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Lightning protection level selector

Turn an IEC 62305-2:2024 risk figure into a lightning protection level and class of LPS. Enter the risk the LPS would reduce and the rest, and see the weakest class that meets the tolerable risk, with the sphere radius, mesh and spacing it sets.

The lightning protection level is the weakest class of LPS whose PLPS brings the risk R within the tolerable risk RT. Enter the risk an LPS would reduce and the rest, and this selector finds that class under IEC 62305-2:2024.

It works from two figures any IEC 62305-2 assessment produces: RAT + RAD + RB, the components a lightning protection system acts on, and the sum of every other component. It tries each class from Table B.3, weakest first, and stops at the first that meets RT. Then it shows what that class means for the design under IEC 62305-3:2024. For background, read the lightning protection levels guide.

The selector

Class of LPS from the risk

Enter yearly risk figures in decimal or e-notation, for example 4e-5. The page opens on the worked example below.

Your risk figures

with P_LPS = 1 and P_S = 1 (Table B.4 NOTE 1)
R_C, R_M, R_U, R_V, R_W and R_Z together
clause 7.3 NOTE 1, unless an authority fixes another
None of these Extensive metal framework Reinforced concrete, rods interconnected and verified Meshed earth termination, no accessible metal in the lightning path
decides whether the framework rows of Table B.3 apply
Weakest class that meets RT
LPL III, class of LPS III
R falls from 4.3 × 10⁻⁵ to 7 × 10⁻⁶ against RT = 1 × 10⁻⁵
Rolling sphere radius
45 m
Mesh size
15 × 15 m
Down conductors every
15 m
ki for separation
0.04
Sphere radius and mesh from IEC 62305-3:2024 Table 2, down-conductor spacing from Table 5 and ki from Table 11. The class brings lightning equipotential bonding at the LPL III to IV tier of IEC 62305-2:2024 Table B.13, P_EB 0.05. The protection angle is a curve against height in Figure 1 of Part 3, so it is not a single figure here.
Class of LPS R with it Within RT?
An LPS IV covers these areas1.1 × 10⁻⁵No
An LPS III covers these areas7 × 10⁻⁶Yes
An LPS II covers these areas5 × 10⁻⁶Yes
An LPS I covers these areas3.8 × 10⁻⁶Yes
Quantity How it was found Value Source
R_LPSR_AT + R_AD + R_B without an LPS, as entered4 × 10⁻⁵IEC 62305-2:2024 (Ed.3) Table 3, equations (B.2) to (B.4)
R_otherevery other component, as entered3 × 10⁻⁶IEC 62305-2:2024 (Ed.3) equation (1)
R_Ttolerable risk1 × 10⁻⁵IEC 62305-2:2024 (Ed.3) clause 7.3 NOTE 1, unless an owner or authority fixed another
R, no LPSR_LPS × 1 + R_other4.3 × 10⁻⁵IEC 62305-2:2024 (Ed.3) Table B.3, no LPS; Lumex™ simplification of equations (B.2) to (B.4), with P_S = 1 by Table B.4 NOTE 1
P_LPSAn LPS III covers these areas0.1IEC 62305-2:2024 (Ed.3) Table B.3
R, with LPSR_LPS × P_LPS + R_other = 4 × 10⁻⁵ × 0.1 + 3 × 10⁻⁶7 × 10⁻⁶Lumex™ simplification of equations (B.2) to (B.4), with P_S = 1 by Table B.4 NOTE 1; judged by clause 7.3
The method

How the risk picks a class

IEC 62305-2:2024 decides whether protection is needed by comparing the risk R with the tolerable risk RT under clause 7.3. R is the sum of the risk components (equation (1)), each a number of dangerous events times a probability of damage times a loss (equation (9), Table 3).

What an LPS changes. A lightning protection system enters the calculation through one probability, PLPS, from Table B.3. It multiplies PAT in equation (B.2), PAD in equation (B.3) and PB in equation (B.4), so it scales three components: RAT and RAD, injury to people from a strike to the structure, and RB, physical damage. Every other component is untouched by PLPS.

The selector's rule. Because those three components carry PLPS as a plain factor, the risk with an LPS of a given class is R = (RAT + RAD + RB) × PLPS + the rest. The selector works that out for each class, weakest first, and picks the first one that meets RT. This is a Lumex™ simplification, labelled as such in the working: it credits the LPS with nothing beyond PLPS, so it never chooses a weaker class than a full assessment would.

From the class to the design. IEC 62305-3:2024 Table 1 maps lightning protection level I to IV onto class of LPS I to IV one for one. The class then fixes the rolling sphere radius and mesh size in Table 2, the preferred spacing of down conductors in Table 5 and the coefficient ki for the separation distance in Table 11. It also sets the lightning equipotential bonding the system brings, which IEC 62305-2:2024 Table B.13 values through PEB.

