Introduction to Lightning System Design- Part One ~ Electrical Knowhow

This is the first Article in this course; Lightning Design Calculations, and of course we must begin with knowing all terms, abbreviations and Symbols used in lightning field and which will be used throughout this course.

First: Lightning and Surge protection terminology

A
Air Termination	Shall mean that part of a lightning protection system designed to capture the lightning strike. Normally is mounted on the roof of the structure and is bonded to the down conductor.
Alternating Discharge Current	The alternating discharge current is the r.m.s. value of the almost sinusoidal power-frequency current that flows through the surge voltage limiter.
Amperage	IEEE has developed waveform guidelines for testing. In this guideline, discharge currents (surge currents) are given as 200A, 500A, and 3000A. These values, when combined with the appropriate transient waveshape represent possible transient activity according to ANSI/IEEE C62.41.
Arc Combustion Voltage	The arc combustion voltage (Vbo) is the instantaneous value of the voltage on the discharge path while an arc discharge is being arrested.
Average steepness of the short stroke current	Average rate of change of current within a time interval t2 – t1. It is expressed by the difference i(t2) – i(t1) of the values of the current at the start and at the end of this interval, divided by t2 – t1.
B
Bond (bonding)	Shall mean a conductor intended to provide electrical connection between a lightning protection system and other metalwork and between various metal parts of a structure or between earthling systems.
Bonding bar	Metal bar on which metal installations, external conductive parts, electric power and telecommunication lines and other cables can be bonded to a Lightning Protection System.
Bonding conductor	Conductor connecting separated conducting parts to a Lightning Protection System.
Bonding network	Interconnecting network of all conductive parts of the structure and of internal systems (live conductors excluded) to the earth termination system.
Burst	This pulse is repeated at certain time intervals.
C
Captive Coupling	Formed by the strength of the voltage field surrounding a conductor. The voltage field causes a current to flow in an adjoining conductor to an area of lower field strength. Proportional to voltage or conductor.
Clamping Voltage	Shall mean the voltage at which the surge arrester starts to conduct. Units typically in RMS Voltage.
Class of LPS	Number denoting the classification of a Lightning Protection System (LPS) according to the lightning protection level for which it is designed.
Combination type SPD	Surge Protective Device (SPD) that incorporates both voltage switching and voltage limiting type components and which may exhibit voltage switching, voltage limiting or both voltage switching and voltage limiting behavior, depending upon the characteristics of the applied voltage (IEC 61643-1:1998).
Connecting component	Part of an external Lightning Protection System, which is used for the connection of conductors to each other or to metallic installations.
Conventional earth impedance	Ratio of the peak values of the earth termination voltage and the earth termination current, which in general, do not occur simultaneously.
Coordinated SPD protection	Set of Surge Protective Devices (SPDs) properly selected, coordinated and installed to reduce failures of electrical and electronic systems.
Common Mode Voltage	The common mode voltage is the voltage that occurs in the case of interference between active conductors and ground.
Continuous Operating Voltage	The continuous operating voltage (Vc) is the highest r.m.s. power-frequency voltage that may be constantly applied to the arrester terminal blocks.
Coupling	Interaction between circuits, during which energy is transmitted from one circuit to the other.
D
Dangerous event	Lightning flash to the object to be protected or near the object to be protected.
Dangerous sparking	Electrical discharge due to lightning, which causes physical damage in the structure to be protected.
Direct Coupling	A physical connection of two or more electrical conductors.
Disconnect Device (Fuse, Breaker)	This is a device that disconnects an arrester from the system when it fails so that fire hazard is prevented and the defective arrester is indicated. Note: the disconnect device is not responsible for providing protection against electric shock by indirect contact.
Down conductor	Shall mean a conductor which connects an earth termination to the (a) System 3000 Dynasphere Terminal or (b) Copper/Stainless Steel air Termination Tape or Finials which are intended to capture lightning discharges and form part of a lightning protection system.
Down conductor system	Part of an external Lightning Protection System which is intended to conduct 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. One or more short strokes may be followed by a long stroke.
