What Is Electrical Noise in Electronics? How Is It Prevented? What Is It?

Electronic circuits and communication lines—the heart of industrial automation systems—are highly sensitive to electrical noise (interference) caused by environmental factors. In electronics, noise refers to any unwanted electrical signal or form of energy that appears randomly or periodically in an electronic circuit or communication line and disrupts the integrity of the useful signal. These unwanted signals can degrade system performance, cause data loss, lead to malfunctions in control systems, and even result in equipment failures.

Interference is generally divided into two main categories: Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI). While EMI encompasses electrical noise across a broad frequency spectrum, RFI refers primarily to noise in the radio frequency range. Both types can propagate in two different ways: conducted and radiated:

• Conducted Noise: Noise transmitted through direct physical contact via power lines, signal cables, or ground paths. For example, noise fed back into the grid by a motor’s switched-mode power supply falls into this category.
• Radiated Noise: Noise that propagates through the air in the form of electromagnetic waves. High-frequency circuits, radios, arc welders, or events such as lightning can cause this type of noise. It is common for cables to act as antennas, radiating or receiving noise.

In industrial environments, sources of noise are quite diverse and are often overlooked. Major sources include: Motors and Variable Frequency Drives (VFDs), Switched-Mode Power Supplies, Relays and Contactors, High-Voltage Lines, Ground Loops, Lightning Discharges, and even nearby radio transmitters. These sources can have devastating effects on PLCs, SCADA systems, sensors, actuators, and other automation components. Proper identification and prevention of interference are critical for the uninterrupted and reliable operation of industrial facilities.

Operating Principle and Technical Data

Noise prevention in electronics requires a multifaceted engineering approach that involves understanding the source of noise, its propagation path, and its effect on the receiver. The fundamental principle is to suppress noise at its source, block its propagation paths, and isolate the receiver from noise. In this context, the Signal-to-Noise Ratio (SNR) is a key parameter in evaluating a system’s performance; a high SNR indicates better signal quality. Interference suppression techniques aim to increase this ratio.

Noise Propagation Mechanisms and Suppression Techniques

Interference propagates through various mechanisms, and specific suppression methods have been developed for each mechanism:

• Capacitive Coupling: The transfer of noise from one circuit to another via unwanted capacitance between two conductors. It is effective for high-frequency noise.
  • Prevention: Increasing the distance between conductors or inserting a grounded shield (based on the Faraday cage principle).
• Inductive Coupling: The transfer of noise caused by the magnetic field generated by a current change in one circuit inducing a voltage in another nearby conductor. This is particularly common between power lines and signal lines.
  • Prevention: Separate the cables; use twisted-pair cables that cancel out magnetic flux; suppress high-frequency currents with ferrite beads.
• Common-Mode Coupling (Ground Loop): This occurs when currents from different circuits flow through a common ground path, creating a voltage drop across this common impedance, which in turn affects the other circuits.
  • Prevention: Use a star-grounding scheme, separate signal and power grounds, and use optical isolators or isolation transformers.
• Radiation (Antenna Effect): The emission of high-frequency currents or voltages into the air as electromagnetic waves by cables or circuit boards.
  • Prevention: Metallic shielding (metal enclosures, shielded cables), minimizing cable lengths, and proper termination.

Technical Data and Applications

Field Considerations

• Comprehensive Grounding System Design and Implementation:

  In industrial automation systems, the most critical step in interference prevention is a proper and robust grounding infrastructure. Star grounding (single-point grounding) is an ideal configuration where all grounding lines converge at a single point; this prevents the formation of ground loops. Different types of grounding—such as power grounding, instrument grounding, and digital grounding—must be kept separate and connected only at a central point. Grounding bars should be made of low-impedance, large-cross-section conductors, and connections must be corrosion-resistant. Grounding resistance measurements should be performed periodically to ensure values remain within specified limits.

• Proper Cabling and Cable Management:

  Cables can act as both a source and a receiver of noise. Power cables (high current and voltage) and signal cables (low current and voltage) must be routed in physically separate channels or at a minimum distance of 15–30 cm from one another. Shielded cables are mandatory, especially for analog signals and high-frequency communication lines. The shield must be continuous along the entire length of the signal cable and grounded at only one end (typically the source end). Multi-point grounding can increase interference by creating ground loops. In digital communication lines (RS-485, Profibus, etc.), twisted-pair cables should be used to minimize noise caused by inductive coupling. Cable ends should be terminated with the correct termination resistors, and unnecessary lengths should be avoided.

• Strategic Use of EMI/RFI Filters:

  Appropriate EMI/RFI filters should be installed at the output of noise sources and at the input of sensitive receivers. In particular, filters should be integrated into the power inputs and outputs of high-noise-generating devices such as Variable Frequency Drives (VFDs), switched-mode power supplies, and large contactors. Filters suppress noise at its source, preventing it from propagating. Ferrite beads can be added in series to signal and power lines to absorb high-frequency noise. The effectiveness of filters should be maximized by ensuring they are properly grounded and using short connection paths.

• Panel and Equipment Shielding:

  Control panels, field enclosures, and electronic equipment housings must be metallic and properly grounded to block electromagnetic noise coming from outside or radiating from inside. Panel covers and access points should be sealed with EMI gaskets to prevent leakage. Shielding material should be selected based on the frequency and intensity of the noise. Internal noise sources (e.g., power supplies) can be isolated using separate metal compartments. During equipment installation, care must be taken to maintain electrical continuity between metal surfaces.

