HEMP DEFINITIONS

ABSORBER. A material which causes the irreversible conversion of the energy of an electromagnetic wave into another form of energy, usually heat as a result of its interactions with matter.

ABSORPTION. In the transmission of signals (electrical, electromagnetic, optical, acoustical), the conversion of transmitted energy into heat or other forms of energy.

ABSORPTION LOSS. The attenuation of an electromagnetic wave as it passes through a shield. This loss is primarily due to induced currents and the associated power loss.

AMBIENT LEVEL. Those levels of radiated and conducted energy existing at a specified location and time when test sample is de-energized. Atmospheric noise signals, both desired and undesired, from other sources and the internal noise level of the measuring instruments all contribute to the “ambient level”.

ANODE. In the context of metal corrosion, the less noble or higher potential member of a pair of metals, upon which oxidation or corrosion occurs. Anode is the opposite of cathode.

ANODIZING. Causing a metal, usually aluminum, to become oxidized on its surface to form a protective coating and prevent further corrosion. Anodizing is caused by an acid bath, usually sulfuric acid.

ANTENNA. Any structure or device used to collect or radiate electromagnetic waves.

APERTURE. An opening in a shield through which electromagnetic energy passes.

APERTURE POINT-OF-ENTRY. Intentional or inadvertent holes, cracks, openings, or other discontinuities in the facility HEMP shield surface. Intentional aperture points-of-entry are provided for personnel and equipment entry and egress and for fluid flow (ventilation and piped utilities) through the electromagnetic (EM) barrier.

ARRESTER. A device to protect an equipment, circuit, subsystem, or system from a voltage or current surge such as may be produced by lightning or an electromagnetic pulse.

ATTENUATION. A general term used to denote a decrease in magnitude of power or fields strength in transmission from one point to another caused by such factors as absorption, reflection, scattering and dispersion. It may be expressed as a ratio, or by extension of the term, decibels.

BOND. The electrical connection between two metallic surfaces, established to provide a low-resistance path between them.

BOND, DIRECT. The electrical connection utilizing continuous metal-to- metal contact between the members being joined.

BOND, INDIRECT. An electrical connection employing an intermediate electrical conductor between the bonded members.

BONDING. The process of establishing the required degree of electrical continuity between the conductive surfaces to be joined.

BREAKDOWN VOLTAGE. The voltage at which an insulating material ceases to insulate and becomes electrically conductive.

BROADBAND EMISSION. An emission which has a broad and continuous spectral energy distribution, so that the response of the measuring receiver does not vary significantly when tuned over a large bandwidth.

BURNOUT. A type of failure which implies the destruction of a component due to a permanent change beyond an acceptable amount in one or more characteristics.

CATHODE. In the context of metal corrosion, the more noble or lower potential member of a metal pair, where reduction and practically no corrosion occurs.

CATHODIC PROTECTION. Cathodic protedion. Reduction or elimination of corrosion by means of direct current (dc) that makes the metal to be protected a cathode, which is connected to another metal that serves as a sacrificial anode.

CHEMICAL BIOLOGICAL, RADIOLOGICAL, (NUCLEAR) EXPLOSIVE (CBRE, CBRNE). The uncontrolled release of chemicals, biological agents or radioactive contamination into the environment or explosions that cause widespread damage. CBRNE events can be caused by accidents or by terrorist acts.

CIRCULAR MIL. A unit of area equal to the area of a circle whose diameter is one mil (1 mil = 0.001 in.). To convert a circular mil to a square mil, multiply by 0.7854 (1 square mil = 10.-6- square in.). The area of a circle in circular mils is equal to the square of its diameter in mils.

CLAMP. A function by which the extreme amplitude of a waveform is reduced to a specified level.

CLIP. To limit voltage or current amplitudes to a predetermined level.

COMMON MODE. The voltage or current which is common to all signal-carrying conductors with respect to ground (also see differential mode).

COMMON-MODE REJECTION. The ability of a device to reject a signal that is common to both its input terminals.

CONDUCTED EMISSION. Electromagnetic emissions propagated along a power or signal conductor, acting as a transmission line with conduit, shield, cable tray, or earth as the return conductor.

