EMI in industry - The Basics

This guide provides a practical overview of Electromagnetic Interference (EMI), its causes, and best-practice mitigation strategies for industrial environments. It synthesizes information from the provided technical documents into a succinct and readable format for technicians and engineers.

1. Understanding the Electromagnetic Environment

To effectively manage EMI, it's crucial to first understand the fundamental concepts and the regulatory landscape.

EMI vs. EMC

• Electromagnetic Interference (EMI) is any undesired electrical disturbance or noise that disrupts the normal operation of an electrical or electronic circuit. It can cause issues ranging from minor data errors to complete system failure.

• Electromagnetic Compatibility (EMC) is the ability of electronic systems to function correctly in their electromagnetic environment without generating or being affected by EMI. Essentially, EMC is the goal: designing systems that are immune to EMI and don't create it for others.

Sources of EMI: Intentional vs. Unintentional

EMI sources are everywhere and can be broken down into two main categories:

• Intentional Sources: These are devices designed to radiate electromagnetic energy to perform their function. Common examples in an industrial setting include Wi-Fi access points, cellular modems, two-way radios (RTs), and wireless sensor networks (e.g., LoRa, wireless power meters).

• Unintentional Sources: These are devices that radiate EMI as an unwanted byproduct of their operation. Industrial environments are filled with potent unintentional sources, such as Variable Frequency Drives (VFDs), Switched-Mode Power Supplies (SMPS), electric motors, arc welders, and the fast-switching digital circuits in modern automation systems.

Governance and Radio Spectrum Management

In New Zealand, Radio Spectrum Management (RSM), a business unit of the Ministry of Business, Innovation and Employment (MBIE), is responsible for managing the radio spectrum. Their role is to ensure that intentional radiators (like radio stations and cell networks) can operate without interfering with each other. While their primary focus is on intentional transmitters, they become involved when unintentional radiators (like faulty industrial equipment) cause harmful interference to licensed radio services.

2. The Grounding Foundation: Safety and EMI Control

Proper grounding is the cornerstone of both electrical safety and EMI mitigation. However, the requirements for each can differ, especially at high frequencies.

The Dual Role of Grounding

A ground system is a network of conductors providing a common reference potential and a controlled, low-impedance path for current. It's crucial to distinguish between its two primary functions:

• Safety Earth (Protective Earth - PE): This system is designed to carry large fault currents safely to the ground, tripping protective devices like circuit breakers to prevent electric shock. Its main characteristic is low resistance.

• EMC Ground (Functional Earth - FE): This system is designed to provide a quiet, stable reference for sensitive electronics and to shunt high-frequency EMI noise away from circuits. Its critical characteristic is low impedance, which is dominated by inductance at high frequencies.

Grounding Topologies: Star vs. Daisy-Chain

• Star Grounding (Single-Point Grounding): In this topology, each piece of sensitive equipment has its own separate ground conductor running directly to a single, common ground point. This is the best practice for sensitive instrumentation areas as it prevents noise currents from one system from interfering with another through a shared ground path. It is most effective at lower frequencies (below 1 MHz). Earthing arrangements in a multi-panel MCC which services multiple areas of plant should always be star-grounded rather than daisy-chained to avoid an accumulation of noise in one area of site propagating to another area.

• Daisy-Chaining: This involves connecting the ground of one instrument to the next, and so on, back to a single ground connection. This method is strongly discouraged for sensitive equipment as it creates common-impedance coupling, where noise from devices further down the chain can affect all other devices in the chain.

• Multi-Point Grounding: For high-frequency systems (>10 MHz), equipment is bonded at multiple points to a low-impedance ground plane (like a chassis or reference grid). This provides short, low-inductance paths for high-frequency noise currents. Most large industrial facilities use a hybrid approach, employing single-point principles for power distribution and multi-point grounding for local high-frequency electronics.

Wiring Specification and Conductor Choice

At the high frequencies associated with EMI, the physical shape of a conductor matters more than its total cross-sectional area due to a phenomenon called skin effect.

• Skin Effect: High-frequency current tends to flow only on the outer surface (or "skin") of a conductor, rendering the central core useless. This increases the effective AC resistance.

• Inductance: The impedance of a ground path at high frequency is dominated by its inductance, not its resistance. Round wires have significantly higher inductance than flat conductors.

For these reasons, flat, wide braided straps are preferable to standard round wires for high-frequency EMI grounding and bonding connections. They offer a much larger surface area (mitigating skin effect) and significantly lower inductance, providing a more effective path for noise currents to ground. A length-to-width ratio of 3:1 or less is often recommended for ground straps to keep inductance low.

VSD and Instrument Shield Termination

The effectiveness of a shielded cable depends entirely on how the shield is terminated.

• Correct Method (360-Degree Termination): The shield must be terminated with a continuous, 360-degree connection directly to the metal chassis or connector backshell at the point of entry. This provides the lowest possible inductance path for noise currents to be shunted to ground.

• Incorrect Method ("Pigtail"): Terminating a shield with a short wire (a pigtail) is strongly discouraged. This wire introduces significant inductance, which creates a high impedance at high frequencies, effectively turning the shield into an antenna that can radiate noise.

• Grounding at One End vs. Both Ends:

  • Instrument screen grounding: For low-frequency interference (<1 MHz), grounding the shield at only one end (typically the source) can prevent ground loops.

  • VSD Screen Grounding: For high-frequency EMI (>1 MHz), grounding the shield at both ends is generally more effective as it provides a low-impedance path for high-frequency noise currents to flow to ground. The VSD screen should remain isolated from protective earth until it terminates at the motor which it supplies. It's not uncommon to find that screen and protective earth get linked together at the motor isolator through an earth terminal. This fixable by swapping for a normal, insulated terminal.

