          ELECTROMAGNETIC PULSE (EMP) AND TEMPEST PROTECTION FOR FACILITIES

                      Table of Contents: http://jya.com/emp.htm

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                                       CHAPTER 3

                        EMP HARDENING CONCEPTS FOR FACILITIES

    3-1. Outline. This chapter is organized as follows:

         3-1. Outline
         3-2. Discussion of general concepts
              a. System functions
              b. Survival confidence
                 (1) Levels of confidence
                 (2) Inherent uncertainties
              c. Critical equipment sensitivities
                 (1) Design margin
                 (2) Coupled energy
              d. Potential HEMP coupling paths
              e. Design verifiability
                 (1) Hardness validation
                 (2) Retrofit designs
                 (3) Designing to facilitate testing
                 (4) Approaches to validation
              f. Physical environment
              g. Other factors
         3-3. Description of HEMP hardening concepts
              a. Shielding
                 (1) Global shielding
                 (2) Tailored shielding
                 (3) Zonal or topological shielding
                 (4) System configuration
                 (5) Cable shielding
                 (6) Grounding
              b. Hardening allocation concept
              c. Shield penetration protection concepts
                 (1) Large access doors
                 (2) Personnel entrances
                 (3) Electrical penetrations
                 (4) Transient suppression devices and filters
                 (5) Electromagnetic isolation
                 (6) Dielectric isolation
                 (7) Isolation switching
         3-4. Cited references
         3-5. Uncited references

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    3-2. Discussion of general concepts. The HEMP environment is defined by DOD-
    STD-2169. This definition includes the classification and specific
    information on field strengths, pulse characteristics, spectral content, angle
    of arrival, range of relative burst locations, and weapon yield.

       a. System functions. Associated with the electronic and electrical
    systems and subsystems to be protected are support functions such as
    utilities, personnel housing, office space, document storage, food facilities,
    and others. Many aspects of a facility are not sensitive to HEMP energy or
    are robust enough that HEMP protection is not required. Some sensitive system
    elements may not be critical to the facility mission. The definition of
    mission-essential functions that must remain in operation will have major
    impact on the choice of hardening concepts.

       b. Survival confidence. The issue of defining "survivability
    requirements" must be specifically addressed and resolved in the concept
    definition phase of each particular HEMP hardening effort. The system user
    should define the required survival confidence level, at least qualitatively,
    since this factor will determine how conservative the design will be. If
    required confidence levels are high, greater safety margins in protection
    levels will be required, producing a need for a high-quality overall shield
    and adequate validation testing.

          (1) Levels of confidence. Survivability confidence may require that a
    facility--

             (a) Experience no HEMP-induced stress greater than the stresses
    occurring in the normal operating environment.

             (b) Experience neither permanent nor operational upset as a result
    of the HEMP.

          (2) Inherent uncertainties. Another survivability issue concerns the
    inherent and analytical uncertainties in quantifying the stress level causing
    malfunction or the stress level experienced by the equipment.

       c. Critical equipment sensitivities. The main factors in determining
    required protection levels are--

          (1) Design margin. The design margin required, which is related to the
    difference between critical equipment sensitivities and coupled transients.

          (2) Coupled energy. The energy level coupled from connected subsystems
    or components.

       d. Potential HEMP coupling paths. Most electronic/ electrical systems to
    be HEMP-hardened and their housing facilities will have to interface with
    external elements such as antennas, utilities, communications lines, and other
    facilities. The complexity of interfacing and possible coupling paths for
    HEMP energy will greatly affect the choice of topological approaches to HEMP
    hardening.

       e. Design verifiability.

          (1) Hardness validation. A key issue in HEMP hardening philosophy and
    associated design concepts is that of hardness validation and required confi-
    dence levels for final acceptance. (Required confidence levels are usually
    specified only qualitatively.) Generally, the more critical the facility is
    to national military security, the more politically and publicly visible it
    will be; for these facilities, higher confidence levels will be required. In
    all cases, design concepts may not be chosen if they cannot be validated with
    acceptable confidence levels. For example, a design concept for a large
    underground facility that depends on a degree of protection from the overbur-
    den and has numerous conducting penetrations through the overburden may have
    hardness uncertainties. Examples include questions about the homogeneity of
    the overburden and difficulties in protecting penetrations when no highly
    conductive shield is present. If the facility is too large to be practically
    subjected to a threat level test by an EMP simulator and no other proven vali-
    dation tests exist, the uncertainties will prevail and hardness confidence
    will be low.