CONDUCTED INTERFERENCE. Undesirable signals that enter or leave an equipment along a conductive path through direct coupling.

CONDUCTED SUSCEPTIBILITY. Measure of the interference signal current or voltage required on power, control, and signal leads to cause an undesirable response or degradation of performance.

CONTINUOUS SHIELD. A shield fabricated from metal sheets or plates joined by welding, brazing, soldering, or other process so that all seams are completely filled with metal to form an electrically continuous joint.

CORROSION. A specific type of deterioration of a material, usually a metal, or its properties as a result of the surrounding environment.

COUNTERPOISE. A system of wires or other conductors usually elevated above and insulated from the ground, forming the lower system of conductors of an antenna. For EMP and power systems this is often an array of buried conductors.

COUPLING. The means by which energy is transferred from one conductor (including a fortuitous conductor) to another.

COUPLING, CONDUCTED. Energy transfer through a conductor/transmission line.

COUPLING, FREE-SPACE. Energy transfer via electromagnetic fields not through a conductor.

CUTOFF FREQUENCY. The frequency below which electromagnetic energy will not propagate in a waveguide without significant attenuation, or the 3 dB attenuation or insertion loss point in electrical filter responses.

DAMAGE (MALFUNCTION) LEVEL. The value of voltage, current, or field strength that causes a permanent malfunction or damages an equipment item.

DEGRADATION. A decrease in the quality of a desired signal (i.e., decrease in the signal-to-noise ratio or increase in distortion), or an undesirable change in the operational performance of equipment as the result of interference.

DELIBERATE ANTENNA. A receiving or transmitting antenna specifically designed to be a part of a system, but which may pick up or receive HEMP energy as well (also see inadvertent antenna).

DIFFERENTIAL MODE. The voltage or current of a conductor with respect to any other conductor (also see common mode).

DISSIMILAR METALS. Any combination of bare metals that are unlike. Metals are dissimilar when two metal specimens are in contact or otherwise electrically bonded together and generate an electric current. This current causes corrosion of one or both of the metal specimens. The more dissimilar the metals, the greater the galvanic attack of the anodic metal.

DOWN CONDUCTOR, LIGHTNING. The conductor connecting the air terminal or overhead ground wire to the earth electrode subsystem.

EARTH ELECTRODE SUBSYSTEM. A network of electrically interconnected rods, plates, mats, or grids installed or connected for the purpose of establishing a low-resistance contact with earth.

ELECTRIC PROTECTION. The use of electrical devices and techniques to protect equipment, facilities, and people against hazardous voltages and currents.

ELECTRIC FIELD. A vector field around an electrically charged body. The field’s strength at any point is the force that would be exerted on a unit positive charge at that point.

ELECTRONIC SURGE ARRESTER (ESA). A transient suppression device generally installed between an electrical terminal and ground. These devices respond to the rate of change and level of a current or voltage to prevent a rise above a predetermined value. The devices may include metal-oxide varistors (MOVs), spark gaps, diodes, and others. ESAs are also known as electrical surge arresters, transient protection devices, and nonlinear devices. ESAs are used in conjunction with linear attenuation devices for electrical point-of–entry protection.

ELECTROMAGNETIC BARRIER. The topologically closed surface created to prevent or limit HEMP fields and conducted transients from entering the enclosed space. The barrier consists of the facility HEMP shield and point-of-entry treatments, and it encloses the protected volume.

ELECTROMAGNETIC COMPATIBILITY (EMC). The ability of telecommunications equipment, subsystems, or systems to operate in their intended operational environments without suffering or causing unacceptable degradation because of electromagnetic radiation or response.

ELECTROMAGNETIC INTERFERENCE (EMI). The phenomenon resulting when electromagnetic energy causes an unacceptable or undesirable response, malfunction, degradation, or interruption of the intended operation of an electronic equipment, subsystem, or system.

ELECTROMAGNETIC PULSE (EMP). The electromagnetic radiation from a nuclear explosion caused by Compton-recoil electrons and photoelectrons from photons scattered in the materials of the nuclear device or in a surrounding medium. The resulting electric and magnetic fields may couple with electrical/electronic systems to produce damaging current and voltage surges. EMP may also be caused by nonnuclear means.

