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 plates, isolated from the structure and metal conduit system on stand-off
insulators. The branch ground cables, No. 2 AWG minimum, attach to the trunk
ground cable, 1/0 AWG minimum, which is then attached to the main ground
plate, which is also isolated electrically from the structure. The main
ground plate is then connected to the earth electrode subsystem ground by
means of a ground cable with a minimum size of 500 cmil for each foot of
required length and limited to less than 200 ft length if possible. Within a
metal shielded enclosure the single-point ground stud or plate is usually
placed in the wall of the shielded enclosure and serves the function of the
main ground plate for TEMPEST low frequency ground networks. It is then
connected from the exterior of the shielded enclosure to the earth electrode
subsystem ground. See Figure 13 for a low frequency ground system in a
shielded enclosure and Figure 14 for a single point ground stud installation
in a bolted enclosure.
2.10
TEMPEST Shielding Requirements. Facilities which contain electronic
equipment which process classified information must be shielded in accordance
with OPNAVINST C5510.93D, Navy Implementation of National Policy of Control of
Compromising Emanations. This type of electromagnetic shielding is known as
TEMPEST shielding and is required to reduce the conducted and radiated
emissions from within the sensitive environment to an undetectable level
outside the shielded enclosure in uncontrolled areas. The typical required
level of shielding effectiveness (SE) is 100 dB to an upper frequency of 10
GHz, measured in accordance with NSA 65-6 procedures, and Figure 15 in this
manual. Special requirements for the TEMPEST shielded enclosures can include
electrical isolation on all six sides (greater than some specified minimum dc
resistance ranging from 1,000 ohms to 15,000 ohms or greater). It includes
single-point grounding of the shielding to the facility ground system,
electrical filters on all conductors penetrating the shielding (including all
power, phases and neutral, signal, telephone, alarm control and sensing). It
includes waveguide attenuator penetrations for all piping penetrations, seen
typically in Figure 3, air vents (intake and exhaust) seen in Figure 2, and
with inserts in piping that exceeds 1/2-in. (12.7 mm) I.D. (to provide
waveguide attenuation for frequencies to 10 GHz), as seen in Figures 5 and 6.
The requirement for electrical isolation of piping, conduits and air handling
system ducting connections to the external sides of the enclosure of 6 in.
(152.3 mm) of isolation within 2 in. of the shielding surface.
2.11
Types of Shielding Systems. There are large varieties of
construction techniques and materials utilized for EMI shielded enclosures.
The basic shielding material and its thickness are generally selected based on
the low magnetic frequency shielding requirements. Shielding effectiveness
measured with low frequency magnetic fields is provided mainly by absorption
rather than reflection losses. This is because the magnetic field wave
impedance is low compared to electric and plane wave field impedances, and it
is the mismatch in impedance between the metal surface (which is very low),
and the wave impedance which results in the reflection loss. The absorption
loss is directly proportional to the thickness of the material and to the
ft.2- of the permeability. The permeability of sheet steel at low frequencies
is over 200 compared to a value of one for copper, aluminum, and other
nonmagnetic shielding materials. NSA 65-6 TEMPEST shielding requires 20 dB of
magnetic shielding effectiveness at 1 kHz, increasing to 90 dB at 100
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