          (2) Retrofit designs. In retrofit designs, another consideration-in
    concept selection may be the ability to validate hardness without disrupting
    the operation of critical systems. Concepts should be chosen to allow
    nondisruptive validation and acceptance testing.

          (3) Designing to facilitate testing. Good design validation requires a
    choice of design concepts that facilitate testing. HEMP hardening management
    must include adequate funding and scheduling for this effort. The difficulty
    and cost of validation will increase with--

             (a) System complexity.

             (b) Topology layer and zone numbers.

             (c) The number of required penetrations.

             (d) The protective design philosophy.

          (4) Approaches to validation. In considering the validation problem
    for concept selection, it is helpful to review the many approaches to valida-
    tion, including laboratory testing, full-scale HEMP threat level field
    testing, partial scale threat-level field testing, current injection testing,
    scale model testing, physical modeling testing, computer modeling evaluations,
    analyses, and radio frequency CW shielding tests.

       f. Physical environment. Various aspects of the facility physical
    environment can greatly affect concept selection, mainly in the degree to
    which corrosion can accelerate aging and degradation of protection.

       g. Other factors. Other factors to be considered in concept selection
    are--

          (1) Complexity of required interactions with facilities.

          (2) Design and construction costs.

          (3) Constructibility.

          (4) Maintenance costs.

          (5) Reliability requirements.

          (6) Flexibility for expansion or system changes.

          (7) New construction versus retrofit.

          (8) Supportability.

    3-3. Description of HEMP hardening concepts.

       a. Shielding. For HEMP-hardened facilities, some kind of EM shielding is
    essential. Shielding theory is discussed in detail in chapter 5 and is
    treated thoroughly in the literature. Shielding involves the use of a barrier
    or series of barriers to reduce the magnitude of the EM energy incident upon
    the electronic or electrical system to be protected. Shielding philosophy can
    be developed around different approaches as discussed in paragraphs (1)
    through (6) below and shown in figure 3-1.

          (1) Global shielding. Global shielding (or hardening) is a protection
    concept that uses an overall shield to encompass the entire facility. In this
    approach, all conducting penetrations and all apertures are protected at the
    shield. The intent is to keep all HEMP fields and HEMP-induced transients
    outside the protected volume. The global shield could be placed on the entire
    outer walls, ceiling, and floor (surface) of the facility, or it could be
    reduced to a smaller volume that contains all sensitive equipment to be
    protected. The most common shield material for global shielding of ground-
    based facilities is sheet steel with welded seams, although other designs can
    provide adequate global HEMP shielding.

             (a) Global shielding may be desirable if there is a requirement to
    be able to modify, reorganize, add to, or move the sensitive equipment without
    changing the shield or protective features.

             (b) A remote, yet possible, disadvantage of global shielding that
    must be considered is that a single protective component or device failure may
    jeopardize the entire facility.

          (2) Tailored shielding. Tailored shielding is a protection concept in
    which shielding is designed and constructed according to specific protection
    requirements for the equipment involved. After defining the system to be
    protected, its possible operating configurations, the expected HEMP
    environment, coupling paths, equipment sensitivities, and subsystem/system
    criticalities, the required protection levels for various subsystems or groups
    of subsystems can be defined. Tradeoff studies may be performed for comparing
    various shielding arrangements to verify that they meet safety margins in
    protection, cost-effectiveness, maintainability, survivability, flexibility,
    and other requirements. The objective is to optimize protection for the
    specific mission-critical system. Tailored shielding options may include
    global shielding, zonal shielding (discussed under (3) below), shielding of
    cabinets or components, or combinations thereof. In a typical tailored
    protection design, discrete protection will be provided to eliminate specific,
    localized deficiencies.

          (3) Zonal or topological shielding. Zonal or topological shielding
    (ref 3-1) is a concept in which a facility is divided into zones, with
    shielding barriers located topologically in a shield within a shield
    configuration. Figure 3-2 shows a generic topological shielding system. The
    outer zone is designated zone 0; zone 1 is inside shield 1 but outside shield
    2. Zones and shields are assigned increasingly larger numbers as they
    progress toward the more deeply nested areas.

             (a) Note that figure 3-2 is a simple schematic to represent the
    zoning concept; although not depicted, each zone could contain more sets of
    subzones. For example, shield 3 could contain 2 or more zones designated as
    zone 4. Further, figure 3-2 shows possible shield types including a site
    housing shield and an interior shielded room, with equipment and component
    housings making up the shields of the next topological orders.

