Why Use EMI Filters in Shielded Rooms and Shielded Enclosure?

Shielded rooms are a crucial EMC component to provide an electromagnetic noise free environment for many critical applications. They offer protection against electromagnetic fields. From MRI rooms, commercial test chamber and secure data centers to government embassies, military facilities, strategic command centers, all require some level of shielding. This may include partial shielding, faraday cage, or a fully shielded room to suppress radiated noise and critical electronic signals.

Shielded rooms employ a full metal construction (common materials are Copper, Steel, Nickel and Brass) with fully welded seams, corners, and joints to avoid any gaps, opening or cracks. The room can either be built out of high strength metal (such as steel) or using standard construction material (wood, foam, sheet rock etc.), and then covering all the side with metal sheets. In either case, continuity of the shield (sheet metal or frame) is critical for the integrity of the room or enclosure. Spring loaded beryllium copper finger gaskets and special latching systems are installed on the metal doors to create full shielding once the door is closed.

A fully shielded construction works two ways:

  • Stops radiated (over the air) electromagnetic waves/noise from entering the room.
  • Stops electromagnetic signals generated inside the room from going out.


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Shielded rooms require various penetration including power and signal cabling which can compromise the shield integrity of the fully enclosed construction. Thus, a line filter is required at the point of entry of each cable to clean conduced EMI on the line and reestablish shielding effectiveness of the enclosure.

Because shielded rooms are used for sensitive applications, it is important that EMI does not impact the processes being carried out within these rooms. To meet this primary objective, shielded room EMI filters must meet these keys requirements:

  • Must have an RF-tight cabinet to meet high insertion loss requirements.
  • Must have a shielded output compartment to isolate dirty input and clean output.
  • High attenuation (80~100dB) in wide frequency spectrum (14KHz/150KHz ~ 10GHz and up).
  • Must provide specified insertion loss when measured in accordance with MIL-STD-220 and CISPR 17.
  • Must demonstrate required shielding effectiveness when measured per applicable standards such as IEEE 299, ASTM E1851-15, MIL-STD-285.
  • Backup connecting bolt for a secondary ground wire.
  • Must be built to comply with all the relevant sections of safety requirements such as UL 1283.
  • Provide “optional” protection such surge protection per UL 1449 or EMP/ HEMP protection per MIL-STD-188-125-1, if requested by the end user.
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Shielded room EMI filters have certain characteristics that make them highly reliable and efficient in blocking electromagnetic frequencies. The primary features include:

  • Multi-stage Circuit: To achieve high insertion loss levels (up to 100dB) in very wide frequency spectrum, these filters often have 2, 3 or more circuit stages. This lowers the cut off frequency and results in higher attenuation performance.
  • High Leakage Current: Leakage current refers to electrical current that flow between the filter and ground. To achieve high common mode insertion loss, these filters typically deploy line to ground capacitance topology. This results in high leakage currents.
  • Bleeder Resistor: Once the power to these filters is disconnected, the accumulated charge in line to ground capacitors can present a hazard for electric shock to anyone touching the filter terminals. To mitigate this safety risk, these filters are often fitted with bleeder resistor to discharge capacitor quickly and a safety warning.
  • Harsh Environment Capable: Shielded room filters can be installed indoors or outdoors (weather protected). Filters come in a rugged cabinet design that allows them to work as intended for many years in even the toughest climates. In addition to an EMI gasket, the compartment covers often include a weather seal/gasket as well.
  • Construction Types: Single box or cabinet & insert design. In single box design, all the lines (1, 2, 3 or 4) are built and potted inside a single compartment, whereas in cabinet - insert type construction, there is one line insert (filter) for each line, and based on the application, 1, 2, 3 or 4 inserts can be mounted inside a shielded cabinet. In latter, the line inserts are replaceable, whereas in former the whole filter must be replaced if one line goes bad.
  • Output Compartment: Must have a shielded output compartment to isolate clean output from the dirty input.
  • Mounting: The cabinets have flanges for stable mounting. This enables users to securely adhere the filter to the shielded room wall both mechanically and electrically (for grounding)
  • Input-Output Isolation: A threaded pipe is typically welded to the output/clean compartment penetrates the shielded room wall isolating clean power (cables) from outside dirty environment. Threaded pipe is an optional item for filter manufacturers as it is often assembled in field during the installation to ensure correct size, location, and length. In many instances, end users prefer to route the power using fully shielded conduits thus making the pipe redundant.


To select an appropriate filter for each application the following information is required:

  • Current Rating
  • Number of Lines: 1 (single line), 2 (single phase), 3 (3-phase delta) or 4 (3 phase wye)
  • Frequency: Most filters are designed for 50/60Hz. When used in 400Hz application, the operating voltage must be reduced to 120/208VAC to restrict excessive leakage current. The 400Hz application for current over 75A might require PFC coils. Some filters are specifically designed for 400Hz operation.
  • Application: Commercial or military/government - certain MIL/Gov application might have country of origin restriction or made in USA requirement. Vehicle mount application might require more robust and compact design. Commercial application of the other hand require more cost effective solutions. 
  • Performance: 100dB performance starts at 14KHz or 150KHz and needs to go up to 10GHz, 18Ghz or 40GHz?

popular shielded room emi filter applications


Shielded room filters are essential in various industrial and military/government applications due to their excellent noise-mitigation feature. These are commonly used for applications that require the absence of external electrical noise, such as EMI testing, signal processing, secure communication etc.

The most popular applications include:

  • Test Chambers
  • Research Facilities
  • Radio Telescopes / Measurement
  • Secure Data Centers
  • Foreign Embassies
  • National Security Agency
  • Important Government Buildings/Offices
  • Tempest Facilities
  • Secure Military Facilities
  • Military Strategic Command Centers
  • Field Deployable Secure Communication Enclosures
  • Telecommunication
  • MRI Rooms
  • Universities


Astrodyne TDI offers 057/01 and 057/02 series shielded room power line filters for commercial applications and F1960 series power line filters for military applications. For signal line applications (phone, data, control, and alarm), we offer 058 series and F15000 series filters.


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About Astrodyne TDI

With more than 60 years of experience as a Power Supply and EMI/EMC/RF Filter manufacturer, we are known for our ability to produce highly reliable and durable products for some of the most demanding industries and end markets. Astrodyne TDI has also been providing EMI filters and Power supplies for a wide range of applications, including medical, MRI / shielded rooms, aerospace, semiconductor, commercial, appliance, industrial and military. Our EMI filter solutions can help your products achieve EMI performance and EMI compliance to relevant EMC standards.

We offer a full range standard, modified and custom EMI filter solutions. We work closely with our customers to determine ideal EMI filtering solutions for their applications. We offer a wide range of filter types, from small IEC inlets, power entry module and single-phase options to high current DC and three-phase filters. We have full engineering and design expertise for developing custom EMI solutions to help your equipment meet applicable EMI requirements, if necessary.

Submit our contact form to get in touch with the experts at Astrodyne TDI today!