NLSI - Providing expert training and consulting for lightning problems

Contact Us »
Site Map »
Home »
Back to Section 6 Contents »

 ~ National Lightning Safety Institute ~

Section 6.4.1

Lightning Terms Glossary

Copyright © 2000, National Lightning Safety Institute

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.

ACCESS WELL: A small covered opening in the earth using concrete, clay pipe or other wall material to provide access to an earth electrode system connection.

ACTION INTEGRAL: Defines the energy in any portion of the current path per ohm resistance. Measured in joules per ohm.

AIR TERMINAL: The lightning rod or intended attachment conductor placed on or above a building, structure, tower, for the purpose of intercepting lightning.

AIR-TERMINATION SYSTEM: Part of the external LPs which is intended to intercept lightning flashes.

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.

ARC: A low-voltage, high-current electrical discharge that occurs at the instant two points, through which a large current is flowing, are separated.

ARRESTER: Components, devices or circuits used to attenuate, suppress or divert excess electrical (surge and transient) energy to ground. The terms arrester, suppressor and protector are used interchangeably except that the term arrester is used herein for components, devices and circuits at the service disconnecting means.

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.

CADWELD: Process of molecular bonding patented by ERICO. Also welded connection.

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.

CIRCUIT: An electronic closed-loop path between two or more points used for signal transfer.

CIRCULAR MIL: A unit of area equal to the area of a circle whose diameter is one mil (1 mil = 0.001 inch).

CLAMPING VOLTAGE: The voltage that appears across surge suppressor terminals when the suppressor is conducting transient current.

CLOUD-TO-CLOUD (CC) LIGHTNING: A lightning stroke between thunderclouds. Typically, CC lightning precedes CG lightning.

CONDUCTOR SHIELDING: An envelope that encloses the conductor of a cable and provides an equipotential surface in contact with the cable insulation.

CONE OF PROTECTION: A conic space around a vertical lightning rod used to define a region of protection. The cone whose height equals the height of the rod and whose base radius is equal to the rod height. Regarded as an obsolete term.

COPPER CLAD STEEL: Steel with a coating of copper bonded on it.

CORONA DISCHARGE: A localized cold discharge in air which forms around grounded objects producing an enhancement in electric field strength to allow ionization growth. Called St. Elmos's Fire by ancient mariners. Precedes an arc or spark.

COULOMB: Current times Time. A measurement of charge in amp-seconds.

COUPLING: Energy transfer between circuits, equipments, or systems.

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.

DATA LINE: A cable carrying information as distinct from power. Examples of data lines are telephone lines, telemetry control and signal lines.

DOWN-CONDUCTOR SYSTEM: Part of the external LPS which is intended to conduct the lightning current from the air-termination system to the earth-termination system.

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.

EARTH: That portion of the earth's crust sufficiently below the surface to act as an infinite sink or source for electric charge. Earth is considered the universal ground or reference zero potential level.

EARTH ELECTRODE SYSTEM (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.

EARTH-TERMINATION SYSTEM: Part of an external LPS which is intended to conduct and disperse the lightning current to the earth.

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 (EMI): 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 MULTIPOINT 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. See IEEE 1100 and FAA 019d.

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.

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: It consists of an air-termination system, a down conduction system and an earth termination system.

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 criteria included in this standard, 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.

FARADAY SHIELD: An electrostatic (E field) shield made up of a conductive or partially conductive material or grid. 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.

FLASHOVER: Arcing or sparking between two or more (isolated) conductors. See thermal sparking.

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.

GROUND: If not otherwise qualified, ground means any electrical connection to earth, either directly through a facility ground network or through some intermediary grounding system such as an instrumentation ground network.

GROUND FLASH DENSITY (Ng): The average annual ground flash density is the number of lightning flashes per square kilometer per year.

GROUND IMPEDANCE: The ground resistance and the inductance/capacitance value of the grounding system. Also called dynamic surge ground impedance.

GROUND LOOP: An undesired potential EMI condition formed when two or more pieces of equipment are interconnected and earthed for shock safety hazard prevention purposes.

GROUND RESISTANCE: The resistance value of a given ground rod or grounding system as measured, usually by a fall of potential (3 stake) method, using a 100-Hz signal source.

GROUNDED, EFFECTIVELY: 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.

GROUNDING: Grounding is the act of effecting optimum electrical continuity between conducting objects and earth.

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.

IMPEDANCE: Nominal impedance of the device. The variation of this impedance with frequency is measured as VSWR.

