Piping and Conduit Penetrations

Custom Search

Back
Figure 2. Preferred Waveguide Air Vent Filter

Up
Radio Frequency Shielded Enclosures

Figure 3. Typical TEMPST Pipe Penetration, Bolted and Welded Enclosure

2.6.4

Piping and Conduit Penetrations. Numerous penetrations in an

enclosure are required for electrical conduits (power, telephone, alarm,

etc.), and for mechanical piping (chilled water, fire sprinkling vents and

drains). The TEMPEST requirement for an electrically isolating section six

inches in length, within two inches from the shielding surface tends to

complicate the selection of the penetrating piping materials. Schedule 80 PVC

with standard fittings are readily available with reasonable temperature and

pressure ratings (2 in., 295 psi at 68 deg. F, or 51 mm, 2034 KPa at 20 deg.

C, for example). For higher temperatures, schedule 80, PVDF can be used

(rated at 300 deg. F or 149 deg. C). The penetration through the shielding

must be metal, and continuously bonded electrically around the periphery of

the penetration to the shielding (welded or brazed) as seen in Figures 3 and

Clamped or threaded penetrations, and those using RF gaskets are not projected

to provide long term electrical continuity because of oxidation and corrosion

of closure contact surfaces under normal environments. These nonpermanent

types of penetrations should be prohibited in the written specifications.

The metal piping penetrations will provide waveguide below cutoff attenuation

for frequencies below their cutoff frequency. For piping larger than 1/2 in.

(12.7 mm) I.D. and with NSA 65-6 requirements for attenuation at 10 GHz, a

waveguide filter insert may be required, as seen in Figure 5. Where the

piping has continuously welded, brazed or soldered joints in the run from the

penetration to the equipment within the shielded enclosure, or where the

piping has tight threaded couplings, the filter insert may not be needed. For

short runs to the penetrations typical of the air vents and roof or floor

drains, the insert is usually required. The waveguide attenuation piping

insert will be a honeycomb comprised of steel or brass, tin-dipped, or else

stainless steel or brass disk with clusters of drilled holes, as seen in

Figure 6. The filter insert unit will be soldered, brazed, or welded to the

piping. Large diameter piping for fire sprinkler systems within the enclosure

are typically assembled with a nonconductive gasketed joint, and will require

a filter insert. Generally the piping penetration in the shielding is first

fitted with a brazed or welded collar of steel to provide a transition between

the typically thin shielding material and the heavier walled piping, as seen

in Figures 3 and 5. The collar also allows for cutting of the necessary

penetration hole in the field for welding on site without close tolerance

requirements on the hole dimensions. The waveguide filter piping insert will

cause a pressure drop in the flow of liquids which must be considered in their

design. National Fire Protection Association regulations may require the

addition of refractory materials around electrical isolation sections of

piping used with TEMPEST requirements (typical of the fire sprinkling

penetration).