Duration of long stroke current (Tlong)	Time duration during which the current in a long stroke is between the 10% of the peak value during the increase of the continuing current and 10% of the peak value during the decrease of the continuing current.
Dynamic Test	Test conducted with normal operating voltage applied.
Dynasphere	Is an air terminal designed to capture a lighting strike with the use of Controlled Streamer Emission (CSE) Technology.
E
Earth Termination (earth termination network)	Shall mean that part of an earthing system that makes contact with, and is intended to discharge currents to the general mass of earth. In the case of a lightning protection system, this includes all parts below any test link.
Earth Conductor	Shall mean the conductor making the final connection to the earth rod or electrode.
Earthing Electrodes	Shall mean those portions of the earth termination, which make direct low resistance / impedance electrical contact with the earth.
Earthing Resistance/Impedance	Shall mean the resistance/impedance of the earthing system to the general mass of earth, as measured from a test point.
Earthing System	Shall mean and include all conductors, piping, electrodes, clamps and other connections and earthing compounds whereby installations are earthed.
Electrical Engineering Consultant	Shall mean an eminent expert in a sub-discipline of Electrical Engineering.
Electromagnetic Compatibility (EMC)	EMC is the ability of a device to function satisfactorily in its electromagnetic environment, without producing interference which cannot be tolerated in this environment or by other devices.
Electromagnetic Interference	Electromagnetic interference is the loss of performance, malfunctions or failure of an electrical or electronic device caused by electromagnetic interference.
Electromagnetic shield	Closed metallic grid-like or continuous screen enveloping the object to be protected, or part of it, used to reduce failures of electrical and electronic systems.
Electronic system	System incorporating sensitive electronic components such as communication equipment, computer, control and instrumentation systems, radio systems, power electronic installations.
Earth termination voltage	Potential difference between the earth termination system and the remote earth.
Electrical system	System incorporating low voltage power supply components and possibly electronic components.
Energy	Maximum allowable energy for a single impulse on a 10/1000mS current waveform. Indicative of the maximum amount of energy that the suppressor can dissipate. This energy is transitory and is dependent upon three (3) variables: 1) voltage, 2) current, and 3) time. Any variation of the three will effect this figure.
Equipment to be Protected	All devices of a structural system or a range which require surge voltage protection or lightning protection.
Equipotential Bonding	This is the elimination of differences in potential between conductive parts, whereby all points take on approximately the same potential. A distinction is made between functional equipotential bonding and protective equipotential bonding.
Equipotential Bonding System	This refers to all the interconnected equipotential bonding conductors, including the conductive parts such as housings or extraneous conductive parts, which work in the same way. The equipotential bonding system can also be a grounding system or part of a grounding system.
Equipotential Conductor	These are electrically conductive connections which serve to produce the equipotential bonding.
External conductive parts	Extended metal items entering or leaving the structure to be protected such as pipe works, cable metallic elements, metal ducts, etc which may carry a part of the lightning current.
External lightning protection system	Part of the Lightning Protection System consisting of an air termination system, a down conductor system and an earth termination system. Typically these parts are outside the structure.
External LPS isolated from the structure to be protected	Lightning Protection System (LPS) whose air termination system and down conductor system are positioned in such a way that the path of the lightning current has no contact with the structure to be protected. In an isolated Lightning Protection System dangerous sparks between the Lightning Protection System and the structure are avoided.
External LPS not isolated from the structure to be protected	Lightning Protection System (LPS) whose air termination system and down conductor system are positioned in such a way that the path of the lightning current can be in contact with the structure to be protected.
F
Failure current (la)	Minimum peak value of lightning current that will cause damage in a line.
Failure of electrical and electronic system	Permanent damage of electrical and electronic system due to LEMP.
Fixing component	Part of an external Lightning Protection System, which is used to fix the elements of the Lightning Protection System to the structure to be protected.
Flash charge (Qflash)	Time integral of the lightning current for the entire lightning flash duration.
