Lightning Protection, Grounding,
& Surge Suppression Glossary of Terms

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Absorption Loss
The attenuation of an electromagnetic wave as it passes through a shield. This loss is due primarily to induced currents and the associated resistance loss.
Air Discharge
A form of lightning discharge similar to a cloud discharge, in which the lightning channel propagates away from a cloud charge center into apparently clear air where it terminates.
Air Terminal
A type of strike termination device intentionally installed for the purpose of intercepting lightning flashes. These items are sometimes referred to as lightning rods.
Ambient Field
The electric field strength of the atmosphere at rest, in clear air and under static-free conditions. Generally thought to be some 150-300v/m at standard temperature and pressure.
Amperage
The unit measure for current flow. One (1) ampere equals one (1) coulomb of electrons passing a point in a circuit in one (1) second.
ANSI
American National Standards Inst
Anvil-to-Ground Lightning
A form of positive lightning, since it emanates from the anvil top of a cumulonimbus cloud where the ice crystals are positively charged. The leader stroke issues forth in a nearly horizontal direction until it veers toward the ground. These usually occur kilometers/miles from (often ahead) of the main storm and will sometimes strike without warning on a sunny day. An anvil-to-ground lightning bolt is a sign of an approaching storm, and may occur in a largely clear sky.
Arc
A low-voltage, high-current electrical discharge that occurs at the instant two points, through which a large current is flowing, are separated.
Atmospheric Electric Field
A term denoting the electric field strength of the atmosphere at any specified point in space and time. In areas of fair weather, the atmospheric electric field near the earth’s surface typically is about 100 volts per meter and is directed vertically in such a sense as to drive positive charges downward to the earth. In areas of fair weather this field decreases in magnitude with increasing altitude, falling for example, to only about 5 volts per meter at an altitude of about 10 km. Near thunderstorms, and under clouds of vertical development, the surface electric field varies widely in magnitude and direction, usually reversing its direction immediately beneath active thunderstorms. It is now believed that thunderstorms, by replenishing the negative charge to the earth’s surface, provide the supply current to maintain the fair-weather electric field in spite of the continued flow of the air-earth current that tends to neutralize that field. During fair weather, a potential difference of 200,000 to 500,000 volts exists between the earth’s surface and the ionosphere, with a fair weather current of about 2×10-12 Amperes/meter2. It is widely believed that this potential difference is due to the worldwide distribution of thunderstorms. Present measurements indicate that an average of almost 1 Ampere of current flows into the stratosphere during the active phase of a typical thunderstorm. Therefore, to maintain the fair weather global electric current flowing to the surface, one to two thousand thunderstorms must be active at any given time, producing on the order of 100 flashes per second.
Ball Lightning
Ball lightning is described as a floating, illuminated ball that occurs during thunderstorms. They can be fast moving, slow moving or nearly stationary. Some make hissing or crackling noises or no noise at all. Some have been known to pass through windows and even dissipate with a bang. Ball lightning has been described by eyewitnesses but rarely recorded by meteorologists.
Battery Return Conductor
A conductor that carries dc return current between a dc power source and the loads it serves. Battery return conductors are grounded conductors, usually bonded by one (or more) dc system grounding conductor(s) to proper point(s) of the grounding system. They are usually designated BR or an equivalent acronym, but are sometimes designated by using a "+" or "-" followed by the nominal system voltage (e.g. +48, -130, etc.).
Bead Lightning
A type of cloud-to-ground lightning which appears to break up into a string of short, bright sections, which last longer than the usual discharge channel. It is fairly rare.
Blue Jets
First recorded by the space shuttle, blue jets differ from sprites in that they project from the top of the cumulonimbus above a thunderstorm, typically in a narrow cone, to the lowest levels of the ionosphere. They are also brighter than sprites and, as implied by their name, are blue in color.
Bond
The electrical connection between two metallic surfaces established to provide a low resistance path between them.
Bonding
The joining of metallic parts to form an electrically conductive path to assure electrical continuity and the capacity to conduct current imposed between the metallic parts. A bond resistance as high as 1 megohm is adequate for static dissipation. For stray current protection, lightning protection, and other electrical systems, the bonding resistance needs to be significantly lower, no more than a few ohms.
Bonding Jumper
A conductor to assure electrical conductivity between metal parts required to be electrically connected.
Breakdown
The process by which electrically-stressed air is transformed from an insulator to a conductor. Breakdown involves the acceleration of electrons to ionization potential in the electric field imposed by the thundercloud, and the subsequent creation of new electrons which avalanche and expand the scale of enhanced conductivity. Breakdown precedes the development of lightning or higher current-carrying processes during lightning flashes.
Capacitance
The capacity of an electric nonconductor that permits the storage of energy when opposite surfaces are maintained at a difference of potential. Measured at 1.0 Hz unless other wise stated.
Capacitor
A device that can store an electrical charge.
Charge Separation
Generally, positive ions (water/ice molecules that have lost their electrons) are moved to the tops of clouds by the strong wind currents in the storm. Meanwhile, electrons gather on heavier ice particles that fall to the bottom of the clouds. This causes the charge separation: the tops of the clouds are positive, while the bottom is negative.
Chimney
A smoke or vent stack not meeting the requirements of a heavy-duty stack.
Circuit
An electronic closed-loop path between two or more points used for signal transfer.
Circuit Breaker
An automatic protective device that will allow current to flow under normal conditions, but will open the circuit under abnormal conditions to prevent damage from excessive current.
Circular Mil
A unit of area equal to the area of a circle whose diameter is one mil (1 mil = 0.001 inch).
Clamping Device
A component whose action is triggered by a pre-determined voltage. A clamping device will activate (turn on) and deactivate (turn off) at specific predetermined voltages.
Clamping Voltage
The voltage that appears across surge suppressor terminals when the suppressor is conducting transient current.
Classification Of Buildings
a) Ordinary Buildings - A building of common or conventional construction used for ordinary purposes, whether commercial, farm, industrial, institutional, or residential.

b) Class I Ordinary Building - A building that is not more than 75 feet (22.9 m)

c) Class II Ordinary Building - A building that is more than 75 feet (22.9 m) high or greater.

d) Metal-Clad Building - A building with either sides or roof made of or covered with sheet metal.