             (b) The zonal concept shown in figure 3-3 is a specific example of
    an underground facility that uses topologically zoned protection. The rock
    and soil overburden above the facility serves as shield 1. Zone 1 is the
    volume between the underground building and the excavated outline of overhead
    rock. In some cases, a shield of this type provides adequate protection for
    robust electrical or electronic equipment. Shield 2 is composed of a sheet
    metal building that may provide only a limited level of shielding. Inside
    this building (zone 2), some systems would be adequately protected. The
    above-ground building and connecting conduit represent an extension of zone 2.
    Shield 3 is then the interior shielded room which provides further protection
    within zone 3. where sensitive, electronic equipment may be operated.

             (c) Figure 3-4 shows another specific example of a zonal or
    topographically shielded facility for which steel-reinforced concrete
    comprises shield 1. This type of shield usually does not provide adequate
    protection and thus the additional shields are necessary.

          (4) System configuration. The term "system configuration" identifies
    which way the cables, wires, equipment, and subsystems are laid out in
    relationship to each other, as well as the relationship of these items to the
    topological boundaries. In some instances, the cables, connectors, and
    equipment casings are actually part of the topological protection. Although
    "system configuration" as defined does not directly attenuate the environment,
    it is an important element in the topological protection concept. The system
    configuration influences protection design requirements since some
    configurations are easier to protect than others ~e.g., collocation of all
    mission-critical equipment). Thus, the system configuration should be
    coordinated with the protection design and the protection topology will be
    optimal for a specific configuration. During the facility life cycle, the
    protection design may be required to accommodate some changes in
    configuration. To ensure that the configuration's design modifications do not
    compromise or defeat the protection, careful configuration management is
    necessary. The topology should be designed to tolerate configuration changes
    that are totally within a boundary. The boundary can never be violated (for
    example, opened)--only extended. All modifications must be subjected to
    review by EMP experts to ensure continual compliance with the HEMP hardening
    requirements.

          (5) Cable shielding. Conductive or metallic cable shielding or conduit
    is used in the zonal/topological protection concept to extend the boundary
    formed by equipment enclosures and thus provide a way to interconnect elements
    while maintaining boundary continuity. Cable shielding is also used to
    protect a wire or wires as they travel from one boundary to another. This
    would be the case with a shielded RF signal traveling from its entrance into a
    building to the RF receiver. From a HEMP standpoint, the shielding attenuates
    coupling of radiated energy within the first boundary as the signal travels to
    the receiver. Of course the shield is somewhat reciprocal in that it also
    prevents signals from radiating out of the cable. The main feature of cable
    shielding stressed here is continuity of the boundary provided by the cable
    shield/connector combination which may require special joints.

             (a) Another way to maintain this continuity and provide cable
    shielding is by using steel conduit to house all wires and cables. The steel
    conduit will provide substantially higher shielding levels than the cable
    shields. Chapter 5 presents conduit system design in detail.

             (b) Both cable shields and conduit connected to a shielded zone must
    have equal or greater shielding effectiveness than the shield.

             (c) Figure 3-5 shows a cable entry vault used to protect cable
    penetrations through a shield. Entry vaults are discussed under shield
    penetrations in paragraph c below.

          (6) Grounding. Some form of grounding is required in any electrical or
    electronic system for protecting personnel from electrical shock, controlling
    interference, proper shunting of transient currents around sensitive
    electronics, and other reasons. (Grounding does not directly provide
    protection against EMP, but must be done properly to prevent creation of more
    serious EMP vulnerabilities.) Ideally, grounding would keep all system
    components at a common potential. In practice, because of possible inductive
    loops, capacitive coupling, line and bonding impedances, antenna ringing
    effects, and other phenomena, large potentials may exist on grounding
    circuits. The choice of grounding concept is therefore important in the HEMP
    protection philosophy.

       b. Hardening allocation concept. The shielding concepts in this chapter
    introduce the concept of hardening allocation in which the overall protection
    philosophy specifies degrees of hardening for each zone. The practicality of
    this concept usually depends on the complexity of the system to be protected.
    If it is determined that an overall SE of 80 decibels is required for the most
    sensitive components, but the remaining elements require only 60 decibels,
    then zones with different SE may be established. The cost-effectiveness of a
    zonal design with a hardening allocation for each barrier must be studied
    carefully on a facility/ system specific basis to determine the practicality
    of this approach.

       c. Shield penetration protection concepts. All shielded zones will
    require penetrations to allow entry of equipment, personnel, electric power,
    communications, and control signals, ventilation, water, fuel, and various
    fluids. Without Protection, these penetrations compromise the shield.