IMPROVED GROUNDING: Inadequate, not-connected, or loose grounding is a major cause of power quality problems as well as personnel safety issues. Well-known techniques, verified by many Codes and Standards, offer remediation and upgrade approaches. Minimum standards are in NEC (NFPA-70) section 250.

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.

ISOKERAUNIC (OR ISOCERAUNIC): Value (number) of thunderstorms measured daily (Td/yr.). I.e. Kampala, Uganda 185 Td/yr.; Florida 110 Td/yr.

JOULES: A unit of energy. One joule for one second is equal to one watt of power. Joules is (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.

LEADER: A preliminary breakdown that forms an ionized path.

LIGHTNING DETECTOR: Device which provide indication of lightning.

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 TO EARTH: Electric discharge of atmospheric origin between cloud and earth consisting of one or more strokes.

LIGHTNING GROUND: A connection between a lightning protection system and a facility ground network or counterpoise.


LIGHTNING PROTECTION SYSTEM (LPS): 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. Includes air terminals, interconnecting conductors, ground terminals, surge protection for data and power lines, shielding and bonding, lightning detectors, and other equipment and techniques to assure that the lightning discharge will be directed safely to earth.

LIGHTNING STROKE: Single discharge in a lightning flash to earth.

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.

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.

NATIONAL ELECTRICAL CODE (NEC): A standard governing the use of electrical wire, cable, and fixtures installed in buildings. The National Fire Protection Association (NFPA-70) sponsors it under the auspices of the American National Standards Institute (ANSI-CI).

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.

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.

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.

PENETRATION: The passage through a structure by a cable, wire, or other conductive object.

POWER: Power is (voltage x current) or a (coulomb/second).

POWER GROUND: A designed connection between a power circuit conductor and a grounding counterpoise.

POWER LINE: A cable carrying AC or DC power.

PRESSURE CONNECTOR: A high-pressure method which uses hydraulic crimpers to create connectivity.

PRIMARY CLOUD-TO-GROUND (CG) 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.

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.

REVERSE STANDOFF VOLTAGE: The maximum voltage that can be applied across surge suppressor terminals with the surge suppressor remaining in a non-conducting state.

RF: Radio frequencies - any and all frequencies that can be radiated as an electromagnetic wave (plane wave).

RING EARTH ELECTRODE: An earth electrode forming a closed loop around the structure below or on the surface of the earth.

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).

SELF-INDUCTANCE: The property of a wire or circuit which causes a back e.m.f. 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.

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

SHIELDING: The process of applying a conducting barrier between a potentially disturbing noise source and electronic circuitry. Shielding may be accomplished by the use of metal barriers, enclosures, or wrappings around source circuits and receiving circuits.

SIGNAL GROUND: A connection between a signal circuit and its zero signal reference plane.

SKIN EFFECT: The gradient conduction and propagation of RF or RF components of a surge on the outer surfaces of conductors.

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.
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.

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.

SUBSEQUENT RETURN STROKES-RESTRIKES: Those strokes occurring after the first return stroke in a multi-stroke flash.

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.)

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 REFERENCE EQUALIZER: A surge protective device used for connecting equipment to external systems whereby all conductors connected to the protected load are routed, physically and electrically, through a single enclosure with a shared reference point between the input and output ports of each system.

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.

SURGES AND SURGE SUPPRESSION: Surges are direct and induced excess energies in a surging waveform. The load is subject to damage by voltages which exceed specifications. Surge protectors can clip off or disperse excess energy using a variety of techniques.

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.

THUNDERCLOUD: A cloud containing a charge density sufficient to allow formation of a lightning stroke.

THUNDERSTORM DAY: A local calendar day on which thunder is heard.

THUNDERSTORM DAYS (Td): The number of thunderstorm days per year obtained from isoceraunic maps.

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.

1. A brief event, usually lasting less than a few milliseconds. In many situations transmission line theory, rather than circuit analysis, must be used to describe the propagation of a transient voltage or current.
2. In mathematical analysis the transient is the part of the system's behavior before the steady state is reached.
3. The work "transient" is often used to indicate a "transient overvoltage".

TURNON VOLTAGE: The voltage required across a transient suppressor terminal to cause the suppressor to conduct current.

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.

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.

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

ZONE OF PROTECTION: The presumed volume of space adjacent to a lightning protection system that is substantially immune to lightning strikes. (This is still subject to debate. The random nature of lightning and its behavior is such that Zone of Protection remains a general and theoretical model. Protection from its effects, therefore, in an Absolute sense, is impossible.)

About NLSI | NLSI Business Services | Lightning Incidents
Personal Lightning Safety | Structural Lightning Safety | Reference Information

National Lightning Safety Institute
Providing expert training and consulting for lightning problems