ELECTROMAGNETIC RADIATION (EMR). Radiation made up of oscillating electric and magnetic fields and propagated with the speed of light. Electromagnetic radiation includes gamma radiation; X-rays; ultraviolet, visible, and infrared radiation; and radar and radio waves.

ELECTROMAGNETIC STRESS. A voltage, current, charge, or electromagnetic field which acts on an equipment. If the electromagnetic stress exceeds the vulnerability threshold of the equipment, mission-aborting damage or upset may occur.

EMISSION. Electromagnetic energy propagated from a source by radiation or conduction.

EQUIPMENT GROUND NETWORK. An electrically continuous network consisting of interconnected grounding plates and structural steel elements.

EQUIPOTENTIAL (GROUND) PLANE. A mass, or masses, of conducting material which, when bonded together, offers a negligible impedance to current flow.

EXCLUSION ZONE. A region, inside a barrier, from which all cables and other conductors are excluded to ensure that they do not interact strongly with the HEMP fields in that region.

FACILITY. A building or other structure, either fixed or transportable in nature, with. its utilities, ground networks, and electrical supporting structures. All wiring and cabling required to be provided are considered to be part of the facility. Any electrical and electronic equipment required to be supplied and installed are also part of the facility.

FACILITY GROUND SYSTEM. The electrically interconnected system of conductors and conductive elements that provides multiple current paths to the earth electrode system. The facility ground system includes the equipment ground network and the equipment racks, cabinets, conduit, junction boxes, raceways, duct work, pipes, and other normally noncurrent-carrying metal elements.

FACILITY HEMP SHIELD. The continuous metallic housing that substantially reduces the coupling of HEMP electric and magnetic fields into the protected volume. The facility HEMP shield is part of the electromagnetic barrier.

FAILURE. The termination of the ability of an item to perform its required function.

FAR-FIELD. The region of the field of a source where the angular field distribution is essentially independent of the distance from the source.

FAULT. In power systems, an unintentional short circuit or partial short circuit between energized conductors or between an energized conductor and ground.

FAULT PROTECTION SUBSYSTEM. This subsystem ensures that personnel are protected from shock hazards and the equipment is protected from damage resulting from faults, including short circuits that may develop in the electrical supply and distribution.

FERROUS SHIELDING. A low electrical resistance and high magnetic permeability material which provides a low reluctance magnetic and high conductivity path.

FIELD STRENGTH. A general term that means either the magnitude of the electric field vector (in volts per meter) or the magnitude of the magnetic field vector (in ampere-turns per meter). As used in the field of EMC/EMI, the term “Field Strength” shall be applied only to measurements made in the far field and shall be abbreviated as FS. For measurements made in the near field, the term “Electric Field Strength” (EFS) or “Magnetic Field Strength” (MFS) shall be used, according to whether the resultant electric or magnetic field, respectively is measured. The EFS shall be expressed as V/M and the MFS as A/M. In this near field region, the field measured will be resultant of the radiation, inductive and quasistatic (1/r, 1/r^2-, and, if present, the 1/r^3) components, respectively of the field where r is the distance from the source.

FILTER. In electronics, a device that transmits only part of the incident energy and may thereby change the spectral distribution of energy.

FLASHOVER. The destructive arcing of electricity over insulators or other components due to an overvoltage condition (e.g. EMP).

FORTUITOUS CONDUCTOR. Any conductor which may provide an unintended path for intelligible signals, or potentially harmful energy from and EMP; for example, water pipes, wire or cable, metal structural embers, and so forth.

FREE-FIELD. An electromagnetic field in which the effects of boundaries are negligible over the region of interest.

GEOMAGNETICALLY INDUCED CURRENTS (GIC). A manifestation at ground level of space weather. During space weather events, electric currents in the magnetosphere and ionosphere experience large variations, which manifest also in the Earth’s magnetic field. These variations induce currents (GIC) in conductors operated on the surface of Earth. Electric transmission grids and buried pipelines are common examples of such conductor systems. GIC can cause problems, such as increased corrosion of pipeline steel and damaged high-voltage power transformers. GIC are one possible consequence of geomagnetic storms

GALVANIC CORROSION. Corrosion caused by placing two dissimilar metals in a corrosive or conductive solution in contact with each other, so that the potential difference allows electrons to flow between them.

