Lightning protection glossary
The symbols and terms used in lightning risk assessment and protection design, each defined in plain words and cited to the clause, table or equation of IEC 62305, NFPA 780 or AS 1768 that sets it.
Lightning risk assessment has its own shorthand, and each standard writes it a little differently.
Jump to a group: risk and its tolerable limits, strike density and collection areas, probabilities and protection credits, protection levels and classes, the parts of the protection system, separation distance and bonding, zones and surges, other standards and methods. For the method that ties the risk terms together, read the IEC 62305-2 risk method.
Risk and its tolerable limits
Risk R
R is the single risk figure IEC 62305-2:2024 (Ed.3) uses to judge a structure. It adds the risk components for injury to people and physical damage across the four sources of damage, equation (1). The structure needs protection when R is higher than the tolerable risk RT, under clause 7.3.
Tolerable risk
RT
Frequency of damage
F is how often per year a strike is expected to cause a failure of internal electrical and electronic systems. IEC 62305-2:2024 builds it from components, each a number of dangerous events multiplied by a probability of damage, equation (13). It is judged against the tolerable frequency FT, separately from the risk R.
Tolerable frequency of damage
FT is the number of internal system failures per year a site can accept. IEC 62305-2:2024 clause 9.3 NOTE 1 gives representative values of 0.1 per year for systems critical to the service and 1 per year for the rest. The owner sets it by the downtime the service can bear.
Risk of loss of human life
R1 is the risk of loss of human life, a term NFPA 780 and AS 1768 still use. NFPA 780-2026 Table L.6.2 gives a default tolerable value of 10⁻⁵ per year, and AS 1768:2021 Table 2.1 gives the same. IEC 62305-2:2024 (Ed.3) no longer splits risk into R1 to R4; it judges one combined risk R.
Risk of loss of service
R2 is the risk that the public loses a service the structure provides. NFPA 780-2026 Table L.6.2 gives a default tolerable value of 10⁻³ per year, and AS 1768:2021 Table 2.1 gives 10⁻³ for its essential services category. In IEC 62305-2:2024 (Ed.3), loss of service to the public is the type of loss L2, counted inside the combined risk R.
Risk of loss of cultural heritage
Risk of loss of economic value
Risk component
A risk component is one part of the total risk, tied to one source and one cause of damage. IEC 62305-2:2024 equation (9) computes each as the dangerous events per year, times the probability of damage, times the loss, written NX × PX × LX. Ed.3 has nine, from RAT and RAD through to RZ.
Sources of damage
IEC 62305-2:2024 sorts strikes into four sources of damage in Table 1. S1 is a flash to the structure, S2 a flash to the ground near it, S3 a flash to a connected line, and S4 a flash near a connected line. Each source has its own collection area, its own number of dangerous events and its own components.
Types of loss
A type of loss names what a strike can take away. IEC 62305-2:2024 (Ed.3) clause 5.3 uses two: L1, loss of human life, and L2, loss of service to the public. The combined risk is the sum of the risk for each, R = RL1 + RL2, which is equation (6) of the standard.
Risk zone
A risk zone is a part of a structure with uniform characteristics, assessed as one unit. IEC 62305-2:2024 clause 8.3 lets you treat a structure as one zone or split it at changes of floor surface, fire compartments or spatial shields. Risk zones are units of the calculation, not the lightning protection zones of IEC 62305-4.
Strike density and collection areas
Ground strike-point density
NSG is the number of points struck on the ground per square kilometre per year. IEC 62305-2:2024 uses it in place of flash density, because one flash can strike the ground at several points. Equation (A.1) gives NSG = k × NG, with k = 2 assumed when the data provider gives no ratio (clause A.1 NOTE 1).
Ground flash density
NG is the number of lightning flashes to ground per square kilometre per year. IEC 62305-2:2024 converts it to strike points with equation (A.1). NFPA 780-2026 multiplies a flash density by 1.7 to reach NSG (L.2.2). AS 1768:2021 still works from flash density, written Ng and read from its map in Figure C.1.
Dangerous events from flashes to the structure
ND is the expected number of direct flashes to the structure per year, source S1. IEC 62305-2:2024 equation (A.5) multiplies the ground strike-point density by the collection area AD and the location factor CD, then by 10⁻⁶ to turn square metres into square kilometres. NFPA 780-2026 screens ND against NC.
Dangerous events from flashes near the structure
NM is the expected number of flashes per year to the ground near the structure, source S2. They can induce surges inside it without touching it. IEC 62305-2:2024 equation (A.7) takes it from the ground strike-point density and the collection area AM, scaled by an averaging factor with k = 2 assumed by its NOTE 1.