          (l) Large access doors. Large access doors are often necessary to
    provide an entry for equipment, supplies, or vehicles into EMP hardened
    facilities. In facilities that require blast overpressure protection, large
    blast doors are used. These doors generally use one or more thick steel
    plates to provide protection. The door's inherent shielding ability is thus
    high, but its large size presents a difficult gasketing problem. If blast
    protection is not required, it is still necessary to design the door with a
    high degree of structural strength. This step is to ensure that the door can
    provide the necessary gasket compression force and that proper mechanical
    alignment of closure contact surfaces is maintained.

          (2) Personnel entrances. Two concepts are commonly used for personnel
    entrances: conventional EMP/RFI shielded doors and personnel tunnels that act
    as waveguides below cutoff. The shielded doors generally use metal
    fingerstock or EMI/RFI gaskets to provide an electromagnetic seal around the
    door jamb periphery. Currently available gasket and fingerstock doors require
    regularly scheduled maintenance and/or replacement to maintain required
    shielding levels. The gaskets are relatively easily damaged and also require
    replacement. Air-expandable doors may also be used, although they typically
    have more maintenance problems. These doors generally use a movable
    subassembly of two shielding plates on a framework that is moved on rollers in
    and out of a steel-framed opening. When closed, air expansion tubes cause the
    two shielding plates to make uniform surface contact with the frame inner
    surfaces.

             (a) Fingerstock doors can provide over 80 decibels of shielding to
    magnetic fields from 100 kilohertz through 30 megahertz and greater SE to
    plane waves and electric fields. Air-expandable doors can provide greater
    than 120 decibels of magnetic field SE from 10 kilohertz to 10 gigahertz.

             (b) Air-expandable doors require an air source and air controls with
    back-up in safety controls. They also require very strong steel frames and,
    as a result, are more expensive than gasketed doors. They are also more
    difficult and costly to maintain. The air-expandable door would thus be used
    only when a large safety margin of HEMP shielding is needed or when equipment
    to be protected is extremely sensitive to HEMP or other EM interference.

             (c) The waveguide entry tunnel acts as a WBC that will typically
    have a cutoff frequency* in the 60-megahertz region. Thus, the higher
    frequencies in the HEMP spectrum will penetrate it. Doors are therefore
    required to prevent the higher frequency signals from penetrating. Since only
    high frequencies can propagate through, doors have good attenuation in this
    range and can easily provide the required attenuation. Maintenance require-
    ments are not as stringent as for doors that must block the entire frequency
    spectrum; thus, the waveguide entry tunnel for personnel access is attractive
    from a life-cycle cost standpoint. When the facility has a TEMPEST
    requirement as well as EMP shielding requirements, the tunnel is usually
    designed with interlocking doors, i.e., a door at each end and interlocked so
    that only one door can be opened at once, thus preventing any leakage of
    classified information during the entry of personnel. The waveguide entry
    tunnel also is highly useful in underground or buried facilities because the
    overburden attenuates the high frequencies, thus acting to complement the
    tunnel attentuation.
    ________________________
    *Cutoff frequency is determined by the relationship Fo = 5900 MHz/W, where W is
    the greatest cross sectional dimension in inches. Below cutoff, the waveguide
    attenuation is a function of the waveguide length. In practice, the length-to-
    width ratio should be 5.

          (3) Electrical penetrations. A common feature for electrical
    penetrations in a global protection approach is a cable entry vault to prevent
    large currents on external conductors from being conducted into the facility.
    Ideally, all penetrations should enter a single vault. In some cases,
    however, it may be necessary to separate the vault into two compartments or to
    use two vaults for penetrations by different types of lines: power, signal
    and control, and antenna. The vault must be connected directly to the
    external facility ground system. (See chapter 5 for details.) The cable
    entry vault serves three purposes: to insure that penetrating conductors do
    not cause conducted HEMP energy to enter the protected topology; to contain
    and divert penetrator-conducted HEMP energy to the boundary exterior; and to
    contain or divert radiant EM energy resulting from the activation of transient
    suppression devices subjected to a conducted pulse. Conductive penetrations,
    such as a conduit, waveguide, or shielded cable, must have a circumferential
    weld or other means of providing good electrical connection at the
    intersection with the entry vault.

