ELECTROMAGNETIC PULSE (EMP) AND TEMPEST PROTECTION FOR FACILITIES
Table of Contents: http://jya.com/emp.htm
------------------------------------------------------------------------------
CHAPTER 4
SYSTEM ENGINEERING REQUIREMENTS
4-1. Outline. This chapter is organized as follows:
4-1. Outline
4-2. Standards and specifications
4-3. Electromagnetic integration
a. Incompatible design approaches
b. Correcting incompatibilities
c. Electromagnetic shielding
d. Surge protection
4-4. HEMP and lightning protection integration
a. Lightning rise time
b. Frequency and current levels
c. Induced transients and injected current
d. Voltage surges
e. Radiated and static fields
f. Magnetic fields
g. Summary
4-5. HEMP/TEMPEST and electromagnetic integration
a. Electromagnetic compatibility (E~CJ
b. Electromagnetic interference (EMI)
(1) Natural radio noise
(2) Purposely generated signals
(3) Man-made noise
c. Achieving electromagnetic compatibility
(1) Frequency ranges
(2) Spectra encompassed
(3) Interference within enclosures
(4) Exceptions
4-6. Environmental requirements
a. Corrosion
b. Groundwater
c. Thermal effects
d. Vibration and acoustics
e. Ground shock
4-7. Cited references
------------------------------------------------------------------------------
4-2. Standards and specifications. Definitive standards and specifications
for hardening facilities against HEMP/TEMPEST do not exist. However, efforts
are underway to develop them and to integrate them with other HEMP/TEMPEST
requirements and with electromagnetic compatibility (EM) standards. Results
of some recent studies have been reported (refs 4-1 through 4-3). Campi et
al. (ref 4-1) compiled a listing of Government and industrial standards,
specifications, and handbooks related to HEMP/TEMPEST mitigation. Most of
these standards pertain to EMC and TEMPEST (table 4-1). However, many of
these specifications and standards may be useful in integrating EMP hardening
requirements. A comprehensive listing of EMP-related standards is available
in reference 4-4.
4-3. Electromagnetic integration. Facilities often are required to be
protected against several EM environments, including HEMP (or other EMP),
electromagnetic interference (EMI), electromagnetic compatibility, and
lightning. The facility may also have TEMPEST requirements that impose the
need for communications security through control of compromising EM
emanations.
a. Incompatible design approaches. Vance et al. (ref 4-2) have examined
70 related standards and specifications and tabulated areas in which the
design approaches are not compatible for all EM protection requirements. Many
of these incompatibilities are related to methods for grounding cable shields
and allowances for penetrating conductors.
b. Correcting incompatibilities. Graf et al. (ref 4-3) have recommended
ways to correct these incompatibilities. In view of these studies and other
programs, unified EM specifications and standards probably will eventually
become available. Meanwhile, designers will find it necessary to integrate
the EM design on a site-, facility-, and system-specific basis.
c. Electromagnetic shielding. Generally, the main method used in EM
protection is EM shielding. The shielding required for HEMP/TEMPEST is
usually more than enough for all other EM protection. A comprehensive
discussion of grounding and bonding technology for all EM protection is in
MIL-HDBK-419A (ref 4-5). MIL-STD-188-124A gives specific grounding and
bonding requirements (ref 4-6).
d. Surge protection. An area in which care must be taken to ensure
compatibility in EM integration is surge protection. Some surge arresters
used for lightning do not clamp fast enough to protect against EMP. Some ESAs
used for EMP may not have great enough current carrying capacity for lightning
protection in all situations. Thus, for compatible lightning and EMP
protection, a carefully selected combination of protection elements will be
required.
4-4. HEMP and lightning protection integration. The EM environment generated
by lightning differs from that of HEMP in energy spectral distribution rise
time, current levels, pulse repetition and coverage area.
a. Lightning rise time. Many early studies indicated that the typical
rise time of lightning was almost three orders of magnitude slower than that
of HEMP. More recent work, however, has shown that radiation fields produced
by lightning can have much faster rise times. Step leaders in the initial
stroke have had measured rise times reportedly approaching 30 nanoseconds.