• Differential Signaling and Optical Isolation:

  Using differential signaling (e.g., RS-485, HART) for the transmission of sensitive analog signals or long-distance digital communications significantly eliminates common-mode noise. Since the potential difference between the two signal lines is measured, noise that affects both lines equally is canceled out. In extremely noisy environments or when establishing communication between systems with different potentials, using optical isolators or fiber-optic cables is the most effective method. This completely cuts off the electrical connection, making noise transfer impossible.

Common Problems and Solutions

In the field of industrial automation, interference-related issues typically manifest in random and difficult-to-diagnose ways. This can lead to production downtime and costly failures. Here are some common issues and expert solutions:

• Random Errors and System Freezes (PLC/DCS):

  Problem: Control systems (PLC, DCS) unexpectedly generate errors, randomly reset, or freeze. General messages such as “memory error” or “CPU error” appear in event logs.
  Causes: Typically, these issues stem from poor grounding, ground loops, sudden voltage drops or spikes (transients) in power lines, or severe EMI generated by nearby high-power switching equipment.
  Solution: First, check the grounding system and ensure it complies with the star grounding principle. Add a high-quality EMI/RFI filter and/or an isolation transformer to the PLC power supply. Ensure that the PLC and associated I/O modules are securely grounded to the metal panel. Use voltage suppressors (surge suppressors) or transient voltage suppressor (TVS) diodes to suppress transients on the power line. Reinforce the metal shielding of the panel housing the PLC and use EMI gaskets at all cable entry/exit points.

• Fluctuations or Incorrect Readings in Sensor Data:

  Problem: Values from analog sensors—such as pressure, temperature, or level—are unstable, fluctuate constantly, or suddenly display incorrect values.
  Causes: Inductive or capacitive coupling resulting from sensor cables running close to power cables, poor sensor shielding, or the sensor being included in the ground loop.
  Solution: Replace the sensor signal cables with shielded twisted-pair cables and ground the shield only on the control panel side. Separate the signal cables from the power cables as much as possible, or run them through separate metal conduits. Use low-pass filters in the PLC or DCS analog input modules, or apply signal filtering (averaging) in the software. If necessary, use signal isolators for the sensor signal to break ground loops and obtain a noise-free signal.

• Industrial Communication Interruptions (RS-485, Profibus, Ethernet):

  Problem: Serial or Ethernet communication between the PLC and the HMI, VFD, or other devices is intermittent; data packets are lost, or the connection frequently drops.
  Causes: Communication cables are affected by nearby EMI/RFI sources, impedance mismatch, missing or incorrect cable termination, or exceeding cable length limits.
  Solution: Replace the communication cables with shielded twisted-pair cables of the appropriate category (e.g., CAT6a for Ethernet). Ensure that the shielding is properly grounded at a single point in the control panel. For serial communication lines such as RS-485 and Profibus, verify that termination resistors are set to the correct value (typically 120 ohms) and are present at both ends of the line. For Ethernet, use industrial-grade fiber-optic converters or industrial Ethernet switches to increase resistance to electrical noise. Ensure that the maximum cable length limits are adhered to.

• Abnormalities in Motor Control (VFDs):

  Problem: Motors controlled by VFDs are experiencing overheating, loud noise, torque fluctuations, or unexpected stoppages.
  Causes: High-frequency switching noise (PWM signal) generated by VFDs propagating through motor cables, inducing current flow in motor bearings (bearing currents), or causing EMI in nearby sensor and control cables.
  Solution: Reduce voltage stress on the motor and radiated noise by adding sine wave filters, dv/dt filters, or output chokes to the VFD output. Be sure to replace the motor cables with shielded, low-capacitance VFD-compatible cables, and ground the shield 360 degrees on both the VFD and motor sides. Ensure that the metal housings of the motor and VFD are securely grounded. Ensure that the VFD’s grounding busbar has low impedance and is directly connected to the main grounding system.

Expert Advice

In industrial automation systems, electronic interference is not just a minor nuisance—it is a serious threat that directly affects production efficiency, system reliability, and operational safety. From an expert’s perspective, electromagnetic interference (EMI) mitigation efforts represent a critical investment in the overall health and longevity of the facility. The fundamental principles and techniques covered in this guide—grounding, shielding, filtering, proper cabling, and isolation—are complementary strategies that must be addressed as part of a holistic approach. No single solution can eliminate all noise issues; the key to success lies in proactively implementing measures at every level of the system by thoroughly understanding the source of noise, its propagation paths, and interaction mechanisms.

Our field experience shows that a significant portion of interference problems stems from errors made during the system design phase or simple details overlooked during installation. Therefore, EMI/RFI compliance and interference prevention principles must be incorporated into the design process from the very beginning of automation projects. Cable routing, panel layouts, equipment selection, and grounding schemes must be planned meticulously, taking into account potential noise sources and sensitive receivers. In existing systems, potential issues should be identified and resolved before they escalate through periodic inspections and noise analyses. Tools such as thermal cameras, spectrum analyzers, and grounding testers can be invaluable aids in identifying noise sources and weak points.

It is important to remember that industrial automation is a dynamic field, and new technologies (such as IoT devices and high-speed communication protocols) can introduce new interference challenges. Therefore, it is vital for the technical team to undergo continuous training and stay up-to-date with industry best practices. Interference prevention is not a one-time process but an ongoing improvement process. Using the information provided in this guide, you can take the necessary steps to enhance the reliability and performance of automation systems in your industrial facilities, thereby preventing unwanted failures and production losses. For a safe and uninterrupted production environment, interference control is an indispensable engineering discipline.

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