GALVANIC SERIES. A listing of metals and alloys, ordered on their tendency to corrode independently in a particular electrolyte solution or other environment. This tendency for dissolution or corrosion is related to the electrical potential of the metal in a conductive medium, such as sea water. Metals closely positioned in the galvanic series will have similar electrical potentials, and corrosion will be minimized.

GAS TUBE. A spark gap with metal electrodes hermetically sealed in an envelope, so that a gas mixture and pressure can be controlled, thereby controlling the breakdown voltage of the device.

GLOBAL SHIELD. A single HEMP shield that encloses an entire building or the entire part of the building containing the mission-critical systems. Global shield is a synonym for overall shield.

GROUND. The electrical connection to earth through an earth electrode subsystem. This connection is extended throughout the facility via the facility ground system, consisting of the signal reference subsystem, the fault protection subsystem, and the lightning protection subsystem.

GROUND LOOP. Circulating common mode currents caused by DC, AC, Audio or RF ground potential differences. Also referred to as Circulating Ground Currents and Ground Current Loops.

GROUND PLANE. A conducting surface, plate or system of conductors used as a common reference point for circuit signal current returns and electrical or signal potentials.

GROUNDING (GROUND). The process of providing a metallic surface with low resistance or low impedance path to ground potential.

GROUND, SINGLE-POINT. A scheme of circuit/shield grounding in which each circuit/shield has only one physical connection to ground, ideally at the same point for a given subsystem. This technique prevents return currents from flowing in the structure.

HARDNESS. A measure of the ability of a system to withstand exposure to one or more of the effects of either nuclear or nonnuclear weapons.

HAZARDOUS ELECTROMAGNETIC RADIATION TO FUEL (HERF). Potential for fuel and petroleum products to be adversely affected by electromagnetic radiation.

HAZARDOUS ELECTROMAGNETIC RADIATION TO ORDNANCE (HERO). Potential for munitions or electroexplosive devices to be adversely affected by electromagnetic radiation.

HAZARDOUS ELECTROMAGNETIC RADIATION TO PERSONNEL (HERP). Potential for electromagnetic radiation to create a hazard for personnel.

HIGH-ALTITUDE ELECTROMAGNETIC PULSE (HEMP). An electromagnetic pulse produced at an altitude above the sensible atmosphere.

HEMP ACCEPTANCE TEST. An acceptance test is a test of a system, subsystem, or component performed to ensure that specified performance characteristics have been met. HEMP acceptance tests, conducted near the conclusion of a hardening construction or installation contract, are tests for the purpose of demonstrating that at least minimum performance requirements of the HEMP protection subsystem have been achieved before the subsystem will be accepted by the Government from the contractor.

HEMP HARDNESS ASSURANCE. Quality assurance measures during fabrication .and installation of the HEMP protection subsystem for maintaining the integrity of the hardened design. HEMP hardness assurance is part of the total hardness assurance, maintenance, and surveillance (HAMS) program.

HEMP HARDNESS CRITICAL ASSEMBLY (HCA). A top-level assembly of HEMP hardness critical items and other components, such as mounting brackets and fasteners, that may not be hardness critical. Hardness maintenance and surveillance actions are normally scheduled, performed, and tracked at the hardness critical assembly level.

HEMP HARDNESS CRITICAL ITEM (HCI). A hardness critical item is an item, usually at the individual component level, having performance requirements for the purpose of providing protection from an explosion or natural disaster. Nuclear hardness critical items provide protection from environments produced by a nuclear event or are specially designed to operate under nuclear stresses. HEMP hardness critical items are the elements of the HEMP protection subsystem.

HEMP HARDNESS CRITCAL PROCESS (HCP). A process, specification, or procedure which must be followed exactly to ensure that the associated HEMP hardness critical item attains its required performance.