Flash duration (T)	Time for which the lightning current flows at the point of strike.
Foundation earthing electrode	Reinforcing steel of foundation or additional conductor embedded in the concrete foundation of a structure and used as an earthing electrode.
Filter Frequency Range	The range of frequency in which a filter operates. This is usually dictated by the -3db points on the low and high ends of the frequency scale.
Follow-Through Current	This is the current which follows the leakage current under the influence of the operating voltage. It is given as a peak value. Note: with some arrester types (e.g. Metal-Oxide-Varistors) the follow-through current is very small (<1 mA).
Frequency (Noise) Attenuation	The range of attenuation for a given frequency range. A larger negative number indicates a greater attenuation.
G
Grid-like spatial shield	Electromagnetic shield characterised by openings. For a building or a room, it is preferably built by interconnected natural metal components of the structure (eg rods of reinforcement in concrete, metal frames and metal supports).
Gas Filled Surge Arrester	The gas filled surge arrester is a discharge path that is filled with another gas than air, generally an inert gas.
Ground	This expression refers to the soil and the ground.
Grounding	Grounding describes all methods and measures utilized.
Grounding Busbar	this is the rail which is used to connect equipment grounding conductors, equipotential bonding conductors and, if necessary, functional grounding conductors with the ground conductor and the ground electrodes.
Ground Conductor	This is a conductor that connects a device to be grounded with a ground electrode. This is only relevant when the ground conductor is not embedded in the ground.
Ground Electrode	This is a conductor that is embedded in the ground and is conductivity connected to it. Parts of feed lines that are in the ground but are not insulated are regarded as parts of the ground electrode.
Ground Flash (Earth Discharge)	Means a lightning flash in which at least one discharge channel reaches the ground.
Ground Resistance	This is the resistance between the grounding system and the reference ground. The amount of ground resistance results from the combined effect of the individual ground electrodes and soil conditions.
I
Injuries of living beings	Injuries, including loss of life, to people or to animals due to touch and step voltages, fire or explosion caused by lightning.
Interconnected reinforcing steel	Steelwork within a concrete structure, which is considered electrically continuous.
Internal lightning protection system	Part of the Lightning Protection System consisting of lightning equipotential bonding and compliance with the separation distance within the structure to be protected.
Internal system	Electrical and electronic systems within a structure.
Inductive Coupling	Formed by the magnetic field surrounding a conductor with a changing current flowing through it. When the magnetic flux lines are cut by another conductor, a voltage is developed on that conductor. The greater the rate of change of the flux lines, the greater the voltage developed.
Impulse Withstand Voltage	The peak value of the highest impulse voltage (Vst) with a preset form and polarity, which will not lead to a disruptive charge under the given test conditions. Note: the surge voltage is equal to or greater than the rated surge voltage.
Input Power Frequency - frequency	Range in which the suppressor operates without causing damage to suppressor or equipment, or interference with the power signal. Applicable to AC circuits.
Intrinsically Safe Circuit	A circuit protected against sparks and ignition and thermal effects (as specified in the DIN VDE 0170/0171 standard which applies to normal operation and specific conditions) which could cause an ignition in an explosive atmosphere.
J
Joint	Shall mean a mechanical and electrical junction between two or more portions of a lightning protection system.
L
LEMP Protection Measures System (LPMS)	Complete system of protection measures for internal systems against LEMP.
Lightning current (i)	Current flowing at the point of strike.
Lightning Electromagnetic Impulse (LEMP)	Electromagnetic effects of lightning current. It includes conducted surges as well as radiated impulse electromagnetic field effects.
Lightning Equipotential Bonding (EB)	Bonding to the Lightning Protection System of separated metallic parts, by direct conductive connections or via surge protective devices, to reduce potential differences caused by lightning current.
Lightning flash near an object	Lightning flash striking close enough to an object to be protected that it may cause dangerous overvoltages.
Lightning flash to an object	Lightning flash striking an object to be protected.
Lightning flash to earth	Electrical discharge of atmospheric origin between cloud and earth consisting of one or more strokes.