Return strokes have been determined to have initial portions with rise time in
the 40- to 200-nanosecond range. A complete lightning flash contains a first
stroke with a downward-moving step leader and usually numerous return strokes
as shown in figure 4-1. The total flash time can be greater than 1 second.
b. Frequency and current levels. A comparison of lightning and HEMP in
the frequency domain shows that radiated lightning energy is higher at low
frequencies and lower at high frequencies as indicated in figure 4-2. The
current levels of lightning return strokes average nearly 35 kiloamps, but may
be less than 10 kiloamps and as high as several hundred kiloamps for so-called
"superbolts."
c. Induced transients and injected current. Hazards common with both HEMP
and lightning are induced transients coupled onto sensitive elements and
injected current from exterior electrical conductors. Lightning also can
strike directly with extreme damage potential. In rare cases, the direct
strike has been known to cause structural damage as well as electrical damage,
even to underground facilities. Thus, facilities need a system of lightning
rods with suitable grounding to divert the extremely high currents (up to
hundreds of kiloamperes peak) away.
d. Voltage surges. Lightning can produce high voltage surges on power
lines without a direct strike. Figure 4-3 shows some typical surge values
versus distance from the stroke.
e. Radiated and static fields. One study has identified radiated fields
associated with lightning (ref 4-7). Figure 4-4 summarizes approximated typi-
cal near-field radiated E-field values. Another study has identified radiated
and static fields associated with lightning (ref 4-8). Figure 4-5 shows
averages for these fields.
f. Magnetic fields. Table 4-2 lists typical values of the H-field close
to a stroke. The close in H-field from lightning thus has higher magnitude
than the HEMP H-field (see table 4-2 for magnitudes); since it has greater
energy content at low frequencies, shield thickness must be greater than for
HEMP.
g. Summary. In summary, integrating HEMP and lightning protection
requires--
(1) Greater shield thickness for lightning if protection from close-in
strokes is required since the H-field magnitude can be greater, although this
is not common practice.
(2) More robust surge arresters for lightning.
(3) Use of lightning rods.
(4) High-frequency protection for HEMP using more sophisticated
transient protection and filtering.
4-5. HEMP/TEMPEST and electromagnetic integration. EMC is defined in ref 4-9
as the ability of communications-electronics equipments, subsystems, and
systems to operate in their intended environments without suffering or causing
unacceptable degradation because of unintentional EM radiation or response.
Electromagnetic interference (EMI) results when EM energy causes unacceptable
or undesirable responses, malfunctions, degrades or interrupts the intended
operation of electronic equipment, subsystems, or systems. RFI is a special
case of EMI for which the radio frequency transmission (usually narrow-band)
causes unintentional problems in equipment operation. For commercial
electronic and electrical equipment, systems, or subsystems, the Federal
Communications Commission (FCC) has regulations defining allowable emission
and susceptibility levels. Military equipment is regulated by MIL STD 461 and
MIL STD 462 (refs 4-10 and 4-11). MIL STD 461 defines allowable emission
levels, both conducted and radiated, and allowable susceptibilities, also both
conducted and radiated. Other specifications exist, but they apply to
specific equipment.
a. Electromagnetic compatibility (EMC). EMC requirements usually apply to
individual equipment as well as to the overall system. Because of equipment
level requirements, the equipment cabinets or racks often must have a degree
of protection, which comprises part of the topological protection.
b. Electromagnetic interference (EMI). The EMI environment has
contributors from three main classes:
(1) Natural radio noise. Natural radio noise originating mainly from
atmospheric disturbances (including lightning) and partly from
extraterrestrial sources.
(2) Purposely generated signals. Signals that are generated purposely
to convey information but that may interfere with the operation of other
equipment.
(3) Man-made noise. Man-made noise such as spectral components
generated incidentally by various electrical and electronic devices, motors,
generators, and other machinery.
c. Achieving electromagnetic compatibility. Achieving EMC involves the
same principles as protection against HEMP/TEMPEST. Generally, a
HEMP/TEMPEST-protected facility will provide EMC protection as well over most
of the desired frequency range. Some exceptions are--
(1) Frequency ranges. EMC encompasses the low frequencies, including
the power frequency spectrum (5 to 400 hertz), and therefore, may have
shielding and filtering requirements different than those for HEMP or TEMPEST
protection.
(2) Spectra encompassed. EMC includes the VHF and microwave spectra as
well as system-specific radiators or susceptibilities requiring special
treatment. Examples are susceptibilities to high power radars beyond the
HEMP/TEMPEST frequency range and switching transients below the HEMP/TEMPEST
frequency range.
(3) Interference within enclosures. EMC also can include interference
between equipment within the same shielded enclosures.
d. Exceptions. Clearly, EMC integration requires that special engineering
attention be given to these stated exceptions. For further guidance, see
references 4-9 and 4-12.
