ELECTROMAGNETIC PULSE (EMP) AND TEMPEST PROTECTION FOR FACILITIES
Table of Contents: http://jya.com/emp.htm
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CHAPTER 9
EMP AND TEMPEST PROTECTION CONCEPTS
9-1. Outline. This chapter is organized as follows:
9-1. Outline
9-2. Introduction
a. Component or room hardening
(1) Equipment separation distances
(2) Component shielding
b. Facility shielding
(1) Waveguide tunnel
(2) Grounding system
(3) Uninterruptible power supply
(4) Conduit runs
c. Zoning
(l) Reason for zoning
(2) Typical zones
d. Global approach
(1) Programing and design
(2) Off-the-shelf products
(3) Performance degradation
(4) Project costs
(5) Envelope
9-3. TEMPEST requirement in relation to HEMP
a. Shielding similarity
b. Peak power comparison
c. Upper frequency range
d. Critical component location
e. Dielectric breaks
f. Common hardening
9-4. Generic facility hardening
a. Overview
b. EMP Protective features
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9-2. Introduction. Critical facilities are very vulnerable to HEMP damage or
upset and, in most cases, these facilities have equipment that processes
classified information that could be compromised. A single nuclear weapon
detonated 300 kilometers above the United States can blanket the entire CONUS
area with HEMP effects. HEMP effects are especially damaging to integrated
circuits and other sensitive low-voltage/current electronic devices on which
facilities rely. It is critical to national security that these facilities
incorporate HEMP and TEMPEST protection measures to prevent compromise of
information and disastrous damage and upset to the electronics equipment.
Generally, there are two concepts to be considered as a methodology for HEMP
and TEMPEST protection and a zoning plan which may be applied to either
methodology as required.
a. Component or room hardening. This method consists of defining a subset
of equipment that is mission-essential and hardening only that equipment and
its required auxiliaries. This method is employed in two circumstances:
(1) Equipment separation distances. Equipment is physically separated
by large distances and it is not realistic to try to shield the entire area.
(2) Component shielding. The mission-essential equipment is in a
facility where it comprises only a small relative part, and thus, it is not
advisable to shield the entire facility. When this method is used, each
system or piece of equipment is considered a separate entity for shielding.
This method employs either small shield rooms or shielded equipment with
waveguides-beyond-cutoff (WBC), electric filters, access panels, and RF doors.
It differs from facility shielding only in scale. The drawbacks with
component shielding are that it is totally inflexible, very expensive, and
very difficult to maintain. Also the system is vulnerable during servicing
when access panels are open. This method is usually cost-effective only in
the circumstances outlined above.
b. Facility shielding. This method is by far the most common for high-
level HEMP and TEMPEST protection. It maximizes flexibility since any
standard equipment can be used inside the shielded facility. Facility
shielding may be low-level or high-level (50- or 100-decibel) attenuation.
HEMP shielding (100-decibel) consists of at least 3/16-inch welded steel (12-
gauge walls and 10-gauge floors are recommended). TEMPEST shielding (50-
decibel) consists of at least 22 to 26 gauge steel walls, floors, and ceiling
with clamped joints. All penetrations are protected by WBC, filter or RF seal
of some kind. Penetrations should be reduced to a minimum and if possible
colocated at one area of the facility in a penetration entry room (PER) or
vault. A PER is a small, shielded room that affords extra protection to the
facility where all, or most, utilities enter. It is placed on the outside
skin of the building using the exterior of the facility shield as an interior
wall. The PER is especially effective in control of all penetrations and
provides a desirable margin of safety for critical facilities.
(1) Waveguide tunnel. A waveguide tunnel shall be provided for large
facilities at the main personnel entry. This long, welded tunnel can provide
up to 40 decibels of attenuation (more at low frequencies). It offers
valuable protection at the weakest point of the shield. RF doors shall be
installed at both ends of the entryway tunnel, interlocked to ensure that only
one may be opened at any time. No conductive lines are permitted in the
tunnel; lighting shall be provided from above the tunnel through a WBC vent in
the ceiling or the lighting circuit shall be protected by a filter.
(2) Grounding system. The grounding system for the facility shall use
an equipotential ground and tie into a welded stud that does not penetrate the
shield. Another stud welded to the opposite side should then run to the
exterior ground system.
(3) Uninterruptible power supply. Generally, an uninterruptible power
supply (UPS) is used to provide power when the commercial source fails during
a HEMP event. The UPS is usually contained inside the shield and is often
used on a daily basis to provide clean power for computers and mission
equipment. Surge arresters shall be used to clamp the HEMP transient pulse on
long commercial power lines; the electrical surge arresters also serve to
protect the filters from the high voltages, currents, and energies in the HEMP
pulse.
(4) Conduit runs. Wherever conduit runs must exit the facility to
access some critical equipment such as an outside shielded generator, heavy
metal rigid conduit shall be welded at the couplings to form an RF-tight
shield. All conduit runs in the PER shall also be heavy metal rigid welded
conduit. The conduit extends the shield to envelope critical shielded
equipment outside the protected facility.
c. Zoning.