Lightning protection designer	Specialist competent and skilled in the design of a Lightning Protection System.
Lightning protection installer	Person competent and skilled in the installation of a Lightning Protection System.
Lightning Protection Level (LPL)	Number related to a set of lightning current parameters values relevant to the probability that the associated maximum and minimum design values will not be exceeded in naturally occurring lightning. Lightning protection level is used to design protection measures according to the relevant set of lightning current parameters.
Lightning Protection Zone (LPZ)	Zone where the lightning electromagnetic environment is defined. The zone boundaries of an LPZ are not necessarily physical boundaries (eg walls, floor and ceiling).
Lightning protective cable	Special cable with increased dielectric strength, whose metallic sheath is in continuous contact with the soil either directly or by the use of conducting plastic covering.
Lightning protective cable duct	Cable duct of low resistivity in contact with the soil (for example, concrete with interconnected structural steel reinforcements or a metallic duct).
Lead Length	The length of leads, whether integral to the unit or added to effect field connection, extending from the suppression device enclosure on a hard wire panel unit. This is an important factor in testing, as specifications should reflect actual installation application.
Leakage Current	The minuscule current flowing through insulators, electronic components which are in a non-conductive state, or any two points which are insulated between each other. A rising leakage current can be a warning of impending insulation or component failure.
Let-through Voltage	Shall mean the voltage appearing on the equipment side of a surge arrester when an impulse voltage-current, with the voltage exceeding the clamping voltage is applied to the input. Surge arresters have non-linear V-1 characteristics, and the let-through voltage increases significantly depending on the peak current in the surge.
Lightning Flash (lightning discharge)	Shall mean an electrical discharge in the atmosphere involving one or more electrically charged regions, and shall include flashes that do or do not reach the ground.
Lightning Protection System	Shall mean a System designed to reduce the injurious and damaging effects of lightning by providing a safe path for capture and conducting lightning energy to ground.
Lightning Strike	Is the term used to describe a lightning flash when the attention is centered on the effects of the flash at the point where it connects to the ground or a structure.
Lightning Stroke	Is the term used to describe an individual current impulse in a complete ground flash.
Long stroke	Part of the lightning flash which corresponds to a continuing current. The duration time Tlong (time from the 10% value on the front to the 10% value on the tail) of this continuing current is typically more than 2ms and less than 1 second.
Long stroke charge (Qlong)	Time integral of the lightning current in a long stroke.
Loss (Lx)	Mean amount of loss (humans and goods) consequent to a specified type of damage due to a dangerous event, relative to the value (humans and goods) of the object to be protected.
Listings	Statement of independent laboratory testing, for safety and/or performance.
Location Categories	Shall mean the categories for the placement of protective equipment as per ANSI C62.41.
M
Metal installations	Extended metal items in the structure to be protected, which may form a path for lightning current, such as pipework, staircases, elevator guide rails, ventilation, heating and air conditioning ducts, and interconnected reinforcing steel.
Multiple strokes	Lightning flash consisting on average of 3 - 4 strokes, with typical time interval between them of about 50ms (events having up to a few tens of strokes with intervals between them ranging from 10ms to 250ms have been reported).
Maximum Continuous Operating Voltage (MCOV) (of a surge arrester)	Shall mean the maximum RMS voltage that can be applied continuously between the terminals of a surge arrester without inhibiting its correct operation.
Mesh method (MM)	Method to determine position of air-termination system
Metallic Sheath (of a cable)	Shall include any conductive sheath, armor or screen.
N
“Natural” component of LPS	Conductive component installed not specifically for lightning protection which can be used in addition to the Lightning Protection System (LPS) or in some cases could provide the function of one or more parts of the Lightning Protection System (LPS). Examples of the use of this term include: – “natural” air termination; – “natural” down conductor; – “natural” earthing electrode.
Node	Point on a service line at which surge propagation can be assumed to be neglected. Examples of nodes are a point on a power line branch distribution at a HV/LV transformer, a multiplexer on a telecommunication line or Surge Protective Device (SPD) installed along the line.