HEMP HARDNESS MAINTENANCE (HM). Preventive maintenance (e.g., adjustments or cleaning) and corrective maintenance (e.g., repairs or replacements) on the HEMP protection subsystem or its hardness critical items and assemblies. These HM activities are intended to eliminate faults or to preserve specified performance levels.

HEMP HARDNESS MAINTENANCE/HARDNESS SURVEILLANCE (HM/HS). The combined preventive maintenance, inspection, test, and repair activities accomplished on a HEMP-protected operational facility to ensure that HEMP hardness is retained throughout the system life cycle. Hardness maintenance and hardness surveillance, along with hardness assurance, constitute a total HAMS program.

HEMP HARDNESS SURVEILLANCE (HS). Inspections and tests of the HEMP protection subsystem or its hardness critical items and assemblies. These HS activities are intended to observe and monitor the condition and performance of the hardening elements and to detect faults.

HIGHER FREQUENCY GROUND. The interconnected metallic network intended to serve as a common reference for currents and voltages at frequencies above 300 kHz and in some cases down to 30 kHz. Pulse and digital signals with rise and fall times of less than 1 microsecond are classified as higher frequency signals. High frequency in this sense should not be confused with the broadcast HF band which covers from 3 to 30 MHz.

HIGH-SPEED GAP. A gas tube with improved response time produced by radioactive doping of the gas medium and the presence of a semiconductor triggering element across the gap.

IMPULSE RATIO (OF A SPARK GAP). The ratio of the actual sparkover voltage from an applied surge to the static sparkover voltage. The ratio is generally greater than or equal to one and increases with the increasing rate of rise of the applied voltage surge.

INADVERTENT ANTENNA. Any physical object, other than deliberate antennas, that can act as a receiving antenna for HEMP energy.

INTEGRATED LOGISTICS SUPPORT (ILS). A composite of all the support considerations necessary to ensure the effective and economical support of a system for its life cycle. It is an integral part of system acquisition and operation.

INTERNAL COUPLING. That part of the overall HEMP energy transfer process which occurs inside the protected volume, where fields penetrating the HEMP harrier induce currents traveling along cables or conductors inside this volume.

INTOLERABLE UPSET. An upset time of greater that n seconds, identified as part of the facility operational mission requirements.

ISOLATION. Physical and electrical arrangement of the parts of an equipment, system, or facility to prevent uncontrolled electrical contact within or between the parts.

LIFE-CYCLE COST (LCC). The total direct, indirect, recurring, nonrecurring, and other related costs incurred or estimated to be incurred in the design, development, production, operation, maintenance, and support of a major system over its anticipated useful life.

LIGHTNING DOWN CONDUCTOR. The conductor connecting the air terminal or overhead ground wire to the earth electrode subsystem.

LIGHTNING PROTECTION SUBSYSTEM. This subsystem provides a non- destructive path to ground for lightning energy and directs these high currents away from susceptible elements and also limits the voltage gradients developed by the high currents to safe levels.

LOWER FREQUENCY GROUND. A dedicated, single-point network intended to serve as a reference for voltages and currents, whether signals, control or power, from dc to 30 kHz and in some cases to 300 KHz. Pulse and digital signals with rise and fall times greater than 1 microsecond are considered to be low frequency signals.

LOW-RISK HEMP HARDENING. A hardening technique that features a high-quality electromagnetic barrier with minimized and protected points-of-entry. Virtually all mission-essential communications-electronics and support equipment are placed in the protected volume enclosed by the barrier and operate in a relatively benign electromagnetic environment, isolated from the external HEMP stresses. The low-risk approach results in a well-defined HEMP protection subsystem configuration with inherent testability.

MAGNETIC FIELD. A vector field set up by a moving charge or current. This field also exerts a force on moving charges or currents within the field.

MISSION-CRITICAL SYSTEM Synonym for mission-essential equipment.

MISSION-ESSENTIAL EQUIPMENT (MEE). Includes all communications-electronics and support equipment required to perform specified missions. In the context of MILSTD-188-125 MEE refers to equipment required to perform missions specified to be hardened against the HEMP environment.

MULTIPOINT GROUND. More than one path to ground.