e) Metal-Framed Building - A building with electrically continuous framing of sufficient size and conductivity to be used as part of the lightning protection system.
Cloud-to-Cloud Lightning
Lightning discharges may occur between areas of cloud having different potentials without contacting the ground. These are most common between the anvil and lower reaches of a given thunderstorm. This lightning can sometimes be observed at great distances at night.
Cloud-to-Ground Lightning
A lightning discharge between a cumulonimbus cloud and the earth initiated by the downward-moving leader stroke. This is the second most common type of lightning, and poses the greatest threat to life and property of all known types.
Commercial Power
Power furnished by an electric power utility company.
Common-Mode Noise (longitudinal)
The noise voltage that appears equally, and in phase, from each current-carrying conductor to ground.
Concrete-Encased Ground Electrode
Also know as a Ufer ground. A grounding electrode completely encased within concrete, located within, and near the bottom of, a concrete foundation or footing or pad, that is in direct contact with the earth.
Conductor
The portion of a lightning protection system intended to transfer lightning discharge currents between strike termination devices and ground or to provide potential equalization between conductive bodies in/on the structure.

a) Main Conductor - A conductor intended to conduct primary lightning currents that interconnects strike termination devices with grounding electrodes.

b) Secondary Conductor - A conductor that connects metal bodies within the zone of protection to the lightning protection system to eliminate electrical potential that may create arcing.
Conductor Combination
The various conductors that are joined by a connection.
Conductor Shielding
An envelope that encloses the conductor of a cable and provides an equipotential surface in contact with the cable insulation.
Connection
A metallic device of suitable electric conductance and mechanical strength used to join conductors.
Connection Thermal Capacity
The ability of a connection to withstand the amount of current required to produce a specified temperature on the control conductor without increasing the resistance of the connection beyond that specified in this standard.
Control Conductor
The conductor that is utilized to measure equivalent changes in temperature, size, etc., that are occurring in at least one of the conductors joined by the connection under test.
Copper Clad Steel
Steel with a coating of copper bonded on it.
Corona
A faint glow enveloping the high-field electrode in a corona discharge, often accompanied by streamers directed toward the low-field electrode.
Coulomb
Named after Charles-Augustin de Coulomb, Coulomb is Current times Time. A measurement of charge in amp-seconds.
Counterpoise
A conductor or system of conductors located on, above or most frequently below the surface of the earth.
Coupling
The association of two or more circuits or systems in such a way that power or signal information may be transferred from one to another.
Coupling Capacitance
The association of two or more circuits with one another by means of capacitance mutual to the circuits.
Crest Factor (of A Periodic Function)
The ration of the peak value of a periodic function (ypeak) to the root-mean-square (rms) value (yrms); cf=ypeak/yrms.
Critical Load
Devices and equipment whose failure to operate satisfactorily jeopardizes the health or safety of personnel, and/or results in loss of function, financial loss, or damage to property deemed critical by the user.
Crowbar
Crowbar is a method of shorting a surge current to ground in surge protection devices. This method provides protection against more massive surges than other types, but lowers the clamping voltage below the operational voltage of the electronic equipment causing noise and operational problems. It also permits a follow current which can cause damage.
Cumulonimbus
Cumulonimbus clouds are also called thunderstorm clouds. They are frequently accompanied by heavy showers, lightning, thunder, and sometimes hail, tornadoes or strong, gusty winds. This cumulus cloud is vertical and often appears with an anvil-shaped cloud on top. A thunderstorm occurs from a cumulonimbus cloud.
Current
The flow of electrons through a conductor. Current is measured in amperes.
Current Loop Cycle
The combination of conductors and connections that carries the current of the circuit under test.
Customer Premises Equipment (CPE)
Any equipment connected by customer premises wiring to the customer side of the demarcation point (network interface). (ANSI T1.318)
Dart Leader
The leader which typically initiates each succeeding stroke of a multiple-stroke flash lightning after the first stroke. The first stroke is initiated by a stepped leader. The dart leader derives its name from its appearance on photographs taken with streak cameras. The dart leader’s brightest luminosity is at its tip which is tens of meters in length, propagating downward at about 10^7 m/s. In contrast to stepped leaders, dart leaders do not typically exhibit branching because the pre-established channel’s low gas density and residual ionization provide a more favorable path for this leader than do any alternative ones.
Data Line
A cable carrying information as distinct from power. Examples of data lines are telephone lines, telemetry control and signal lines.
DC Equipment Grounding Conductor (DCEG)
The conductor that bonds an equipment frame, cabinet, or other enclosure to the site grounding system. The DCEG may also bond an equipment unit within a frame, cabinet, or enclosure to the site grounding system.
DC System Grounding Conductor (DCG)
A conductor or conductive path used to provide a connection between one side of a dc source and a point on the site grounding system. The dc source can be a battery plant, a converter plant, or one or more individual dc/dc converters.
Differential-Mode Noise
See: Transverse-Mode Noise
Direct Coupling
The association of two or more circuits by means of self-inductance, capacitance, resistance, or a combination of these that is common to the circuits.
Direct-Reading Ammeters
Ammeters that employ a shunt and are connected in series and carry some of the line current through them for measurement purposes. They are part of the circuit being measured.
Displacement Power Factor
(a) The displacement component of power factor.

(b) The ratio of the active power of the fundamental wave, in watts, to the apparent power of the fundamental wave, in volt-amperes.
Distortion Factor
The ratio of the root square value of the harmonic content to the root square value of the fundamental quantity, expressed as a percent of the fundamental. See also: Total Harmonic Distortion. (IEEE Std 519™)
Doppler Radar
Weather radar that measures direction and speed of a moving object, such as drops of precipitation moving toward or away from the radar. Using the Doppler effect, it measures the speed of particles. It was named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away.
Down Conductor
That part of the external LPS which is intended to conduct the lightning current from the air-termination system to the earth-termination system.
Downdraft
A sudden descent of cool or cold air to the ground, usually with precipitation, and associated with a thunderstorm or shower.
Downward Flash
Lightning flash initiated by a downward leader from cloud to earth. A downward flash consists of a first short stroke, which can be followed by subsequent short strokes and may include a long stroke.
Dropout
A loss of equipment operation (discrete data signals) due to noise, voltage sags, or interruption. (IEEE std 1159™)
Dry Lightning
A term in the United States for thunderstorms which produce no precipitation at the surface. This type of lightning is the most common natural cause of wildland fires.