Number of dangerous events due to flashes near a service (NI)	Expected average annual number of dangerous events due to lightning flashes near a service.
Number of dangerous events due to flashes near a structure (NM)	Expected average annual number of dangerous events due to lightning flashes near a structure.
Number of dangerous events due to flashes to a service (NL)	Expected average annual number of dangerous events due to lightning flashes to a service.
Number of dangerous events due to flashes to a structure (ND)	Expected average annual number of dangerous events due to lightning flashes to a structure.
Nominal Voltage	Normal operating voltage or average voltage at which equipment operates.
Normal Mode Voltage	Normal mode voltage is voltage interference between two conductors of a circuit. (Line to Line)
O
Object to be protected	Structure or service to be protected against the effects of lightning.
P
Peak Surge Current	Maximum current allowed for a single 8x20mS impulse waveform with continuous voltage applied. The higher the number, the stronger the unit.
Phase Angle	The point on the sine wave at which a transient occurs. IEEE states that transients can occur at any phase angle. It is important to be able to see suppression device response to transients at varying phase angles.
Physical Dimensions	The length, width and height or depth or the suppression device. When considering space constraints in any application, this specification affords quick determination of acceptability.
Physical damage	Damage to a structure (or to its contents) or to a service due to mechanical, thermal, chemical or explosive effects of lightning.
Point of strike	Point where a lightning flash strikes the earth, or a protruding object (eg structure, Lightning Protection System, service, tree, etc). A lightning flash may have more than one point of strike.
Probability of damage (PX)	Probability that a dangerous event will cause damage to or in the object to be protected.
Protection measures	Measures to be adopted in the object to be protected to reduce the risk.
Protection angle method (PAM)	Method to determine position of air-termination system
Positive or Negative Polarity	Indicates direction in which the surge occurs.
Power	Shall mean electrical power, (e.g., power apparatus refers to electrical power apparatus.)
Power-Frequency Withstand Voltage	This is the r.m.s. value of the highest sinusoidal voltage with system frequency that will not lead to a disruptive charge under the given test conditions.
Pressure Relief Device	Relieves the arrester of internal over-pressure in the case of overload.
Protection Modes	Protection mode indicates suppressor’s ability to protect different paths of transient activity. Normal mode = line to neutral and/or line to line. Common mode = line to ground and/or neutral to ground.
R
Rated impulse withstand voltage (UW)	Impulse withstand voltage assigned by the manufacturer to the equipment or to a part of it, characterising the specified withstand capability of its insulation against overvoltages. For the purpose of BS EN 62305, only withstand voltage between live conductors and earth is considered. [IEC 60664-1:2002]
Ring conductor	Conductor forming a loop around the structure and interconnecting the down-conductors for distribution of lightning current among them.
Ring earthing electrode	Earthing electrode forming a closed loop around the structure below or on the surface of the earth.
Risk (R)	Value of probable average annual loss (humans and goods) due to lightning, relative to the total value (humans and goods) of the object to be protected.
Risk component (RX)	Partial risk depending on the source and the type of damage.
Rural environment	Area with a low density of buildings. ”Countryside” is an example of a rural environment.
Rolling sphere method (RSM)	Method to determine position of air-termination system
Rated Voltage	Rated voltage is the desired normal continuous operating voltage at which surge components are specified.
Residual Voltage	The rated peak voltage reached while surge current is flowing through the surge arrester.
Response Time	The time in which a suppression device responds to a transient.
S
Separation distance	Distance between two conductive parts at which no dangerous sparking can occur.
Service to be protected	Service connected to a structure for which protection is required against the effects of lightning in accordance with this standard. The service to be protected comprises the physical connection between: – the switch telecommunication building and the user’s building or two switch telecommunication buildings or two user’s buildings, for the telecommunication (TLC) lines; – between the switch telecommunication building or the user’s building and a distribution node, or between two distribution nodes for the telecommunication (TLC) lines; – the high voltage (HV) substation and the user’s building, for the power lines; – the main distribution station and the user’s building, for pipes.