          (4) Transient suppression devices and filters. Transient suppression
    devices fill a critical gap in the concept of topological protection. The
    necessity of supplying power to a facility and of communicating over cables or
    antennas are two major factors contributing to their use. Power lines
    entering a facility are typically connected to an unshielded power grid so
    that large, conducted currents must be bled off to prevent their entry into a
    facility.* These currents are diverted to the exterior boundary of the
    topology. This boundary can be an overall external shield or an enclosed en-
    trance vault. Antennas, such as for high-frequency (HF) communications, are
    designed to gather EM signals (at wavelengths in the EMP frequency spectrum)
    and to apply these signals to the center conductor of a shielded cable. The
    EMP transients associated with an HF antenna can be, by far, the largest
    single signal entering a facility. Transient suppressors often are used in
    conjunction with filters. Filters are frequency-selective whereas surge
    suppressors are amplitude-selective. Filters often are used to attenuate
    transients associated with the nonlinear operation of surge arresters. They
    also are used for selectively passing (or stopping) frequency bands as in the
    case of antenna cable penetrations. Transient suppressors are an integral
    part of the EM topology, demanding specific installation techniques as will be
    seen later. A spark gap is a surge suppressor that provides a conducting path
    to ground when the voltage across the device exceeds the gap breakdown level.
    Spark gaps with a high current capacity do not operate quickly enough to block
    all HEMP energy transients entering the vault. For this reason, it may be
    necessary to use other protection devices in conjunction with the spark gap.
    ________________________
    *Within a facility, inside shield 1, power lines are often
    routed through steel conduits to provide shielding.

          (5) Electromagnetic isolation. The electromagnetic isolation concept
    involves the use of elements either immune to interaction with EM radiation or
    that provide a current path interruption. Optical fibers are examples of
    elements immune to EM radiation that can be used to reduce the number of
    conductive penetrations. For practical purposes, optical fibers can be used
    for long communications links without signal interference from HEMP. Further,
    they can be used to enter shielded zones through waveguide below cutoff
    penetrations without compromising the EM shielding effectiveness, as figure 3-
    6 shows. Where possible, optical fibers are recommended for--

             (a) Voice and data communications lines.

             (b) Energy monitoring and control systems (EMCS).

             (c) Intrusion detection systems.

             (d) Other security systems.

             (e) Control systems.

             (f) Any other use where possible and practical.

          (6) Dielectric isolation. Other isolation techniques include using
    dielectric isolators for shield penetration when external metallic EM energy
    collectors are involved. Examples are control rods* or cables (normally
    metallic), piping systems for fluids, and metallic duct systems for air.
    Dielectric sections are installed at or near the shield to prevent the energy
    induced on the external metallic part from being conducted through the shield.
    Dielectric control rods can enter through a shield in the same way as optical
    fibers, that is, through a waveguide-below-cutoff section. Dielectric isola-
    tion concepts for metallic piping systems and air ducts are discussed in
    chapter 5.
    _________________________
    * Rods that must be mechanically rotated or pulled to control switches, valves,
    and other components.

          (7) Isolation switching. Although not recommended now, isolation
    switching has been provided at facilities so they can use commercial electric
    power during routine operation, but can switch to internal generators or power
    systems in the event of an emergency such as nuclear attack. Since the
    commercial power wiring is a source of significant HEMP energy injection
    through a shield, switching to internally generated power is an obvious
    advantage when advance warning of impending nuclear attack is received and
    throughout the entire nuclear attack cycle. This concept applies to
    communications lines and control lines as well as power lines. Switching used
    in past facility designs has been called "alert attack" switching. Such
    switching must provide adequate switch contact separation to prevent arcing,
    and must be designed to reduce coupling interactions between wiring and switch
    contacts to acceptable levels. It should be noted that advance notice of a
    HEMP attack is not always provided.

    3-4. Cited reference.

        3-1.      Vance, E. F., Shielding and Grounding Topology for Interference
                  Control, Interaction Note 306 (Air Force Weapons Laboratory
                  [AFWL], APril 1977).

    3-5. Uncited references.

            Bailey, D. T., et al., EMP Hardening Guidelines: System Life Cycle
            Cost Design Considerations, AWFL-TR-79-161 (AWFL, May 1980).

            BDM Corporation, Defense Nuclear Agency (DNA) EMP Course (Draft),
            BDM/W-82-305-TR (DNA, April 1983).

            Mindel, I. N., DNA EMP Awareness Course Notes, Third Edition, DNA
            2772T (October 1977).

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    [End Chapter 3]