(1) Reason for zoning. Zoning is a method for control used when
differing levels of protection are required. For example, the rugged
generator set may operate without problems in a very low-level attenuation
area, perhaps only under an earth rebar structure. A more sensitive UPS and
communication room may require a low to medium protection level of 60
decibels, and a very sensitive control and computer room may require a high
level (100-decibel) protection area.
(2) Typical zones. Protected areas can be designated Zone 0 for
outside, Zone 1 for generator (40 decibels) area, Zone 2 for UPS and
communication area, and Zone 3 for the highly sensitive control room. The
zones can be drawn schematically and usually are nested one inside the other
so that the highest attenuation area is centered inside the other areas. This
method ensures that no potential compromise is overlooked, and the potential
savings from nested/layered shielding can be realized. Zoning can be used for
component or facility shielding and serves as an excellent tool for deciding
what is critical and how it should be protected.
d. Global approach. In global shielding, a single requirement (e.g.,
100 dB reduction or 50 dB reduction) is established as the protection level
the design must meet. The approach has some advantages, including the
following:
(1) Programing and design. Facility programing and design can
proceed without an in-depth knowledge of the emanation and susceptibility
profiles of the equipment to be housed within the building. Thus, the project
can go forward in parallel with development of the mission hardware.
(2) Off-the-shelf products. Construction components and materials
are generally off-the-shelf commercial products. Processes used to assemble
the HEMP/TEMPEST protection subsystem are common in the design and
construction trades.
(3) Performance degradation. This approach minimizes the potential
for performance degradation of the subsystem and also minimizes the need for
routine maintenance.
(4) Project costs. Overall project cost to the government may be
reduced because no extraordinary electromagnetic susceptibility requirements
need to be levied on the mission equipment manufacturers. Furthermore,
hardware developed for non-HEMP/TEMPEST applications can normally be used
without modifications.
(5) Envelope. This approach creates a protected envelope within
which equipment and configuration changes can be made without modifying the
isolation subsystem.
9-3. TEMPEST requirement in relation to HEMP.
a. Shielding similarity. Shielding and penetration protection techniques
are efficient and effective for limiting the passage of electromagnetic energy
in either direction--inward in the case of HEMP and in an outward direction
for TEMPEST isolation requirements. A single electromagnetic barrier can
perform both functions. This approach avoids costs and potential interaction
effects associated with double shielding or double filtering.
b. Peak power comparison. The peak power of the HEMP environment is much
greater than that in a potentially compromising TEMPEST emanation. Therefore,
HEMP protection devices are constructed to survive greater stresses than
TEMPEST protection devices.
c. Upper frequency range. TEMPEST protection extends to an upper design
protection frequency, typically 1 to 10 gigahertz. This range will require
the shield penetrations such as waveguides-beyond-cutoff and filter assemblies
to Provide protection at this frequency.
d. Critical component location. The conflicts are found when requirements
of the two disciplines are compared. HEMP survivability considerations
dictate that mission-critical modulator/demodulators (MODEMs) and radio
transmitters be afforded the protection provided by the shielding and
penetration subsystem. TEMPEST guidelines, in contrast, indicate that these
BLACK devices be placed external to the shield. To meet both requirements,
the MODEMs and transmitters can be located within the shielded enclosure,
provided that RED/BLACK isolation procedures are followed and all associated
electrical lines penetrating the shield are filtered properly or otherwise
isolated.
e. Dielectric breaks. The second area of conflict relates to the
nonconductive section required by TEMPEST considerations in piping,
ventilation, and some electrical penetrations. From a HEMP protection
standpoint, such dielectric breaks are undesirable. The nonconductive
sections should be eliminated where the HEMP threat is increased by their
inclusion (such as electrical conduit runs).
f. Common hardening. The facility hardening as provided for HEMP in all
other areas of this Pamphlet will also provide protection for TEMPEST.
9-4. Generic facility hardening.
a. Overview. Provide a welded facility shield that attenuates the EMP to
an acceptable level (usually 80 to 100 decibels, depending on the
susceptibility of equipment to a HEMP event). All conductive utility lines
are circumferentially welded to the shield and PVC or other nonconductive
lines are used in conjunction with WBC-type entries. Telephone lines should
be fiber optic (preferred) or filtered. Power lines and antenna lead-ins must
be filtered, preferably with electric surge arresters to protect the more
expensive filters. The shield is provided with a grounding grid to ensure a
good path to ground. Air-conditioning vents and ducts are provided with
honeycomb WBC filters. This system provides protection for most of the
equipment; however, certain items (such as computers) also may need separate
shielded enclosures to attenuate the EMP to a tolerable level. Finally,
on-site generators usually exist to sustain mission-essential equipment until
commercial power can be restored and to isolate the site from the power lines.
b. EMP protective features. To protect susceptible mission equipment from
upset or damage due to the HEMP free-field environment or coupled transients,
the following HEMP protective features will ensure a hardened facility:
(1) Welded facility shield.
(2) RFI doors (fingerstock).
(3) Waveguide entries.
(4) Waveguide vents.
(5) Waveguide-beyond-cutoff.
(6) Dielectric inserts.
(7) Fiber optic signal and communication lines.
(8) Filters and surge arresters.
(9) RFI-tight conduit runs and grounding system.
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[End Chapter 9]