The Cable Tray Trap

Metallic cable trays are designed for mechanical support and cable routing, not as electrical conductors. Cable trays should never be used as the primary equipotential bonding or safety earth conductor. They are often constructed from sections with unreliable electrical connections and can carry significant EMI "noise" currents from various sources. Using them as a ground reference can inject this noise directly into sensitive equipment connected to it through inductive coupling. A separate, dedicated grounding conductor should always be run with the cables.

3. How Small Problems Escalate

EMI issues often follow a pattern of frequency escalation, where low-frequency power disturbances create higher-frequency problems.

The Source: Power Electronics and Fast Switching

Modern devices like VFDs and SMPS use power transistors that switch on and off thousands of times per second to control power efficiently. These rapid switching actions create sharp-edged voltage and current waveforms, characterized by a high rate of change (high dV/dt and di/dt). According to Fourier analysis, these sharp edges are rich in high-frequency harmonic content, extending far beyond the fundamental switching frequency.

The Escalation Ladder

1. Low-Frequency Power Quality Issues (Harmonics): The non-sinusoidal currents drawn by non-linear loads like VFDs distort the main 50 Hz power supply. This creates low-frequency harmonics (e.g., up to the 40th or 50th order, ~2.5 kHz) that cause issues like overheating and inefficiency.

2. Mid-Frequency Conducted EMI (up to 30 MHz): The high-frequency energy from the sharp switching edges and their associated harmonics propagates along power cords and other cables as conducted EMI. This is the primary mode of interference at lower frequencies, typically up to 30 MHz.

3. High-Frequency Radiated EMI (30 MHz+): When conducted noise currents flow on a cable, the cable acts as an unintentional antenna, radiating the noise into the environment as an electromagnetic wave. At higher frequencies (generally above 30 MHz), even short lengths of wire or PCB traces can become efficient radiating antennas.

This chain reaction shows how a "power quality" problem at 50 Hz can ultimately cause a "radiated EMI" problem interfering with a Wi-Fi system at 2.4 GHz.

Coupling Mechanisms and Fields Explained

Noise gets from a source to a victim via coupling paths.

• E-Field (Electric Field): Created by voltage. An E-field exists between two conductors at different potentials. In simple terms, think of it as the "pressure" part of electricity. High-voltage, low-current sources (like an unterminated wire with high voltage) create dominant E-fields. Noise that couples this way is called capacitive coupling.

• H-Field (Magnetic Field): Created by current. An H-field is generated by the flow of electrical current. In simple terms, think of it as the "flow" part of electricity. High-current, low-voltage sources (like a loop of wire carrying significant current) create dominant H-fields. Noise that couples this way is called inductive coupling.

4. Recognizing the Symptoms of Conducted EMI

The presence of conducted EMI often manifests as perplexing and intermittent issues. Common symptoms include:

• Instrumentation and Control: Inaccurate or unstable analog sensor readings (e.g., 4-20mA or 0-10V signals drifting), encoders reporting incorrect positions, and unexplained PLC or VFD malfunctions or shutdowns.

• Communications: Data corruption or loss in industrial networks like Ethernet or serial communications (RS-485, RS-232).

• Visual and Audible: Flickering lights or distorted equipment displays, and audible buzzing or humming from speakers or intercoms.

• General Electronics: Unexplained system lockups, processor resets, and critical system alarms failing to activate or triggering falsely.

5. A Three-Tiered Approach to EMI Investigation

A structured, three-tiered approach can be used to efficiently troubleshoot EMI problems.

Tier 1: Initial Survey (Low-Cost / Basic)

This is a quick, qualitative check to see if strong EMI sources are present.

• Equipment: A simple handheld AM radio.

• Method: Tune the radio to a quiet spot on the AM band and walk around the facility or near suspect equipment. The radio will produce buzzing, clicking, or whining sounds when near a significant EMI source like a VFD or SMPS.

• Limitations: This method is not quantitative, has a limited frequency range, and cannot be used for compliance or detailed diagnostics. It is only for initial "sniffing".

Tier 2: Engineering / Pre-Compliance Diagnostics

This stage is for pinpointing the physical source of EMI and characterizing its nature.

• Equipment:

  • An Oscilloscope with Fast Fourier Transform (FFT) capability to view signals in the frequency domain.

  • Near-Field Probes (E-field and H-field) to "sniff" out specific leaky spots on PCBs, enclosure seams, or cables.

  • RF Current Probes to clamp onto cables and measure the noise currents flowing on them.

• Method: Use the probes connected to the oscilloscope or a spectrum analyzer to identify which components, traces, or cables are emitting the most noise. This allows for targeted fixes (e.g., adding a ferrite, improving a ground connection) and provides relative "before and after" measurements.

• Limitations: Near-field measurements are relative and do not correlate well quantitatively with far-field compliance test results. This tier is for diagnostics, not formal compliance.

Tier 3: Full Compliance Testing

This is the formal, quantitative process for verifying compliance with regulatory standards (e.g., CISPR, IEC).

• Equipment:

  • A high-performance Spectrum Analyzer or EMI Receiver.

  • Line Impedance Stabilization Network (LISN) for conducted emissions testing.

  • Calibrated Antennas (biconical, log-periodic, horn) for radiated emissions testing.

  • A controlled environment like a Semi-Anechoic or Fully Anechoic Chamber to isolate the test from external ambient noise.

• Method: The equipment is set up and operated according to strict standards (e.g., IEC 61000-6-4, CISPR 11). Precise measurements are taken and compared against the legal limits.

• Limitations: This process is expensive, requires specialized equipment and facilities, and is typically performed by accredited test laboratories. It is used for final verification, not initial troubleshooting.