Earth Electrode System
See: Ground electrode system
Earth Termination System
See: Ground termination system
Earthing
See: Grounding
Effective Resistivity
A factor such that the conduction-current density is equal to the electric field in the material divided by the resistivity.
Effectively Grounded
Permanently connected to earth through a ground connection of sufficiently low impedance and having sufficient current carrying capacity that ground fault current which may occur cannot cause a voltage build up dangerous to personnel.
Efficiency
The output real power divided by the input real power.
Electric Potential
The potential difference between the point and some equipotential surface, usually the surface of the earth, which is arbitrarily chosen as having zero potential (remote earth).
Electrical Noise
Unwanted electrical signals that produce undesirable effects in the circuits of the control systems in which they occur.
Electromagnetic Compatibility (EMC)
The capability of equipments or systems to be operated in their intended environment, within designated levels of efficiency, without causing or receiving degradation due to unintentional electromagnetic interference. EMC is the result of an engineering planning process applied during the life cycle of the equipment. The process involves careful considerations of frequency allocation, design, procurement, production, site selection, installation, operation, and maintenance
Electromagnetic Interference
Any electrical or electromagnetic phenomenon, manmade or natural, either radiated or conducted, that results in unintentional and undesirable responses from, or performance degradation or malfunction of electronic equipment.
Electron Avalanche
An electron multiplication process due to electron-impact ionization of gas molecules. This is the initial stage in the development of an electrical discharge in air, e.g. a corona or streamer.
Electronic Multi-point Ground System
An electrically continuous network consisting of interconnected ground plates, equipment racks, cabinets, conduit, junction boxes, raceways, duct work, pipes and other normally non-current-carrying metal elements for electronic signals. It includes conductors, jumpers and straps that connect individual electronic equipment components to the electronic multipoint ground system.
Electronic Single-point Ground System
A single point ground system provides a single point reference in the facility for electronic signals. The single point ground system shall be installed in a trunk and branch arrangement to prevent conductive loops in the system. It shall be isolated from all other ground systems except for an interconnection, where applicable, to the multipoint ground system at the main ground plate. The single point ground system consists of insulated conductors, copper ground plates mounted on insulated stands, and insulated ground plates, buses, and/or signal ground terminals in the electronic equipment which are isolated from the frame of the equipment.
Elves
An acronym for Emissions of Light and Very Low Frequency Perturbations From Electromagnetic Pulse Sources, Elves often appear as a dim, flattened, expanding glow around 250 miles in diameter that lasts for, typically, just one millisecond. They occur in the ionosphere over thunderstorms.
EMI/RFI
The interference in a signal transmission or reception resulting from the radiation of undesirable electrical or magnetic and electrical fields.
Equalizer
A device to provide equipotential planes for resistance measurements.
Equipment Ground
A connection between a unit of electrical equipment and the facility ground network.
Equipment Grounding Conductor
The conductor used to connect non-current-carrying metal parts of equipment, raceways, or other enclosures to the system grounded conductor and/or grounding electrode conductor at the service entrance or at the source of a separately derived system.
Equipment Grounding Conductor (EGC)
The conductor used to connect the non-current-carrying parts of conduits, raceways, and equipment enclosures to the grounding electrode at the service equipment (main panel) or secondary of a separately derived system (e.g. isolation transformer).
Equipotential Line Or Contour
The locus of points having the same potential at a given time.
Equipotential Signal Reference Plane
An equipotential conducting plane designed to maintain a number of electrical/electronic units having a common signal reference at the same potential.
External Lightning Protection System
A system consisting of an air-termination system, a down conduction system and an earth termination system.
Extraterrestrial Lightning
Lightning requires the electrical breakdown of a gas, so it cannot exist in a visual form in the vacuum of space. However, lightning has been observed within the atmospheres of Venus, Jupiter and Saturn.
Facility
A building or other structure, either fixed or transportable in nature, with its utilities, ground networks, and electrical supporting structures. All wiring, cabling as well as electrical and electronic equipment are also part of the facility.
Facility Ground Network
The electrically conductive network, including all structures and grounding cables bonded to the earth grounding counterpoise but excluding the instrumentation ground network and electrical enclosures, conduit, and raceway systems. In steel frame structures, the structural members may be bonded together and connected to the earth grounding counterpoise to form the basic network. In buildings using nonconductive structural methods and materials such as masonry and in outside facility areas such as gas, propellant, or oxidizer service facilities, the facility ground network consists of conductors, sized according to established criteria, bonded to an earth grounding counterpoise and extending to all areas containing equipment to be grounded.
Facility Ground System
The electrically interconnected system of conductors and conductive elements that provides multiple current paths to earth. The facility ground system includes the earth electrode subsystem, lightning protection subsystem, signal reference subsystem, fault protection subsystem, electronic multipoint ground system, electronic single point ground system, as well as the building structure, equipment racks, cabinets, conduit, junction boxes, raceways, duct work, pipes, and other normally noncurrent-carrying metal elements.
Failure Mode
The effect by which a failure is observed to occur.
Faraday Shield
An electrostatic (E field) shield made up of a conductive or partially conductive material or grid. A Faraday cage or screen room is effective for protecting inside equipment from outside radiated RF energies.
First Return Stroke
That current flow along the previously ionized path occurring when that path is complete from cloud to ground
Flash
The total lightning discharge.
Flash-bang Rule
A rough calculation to determine how close lightning is: When you see the flash, begin to count the seconds until you hear the thunder. Divide this number by 5. The number you get is the approximate distance of the lightning in miles.
Flashover
Arcing or sparking between two or more (isolated) conductors. See "thermal sparking".
Flicker
A variation of input voltage, either magnitude or frequency, sufficient in duration to allow visual observation of a change in electric light source intensity.
Follow Current (Power)
The current from the connected power source that flows through an arrester during and following the passage of discharge current.
Forked Lightning
The common form of cloud-to-ground (CG) discharge always visually present to a greater or lesser degree which exhibits downward-directed branches from the main lightning channel. In general, of the many branches of the stepped leader, only one is connected to ground defining the primary, bright return stroke path, and the other incomplete channels decay after the ascent of the first return stroke.
Form Factor (Periodic Function)
The ratio of the root square value to the average absolute value, averaged over a full period of the function.
