturbance with the PLC problem.

For example:

VFD starts → electrical disturbance occurs → sensor signal changes → PLC input changes → machine stops

This type of correlation is much stronger evidence than simply observing that the PLC occasionally fails.

PLC Electrical Noise vs PLC Hardware Failure

An intermittent PLC problem does not automatically mean the PLC CPU is defective.

Consider the following pattern:

Possible EMI problem

• PLC works normally most of the time

• Fault occurs during motor/drive operation

• Problem disappears when equipment stops

• Inputs or analog values fluctuate

• Communication temporarily drops

• Problem is difficult to reproduce

Possible hardware problem

• Same module repeatedly reports a hardware fault

• Fault remains after power and wiring checks

• One specific channel consistently fails

• Module diagnostics identify hardware failure

• Replacement with a known-good module eliminates the problem

Proper troubleshooting should separate environmental/electrical problems from actual PLC hardware failures.

Should You Replace the PLC?

Do not replace a PLC immediately just because it has intermittent faults.

Before replacing the CPU or I/O module, check:

1. Power supply

2. Grounding

3. Wiring

4. Shielding

5. Cable routing

6. VFD/servo installation

7. Communication network

8. Sensors

9. I/O modules

10. PLC diagnostics

11. Program configuration

12. Environmental conditions

If the PLC continues to fail after the surrounding system has been verified, then repair or replacement may be appropriate.

PLC Repair vs Replacement vs EMI Correction

There are three different problems that can look similar.

Problem 1: PLC hardware failure

The PLC itself may require:

• Repair

• Refurbishment

• Replacement

Problem 2: PLC program problem

The hardware may be functional, but the software or configuration requires correction.

Problem 3: Electrical noise problem

The PLC may be completely functional, but EMI is disrupting the system.

Replacing the PLC without correcting an EMI problem may simply cause the new PLC to experience the same problem.

How to Prevent PLC Electrical Noise Problems

A preventive strategy should include:

1. Proper cabinet design

Separate high-power equipment from sensitive control equipment when practical.

2. Proper cable routing

Keep high-power and sensitive signal cables appropriately separated.

3. Correct grounding

Follow equipment manufacturer requirements and applicable electrical standards.

4. Proper shielding

Use appropriate shielded cables and termination methods.

5. VFD installation practices

Follow the VFD manufacturer's EMC installation requirements.

6. Suppression of inductive loads

Use appropriate suppression devices for coils, relays, contactors, and solenoids.

7. Maintain documentation

Document:

• Cable routes

• Grounding points

• Shield termination

• PLC wiring

• Network topology

• Drive installation

8. Monitor recurring faults

Track:

• PLC faults

• Communication errors

• Sensor errors

• VFD faults

• Machine downtime

Repeated patterns can help identify electrical noise problems before they become major production issues.

Common Mistakes When Troubleshooting PLC EMI

Avoid these mistakes:

Mistake 1: Replacing the PLC first

The PLC may not be the root cause.

Mistake 2: Ignoring VFDs

VFDs are an important source to investigate when intermittent problems begin after drive installation or modification.

Mistake 3: Ignoring cable routing

Even a good PLC can experience signal problems when sensitive cables are improperly routed.

Mistake 4: Assuming a shield automatically solves EMI

Shielding must be properly selected, routed, and terminated.

Mistake 5: Resetting the PLC before collecting diagnostics

The original fault information may be lost.

Mistake 6: Only checking the system when it is working

Intermittent problems require observation during the actual failure event whenever possible.

Mistake 7: Changing multiple things simultaneously

If several components are changed at once, it becomes difficult to identify the actual root cause.

When to Call an Industrial Automation Specialist

Consider professional support when:

• PLC faults cannot be reproduced reliably

• EMI appears to affect multiple devices

• VFD/servo interference is suspected

• Analog signals are unstable

• Industrial Ethernet repeatedly disconnects

• PLC hardware may be damaged

• Machine downtime is expensive

• PLC programs or documentation are missing

• The system contains obsolete equipment

• Electrical modifications are required

A qualified automation technician or controls engineer can analyze the complete signal path rather than replacing components based only on symptoms.

MaintenanceFixer Can Help

When a PLC problem is difficult to diagnose, finding the right industrial automation resource can be as important as finding the replacement part.

MaintenanceFixer can connect manufacturing companies with resources for:

• PLC troubleshooting

• PLC repair

• PLC programming

• PLC I/O repair

• VFD repair

• Servo drive repair

• HMI troubleshooting

• Industrial Ethernet troubleshooting

• Sensor troubleshooting

• Controls engineering

• Industrial automation parts

• Emergency maintenance support

If your plant is experiencing an intermittent PLC problem, you can use MaintenanceFixer to find an appropriate supplier, repair company, or technical service provider.

Frequently Asked Questions

Can electrical noise damage a PLC?

Electrical disturbances can contribute to abnormal operation or, under severe conditions, equipment damage. Proper power quality, grounding, shielding, protection, and installation practices help reduce risk.

Can a VFD cause PLC communication problems?

Yes. VFD installations can introduce electrical interference that affects nearby signal and communication wiring if the installation is not properly designed.

Why does my PLC input randomly turn ON?

Possible causes include electrical noise, sensor problems, wiring problems, incorrect input configuration, grounding issues, or a defective input module.

Why does my PLC work normally and then fail randomly?

Intermittent faults can be caused by electrical noise, temperature, vibration, loose connections, communication problems, power disturbances, software conditions, or failing hardware.

Can EMI cause analog PLC readings to fluctuate?

Yes. Analog signals can be sensitive to electrical interference, especially when signal wiring is improperly routed or shielded.

Should I replace my PLC if it has intermittent faults?

Not necessarily. Verify the power supply, wiring, grounding, shielding, communications, I/O, and environmental conditions before replacing the PLC.

How can I find the source of PLC electrical noise?

Start by correlating the PLC fault with machine events such as VFD starts, motor switching, servo movement, solenoid activation, or welding. Then investigate power, grounding, cable routing, shielding, and signal behavior during the event.

Final PLC Electrical Noise Checklist

Before replacing a PLC because of an intermittent problem:

• Check PLC power

• Check 24 VDC power supply

• Check grounding

• Check cabinet bonding

• Check VFD installation

• Check servo installation

• Check motor cable routing

• Check signal cable routing

• Check shielding

• Check sensor wiring

• Check analog signals

• Check Ethernet cables

• Check communication diagnostics

• Check solenoid/relay suppression

• Check PLC diagnostics

• Record the exact time of failures

• Check recent electrical modifications

• Correlate failures with machine events

• Test with appropriate diagnostic equipment

• Confirm the root cause before replacing hardware

A PLC that appears to have a random problem may actually be responding to an electrical environment problem. Identifying the source of the interference can prevent repeated PLC failures, unnecessary hardware replacement, and costly manufacturing downtime.

Related PLC Troubleshooting Articles

• PLC Repair: How to Diagnose a PLC That Will Not Power On

• PLC CPU Fault: Common Causes and Troubleshooting Guide

• PLC Input Not Working: Causes and Troubleshooting

• PLC Output Not Working: Common Causes and Solutions

• PLC Communication Failure: Troubleshooting Ethernet, I/O and      HMI Problems

• PLC Program Errors: Common Causes and Troubleshooting Guide

• PLC Program Backup: Why Manufacturing Plants Need a Backup      Strategy

• PLC I/O Module Failure: Symptoms, Causes and Repair Options

• PLC Analog Input Problems: 4–20mA and 0–10V Troubleshooting

• PLC Overheating: Causes, Symptoms and Prevention