Shielding wire	Metallic wire used to reduce physical damage due to lightning flashes to a service.
Short stroke	Part of the lightning flash which corresponds to an impulse current. This current has a time to the half value T2 typically less than 2ms.
Short stroke charge (Qshort)	Time integral of the lightning current in a short stroke.
SPD tested with a combination wave	Surge Protective Devices (SPDs) that withstand induced surge currents with a typical waveform 8/20μs and require a corresponding impulse test current Isc. For power lines a suitable combination wave test is defined in the Class III test procedure of IEC 61643-1 defining the open circuit voltage Uoc 1,2/50μs and the short circuit current Isc 8/20μs of a 2Ω combination wave generator.
SPD tested with (Iimp)	Surge Protective Devices (SPDs) which withstand the partial lightning current with a typical waveform 10/350μs require a corresponding impulse test current Iimp. For power lines, a suitable test current Iimp is defined in the Class I test procedure of IEC 61643-1.
SPD tested with (In)	Surge Protective Devices (SPDs) which withstand induced surge currents with a typical waveform 8/20μs require a corresponding impulse test current In. For power lines a suitable test current In is defined in the Class II test procedure of IEC 61643-1.
Specific energy (W/R)	Time integral of the square of the lightning current for the entire flash duration; it represents the energy dissipated by the lightning current in a unit resistance.
Specific energy of short stroke current	Time integral of the square of the lightning current for the duration of the short stroke. The specific energy in a long stroke current is negligible.
Structure to be protected	Structure for which protection is required against the effects of lightning in accordance with BS EN 62305. A structure to be protected may be a part of a larger structure.
Structures dangerous to the environment	Structures which may cause biological, chemical and radioactive emission as a consequence of lightning (such as chemical, petrochemical, nuclear plants, etc).
Structures with risk of explosion	Structures containing solid explosives materials or hazardous zones as determined in accordance with IEC 60079-10 and IEC 61241-10. For the purposes of BS EN 62305 structures with hazardous zones type 0 or containing solid explosive materials are considered.
Suburban environment	Area with a medium density of buildings. ”Town outskirts” is an example of a suburban environment.
Surge	Transient wave appearing as overvoltage and/or overcurrent caused by LEMP. Surges caused by LEMP can arise from (partial) lightning currents, from induction effects in installation loops and as a remaining threat downstream of a Surge Protective Device (SPD).
Surge Protective Device (SPD)	Device that is intended to limit transient overvoltages and divert surge currents. It contains at least one non-linear component (see IEC 61643 series).
Side Flash	Shall mean a discharge occurring between nearby metallic objects or from such objects to a lightning protection system or to earth.
Sparkover Voltage	The voltage at which a surge arrester becomes conductive.
Stage (of protection)	Describes the configuration of circuit elements of a surge protective device. Each phase or line of a typical protective device might be composed of the following elements:
Static Test	Test conducted with no normal operating voltage applied.
Structure	Shall mean any building or construction, process plant, storage tank, tree, or similar, on or in the ground.
Surge Arrester (Shunt Diverter)	shall mean a protective device, usually connected between any conductor of a system and earth, tested IEC1643 and ANSI C62.1, which limits surge voltages by diverting surge current to earth when a given voltage is exceeded. The let-through voltage must be within a certain tolerance acceptable to the equipment being protected.
Surge Arresting Capacity	This is the rated maximum current a surge arrester is capable of shunting to ground.
Surge Arrestors	Consist mainly of voltage dependent resistors and/or discharges. The two elements are connected either in series or in parallel, but each can also be used on its own. Arresters are used to protect other electrical equipment and electrical systems against inadmissible surge voltages.
Surge Current	The surge current is the current flowing through the arrester after it has sparked over. It is given as a peak value.
Surge Current Rating	Shall mean the peak value of a standard one-shot 8/20 microsecond impulse current wave that a surge arrester can absorb without damage.