Forward Transfer Impedance
An attribute similar to internal impedance of a power source, but at frequencies other than the nominal (e.g. 60 Hz power frequency). Knowledge of the forward transfer impedance allows the designer to assess the capability of the power source to provide load current (at the harmonic frequencies) needed to preserve a good output voltage waveform. Generally, the frequency range of interest is 60 Hz to 3 kHz for 50 Hz to 60 Hz power systems, and 20 KHz to 25 kHz for 380 Hz to 480 Hz power systems.
Foundation Earth Electrode
Reinforcement steel of foundation or additional conductor embedded in the concrete foundation of a structure and used as an earth electrode. Also called "UFER" ground.
Gas Tube
A surge suppression component that is made up of two or three electrodes in a sealed envelope that contains a rarified gas, or a gas tube is a crowbar device.
Grid System
A system consisting of interconnected bare conductors buried in the earth or in concrete to provide a common ground for electrical devices and metallic structures.
Ground
A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth.
Ground Current
Current flowing in the earth or in a grounding connection.
Ground Electrode
A conductor or group of conductors in intimate contact with the earth for the purpose of providing a connection with the ground.
Ground Electrode Resistance
The ohmic resistance between the grounding electrode and a remote grounding electrode of zero resistance.
Ground Flash Density
The average annual ground flash density is the number of lightning flashes per square kilometer per year.
Ground Grid
A system of grounding electrodes consisting of interconnected bare cables buried in the earth to provide a common ground for electrical devices and metallic structures.
Ground Impedance
The ground resistance and the inductance/capacitance value of the grounding system. Also called dynamic surge ground impedance.
Ground Impedance Tester
A multifunctional instrument designed to detect certain types of wiring and grounding problems in low-voltage power distribution systems.
Ground Loop
A potentially detrimental loop formed when two or more points in an electrical system that are nominally at ground potential are connected by a conducting path such that wither or both points are at the same ground potential.
Ground Mat
A system of bare conductors, on or below the surface of the earth, connected to a ground or a ground grid to provide protection from dangerous though voltages.
Ground Resistance
The resistance value of a given ground rod or grounding system as measured, usually by a fall of potential (3 point) method.
Ground Resistivity
See: Ground resistance.
Ground Termination System
The portion of an external lightning protection system which is intended to conduct and disperse the lightning current to the earth.
Ground Window
The area through which all grounding conductors, including metallic raceways, enter a specific area. It is oftern used in communications systems through which the building grounding system is connected to an area that would otherwise have no grounding connection.
Ground-Return Circuit
A circuit in which the earth is utilized to complete the circuit.
Ground-to-Cloud Lightning
Ground-to-cloud lightning is a lightning discharge between the ground and a cumulonimbus cloud from an upward-moving leader stroke.
Grounded
Connected to earth, or to a conductive material that is connected to earth, so that electric charges are distributed freely to the earth.
Grounding
The act of effecting optimum electrical continuity between conducting objects and earth.
Grounding Conductor
The conductor that is used to establish a ground and that connects an equipment, device, wiring system, or another conductor (usually the neutral conductor) with the grounding electrode or electrodes.
Grounding Connection
A connection used in establishing a ground and consists of a grounding conductor, a grounding electrode and the earth (soil) that surrounds the electrode or some conductive body which serves instead of the earth.
Grounding Electrode
That portion of a lightning protection system extending into the earth, such as a ground rod, ground plate, or conductor, serving to bring the lightning protection system into electrical contact with the earth.
Grounding Electrode System
A network of electrically interconnected rods, plates, mats, piping, incidental electrodes (metallic tanks, etc.) or grids installed below grade to establish a low resistance contact with earth.
Grounding System
Consists of all interconnected grounding connections in a specific area.
Harmonic Distortion
The mathematical representation of the distortion of the pure sine waveform. See also: Distortion Factor
Heat Lightning
Heat lightning appears as a glowing flash on the horizon. It is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard.
High Frequency
All electrical signals at frequencies greater than 100 kilohertz (kHz). Pulse and digital signals with rise and fall times of less than 10 microseconds are classified as high frequency signals.
IEEE
Institute of Electrical and Electronic Engineers.
Impedance
A unit of measure, expressed in Ohms, of the total opposition (resistance, capacitance and inductance) offered to the flow of alternating current.
Impulse
See: Transient
Induced (bound) Charge
Induced charge, also called bound charge, may be caused by nearby storm cloud discharges or the flow of product into open floating roof tanks (commonly used for fuel and chemical storage) and can cause ignition when they discharge suddenly (arc) across air gaps.
Induced Ground Charge
Generally, a positive charge is induced in the ground below the clouds; the repulsive force of the negatively charged cloud pushes the electrons on the ground down and away from the surface, leaving the surface of the ground positively charged. The potential difference between the clouds and the ground can be as high as a billion volts.
Inductance
The ability of a coil to store energy and opposite changes in current flowing through it.
Inductive Coordination Practice
The interrelation of neighboring electric supply and communication circuits by electric or magnetic induction, or both.
Inductive Coupling (Communication Circuits)
The association of two or more circuits with one another by means of inductance mutual to the circuits or the mutual inductance that associates the circuits.
Inductor
A number of turns of wire wrapped around a core used to provide inductance in a circuit. Also called a coil.
Input Power Factor
The ration at the input of active power (measured in watts or kilowatts) to input apparent power (measured in volt-amperes or kilovolt-amperes) at rated or specified voltage and load. See also: power factor, displacement; power factor, total.
Input Voltage Range
The range of input voltage over which the system can operate properly. (ANSI C84.1)
Inrush
The amount of current that a load or device draws when first energized.
Insulated Equipment Ground
An insulated equipment grounding conductor (EGC) runs in the same conduit or raceway as the supply conductors. This conductor is insulated from the metallic raceway and all ground points throughout its length. It originates at an insulated (isolated) grounding receptacle or equipment input terminal block and terminates at the point where neutral and ground are bonded at the power source.
Insulated Grounding Receptacle (IGR)
A receptacle in which the grounding terminal is purposely insulated from the receptacle mounting means, for the reduction of electrical noise (electromagnetic interference) on the grounding circuit. The receptacle grounding terminal shall be grounded by an insulated (isolated) EGC run with the circuit conductors.