Dangerous events from flashes to a line
NL is the expected number of flashes per year to a service line entering the structure, source S3. IEC 62305-2:2024 equation (A.9) works it from the ground strike-point density, the line's collection area AL, and factors for its installation (CI), type (CT) and environment (CE). Each line is assessed separately, under clause 8.2.
Dangerous events from flashes near a line
NI is the expected number of flashes per year to the ground near a connected line, source S4. They induce surges that travel along the line into the structure. IEC 62305-2:2024 equation (A.11) uses the same line factors as NL, with the wider collection area AI and the averaging factor applied to nearby flashes.
Dangerous events from flashes to an adjacent structure
NDJ is the expected number of flashes per year to an adjacent structure at the far end of a connected line. A strike there can send current back along the line. IEC 62305-2:2024 equation (A.6) works it from the adjacent structure's own collection area, its location factor CDJ and the line type factor CT.
Collection area for direct flashes
AD is the ground area that collects direct flashes to a structure. For an isolated rectangular building, IEC 62305-2:2024 equation (A.3) adds the footprint to a band three times the building height wide around it, with rounded corners. A 40 by 20 by 6 m block therefore collects about 3,978 m², far more than its 800 m² footprint.
Collection area for flashes near the structure
AM is the ground area from which a nearby flash can induce damaging surges in the structure's internal systems. IEC 62305-2:2024 equation (A.8) draws it as a band of width rM around the structure, where rM = 350 / UW, so equipment with a lower withstand voltage collects from a wider area.
Collection area for flashes to a line
AL is the ground area along a service line that collects flashes striking the line itself. IEC 62305-2:2024 equation (A.10) sets AL = 40 × LL, with LL the line length in metres. The area grows in direct proportion to the length, so a long overhead feed collects many more strikes.
Collection area for flashes near a line
AI is the ground area along a service line from which a nearby flash induces a surge on it. IEC 62305-2:2024 equation (A.12) makes it a strip running the length of the line, twice rI wide, where rI = 2000 / UW1.8. It is usually far larger than the line's own collection area AL.
Location factor
CD adjusts the number of direct flashes for what surrounds a structure. IEC 62305-2:2024 Table A.1 gives 0.25 when it is surrounded by taller objects, 0.5 among objects of the same height or smaller, 1 when isolated, and 2 for an isolated structure on a hilltop or knoll. The same table gives CDJ for adjacent structures.
Environmental factor
CE adjusts the strikes collected by a service line for the surroundings it runs through. IEC 62305-2:2024 Table A.4 gives 1 for rural, 0.5 for suburban, 0.1 for urban, and 0.01 for urban areas with buildings taller than 20 m. Taller surroundings shield the line, so fewer dangerous events reach it.
Rated impulse withstand voltage
UW is the impulse voltage, in kV, that an internal system can withstand without damage. In IEC 62305-2:2024 it sets the reach of nearby flashes through rM = 350 / UW, equation (A.8), and rI = 2000 / UW1.8, equation (A.12). Sensitive electronics with a low UW widen both collection areas.
Probabilities and protection credits
Probability of damage with an LPS
PLPS is the probability that a direct flash causes physical damage despite a lightning protection system. IEC 62305-2:2024 Table B.3 gives 1 for no protection, then 0.2, 0.1, 0.05 and 0.02 for an LPS of Class IV, III, II and I, with lower values for Class I systems that use the structure's metal framework.
Probability of damage with SPDs
PSPD is the probability that a surge still damages internal systems with coordinated surge protective devices fitted. IEC 62305-2:2024 Table B.7, for the power system, gives 1 with no SPDs, 0.05 for LPL III to IV, 0.02 for LPL II and 0.01 for LPL I. Table B.8 covers telecom systems, and Annex D allows a detailed evaluation.
Thunderstorm warning system factor
PTWS is the credit IEC 62305-2:2024 gives a thunderstorm warning system to IEC 62793. It counts only on a declared condition: a quick and full evacuation of the exposed area, or for flashes near a line, immediate disconnection of the external lines. Without that condition, or without a declared warning performance, PTWS = 1 and no credit is given.
Protection levels and classes
Lightning protection level
A lightning protection level is the set of lightning current parameters a protection system is designed to handle. IEC 62305-1 defines four, LPL I to LPL IV, from most to least demanding. You do not choose it by preference: the IEC 62305-2 risk assessment arrives at the lightest level that brings the risk below the tolerable value.
Class of LPS
The class of a lightning protection system is the physical counterpart of the protection level. IEC 62305-3:2024 Table 1 maps LPL I to IV onto Class I to IV one for one, and clause 4.1 makes the class set the rolling sphere radius, the mesh size, the down conductor spacing and the separation distance coefficient ki.