Surge Reduction FilterÔ (SRFÔ)	Shall be an in-line series filter designed to remove all surges and transients to an acceptable level produced by a lightning strike.
Surge Voltage	The surge voltage is the permanent or temporary voltage between conductors or between conductor and ground in functioning systems (also in switched off systems) which can endanger people and have damaging effects on cables and connected devices.
Surge Voltage Arrester	This is a device for limiting the surge voltage between two parts within the volume to be protected, e.g. spark gaps, valve arresters or semiconductors.
Surge Voltage Category	Allocation of an electrical device to the expected surge voltage.
Surge Voltage Limiter	Surge voltage limiters are components and protective circuits, which limit surge voltages in systems or devices to permissible values.
Surge Voltage Protection Appliances	Surge voltage protection appliances are surge voltage limiters together with all the appliances in telecommunications systems including the cables for surge voltage protection.
Surge Voltage Protection Arrangement	This is an element, a group or an appliance that limits the expected surge voltage.
Switching Surge Voltage	This is surge voltage caused by a switching operation.
T
Test joint	Joint designed to facilitate electrical testing and measurement of Lightning Protection System components.
Time to peak value of short stroke current (t1)	Virtual parameter defined as 1.25 times the time interval between the instants when the 10% and 90% of the peak value are reached.
Time to half value of short stroke current (t2)	Virtual parameter defined as the time interval between the virtual origin O1 and the instant at which the current has decreased to half the peak value.
Tolerable risk (RT)	Maximum value of the risk, which can be tolerated for the object to be protected.
To Ground	To ground means to connect an electrically conductive component (e.g. the lightning protection system) with the ground via a grounding system.
Transient	These are irregular and relatively short positive and/or negative voltage or current changes between two stationary states.
Test Link	Shall mean a joint designed and situated so as to allow resistance/impedance or continuity measurements to be made.
Thunderday	Shall mean a calendar day during which thunder is heard at a given location. The international definition of lightning activity is given as the number of thunderdays per year (also called isokeraunic level)
Transient Earth Clamp (TEC)	Shall mean a device placed in the bond between the lightning earth and the MEN earth to equalize potentials during a lightning strike.
U
Upward flash	Lightning flash initiated by an upward leader from an earthed structure to cloud. An upward flash consists of a first long stroke with or without multiple superimposed short strokes. One or more short strokes may be followed by a long stroke.
Urban environment	Area with a high density of buildings or densely populated communities with tall buildings. ”Town centre” is an example of an urban environment.
V
Virtual origin of short stroke current (O1)	Point of intersection with time axis of a straight line drawn through the 10% and the 90% reference points on the stroke current front; it precedes by 0.1T1 that instant at which the current attains 10% of its peak value.
Voltage switching type SPD	SPD that has a high impedance when no surge is present, but can have a sudden change in impedance to a low value in response to a voltage surge. Common examples of components used as voltage switching devices include spark gaps, gas discharge tubes (GDT), thyristors (silicon controlled rectifiers) and triacs. These SPD are sometimes called “crowbar type“. A voltage switching device has a discontinuous voltage/current characteristic. (IEC 61643-1:1998)
Voltage limiting type SPD	SPD that has a high impedance when no surge is present, but will reduce it continuously with increased surge current and voltage. Common examples of components used as non-linear devices are varistors and suppressor diodes. These SPDs are sometimes called “clamping type“. A voltage-limiting device has a continuous voltage/current characteristic. (IEC 61643-1:1998)
Varistors	The varistor is a bipolar non-linear resistor with a symmetrical voltage/current characteristic curve whose resistance value decreases as the voltage increases.
Voltage	the peak transient voltage which is applied to unit under test according to ANSI/IEEE C62.41 1980, section 4.1: “...This 6 KV level, therefore, can be selected as a typical cutoff for the occurrence of surges in indoor power systems.”
W
Weight	Weight of suppression device. Useful in determining application and installation requirements.
Z
Zone of a structure (ZS)	Part of a structure with homogeneous characteristics where only one set of parameters is involved in assessment of a risk component.