Power-Line Filters for EMC Shielded Rooms: Beyond Insertion-Loss Curves

Insertion-loss curves are an important starting point when selecting power-line filters for an EMC shielded room, but they describe performance under particular test conditions. The practical question is whether the power path will provide the required interference suppression and operate safely with the project's supply, loads and installation.

1. Read the conditions behind the curve

Before comparing curves, check the frequency range, common-mode or differential-mode configuration, source and load impedances, and whether voltage or current bias was applied. The IEC's official description of CISPR 17:2011 identifies circuit impedance, operating voltage, current and ambient temperature as variables affecting filter performance. Insertion loss measured in a 50 Ω system helps compare components, but does not directly represent the impedances of the installed supply and equipment. An unbiased curve should not be treated as the result for every operating load. See the official CISPR 17 description[1] and guidance on impedance conditions[4].

2. Assess current ratings with the load and temperature

The selection specification should identify single- or three-phase operation, whether a neutral is present, AC or DC supply, supply frequency, and line-to-line and line-to-earth voltages. A dielectric test voltage is not a continuous operating-voltage rating. Nor should current selection rely only on a simple conversion from equipment power. Confirm continuous current in each conductor, starting inrush and the manufacturer's short-duration overload capability, then check derating for the actual ambient temperature. With loads such as frequency converters, also assess possible magnetic-component saturation caused by pulse currents. See the electrical-rating guidance[2] and temperature-derating explanation[3].

3. Check leakage current and protective arrangements together

Capacitance to earth helps suppress interference but also affects leakage current. Check the voltage, frequency and connection conditions associated with the leakage-current figure. An electrical designer should confirm compatibility with the earthing system and residual-current protection. Never disconnect protective earth or remove required protection to prevent tripping. The arrangement must follow the selected filter's documentation and the requirements applicable to the project. See the earthing, safety and leakage-current guidance[6].

4. Include the installation interface in selection

Low-impedance bonding between the filter enclosure and the shield, the wall penetration and terminal-compartment seals can all affect installed performance. Keep input and output wiring adequately separated to avoid coupling that bypasses the filter. A long earth wire is not a substitute for effective high-frequency bonding, and temporary connections should not bypass the intended filtering boundary. See the installation guidance[4] and wiring and bonding principles[5].

5. Retain evidence through acceptance

Keep the curves and test conditions, load schedule, temperature derating, leakage-current and protection review, installation drawings and site-verification records together. Appropriately qualified personnel should perform site checks under an agreed procedure, distinguishing component measurements from installed-system results. Selection is complete when the applicable conditions are defined and the installation is verified, not when the highest attenuation curve has been found.

Sources

• [1] IEC: CISPR 17:2011, Edition 2.0 (2011) (https://webstore.iec.ch/en/publication/65)
• [2] SCHURTER: Industrial Mains Filters, online technical guidance (no revision stated) (https://www.schurter.com/en/knowledge/industrial-mains-filters)
• [3] SCHURTER: Power Dissipation in Power Entry Modules (11 April 2024) (https://www.schurter.com/en/news/power-dissipation-in-power-entry-modules)
• [4] MPE: Power Line EMC Filters Overview, online technical guidance (no revision stated) (https://www.mpe.co.uk/products/power-line-emc-filters/power-line-emc-filters-overview/)
• [5] TDK: General Technical Information of Power Line EMC Filters (no revision stated) (https://product.tdk.com/system/files/dam/doc/product/emc/emc/power-line/general_tech_info/generaltec_power-line_en.pdf)
• [6] MPE: Power Line Filters, Issue 12 (April 2022) (https://www.mpe.co.uk/wp-content/uploads/2024/04/Power-Line-Filters-iss-12.pdf)

Sources checked: 24 October 2026.