Architectural shielding--Cabinets, cubicles and chambers

Shielding at the level of an installation, usually called architectural shielding, is not easy to do at low-cost. In general it is much better to purchase apparatus that meets its specifications for electromagnetic performance, i.e. it withstands its electromagnetic environment with acceptable performance degradation, and it does not emit electrical disturbances that cause problems for other apparatus. New apparatus could use any of a number of techniques to achieve this, including if necessary shielded cabinets and shielded cables, and so would not need a shielded room.

Nevertheless, architectural shielding is sometimes required (e.g. for X-ray rooms, to help protect other apparatus). A number of shielding techniques exist, but all are based upon attempting to encase the entire room volume inside a seamless conductive wall.

--- Screened enclosure maintenance

Shielding entire rooms against very low frequencies (say, 50 or 60Hz) is generally regarded as impractical except for high-value military or scientific projects where cost is no object. At frequencies exceeding 10kHz, practical shielding is becoming possible, and by 1MHz good results may be had with quite thin metal shielding materials. "EMC wallpaper" is a thin copper foil with a building paper backing, which it is relatively easy to glue to existing surfaces. Metallized fabrics and expanded metal have also been successfully used as a form of shielding wallpaper, with the advantage that they allow air to pass through them to some degree.

Making a framework for a room from timber studding, and cladding it on both sides with metallized fabric or expanded metal, can provide quite acceptable values of shielding effectiveness provided that the seams between the metallized sheets are properly dealt with. However, for the best shielding performance there is no substitute for two layers of seam-bonded metal plates, and this is how the basic structure of most EMC test chambers is made.

Apertures

The real problem with shielding a large volume, instead of an electronic unit or equipment cabinet, is that its shielding effectiveness is always compromised to a great extent by all the necessary apertures: e.g. windows, doors, cable entries, ventilation and air conditioning. Accidental apertures, especially imperfections in the joints of the supposedly seamless metal skin, also compromise shielding.

A metal room that ought to achieve 80dB shielding effectiveness at 200MHz can be reduced to 10dB or less by the penetration of the cable that powers the lights, or can be reduced to 40dB by a single hole not much wider than one could put a finger through.

All the real expertise in achieving a screened room is in the details of how to make very long metal joints effectively, and how to get people, air, light, services, goods, (and sometimes vehicles), in and out of the room. This requires a fair degree of specialization.

Most of the best practices described in this guide do not need special expertise to implement, and typical installation engineers can easily cope with them. But constructing a shielded room is not something that should be attempted by the non- expert. The best approach is to contact one of the many companies that offers to design and build shielded rooms and subcontract them to do it all for you.

There are a number of such companies, offering a range of services at a range of prices. The most important thing for the installation's project manager to do is to make sure that the contract specifies:

++ The shielding performance target for the frequency range of interest, and how it is to be measured and specified;

++ The air and light quality within the room, and any movement of people, goods, services, and vehicles in and out of the shielded volume;

++ The verification methods to be employed and the minimum results they must achieve ( e.g., using MIL-STD-285);

++ That it is the contractor's responsibility to achieve those results for the agreed price and in the agreed timescale. Holding a substantial part (or all) of the price of the shielded room back, until verification has been satisfactorily completed, is a powerful way to get the desired performance from an experienced contractor. (Don't even think of using a contractor with no proven track record for this work.) Remember that a shielded room is not shielded when any of its doors are open.

Interconnecting shielded enclosures or rooms

--- attempts to show how two shielded enclosures are connected together and to the rest of the installation, without compromising their shielding effectiveness. These enclosures could be any size: equipment cabinets, or large rooms. The screened cables that enter or leave them have their screens terminated by 360-degree electrical contacts in shielding glands (or connectors) at the enclosure walls.

The unscreened cables that enter or leave them only enter the cabinets via filters mounted to the cabinet walls. Bulkhead-mounted filters built into the wall of the enclosure are the best for this purpose. A number of companies make "room filters" for EMC test chambers that are also suitable for use in architecturally shielded rooms or buildings.

Metal-free fiber optic cables would be better than metallic conductors for such rooms, because they may be passed through waveguide tubes so that the small aperture they make in the metal wall does not cause any leakage. Unfortunately it is still impossible to transfer substantial amounts of electrical power into a room by the use of fiber optics (although it has been done at levels of a few tens of watts), so we are still left with metallic power cables and their filters.

APEC (e.g. at least one cable tray, duct, conduit, or item of structural steelwork) is bonded to each cabinet and runs between them, carrying all the cables between the cabinets whilst maintaining their segregation by Class of cable.

--- Installing connections between shielded enclosures

Next: Coupling to, from and within cables

Prev: Shielding techniques

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