Electromagnetic shielding effectiveness requirements for optical cables

Shielding effectiveness (SE) for optical cables is typically measured using standards such as IEEE 299, IEC 62153 series, and emerging IEEE P2855, which define procedures to quantify electromagnetic attenuation across a broad frequency range.Key StandardsIEEE 299: This standard, titled Standard Method for Measuring the Effectiveness of Electromagnetic Shielding Enclosures, provides guidelines for evaluating shielding performance over frequencies from 9 kHz to 18 GHz, with extensions up to 100 GHz. While originally developed for shielded rooms and enclosures, its principles are applied to cable assemblies to determine attenuation levels and EMI protection . IEC 62153-4-3 and IEC 62153-4-21: These standards focus on transfer impedance (Zt) and shielding effectiveness of cable/connector assemblies. Zt measures the voltage induced on the inner conductor per unit current on the shield, providing a length-dependent metric of shielding quality. SE is then calculated by comparing power received in shielded versus unshielded conditions, often expressed in decibels (dB), . IEEE P2855 (Draft): This emerging standard provides recommended practices for characterizing cable and connector assembly shielding effectiveness from DC up to 40 GHz. It formalizes methods for both transfer impedance and SE measurements, specifically addressing modern high-speed and optical hybrid cables .Measurement ConceptsShielding Effectiveness (SE): Defined as the ratio of electromagnetic energy transmitted on one side of the shield to that transmitted to the opposite side, expressed in dB. Higher SE values indicate better shielding; for example, 80 dB provides stronger EMI protection than 60 dB .Transfer Impedance (Zt): Reciprocal to SE in concept, Zt is measured using triaxial fixtures or line injection methods, with units of Ohms per meter. Lower Zt indicates better shielding performance .Frequency Considerations: SE varies with frequency; optical cables may require broadband shielding from low kHz to GHz ranges depending on application, including industrial, military, or medical environments .Practical Shielding Types for Optical CablesBraided Shields: Copper or silver-plated braids provide robust EMI protection, typically achieving 40–85 dB SE up to 18 GHz .Helically-Wrapped Foils: Metalized polymer foils or polyimide layers offer high flexibility and moderate SE, often used in hybrid optical-electrical cables .Superscreened Designs: Incorporating Mu-metal wraps and optimized braids, these designs enhance low-frequency SE and reduce transfer impedance for critical applications .SummaryFor optical cables, SE standards combine IEEE 299, IEC 62153 series, and emerging IEEE P2855 guidelines. Measurement involves evaluating attenuation across relevant frequencies, using SE in dB and Zt in Ohms per meter. Shielding design—braids, foils, or superscreened constructions—directly impacts performance, and the choice depends on the electromagnetic environment and application requirements .