4-6. Environmental requirements. HEMP/TEMPEST protection must withstand
adverse environmental conditions that may occur at the facility. The major
concern is corrosion of buried grounding or shielding system elements,
including exterior steel sheets and buried water pipe or conduit. Other
environments of concern include those with high temperatures, excessive
vibration, and potential ground shock.
a. Corrosion. Design details and the materials used for external
grounding systems and underground shielding elements will affect the corrosion
of all exterior exposed metal installed underground throughout the facility
complex. Galvanic cells are the main cause of corrosion associated with
grounding system and adjacent underground metal objects. A galvanic cell is
produced when two dissimilar metals are immersed in an electrolyte and the
potential difference between electrodes causes a current to flow in a low-
resistance path between them. For HEMP/TEMPEST-protected facilities, the many
grounding connections between steel objects, including shielding and
reinforcing bars in contact with the shield, and the external grounding system
provide a low-resistance conductive path between interconnected metals in the
soil. Current will flow from cathodic material, such as copper or concrete-
encased steel, through these connections to bare steel, such as pipes and
conduits (anodic material). The current flow carries ferrous ions into the
earth electrolyte, resulting in galvanic corrosion of the pipes and conduits.
Conventional design practice for corrosion protection is to electrically
isolate the ferrous metal to be protected from buried copper and concrete
embedded steel. The protected metal often is coated with a dielectric
material. Conventional procedures must be modified to meet the restrictions
and limitations imposed by HEMP/TEMPEST requirements for electrically
continuous and grounded pipes, conduit, and electrical equipment. Close
coordination is required between grounding system design and that for
corrosion protection. Through such coordination, it is often possible to
design grounding systems that avoid corrosion problems, reduce corrosion
protective requirements, and simultaneously improve the grounding system.
b. Groundwater. In areas with high water tables, groundwater presents a
threat to underground shielding elements. Careful design is required to
obtain water-tight penetrations of the floor, roof, and exterior walls. This
includes piping, conduit, and utilitY or access tunnel connections.
c. Thermal effects. If the metallic shield is subjected to temperatures
somewhat higher than adjacent concrete, the sheets will tend to buckle
outward. This condition could occur during construction or during building
operation. Shield buckling is undesirable because welds can be damaged,
compromising the shield and possibly the steel envelope's structural
integrity. To eliminate buckling, provisions for expansion, temperature
control, and/or securing the plates must be included in shielding design.
d. Vibration and acoustics. Shielded rooms in which the audible noise
level is high should be studied for possible acoustical treatment because of
steel's low sound absorption. Likewise, shielded rooms that have vibrating
equipment should be given special consideration to avoid resonant vibration of
shield panels or shielding elements. Excessive panel vibration could
eventually damage welded seams, thus compromising the shielding.
e. Ground shock. If the hardened facility will be in an area of high
seismic activity, or if it must withstand nuclear strikes with high
overpressures, requirements will be defined for ground shock resistance.
Expansion joints may be required between linear plate shielded structures to
protect against differential motion from ground shock. Design for ground
shock protection should be delegated to structural engineers who have
appropriate experience and expertise.
4-7. Cited references.
4-1. Campi, M., G. L. Roffman, and J. R. Miletta, Standardization for
Mitigation of High Altitude Electromagnetic Pulse (HEMP), HDL-
TM-80-33 (U.S. Army Electronics Research and Development
Command, Harry Diamond Laboratories, December 1980).
4-2. Vance, E. F., W. Graf, and J. E. Nanevicz, Unification of
Electromagnetic Specifications and Standards Part I -- Evaluation
of Existing Practices, SRI International AFWL Interaction Note
420 Defense Nuclear Agency [DNA], July 1981).
4-3. Graf, W., J. M. Hamm, and E. F. Vance, Nitrification of
Electromagnetic Specifications and Standard Part II:
Recommendations for Revisions of Existing Practices, DNA 5433F-2
(DNA, February 1983).
4-4. Schulz, R. B., EMC Standards Manual, ECAC-HDBK-82-043 (U.S.
Department of Defense [DOD], November 1982).
4-5. MIL-HDBK-419A, Grounding, Bonding, and Shielding for Electronic
Equipments and Facilities (DOD, 21 January 1982).
4-6. MIL-STD-188-124A, Grounding, Bonding, and Shielding (DOD, 2
February 1984).
