Architectural Electromagnetic Shielding

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The use of radio frequency shielded enclosures is now quite common in our moddern, increasingly electromagnetically-crowded environment. Until recently, the design, manufacture, and installation of shielding was the exclusive territory of a few specialty contractors. Two basic systems were developed by the shielding industry: the modular or clamp-up enclosure, and the welded structural-steel enclosure, with the latter being built in place. Only a few attempts at other types of shielding have been made. The most notable was the aluminum foil system developed by the National Security Agency as described in their detailed specification NSA 73-2A. The need for shielding has become so widespread that a number of companies have developed a variety of single-skin shielded enclosures.

The purpose of this article is to provide you with a very general understanding of architectural electromagnetic shielding. You may then be able to better-instruct architectural and facility-engineering personnel with necessary information which will permit them to make informed decisions on the different types of shielding and how to properly specify them sufficiently so that the constructed shield performs its intended purpose. The completed enclosure must be properly tested to ensure compliance with the desired level of performance. Another purpose of this article is to point out the performance differences between the various shielding systems so that intelligent choices are possible. In the past, it has been common practice by the shield-buying community to request the highest performance specifications available, thus ensuring a safe decision. However, in many instances, you may be paying too much for protection you don't need.

Currently, the most common shielding systems provided by the established shielding manufacturers are prefabricated modular or "clamped-up" rooms and welded enclosures which are built on site. These products have evolved with time, and are proven systems available from a number of large and small companies worldwide. A newer class of shielded enclosure is the one that's built into the structure, i.e., within the walls of a building. This later class shielding systems are usually installed by a general contractor either as described by the drawings or by subcontracting to a specialty contractor. The key to success in these installations is how well they are detailed in the building drawings. Since the shielding is built in, it must be done right the first time because retrofitting in the field is extremely costly and time consuming. If the information given in this handbook is carefully considered and implemented, it will successfully aid those who have the responsibility to implement shielding in their facilities or design/specify them for a client. The selection of the basic enclosure is very important to the overall performance of the facility, but a shield is only as good as the treatment of the penetrations through it. Also important is how the designer selects methods of providing doors, vents, piping, power, communications, and security to a shielded enclosure . Just how badly an improperly installed penetration can degrade an enclosure can't be over-stressed.

Another very important requirement is that of testing the facility while in the process of construction and finally on completion. The latter is absolutely essential. After all tradespersons have completed their work, the shielded enclosure must be tested to the applicable shielding effectiveness specifications. The contrator must be knowledgable about the common specifications, how to use them effectively, and how to write a test plan that will ensure that the completed facility will perform its intended function. In specialty engineering manuals, each type of test is described, along with the common problems encountered in field testing. Accomplishing an acceptable grounding system for a shielded enclosure is often a problem in the field. Signal security and electrical safety requirements don't always appear to be compatible. Using MIL-HB-419A as a guide, guidelines are given on how the various types of grounds can be designed to obtain a harmonious installation, one which is safe for personnel and also meets the needs of the security community. Also refer to MIL-HB-232 for grounding information. A good testing procedure may incorporate a series of design checklists for the three types of shielded enclosures, so the architectural electromagnetic engineer (contractor) can draw a complete enclosure specification together for the various forms of shielding.

Architectural Electromagnetic Shielding draws heavily from the US Government's MIL-HB-419A, Grounding, Bonding, and Shielding for Electronic Equipments and Section 1.2 Radio Frequency Shielding Definitions and Terminology 3 Facilities. This handbook is recommended reading for all who are concerned with large industrial/government facilities. It's also useful for designing or specifying a single room.

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Updated: Friday, 2007-11-16 17:22 PST