Internal Lightning Protection System
All measures additional to those mentioned under external lightning protection system including the equipotential bonding, the compliance of the safety distance and the reduction of the electromagnetic effects of lightning current within the structure to be protected, including shielding and surge protection devices.
Interruption
The complete loss of voltage for a time period.
Isokeraunic Chart
A graphical representation (map) indicating the value (number) of thunderstorms measured daily (Td/yr.).
Isolated Bonding Network
(A) A bonding network that has a single-point connection (single-point ground) to either the CBN or another isolated bonding network.

(B) Typically a system-level grounding topology used by the original equipment to suspected or known site environmental issues such as power fault and surge, lightning, and grounding potential rise. The IBN requires the use of a single-point connection window (SPCW) (also known in the telephone industry as a ground window) to interface the rest of the building metallics (the CBN).
Joule
The unit of measure of energy equal to one watt second. 3,600,000 joules equal one kilowatt hour.
Joules
A unit of energy. One joule for one second is equal to one watt of power. Measured as current x time x voltage.
Landline
Any conductor, line or cable installed externally above or below grade to interconnect electronic equipment in different facility structures or to connect externally mounted electronic equipment.
Laser-triggered Lightning
Lightning strikes triggered by the use of ultra-violet lasers, which create a channel of ionized gas through which the lightning may be conducted to ground.
Leader
A preliminary breakdown that forms an ionized path.
Lightning
Lightning is a sudden and visible discharge of electricity produced in response to the build up of electrical potential between cloud and ground, between clouds, within a single cloud, or between a cloud and surrounding air. The lightning present in all thunderstorms is an electrical discharge that balances the difference between positive and negative charges within a cloud, between two clouds, or between a cloud and the ground.
Lightning Channel
The irregular path through the air along which a lightning discharge occurs. The lightning channel is established at the start of a discharge by the growth of a leader, which seeks out a path of least resistance between a charge source and the ground or between two charge centers of opposite sign in the thundercloud or between a cloud charge center and the surrounding air or between charge centers in adjacent clouds.
Lightning Electromagnetic Pulse (LEMP)
Voltages or currents induced into cables and other conductors by the radiated field from a lightning flash some distance away.
Lightning Flash
The total observed lightning discharge generally has a duration of less than one second. A single flash is usually composed of many distinct luminous events (strokes) that often occur in such rapid succession that the human eye cannot resolve them.
Lightning Ground
A connection between a lightning protection system and a facility ground network or counterpoise.
Lightning Polarity
Lightning may be of a negative or positive polarity depending on the direction of the current flow between a cloud and the ground. Most lightning flashes are negative in polarity (negative cloud to positive ground).
Lightning Protection (LP) System
The complete system used to protect a structure and its contents against the effects of lightning. Commonly it consists of both external and internal lightning protection systems, Including air terminals, interconnecting conductors, ground terminals, surge protection for data and power lines, shielding and bonding, and other equipment and techniques to assure that the lightning discharge will be directed safely to earth.
Lightning Stroke
In a cloud-to-ground discharge, a leader plus its subsequent return stroke. In a typical case, a cloud-to-ground discharge is made up of three or four successive lightning strokes, most following the same lightning channel.
Lightning-induced Magnetism
The movement of electrical charges produces a magnetic field. The intense currents of a lightning discharge create a fleeting but very strong magnetic field. Where the lightning current path passes through rock, soil, or metal these materials can become permanently magnetized. This effect is known as lightning-induced remnant magnetism, or LIRM.
Linear Load
A load that draws a sinusoidal current wave when supplied by a sinusoidal voltage source.
Load
A device that receives electrical energy from a source, draws current and/or provides opposition to current, requires voltage, or dissipates power. Resistors, light bulbs and electric motors are examples of loads.
Loop Conductor
A conductor that (a) encircles a structure; and (b) that is used to interconnect grounding electrodes, main conductors, or other grounded bodies.
Low Frequency
Includes all voltages and currents, whether signals, control, or power, from DC through 100 kHz. Pulse and digital signals with rise times of 10 s or greater are considered low frequency signals.
Magnetic Field
A vector field produced by a continuous flow of charge.
Main
The AC power source available at the point of use in a facility. It consists of the set of electrical conductors (referred to by terms including "service entrance," "feeder," or "branch circuit") for delivering power to connected loads at the utilization voltage level.
Main Conductor
A conductor intended to conduct primary lightning currents that interconnects strike termination devices with grounding electrodes.
Material Resistivity
A factor such that the conduction-current density is equal to the electric field in the material divided by the resistivity.
Maximum Continuous Operating Voltage (MCOV)
The maximum designated root-mean-square (rms) value of power-frequency voltage that may be applied continuously between the terminals of the arrester.
Measured Limiting Voltage
The maximum magnitude of voltage that is measured across the terminals of the surge protective device (SPD) during the application of a series of impulses of specified wave shape and amplitude.
Metal Body
a) Grounded (Non-Isolated) Metal Body - A metal body having a ground path independent of the lightning protection system.

b) Isolated (Ungrounded) Metal Body - A metal body having no ground path.
Metal Oxide Varistor (MOV)
A solid state surge suppression component that can handle large amounts of current and reacts in the low nanosecond time range.
Microburst
Microbursts are also called downbursts. Severe thunderstorms can create incredible violence as they pass over an area. Among their most devastating aspects are down bursts. A downburst is a severe localized wind blasting down from a thunderstorm. These strong downward currents are classified by meteorologists as microbursts if the downburst covers an area less than 2.5 miles in diameter and as macrobursts if the down burst covers an area of at least 2.5 miles in diameter.
Momentary Interruption
(A) A type of short duration variation.

(B) The complete loss of voltage (<0.1 pu) on one or more phase conductors for a time period between 0.5 cycles and 3 s. (IEEE Std 1159)
Multiple Strokes
Lightning flash consisting in average of 3-4 strokes, with typical time interval between them of about 50 ms.
Mutual Inductance
The property of a circuit whereby a voltage is induced in a loop by a changing current in a separate conductor.
Mutual Resistance Of Grounding Electrodes
Equal to the voltage change in one of them produced by a change of one ampere of direct current in the other, and is expressed in ohms.
National Electric Code (NEC)
A standard governing the use of electrical wire, cable, and fixtures installed in buildings.