Two rows need the building's own frame. Table B.3's strongest rows count a continuous metal or reinforced concrete framework as natural down conductors. The selector offers them only when you declare such a framework, the same rule a full Lumex™ assessment applies. Choose a framework above to include them.
Worked example

RAT + RAD + RB = 4 × 10⁻⁵, the rest 3 × 10⁻⁶

Illustrative figures, not a real site, judged against RT = 1 × 10⁻⁵. The result is computed live from the same rows the engine reads.

Without an LPS the risk is 4.3 × 10⁻⁵, so protection is needed under clause 7.3. The weakest class that meets RT is class III, with PLPS = 0.1, which brings R down to 7 × 10⁻⁶. The design then follows LPL III.

Quantity How it was found Value Source
R_LPSR_AT + R_AD + R_B without an LPS, as entered4 × 10⁻⁵IEC 62305-2:2024 (Ed.3) Table 3, equations (B.2) to (B.4)
R_otherevery other component, as entered3 × 10⁻⁶IEC 62305-2:2024 (Ed.3) equation (1)
R_Ttolerable risk1 × 10⁻⁵IEC 62305-2:2024 (Ed.3) clause 7.3 NOTE 1, unless an owner or authority fixed another
R, no LPSR_LPS × 1 + R_other4.3 × 10⁻⁵IEC 62305-2:2024 (Ed.3) Table B.3, no LPS; Lumex™ simplification of equations (B.2) to (B.4), with P_S = 1 by Table B.4 NOTE 1
P_LPSAn LPS III covers these areas0.1IEC 62305-2:2024 (Ed.3) Table B.3
R, with LPSR_LPS × P_LPS + R_other = 4 × 10⁻⁵ × 0.1 + 3 × 10⁻⁶7 × 10⁻⁶Lumex™ simplification of equations (B.2) to (B.4), with P_S = 1 by Table B.4 NOTE 1; judged by clause 7.3
The limits

What the selector leaves to the full assessment

The selector needs a risk assessment to start from: it does not compute RAT, RAD or RB itself. It also judges R only. The frequency of damage F of clause 9.3 is built from surges, an LPS does not lower it, and a structure with sensitive internal systems often fails FT long after R is tolerable. Finally, it looks at one measure. The cheapest way to meet RT is often a smaller LPS plus surge protection or fire provisions, which only a search across every measure finds.

Run the full assessment. Lumex™ computes every risk component and F under IEC 62305-2:2024, searches the protection measures for the combination that meets both, and sets out the class of LPS and its Part 3 design values in the report. Start a free trial, check the collection area calculator for ND, or see how Lumex™ runs IEC 62305. Lumex™ is independent of the IEC: buy the standard for design and certification.
FAQs

Questions answered

How do I choose the lightning protection level?

Under IEC 62305 the level comes out of the Part 2 risk assessment. You compute the risk R of the structure and compare it with the tolerable risk R_T of clause 7.3. If R is too high, you try a lightning protection system of a given class, which lowers the components it acts on through its P_LPS from Table B.3, and pick the weakest class that brings R within R_T. Part 3 Table 1 then maps that class to the same lightning protection level.

What is the difference between LPL and class of LPS?

The lightning protection level (LPL I to IV) describes the lightning current parameters a protection system is designed for. The class of LPS (I to IV) describes the system built to that level. IEC 62305-3:2024 Table 1 maps them one for one, so LPL II means a class II LPS. Class I is the strictest: the smallest rolling sphere, the finest mesh and the closest down conductors.

Which risk components does an LPS reduce?

In IEC 62305-2:2024 the probability P_LPS from Table B.3 multiplies three components from a direct strike to the structure: R_AT, injury from touch and step voltages, by equation (B.2); R_AD, injury from a direct strike to people on the structure, by equation (B.3); and R_B, physical damage, by equation (B.4). The other components, mainly from surges, need other measures such as coordinated SPDs.

Why does the selector ask for R_B with P_S = 1?

Table B.4 of IEC 62305-2:2024 gives P_S, a credit for the structure itself, but its NOTE 1 says that once an LPS to IEC 62305-3 is installed, P_S is taken as 1 and the benefit is carried by P_LPS alone. So the figure the selector scales by P_LPS must be the one with P_S = 1, or the LPS would be credited twice.

Can I use a different tolerable risk?

Yes. Clause 7.3 NOTE 1 of IEC 62305-2:2024 gives 1 × 10⁻⁵ per year as representative, and a national or local authority can fix another value. Enter the value that applies to your structure. A full Lumex™ assessment resolves it from the jurisdiction you choose.

Does an LPS reduce the frequency of damage F?

No. The frequency of damage F of clause 9.3 counts failures of internal systems from surges, and an LPS on its own does not lower the components it is built from. A structure can meet R_T with an LPS and still fail F_T, which needs surge protection or shielding. This selector judges R only.

Is this the same answer as a full Lumex™ assessment?

The values are the same: the selector reads Table B.3 and the Part 3 class parameters from the same rows the Lumex™ engine uses. The method is simpler. It credits the LPS only through P_LPS, so it never picks a weaker class than the full assessment would, but the full assessment can find a cheaper mix of measures and also judges F.

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