Structural Penetrations

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Figure 6. RF Bolted Floor Crain Penetration with Attenuator Insert

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Radio Frequency Shielded Enclosures

Figure 7. RF Shielding of Interior Steel Column Penetration

2.6.5

Structural Penetrations. A major shielding penetration is the

supporting column for ceiling or roof beams. This can be treated by several

different methods. For TEMPEST enclosures the best electrical isolation

between the shielding and the column is obtained by either isolating the

column within the shielding, as seen in Figure 7, or by totally enclosing the

penetrating column in the shielding material with sufficient distance or

nonconducting material provided to result in the necessary electrical

isolation as shown in Figures 8 and 9. A minimum of 1-in. of air space is

recommended between the column and the enclosing shield. If single point

grounding of the shield is not a requirement, the penetrating column may be

treated by continuously welding penetrating members such as reinforcing steel

or steel beams to the floor or ceiling shielding sheets. Transition collars

or sections are typically required to weld thin shielding sheets to heavier

metal beam penetrations as shown in Figure 7. With the large shielded

enclosures built as an outer or inner liner for the building construction, the

beam structure members are typically required to penetrate the shielding in

many places. The design of the shielding must accommodate the expansion and

contraction of large metal surfaces pinned at the penetrations to large

structural members that are fixed in position. Expansion joints may be

required in the shielding membrane for both vertical and horizontal motion so

that connection with heavier members don't result in cracking of welds during

thermal expansion and contracting, especially while the construction is

exposed to the exterior environment.

2.7

Conducted EMI Isolation. As stated earlier, the shielded enclosure

must provide attenuation of both radiated and conducted electromagnetic field

energies. Shielding surface discontinuities such as cracks, holes, poorly

tightened bolted seams, or metal piping penetrations may act as slot antennas,

loop sources or transmission lines radiating or conducting electromagnetic

energies from the inside of the shielding membrane to the outside. The

discontinuities required in the construction of a six sided enclosure with its

necessary entrances and penetrations are the usual source of electromagnetic

leaks. These leaks will cause differences of potential to occur on the outer

surface of the shielding. When metal penetrations such as piping and conduits

are not provided with an electrical isolating section on the exterior of the

shielding which disrupts electrical connection to possible ground returns, the

differences of potential caused by the leaks will cause currents to flow in

the attached conductors. These induced currents may then be transported or

propagated into uncontrolled areas where unauthorized access is possible.

Where conduits carrying wiring pass from one shielded enclosure into adjacent

enclosures and pass through uncontrolled space, they should be provided with

electrically isolating sections on at least one end, and electrical filters on

each shielding penetration. If the conduit with contained wires passes from

one shielded enclosure to another through controlled space, it may be possible

to use conduit with welded joints, no electrical filters, but a single point

ground on only one of the enclosures.