Negative Lightning
A cloud-to-ground discharge which begins in a negative region of the cloud and effectively lowers some tens of coulombs of negative charge to the earth. Over 90% of all cloud-to-ground lightning strikes are of this type.
NEMA
National Electrical Manufacturers Association.
Neutral
The ac power system conductor which is intentionally grounded on the supply side of the first service disconnect (ing) means. It is the low potential (white) side of a single-phase ac circuit or the low potential fourth wire of a three-phase wye distribution system. The neutral (grounded conductor) provides a current return path for ac power currents whereas the grounding (or green) conductor does not, except during fault conditions.
Nominal System Voltage
A nominal value assigned to designate a system of a given voltage class.
Nonlinear Load
A load that draws a nonsinusoidal current wave when supplied by a sinusoidal voltage source. (IEEE Std 519)
Notch
A switching (or other) disturbance of the normal power voltage waveform, lasting less than a half-cycle, which is initially of opposite polarity than the waveform, and is thus subtractive from the normal waveform in terms of the peak value of the disturbance voltage. This includes complete loss of voltage for up to a half-cycle. See also: Transient.
Oceanic Lightning
Lightning rarely strikes the open ocean, although some sea regions are lightning "hot spots." Winter storms passing off the east coast of the United States often erupt with electrical activity when they cross the warm waters of the Gulf Stream which has roughly as many lightning strikes as the southern plains of the USA.
Ohm
The unit of measure of resistance equal to the resistance in a conductor in which one volt of potential produces a current flow of one ampere.
Onset Field
The electric field strength above which ionization occurs, generally thought to be 2.6kV/m in dry air at standard temperature and pressure.
Open-Circuit Voltage (OCV)
The voltage available from the complete test set-up as configured (surge generator, coupling circuit, back filter, connecting leads), prior to connecting the equipment under test (EUT), at the terminals where the surge protective device (SPD) under test will be connected.
Oscillatory Surge
A surge that includes both positive and negative polarity values.
Output Impedance
Similar to forward transfer impedance, but it describes the characteristics impedance of the power source as seen from the load, looking back at the source. See also: Forward Transfer Impedance.
Overshoot Voltage
The fast rising voltage that appears across surge suppressor terminals before the suppressor turns on (conducts current) and clamps the input voltage to a specified level.
Overvoltage
When used to describe a specific type of long duration variation, refers to an rms increase in the ac voltage, at the power frequency, for a period of time greater than 1 min. Typical values are 1.1 pu to 1.2 pu. See also: swell; transient. (IEEE Std 1159)
Pathway
A facility for the placement of telecommunications. (TIA/EIA 607)
Peak Current
The maximum current measured in kilo Amperes (kA) of the lightning discharge is typically in the 20 to 30 kA range with a maximum of 310kA.
Permanent Connection
A grounding connection that will retain its electrical and mechanical integrity for the design life of the conductor within limits established by this standard.
Phase
Term used to describe the hot line or lines in AC Power.
Phase Shift
The displacement between corresponding points on similar wave shapes; it is expressed in degrees leading or lagging.
Pico
Metric prefix meaning one trillionth or 10-12. The abbreviation is p.
Positive Lightning
Positive lightning makes up less than 5% of all lightning. It occurs when the leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is a discharge of positive charges to the ground.
Potential Difference
A measure of force produced between charged object that moves electrons. See Volt.
Potential Profile
A plot of potential as a function of distance along a specified path.
Power Disturbance
Any deviation from the nominal value (or from selected thresholds based on load tolerance) of the input ac power characteristics.
Power Disturbance Monitor
Instrumentation developed specifically to capture power disturbances for the analysis of voltage and current measurements.
Power Ground
A designed connection between a power circuit conductor and a grounding counterpoise.
Power Quality
The concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment.
Power Quality Monitoring
See: Momentary Interruption
Pressure Connector
A high-pressure method which uses hydraulic crimpers to create connectivity.
Primary Cloud-to-ground Lightning Stroke
The initial discharge between the thundercloud and ground which generally is associated with a stepped leader propagation. Sometimes referred to as the initial stroke or simply the lightning flash.
Prospective Overvoltage
The theoretical voltage that a lightning event might couple or induce into a power system if it were not truncated by flashover of insulation or action of a surge protective device (SPD).
Pulse Life
The number of surges of specified voltage, current amplitudes, and wave shapes that may be applied to a device without causing degradation beyond specified limits. The pulse life applies to a device connected to an AC line of specified characteristics and for pulses sufficiently spaced in time to preclude the effects of cumulative heating.
Radial Ground
A conductor connection by which separate electrical circuits or equipment are connected to earth at one point. Sometimes referred to as a star ground.
Radio Frequency (RF)
Radio frequencies - any and all frequencies that can be radiated as an electromagnetic wave (plane wave).
Radio Frequency Interference (RFI)
RFI is manmade or natural, intentional or unintentional electromagnetic propagation which results in unintentional and undesirable responses from or performance degradation or malfunction of, electronic equipment.
Rated Peak Single-Pulse Transient Current
Maximum peak current that may be applied for a single 8/20 µs impulse, with rated line voltage also applied, without causing device failure.
Rated Single-Pulse Transient Energy
Energy that may be dissipated for a single impulse of maximum rated current at a specified wave shape, with rated root-mean-square (rms) voltage or rated DC voltage also applied, without causing device failure.
Rated Standby Power Dissipation
The power dissipated in a protective device while connected to an AC line having a voltage and frequency equal to the rating of the device and with no load current flowing and no surges applied.
Rated Voltage
The designated maximum permissible root-mean-square (rms) value of power-frequency voltage between its line and earth terminals at which it is designated to operate correctly.
Rating
The designation of an operating limit for a device.
Re-strike
High speed videos show that most lightning strikes are made up of multiple individual strokes. A typical strike is made of 3 to 4 strokes although there may be more. Each re-strike is separated by around 40 to 50 milliseconds, causing a noticeable "strobe light" effect.
Recovery Time
Specifies the time needed for the output voltage or current to return to a value within the regulation specification after a step load or line change.
Recovery Voltage
The voltage that occurs across the terminals of a pole of a circuit-interrupting device upon interruption of the current. NOTE: For an arrester, this occurs as a result of interruption of the follow current.