NATIONAL ELECTRICAL CODE (NEC). A standard governing the use of electrical wire, cable, and fixtures installed in buildings; developed by a committee of the American National Standards Institute, sponsored by the National Fire Protection Association (NFPA), identified by the description ANSI/NFPA 70, and adopted by the Federal Government.

NEAR-FIELD. The region of the field of an antenna between the close-in reactive field region and the far-field region. The angular field distribution is dependent upon distance from the antenna in the near field.

NEUTRAL. The AC power system conductor which is intentionally grounded on the supply side of the service disconnecting means. It is low potential (white) side of a single phase AC circuit or low potential fourth wire of a three-phase WYE distribution system. The neutral conductor provides a current return path for AC power currents whereas the ground (or green) conductor should not provide a current return path except during fault conditions.

NOISE. An undesired disturbance within the useful frequency band. Noise is the summation of unwanted or disturbing energy introduced into a communications system from man-made and natural sources.

NORMS. Scalar quantities which characterize the features of a complicated waveform. Norms used as pass/fail criteria for pulsed current injection test residual internal stresses are peak current, peak rate of rise, rectified impulse, and root action.

OPERATIONAL UPSET. Usually implies temporary impairment of operation that will not result in permanent damage, such as a significant disturbance or perturbation to the normal operation.

OVERALL SHIELDING. As used in military handbooks, the protection of an entire facility by use of a single shielded enclosure. An overall shield is a central requirement of the low-risk hardening approach.

PEAK CURRENT. The peak current norm of a current waveform I(t), in units of amperes, is the maximum absolute value of I(t) over times from t =0 to t =5 X 10-3s. At the start of the pulsed current injection drive pulse, t = o.

PEAK RATE OF RISE. The peak rate of rise norm of a current waveform I(t), in units of amperes per second, is the maximum absolute value of dI/dt over times from t = 0 to t = 5 X 10-3s. At the start of the pulsed current injection drive pulse, t = 0.

PENETRATING CONDUCTOR. Any electrical wire or cable or other conductive object, such as a metallic rod, which passes through the electromagnetic barrier. Penetrating conductors are also called conductive points-of-entry.

PENETRATION. The passage through a partition or wall of an equipment or enclosure by a wire, cable, or other conductive object.

PENETRATION ENTRY AREA (PEA). That area of the electromagnetic barrier where long penetrating conductors (such as an electrical power feeder) and piping points-of-entry are to be concentrated.

PERMEABILITY. A general term used to express various relationships between magnetic induction and magnetizing force; the magnetic analog of electrical permittivity. Either absolute permeability or relative permeability may be used. The permeability of free space (magnetic constant) is made up of corresponding values of magnetizing force and flux density.

PERMITTIVITY. The scalar that relates the electric field strength to the electric flux density. Permittivity is also known as dielectric constant. Permittivity is analogous to magnetic permeability, and it specifies the ease with which electric flux is permitted to pass through a given dielectric material.

PLANE WAVE. An electromagnetic wave that predominates in the far-field region of an antenna, and with a wavefront that is essentially in a fiat plane. In free space, the impedance of a plane wave is 377 ohms.

POINT(S) OF ENTRY (POE). A location on the electromagnetic barrier where the shield is penetrated and HEMP energy may enter the protected volume unless an adequate POE protective device is provided. POEs are classified as aperture POEs or penetrating conductors according to the type of penetration. They are also classified’ as architectural, mechanical, structural, or electrical POEs according to the architectural-engineering discipline in which they are usually encountered.

POE PROTECTIVE DEVICE OR POE TREATMENT. The protective measure used to prevent or limit HEMP energy from entering the protected volume at a POE. Common POE protective devices include waveguides-below-cutoff (WBCs) and closure plates for aperture POEs, and filters and ESAs on penetrating conductors.

PULSED CURRENT INJECTION (PCI). A test method for measuring performance of a POE protective device on a penetrating conductor. A HEMP threat-relatable transient is injected on the penetrating conductor at a point outside the electromagnetic barrier, and the residual internal transient stress is measured inside the barrier.

RADIATION. The emission and propagation of electromagnetic energy through space.