4-7. Uman, M. A., M. J. Master, and E. P. Krider, "A Comparison of
Lightning Electromagnetic Fields With Nuclear ELectromagnetic
Pulse in the Frequency Range 104-10-7Hz," IEEE Transactions on
Electromagnetic Compatibility, EMC-24 (4) (Institute of
Electrical and Electronic Engineers [IEEE], November 1982).
4-8. Cianos, N., and E. T. Pierce, A Ground-Lightning Environment for
Engineering Usage, Technical Report 1 (Stanford Research
Institute, August 1972).
4-9. Engineering Design Handbook, Electromagnetic Compatibility,
DARCOM Pamphlet P 706-410 (U.S. Army Materiel Command [AMC],
March 1977).
4-10. MIL-STD-461B, Electromagnetic Emission and Susceptibility
Requirements for the Control of Electromagnetic Interference
(DOD, 1 April 1980).
4-11. MIL-STD-462, (U) Measurement of Electromagnetic Interference
Characteristics (DOD, 9 February 1971). (C)
4-12. USAF Design Handbook DH-1.
4-13. NACSEM 5204, (U) Shielded Enclosures (National Security Agency,
May 1978). (C)
4-14. NACSEM 5203, (U) Guidelines for Facility Design and Red/Black
Installation, (National Security Agency, June 1982). (C)
4-15. MIL-HDBK-232A, (U) Red/Black Engineering Installation Guidelines
(Draft). (C)
------------------------------------------------------------------------------
Table 4-1. HEMP/TEMPEST-related standards and specifications. (Sheet 1 of 3)
____________________________________________________________________________
Specifications
and Standards Issuer Superseded by Short title.
____________________________________________________________________________
AFSC DM 1-4 USAF - Electromagnetic Compact
AFSC DH2-7 USAF - Sys Survivability
AFSCN 500-6 USAF - EMP Ef on Air Force
AIR-STD-20/16 USAF - Des Gde Haz of EMR-Argon
Wpn Sys
AIR 1221 SAE - EMC Sys Des Require
AIR 1255 SAE Spect An for EMI Mgmt
AIR 1173 SAE - Test Proc-Mar RF Shldng Char
AIR 1404 SAE - DC Resis vs. RF IMP-EMI Gask
AIR 1500 SAE - Bib Lossy Filters
AN-J-1 USN/USAF MS 2508 Bonding Jumpers
ANS C63.2 ANSI IF RI-FI Meters < 30 MHz
ANS C63.3 ANSI IF Msrmts, < 25 MHz
ANS C63.5 ANSI IP Msrmt 20 MHz-1 GHz
ANS C63.8 ANSI IP Msrmt < 30 MHz
ANS C63.9 ANSI IP RI-FI Meters 0.01-15 kHz
ARP 935 SAE - Sugg EMI Cntl Plan Outline
ARP 936 SAE - EMI 10-microF Capacitor
ARP 958 SAE IF Antenna Factors
ARP 1172 SAE - Filt. Conv EMI Gen Spec
DCA-330-190-1 DCA - Equip Performance
DCAC-330-175-2 DCA - DCS Engr Installation
DIAM-50-3A DIA - Phy Security Stds for
Sensitive Compartmented
Information Facilities
DNA 2114H-l DNA - EMP Hdbk, Des Principles
DNA 2114H-2 DNA - EMP Hdbk, Anal & Treating
DNA 2114H-3 DNA - EMP Hdbk, Env & Applications
DNA 2114H-4 DNA - EMP Hdbk, Resources
DNA 3286-H DNA - EMP Preferred Test Proc.