Red Sprites
A relatively new discovery, but now well-documented, these are electrical discharges that occur high above some types of thunderstorms. They appear as luminous reddish-orange or greenish-blue, plasma-like flashes, last longer than normal lower stratospheric discharges, and are triggered by the discharges of positive lightning between the thundercloud and the ground. Sprites often occur in clusters of two or more, and typically span the distance from 50 miles to 90 miles above the earth, with what appear to be tendrils hanging below, and branches reaching above.
Repetitive Surge And Follow-Current Withstand
The number of surges of specified voltage and current amplitudes and wave shapes that may be applied to a device without causing degradation beyond specified limits. The repetitive surge and follow-current withstand applies to a device connected to an AC line of specified characteristics and for pulses applied at specified rates and phase angles. The effects of any cumulative heating that may occur are included.
Resistive Coupling
The association of two or more circuits with one another by means of resistance mutual to the circuits.
Return Strokes
Those strokes occurring after the first return stroke in a multi-stroke flash. Also called re-strikes.
Reverse Standoff Voltage
The maximum voltage that can be applied across surge suppressor terminals with the surge suppressor remaining in a non-conducting state.
Ribbon Lightning
Ribbon lightning occurs in thunderstorms with high cross winds and multiple return strokes. The wind will blow each successive return stroke slightly to one side of the previous return stroke, causing a ribbon effect.
Ring Earth Electrode
An earth electrode forming a closed loop around the structure below or on the surface of the earth.
Ring Wave
An open-circuit voltage wave characterized by a rapid rise to a defined peak value, followed by a damped oscillation in which every successive peak has an amplitude of about 60% of the value of the peak that preceded it. For the 100 kHz Ring Wave, the rise time is nominally 0.5 µs and the ringing frequency is approximately 100kHz. No short-circuit current waveform is defined; the exact wave that is delivered is determined by the instantaneous impedance to which the Ring Wave is applied.
RMS Voltage
Applied to an AC sine wave, the RMS value is also known as the effective voltage and is .707 times the peak voltage.
Rocket Lightning
One of the rarest of cloud discharges, it is a form of cloud discharge, generally horizontal and at cloud base, with a luminous channel appearing to advance through the air with visually resolvable speed, often intermittently, with a movement that has been compared to a skyrocket.
Safety Distance
Minimum distance between two conductive parts within the structure to be protected between which no dangerous sparking can occur. See "flashover".
Safety Ground
The local earth ground. The earth ground which grounds the neutral return. The wire may be green or bare and can be through a metal conduit. It may be earth grounded as many times as needed. (Neutral must only be grounded once at the entry location)., See also: Equipment Ground Conductor
Sag
An rms reduction in the ac voltage, at the power frequency, for durations from a half-cycle to a few seconds. See also: Notch; Undervoltage.
Secondary Conductor
A conductor that connects metal bodies within the zone of protection to the lightning protection system to eliminate electrical potential that may create arcing.
Self-inductance
The property of a wire or circuit which causes an electromagnetic field to be generated when a changing current flows through it.
Separately Derived System
A premises wiring system whose power is derived from a battery, a solar photovoltaic system or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.
Series Gap
An intentional gap between spaced electrodes: It is in series with the valve or expulsion element of the arrester, substantially isolating the element from line or ground, or both, under normal line-voltage conditions.
Service Equipment
The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuses and their accessories, connected to the load end of service conductors to a building or other structure, or an otherwise designated area, and intended to constitute the main control or cutoff of the supply.
Service Voltage
The voltage at the point where the electric system of the supplier and the electric system of the user are connected.
Sheet Lightning
Not really a unique form of lightning, but a diffused, sometimes fairly bright, illumination of those parts of a thundercloud that surround and obscure the path of a normal lightning flash, particularly a cloud discharge or cloud-to-cloud discharge.
Shield
Braid copper, metallic sheath, or metallic-coated polyester tape (usually copper or aluminum) applied over the insulation of a conductor or conductors for the purpose of reducing electrostatic coupling between the shielded conductors and others that may be either susceptible to, or generators of, electrostatic fields (noise). When electromagnetic shielding is intended, the term electromagnetic is usually included to indicate the difference in shielding requirements and material.
Shielding
The process of applying a conducting barrier between a potentially disturbing noise source and electronic circuitry. Shielding is used to protect cables (data and power) and electronic circuits. Shielding may be accomplished by the use of metal barriers, enclosures, or wrappings around source circuits and receiving circuits.
Short-Circuit Current (SCC)
The current that the test set-up (surge generator, coupling circuit, back filter, connecting leads) can deliver at the terminals where the surge protective device (SPD) under test will be connected, with the SPD replaced by bonding the two lead terminals. (Sometimes abbreviated as "SCI.")
Signal Ground
A connection between a signal circuit and its zero signal reference plane.
Signal Reference Structure (SRS)
A system of conductive paths among interconnected equipment that reduces noise-induced voltages to levels that minimize improper operation. Common configurations include grids and planes.
Silicon Avalanche Diode
A solid state surge suppression component that is extremely fast, but lacks the ability to handle heavy current.
Single-Point Connection (SPC)
Unique location where an IBN is connected to the CBN, usually to the grounding electrode system or its extension.
Single-Point Connection Bar
A copper bus bar located entirely within the single-point connection window (SPCW) and serving as a means for bonding the metallic conductors associated with the isolated (insulated) bonding network (IBN) to ground. After passing through the SPCW, these conductors must remain insulated from the common bonding network (CBN); no additional paths to ground, intentional or unintentional, are permitted inside the IBN.
Single-Point Connection Window (SPCW)
The interface or transition region between an IBN and CBN, typically envisaged as a sphere with a diameter of 2 m (6 ft.).
Skin Effect
The gradient conduction and propagation of RF or RF components of a surge on the outer surfaces of conductors.
Slew Rate
Rate of change of (ac voltage) frequency.
Spark
A spark results from the sudden breakdown of the insulating strength of a dielectric (such as air) that separates two electrodes of different potentials.
Staccato Lightning
A leader stroke with only one return stroke.
Static Ground
A functional term describing a connection between conductive objects and a facility ground network or counterpoise for the purpose of dissipating static electricity.
Streak Lightning
An ordinary cloud-to-ground discharge that appears to be entirely concentrated in a single, relatively straight lightning channel.
Streamer
A sinuous channel of very high ion density which propagates itself though the air by continual establishment of an electron avalanche just ahead of its advancing tip.