RADIATION RESISTANCE. The resistance which, if inserted in place of an antenna, would consume the same amount of power that is radiated by the antenna.

RADIATION HAZARDS TO PERSONNEL (RADHAZ). Potential for personnel to be adversely affected by electromagnetic radiation.

RADIATED EMISSION. Desired or undesirable electromagnetic energy which is propagated through space. Such an emission is called “radiated interference” if it is undesirable.

RADIATED SUSCEPTIBILITY. A measure of the radiated interference field required to cause equipment degradation.

RADIO FREQUENCY (RF). Those frequencies of the electromagnetic spectrum normally associated with radio wave propagation.

RADIO FREQUENCY INTERFERENCE (RFI). Synonym for electromagnetic interference.

REFLECTION COEFFICIENT. The ratio of the phasor magnitude of the reflected wave to the phasor magnitude of the incident wave under specified conditions.

REFLECTION LOSS. The portion of the transition loss, expressed in dB, that is due to the reflection of power at a barrier or shield. Reflection loss is determined by the magnitude of the wave impedance inside the barrier relative to the wave impedance in the propagation medium outside the barrier.

RF GASKET. A material that when compressed between two conductive surfaces creates a seal that will significantly reduce the passage of RF energy, such as that found in a HEMP pulse.

RF-TIGHT. Offering a high degree of electromagnetic shielding effectiveness.

RESIDUAL INTERNAL STRESSES. The electromagnetic fields, voltages, currents, or charges which originate from the HEMP environment and penetrate into the protected volume after attenuation by elements of the electromagnetic barrier.

RETROFIT HEMP HARDENING. A retrofit action is an action taken to modify in-service equipment. Retrofit HEMP hardening is the installation or substantial upgrade of the HEMP protection subsystem for an existing facility or equipment.

SHIELD. A housing, screen, or cover that substantially reduces the coupling of electric and magnetic fields into or out of circuits or prevents the accidental contact of objects or persons with parts or components operating at hazardous voltage levels.

SHIELDING EFFECTIVENESS (SE). A measure of the reduction or attenuation in the electromagnetic or electrostatic field strength at a point in space, caused by the insertion of a shield between the source and that point.

SHIELDED ENCLOSURE. An area (room or box) specifically designed to attenuate electromagnetic radiation, or electromagnetic radiation and acoustic emanation, originating either inside or outside the area. Necessary openings in shielded enclosures, such as doors, air vents, and electrical feedthroughs, are specially designed to maintain this attenuation.

SIGNAL GROUND BUS. A component of the signal ground system which is utilized primarily to connect Signal Ground Reference Planes to the Signal Ground Planes.

SIGNAL GROUND PLANE. An intermediate grounding point insulated from and installed in, on, or near the Main Distribution Frame, Intermediate Distribution Frame, Classified Intermediate Distribution Frame and connected to the Signal Ground Point.

SIGNAL GROUND POINT. A single designated point in a station to which all RED/BLACK grounds are either directly or indirectly connected. This point serves as the common zero potential reference for the station.

SIGNAL GROUND REFERENCE PLANE. An intermediate focal point between an equipment and the Signal Ground Plane for terminating an equipment’s or Terminal System’s ground circuits. The Signal Ground Reference Plane is isolated from the equipment’s AC Protective Ground and is connected to the Signal Ground Plane by a Signal Ground Bus.

SIGNAL GROUND REFERENCE POINT. Same as a Signal Ground Reference Plane but serving one of several Limited Exclusion Areas vice equipment or Terminal Systems.

SIGNAL REFERENCE SUBSYSTEM. This subsystem provides the reference points for all signal grounding to control static charges, noise and interference. It may be comprised of any one or any combination of the lower frequency network, higher frequency network, or hybrid signal reference network.

SIGNAL RETURN. A current-carrying path between a load and the signal source. It is the low side of the closed loop energy transfer circuit between a source-load pair.

SINGLE POINT GROUND. The basic technique used in which separate ground conductors are used for the various grounding functions (signal, power, hazard, and so forth) with each conductor connected directly or indirectly to a single point (Signal Ground Point).

SIMULATION EQUIPMENT. The equipment used to simulate the threat environment, including pulsers and current drivers.