Lightning protection system
A lightning protection system is the complete installation that protects a structure against physical damage from a direct flash. Under IEC 62305-3:2024 it has an external part, made up of the air termination, down conductors and earth termination, and an internal part, the equipotential bonding and separation distance that stop dangerous sparking inside the structure.
The parts of the protection system
Air termination
The air termination is the part of a lightning protection system that intercepts the flash. IEC 62305-3:2024 clause 5.2.1 builds it from rods, including free-standing masts, catenary wires and meshed conductors, and rules out radioactive terminals. It is positioned by the rolling sphere, mesh or protection angle method for the class of LPS.
Rolling sphere radius
The rolling sphere radius is the size of the imaginary sphere rolled over a structure to find where air terminations are needed. IEC 62305-3:2024 Table 2 gives 20, 30, 45 and 60 m for Class I to IV. AS 1768:2021 Table D.1 uses the same four radii, and NFPA 780-2026 uses at most 150 ft (45 m).
Mesh method
The mesh method protects a roof with a grid of conductors bonded into closed loops, so any strike to the roof meets a nearby conductor. It suits flat and gently pitched roofs. IEC 62305-3:2024 Table 2 sets the mesh size for each class of LPS, finer for the more demanding classes. NFPA 780-2026 has no mesh method.
Protection angle method
The protection angle method treats the space below a rod or mast, inside a cone set at an angle from the vertical, as protected. IEC 62305-3:2024 Table 2 sets the angle by the height of the air termination and the class of LPS, and the angle narrows as either rises. It suits simple shapes and single rooftop items.
Down conductor
A down conductor carries the lightning current from the air termination to the earth termination. IEC 62305-3:2024 clause 5.3 sets how many are needed and how they run, and Table 5 gives the preferred spacing between them for each class of LPS. More down conductors share the current, which lowers kc and the separation distance.
Earth termination
The earth termination is the part of a lightning protection system that disperses the current into the ground. IEC 62305-3:2024 clause 5.4 describes two arrangements, Type A and Type B, and clause 5.4.1 recommends an earth resistance below 10 ohms where possible. For structures with explosive materials or hazardous areas, clause C.2.3 makes 10 ohms a firm maximum.
Type A and Type B earthing
Type A earthing uses separate horizontal or vertical electrodes, one connected to each down conductor. Type B is a closed loop: a ring electrode around the structure or a foundation earth electrode in its concrete. IEC 62305-3:2024 clause 5.4 describes both. Type B gives a more even potential and suits larger buildings and sensitive internal systems.
Strike termination device
Strike termination device is the NFPA 780 term for the parts that receive a flash: air terminals, masts, overhead ground wires, and metal parts with a thickness of 4.8 mm (3/16 in) or more. NFPA 780-2026 defines it in 3.3.42 and sets the devices out in Section 4.5. IEC 62305-3 calls the same part of the system the air termination.
Separation distance and bonding
Separation distance
The separation distance s is the minimum gap between a lightning protection system and unbonded conductive parts, so a strike cannot spark across. IEC 62305-3:2024 clause 6.3.2, equation (6), gives s = (ki / km) × kc × l. A part that cannot be kept further away than s is bonded to the system instead.
Class coefficient
ki is the coefficient in the separation distance formula that depends on the class of LPS. IEC 62305-3:2024 Table 11 gives 0.08 for Class I, 0.06 for Class II, and 0.04 for Classes III and IV. A more demanding class is designed for a more severe strike, so it needs a wider gap.
Current sharing coefficient
kc is the fraction of the lightning current carried by the conductor being checked. IEC 62305-3:2024 Table 13 gives 1 for one down conductor on an isolated system, 0.66 for two and 0.44 for three or more. Annex B gives more precise values from the layout, and uneven Type A electrodes force kc = 1.
Material coefficient
km is the coefficient for the insulating material in the gap between the system and the unbonded part. IEC 62305-3:2024 Table 12 gives 1 for air and 0.5 for concrete, brick or wood, and its NOTE 1 allows 0.7 for an FRP, PE or PVC stand-off at least 0.5 m long. A lower km means a wider gap.
Isolated LPS
An isolated lightning protection system is held clear of the structure it protects, so the lightning current does not flow through the structure itself. IEC 62305-3:2024 sets it apart from an attached system and from an electrically insulated one. Table 13 allows a single down conductor, with kc = 1, only on an isolated system.
Lightning equipotential bonding
Lightning equipotential bonding connects metal parts and incoming services to the lightning protection system, directly or through surge protective devices, so they rise and fall together during a strike. With no voltage difference left between them, nothing can spark across. IEC 62305-3:2024 clause 6.2 sets the rules, and bonding is the alternative to keeping a separation distance.