Striking Distance
The distance covered by the final leader step of a downward propagating primary lightning stroke in making contact with a grounded object. This distance varies with the type and intensity of the lightning stroke.
Stroke
A component discharge of a lightning flash, which follows a leader.
Supercell
A severe thunderstorm characterized by a rotating, long-lived, intense updraft. Although not very common, they produce a relatively large amount of severe weather, in particular, extremely large hail, damaging winds, much lightning, and practically all violent tornadoes. They are the most violent and dangerous thunderstorms.
Surface-Potential Gradient
The slope of a potential profile, the path of which intersects equipotential lines at right angles.
Surge
A type of electrical overstress. In the absence of protective devices, the magnitude of the peak voltage of a surge is usually understood as at least twice the normal system voltage, and the duration of the overvoltage is less than a few milliseconds. (The word "surge" is also used by some engineers and technicians to indicate what should properly be called a swell.) See also: Transient
Surge Let-Through
The part of the surge that passes by a surge protective device (SPD) with little or no alteration. See also: surge remnant.
Surge Protection Device (SPD)
A device designed to protect electrical apparatus from high transient voltage and to limit the duration and the amplitude of follow-current. Device that is intended to limit transient overvoltages and divert or absorb surge currents. Replaces TVSS terminology
Surge Remnant
The part of an applied surge that remains downstream of one or several protective devices. See also: surge let-through.
Surge Response Voltage
The voltage profile appearing at the output terminals of a surge protective device (SPD) and applied to downstream loads, during and after a specified impinging surge, until normal, stable conditions are reached.
Surge Suppression
The process by which transient voltage surges are prevented from reaching electrical or electronic equipment.
Surge Suppressor
Component (s), device (s) or circuit (s) designed to attenuate, suppress or divert conducted transient(s) and surge energy to ground to protect electronic equipment.
Swell
A momentary increase on the power-frequency voltage delivered by the mains, outside of the normal tolerances, with a duration of more than one cycle and less than a few seconds. See also: surge.
System (Circuit) Voltage
The root-mean-square (rms) power-frequency voltage from line to line as distinguished from the voltage from line to neutral.
Telecommunications Equipment Room (TER)
Any transmission, emission, and reception of signs, signals, writings, images, and sounds, i.e. information of any nature by cable, radio, optical, or other electromagnetic systems. (TIA/EIA 607)
Thermal Sparking
Occurs when a very high current is forced to cross a joint between two conducting materials which have an imperfect bonding or mating between their surfaces.
Total Power Factor
The ratio of the total power input, in watts, to the total volt-ampere input.
Total Surge Energy
Total sum of surge energy for all lines of a protector unit. Measured in joules. The minimum total energy which results in the failure of the unit.
Transfer Impedance
Referring to coax, is the impedance to transfer into or outside the coax at various frequencies usually below 1 MHz. Due to loss of skin effect attenuation or shielding at these low frequencies, coax can be susceptible to interference and noise as well as the radiation of such signals.
Transfer Inductance
The property of a circuit whereby a voltage is induced in a loop by a changing current in another circuit, some part of which is included in the loop.
Transfer Time
The time that it takes an uninterruptible power supply (UPS) to transfer the critical load from the output of the inverter to the alternate source, or back again.
Transient
A subcycle disturbance in the ac waveform that is evidenced by a sharp, brief discontinuity of the waveform, which may cause signal impairments, loss of memory or physical damage to electrical and electronic equipment.. May be of either polarity and may be additive to, or subtractive from, the nominal waveform. See also: Notch; Overvoltage; Swell.
Transient Voltage Surge Suppressor (TVSS)
A device that functions as an SPD or surge suppressor.
Transverse-Mode Noise
With reference to load device input ac power: Noise signals measurable between or among active circuit conductors feeding the subject load, but not between the EGC or associated SRS and the active circuit conductors.
Turn-on Voltage
The voltage required across a transient suppressor terminal to cause the suppressor to conduct current.
Undervoltage
When used to describe a specific type of long duration variation, refers to an rms decrease in the ac voltage, at the power frequency, for a period of time greater than 1 min. Typical values are 0.8 pu to 0.9 pu. (IEEE Std 1159)
Uninterruptible Power Supply (UPS)
An apparatus that supplies continuous power to a load, despite disturbances and outages in the mains. A UPS contains a bank of rechargeable batteries that supply power in the absence of acceptable supply voltage.
Updraft
An updraft is a small scale current of air with vertical motion. If there is enough moisture, then it may condense, forming a cumulus cloud which is the first step towards thunderstorm development.
Upward Flash
Lightning flash initiated by an upward leader from an earthed structure to a cloud. An upward flash consists of a first long stroke with or without multiple superimposed short strokes, which can be followed by subsequent short strokes possibly including further long strokes.
Utilization Voltage
The root-mean-square (rms) phase-to-phase or phase-to-neutral voltage at the line terminals of utilization equipment.
Volcanically-triggered Lightning
Extremely large volcanic eruptions, which eject gases and material high into the atmosphere, can trigger lightning.
Volt
The unit of measure of potential or electromotive force. One volt is the force required to cause one amp to flow through a conductor with a resistance of one ohm. The abbreviation is V.
Voltage
Potential energy difference (electrical pressure).
Voltage Distortion
Any deviation from the nominal sine waveform of the ac line voltage
Voltage Rating
The designated maximum permissible operating voltage between its terminals at which an arrester is designed to perform its duty cycle. It is the voltage rating specified on the nameplate.
Voltage Regulation
The degree of control or stability of the rms voltage at the load. Often specified in relation to other parameters, such as input voltage changes, load changes, or temperature changes.
Voltampere (VA)
Base unit of apparent power; 120 electrical degrees.
Watt
The unit of measure of power equivalent of one joule per second. A watt is equal to the power in a circuit in which a current of one amp flows at a potential of one volt. The abbreviation is W.
Wave Impedance
The ratio of the electric field strength to the magnetic field strength at the point of observation.
Waveform
The graphic depiction of a progressive disturbance propagated from point to point.
Zigzag Lightning
A normal cloud-to-ground discharge that appears to have a single, but very irregular, lightning channel.
Zone Of Protection
The presumed volume of space adjacent to a lightning protection system that is substantially immune to lightning strikes.