SPARK GAP. A voltage limiting or clamping device (an ESA) consisting of two or more electrodes separated by a dielectric. An electric arc develops whenever the voltage between two electrodes exceeds the sparkover voltage. Examples are the carbon-block gap and the gas tube.

SPECIAL PROTECTIVE MEASURE (SPM). All HEMP hardening measures required in addition to implementation of the electromagnetic barrier. Special protective measures are necessary for MEE outside the barrier, for MEE which is within the protected volume and experiences damage or upset during verification testing, and in cases requiring a special protective volume.

SPECIAL PROTECTIVE VOLUME (SPV). A region within the electromagnetic barrier and a special protective barrier (SPB), where electromagnetic stresses due to HEMP may exceed the residual internal stress limits for the protected volume. The SPB may be a separate shield with protected penetrations; more commonly, shielded cables or conduits and equipment cabinets and closed piping systems are used to provide the needed electromagnetic isolation from the protected volume.

SPURIOUS SIGNALS. Undesirable signals appearing external to equipment or a circuit. They may be harmonics of existing desired signals, high frequency components of complex wave shapes, or signals produced by incidental oscillatory circuits.

SUSCEPTIBILITY. The degree to which an electronic equipment, subsystem, or system evidences undesirable responses when subjected to electromagnetic interference.

STRENGTH. The electromagnetic strength of an electronic subsystem or equipment is the peak value of an electromagnetic stress, such that the subsystem/equipment will continue to operate without damage or intolerable upset. The difference between equipment strength and electromagnetic stress is known as the strength margin. Tested margins are applicable only in the hardening of MEE outside the low-risk barrier.

SYSTEM STATE. A particular configuration of a system by virtue of the position or state of each switch, circuit breaker, solid-state digital device, or other multistate circuit device; or by virtue of the mechanical configuration of doors, equipment, or machines that make up the system. External states include solid-state logic outside the facility barrier, the position of external switches, and configurations of mechanical devices outside the HEMP barrier. Internal states are determined by the configurations of mechanical devices inside the HEMP barrier or by particular circuit connections realized when such things as switches, circuit breakers, thermostatic control, pressure controls, and door interlocks are in a particular on/off arrangement, and electronic states occurring within the system.

TRANSIENT. Short-time variation outside of steady state conditions in the characteristics of power delivered.

TRANSIENT UPSET. A term used to describe an undesired system effect or degradation induced by a short-duration or transient excitation. The term frequently is used to cover all types of such undesired HEMP effects that are not considered to be permanent damage.

UPSET. The impairment of proper system operation that is not due to burnout or other permanent damage to one or more components. Systems that have been upset may return spontaneously to proper operation or may require some operator action, such as resetting a circuit breaker or reloading information into memory.

VARISTOR. A nonlinear resistance device (e.g. ESA) in which current varies as a function of the applied voltage, thereby acting as a voltage limiter. Examples are the silicon carbide resistor and the metal oxide varistor.

VERIFICATION TESTING. Tests conducted for demonstrating that the installed HEMP protection subsystem provides the required HEMP hardness. They are performed after the construction and acceptance testing are complete and after the equipment is installed and functioning, to determine if the operational system suffers missionaborting damage or upset due to simulated HEMP excitations. Verification is normally a Government-conducted test, and is not part of a facility construction’ contract.

WAVEGUIDE-BELOW-CUTOFF (WBC). A metallic waveguide whose primary purpose is to attenuate electromagnetic waves at frequencies below the cutoff frequency (rather than propagating waves at frequencies above cutoff). The cutoff frequency is determined by the transverse dimensions and geometry of the waveguide and properties of the dielectric material in the waveguide.

WAVEGUIDE CUTOFF FREQUENCY. Waveguide cutoff frequency. The frequency below which electromagnetic energy will not efficiently propagate in a waveguide.

WAVE IMPEDANCE. The ratio of the electric field strength to the magnetic field strength at the point of observation.

ZENER DIODE. A reverse-breakdown diode whose breakdown voltage is caused by tunneling or field emission of charge carriers in the depletion layer. These diodes are sometimes used as ESAs.