Bonding distance
The bonding distance D is the NFPA 780 counterpart to the IEC separation distance. NFPA 780-2026 equations 4.15.2.5.1 and 4.15.2.6.1 give D = (l / 6n) × Km, with l the conductor length, n a factor for the down conductors sharing the current, and Km 1 through air or 0.50 through dense material. Grounded metal closer than D is bonded.
Zones and surges
Lightning protection zone 0A
LPZ 0A is the zone exposed to direct flashes and to the full, unweakened electromagnetic field of lightning, so anything in it may carry the full lightning current. In IEC 62305-4:2024 clause 4 it is the outermost zone, the starting point from which each inner zone steps the threat down.
Lightning protection zone 0B
LPZ 0B is the zone outside the structure that is protected against direct flashes, for example under the cover of an air termination, but still exposed to the full electromagnetic field of lightning. IEC 62305-4:2024 clause 4 defines it. Equipment here will not take a direct hit, yet surges and fields can still damage it.
Lightning protection zone 1
LPZ 1 is the first protected zone inside a structure. The surge current entering it is limited by current sharing and by surge protective devices at its boundary, and spatial shielding can weaken the electromagnetic field. IEC 62305-4:2024 clause 4 defines it, and it is usually the space inside a building with a lightning protection system.
Lightning protection zone 2 and higher
LPZ 2 and any higher zones lie further inside, where more surge protective devices and more shielding cut the surge current and the field again. IEC 62305-4:2024 clause 4 defines them. A shielded equipment room or cabinet inside an LPZ 1 building often forms an LPZ 2, with its own SPDs at the boundary.
Lightning electromagnetic impulse
LEMP, the lightning electromagnetic impulse, is the electromagnetic effect of lightning current: the radiated field and the surges that field and the strike current induce on conductors. It can destroy electronics in a building the flash never touched. IEC 62305-4 protects internal systems against it with zones, shielding, bonding and coordinated SPDs.
Surge protective device
A surge protective device limits surge voltages and diverts surge current away from the equipment behind it. Type 1 SPDs, tested with a 10/350 impulse, sit at the service entrance; Type 2, tested with 8/20, at distribution boards; Type 3 close to sensitive equipment. In IEC 62305-2, coordinated SPDs lower PSPD and so the risk.
Other standards and methods
NFPA 780 Annex L
Annex L is the lightning risk assessment of NFPA 780-2026. It is informative, so it binds only when an authority, insurer or contract adopts it. Section L.5 is a quick screen of ND against NC, and Section L.6 is the detailed method, which builds the relevant risks R1 to R4 and tests each against its own tolerable value.
Tolerable lightning frequency
NC is the tolerable strike frequency in the simplified screen of NFPA 780-2026 Annex L. Equation (L.5.1.1) sets NC = 1.5 × 10⁻³ / C, where C is the product of four coefficients for construction, contents, occupancy and consequence, C2 to C5. An LPS is recommended when ND is higher than NC (L.5.2.1).
AS 1768 Lightning Risk Assessment Tool
The AS 1768:2021 Lightning Risk Assessment Tool v5.0 is the spreadsheet Standards Australia supplies with the standard to run its Section 2 and Appendix B method. Appendix B gives the shape of the method and the tool holds the detail. The standard's worked examples, Figures C.2 to C.8, were produced with it.
Distributed facility
A distributed facility, in AS 1768:2021 clause 2.5.4.2, is a group of standalone structures joined by conductive services inside one site boundary, such as a ground-mounted solar farm. The spreadsheet does not apply to the site as a whole. The site is estimated from its area and Ng, then each standalone structure is assessed on its own.
Early streamer emission terminal
An early streamer emission terminal is an air terminal whose maker claims it launches an upward streamer earlier than a plain rod and so protects a wider radius. The method comes from national standards such as NF C 17-102 in France. IEC 62305-3:2024 clause 5.2.1 grants no enlarged radius, and NFPA 780-2026 leaves ESE out of scope (1.1.3).
From a definition to a worked calculation
The definitions above are the building blocks. To see them work together, read how an IEC 62305 risk is calculated, which carries one building from its inputs to a verdict, and the separation distance calculation, which puts ki, kc and km through a worked example.
For the other two standards, read NFPA 780 Annex L, section by
section and what AS 1768 is
The tolerable risk in IEC 62305-2 is not a fixed constant.
Clause 7.3 NOTE 1 gives RT = 1 × 10⁻⁵ per year as a
representative value of tolerable risk and adds that another value may be set once
the case has been investigated in detail. Printed p.12 then lets national or local regulations fix
RT, the tolerable frequency of damage FT
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