D65/9371 BSI - RFI Aircraft Require
FED-STD-222 All Feds NACSEM-5100 Info Process Emissions
FED-STD-1030A DCA/NCS Proposed Balanced Dig. Interface Ckts
FED-STD-1030A DCA/NCS Proposed Unbalanced Dig Interface Ckts
FED-STD-1040 DCA/NCS Proposed Data Term, Data Ckt Interface
JAN-I-225 USA/USN MIL-I-6181 Interfer Cntl/Test
J551 SAE J551A Vehicle RFI
J551A SAE IF Vehicle RFI
MIL-B-5087B(ASG) USN/USAF Amend #2 Aerospace Bonding
MIL-C-11693A USANAR MIL-C-11693B R-I Feedthru Capacitor
MIL-C-11693B USANAF IF R-I Feedthru Capacitor
MIL-C-12889 USA SC MIL-C-12889A R-I Bypass Capacitors
MIL-C-12899A USANAF IF R-I Bypass Capacitors
MIL-C-19080 USAN SHIPS MIL-C-11693B R-9 Bypass Capacitors
MIL-C-39011 USANAF IF Feedthru Capacitors
MIL-E-4957A USAF MIL-E-4957A(ASG) EMI Shielded Enclosure
MIL-E-4957(ASG) USN/USAF Cancelled EMI Shielded Enclosure
MIL-E-55301(EL) USA MIL-STD-461/462 EM Compatibility
MIL-E-6051C USANAF MIL-E-6051D Sys EMC Require
MIL-E-6051D USANAF IF Sys EMC Require
MIL-E-8669 USN BuA MIL-E-4957A(ASG) EM Shielded Enclosure
MIL-E-8881 USANAF IF Shielded EnclosureMIL-F-
15733C USANAF NIL-F-15733D Radio Interf Filters
MIL-F-15733D USANAF NIL-F-15733E Radio Interf Filters
MIL-F-15733G USANAF IF Radio Interf Filters
MIL-F-18327C USANAF - Filter Specs
MIL-F-18344A USN MIL-F-15733C Radio Interf Filters
MIL-HDBK-232A USANAF - RED/BLACK Engr Instal Gdlines
MIL-HDBK-411 USANAF - Long Haul Comm & Env Cntl
MIL-HDBK-419A USANAF IP GBS for Telecomm Facilities
MIL-I-6051 USANAF MIL-I-6051C Aircraft EMI Limits
MIL-I-6051A USAF MIL-E-006051B Aircraft EMI Limits
MIL-I-006051B USAF MIL-E-6051C Sys EMC Require
MIL-I-6181 USANAF MIL-I-6181B EMI Cntl Aircraft
MIL-STD-188-124A DOD - Grounding, Bonding and
Shielding
MIL-STD-202A DOD - Test Methods for Electronic
and Electrical Component
Parts
MIL-STD-220A DOD - Method of Insertion--
Less Measurement
MIL-STD-248C DOD - Welding and Brazing Procedure
and Performance Qualifi-
cation
MIL-STD-285 DOD - Attenuation Measurements for
Enclosures, etc. Methods
MIL-STD-461C DOD - Electromagnetic Emission and
Susceptibility Requirements
for Control of EMT
MIL-STD-1542 DOD - EMC and Grounding Reqmts
for Space Sys Facilities
NACSEM 5109 NSA - Tempest Testing Fundamentals
NACSEM 5110 NSA - Facilities Evaluation Cri-
teria--TEMPEST
NACSEM 5201 NSA - TEMPEST Guidelines for
Equipment/System Design
NACSEM 5204 NSA - Shielding Enclosures
NACSI 5004 NSA - TEMPEST Countermeasures for
NASCI 5005 NSA - TEMPEST Countermeasures for
Facilities Outside the U.S.
NACSIM 5000 NSA - TEMPEST Fundamentals
NACSIM 5100A NSA - Compromising Emanations
Laboratory Test Reqmts,
Electromagnetics
NACSIM 5203 NSA - Guidelines for Facility
Design and RED/BLACK
Installation
NSA 65-5 NSA - NSA Specification for RF-
Shielded Acoustical
Enclosures for Communica-
tions Equipment
NSA 65-6 NSA - NSA Specification for RF-
Shielded Enclosure for
Communications Equipment
NSA 73-2A NSA - NSA Specification for Foil
RF-Shielded Enclosure
------------------------------------------------------------------------------
Table 4-2. Peak magnetic field values for close lightning strokes.
_____________________________________________________________
Magnetic fields
(amps/meters)
Peak _______________________________________________
current 10 m 100 m 10 km
(kA) from flash from flash from flash
_____________________________________________________________
10 1.6 x 10^2 16 1.9 x 10^-2
20 3.2 x 10^2 32 3.8 x 10^-2
30 4.8 x 10^2 48 5.8 x 10^-2
70 1.1 x 10^3 1.1 x 10^2 1.3 x 10^-2
100 1.6 x 10^3 1.6 x 10^2 19 x 10^-2
140 2.2 x 10^3 2.2 x 10^2 27 x 10^-2
200 3.2 x 10^3 3.2 x 10^2 38 x 10^-2
[Figures 4-1 through 4-5 not digitized here.]
------------------------------------------------------------------------------
[End Chapter 4]