Electromagnetic Interference (EMI) presents a significant challenge to the reliable operation of industrial machinery and control systems. While robust design aims for EMI immunity, industrial cable assemblies themselves can often be pathways or sources of interference. Understanding the key factors causing EMI within these critical links is essential.
Here are the primary factors contributing to EMI issues in industrial cable assemblies:
- Conductor Type and Configuration: Using untwisted pairs significantly increases susceptibility to magnetic field coupling (H-field interference). Twisted pairs are essential for rejecting this common noise source, especially for low-level signal transmission.
- Inadequate or Improper Shielding:
- Poor Coverage: Gaps in shield braid or foil coverage (especially at connectors), low shield braid density, or damaged shields create direct paths for electromagnetic fields (both E-field and H-field) to enter or escape the cable assembly.
- Incorrect Grounding: Not terminating the shield effectively (e.g., using “pigtails”) creates a high-impedance path to ground, drastically reducing shielding effectiveness across the frequency spectrum. Lack of a true 360-degree shield termination at connector interfaces creates leakage points.
- Defective or Poorly Designed Connectors: Connectors that lack continuous EMI shielding, have inadequate backshell shielding, or use insulating coatings on metallic shells disrupt the shield integrity of the overall cable assembly, turning the connector area into a significant EMI leakage point.
- Filtering Component Omission/Failure: Cable assemblies incorporating filters (like feedthrough capacitors or filters within connectors) can suffer EMI issues if these components fail, are under-specified for the frequency range of concern, or were omitted entirely from the design when needed for noise suppression.
- External EMI Sources Proximity: Running cable assemblies in close proximity to powerful EMI radiators (large motors, variable frequency drives (VFDs), switch-mode power supplies, welding equipment, radio transmitters) dramatically increases the induced common mode noise on the cable’s shield or conductors.
- Incorrect Cable Routing: Bundling power cables carrying high currents (especially AC or switched DC) with low-level signal cables (analog sensors, communications) induces cross-talk via inductive and capacitive coupling. Running cable assemblies parallel to major EMI sources over significant distances increases coupling.
- Ground Loops: Creating unintended conductive paths between different grounding points with a potential difference can force large common mode currents to flow on cable shields, converting them into unintentional radiators and causing ground noise problems.
- Lack of Ferrites/Absence: Omitting ferrite beads or cores, particularly at cable entry/exit points of enclosures, misses an effective method of suppressing high-frequency common-mode noise currents flowing along the cable. Proper placement and material selection are critical.
- Long Cable Lengths: Longer cables inherently have higher parasitic capacitance and inductance, making them more efficient antennas for both picking up (susceptibility) and radiating (emissions) electromagnetic energy over a wider frequency range.
