BS EN 62305 in the UK
What the British Standard is, how it relates to the IEC original, what the UK national annexes add on top, and what the March 2025 corrigendum did and did not change.
BS EN IEC 62305-2:2024 is the UK implementation of EN IEC 62305-2:2024, and its national foreword states that it is identical to IEC 62305-2:2024. The engineering does not change at the border. What the UK adds sits in a national foreword and in national annexes at the back.
That single sentence answers most of what a UK engineer wants to know, and it is the part most pages about BS EN 62305 get roughly right. The part they get wrong, or leave out, is what the UK actually contributes: three national annexes, a UK ground strike density figure, a changed parameter that moves a computed result, and a corrigendum whose cover wording suggests far more than it did.
This page is written from the BSI consolidated edition of BS EN IEC 62305-2:2024, the one marked as incorporating the corrigendum of March 2025. Every claim below names the clause, equation, table or annex it comes from, so you can check it against your own copy. It does not reproduce the standard, and Lumex is independent of BSI, CENELEC and the IEC.
How one standard picks up three prefixes
The full designation of the current risk management part is BS EN IEC 62305-2:2024, and each element of that name is doing a job. The confusion people run into is treating BS EN 62305 and IEC 62305 as two standards to compare. They are one technical text with three publishers in a chain.
IEC technical committee 81 prepared the text as edition 3 of IEC 62305-2. CENELEC put the same text to a parallel vote and approved it as EN IEC 62305-2:2024 on 17 October 2024. The endorsement notice printed in the European foreword is unusually blunt about how little was altered: the International Standard was adopted by CENELEC unchanged as a European Standard, in its words, "without any modification". BSI then published that European Standard in the UK as BS EN IEC 62305-2:2024, and its national foreword repeats the point in national terms, calling the British Standard the UK implementation of the European one and identical to the IEC one.
So BS tells you who published the copy in your hands, EN tells you it came through the European route rather than direct from the IEC, and IEC 62305-2:2024 tells you which technical text and which edition you are computing. Only the last part affects a calculation. UK participation in preparing the standard was entrusted to BSI technical committee GEL/81, Protection against lightning.
One caveat the BSI copyright page states plainly, and it is worth carrying into any conversation with a client: compliance with a British Standard cannot confer immunity from legal obligations. The standard is the method. Whether it is required of you, and what else is required alongside it, is a separate question for the code, the authority and the specification on your project.
The three national annexes
The identical technical text is followed by UK material that a UK assessment is expected to use. This is the substance of the British adoption, and it is where a UK job genuinely departs from a bare IEC one.
Probability of damage
Reproduces the IEC Annex B assessment of the probability PX of damage, with UK interpretations applied. Its clauses and equations are numbered NB.1 to NB.12 against the IEC B.1 to B.12, so a UK reference maps one for one onto the international one.
Assessment of loss
Reproduces the IEC Annex C assessment of loss LX, again with UK interpretations. Table NC.2 carries the typical mean loss values for LT, LD, LF1, LF2, LO1 and LO2 that a UK zone is assigned from.
UK ground strike density
Two maps, a south sheet and a north sheet, letting you read the ground strike point density NSG at any point in the UK. Annex NG.1 records that they were computed from three lightning location systems using ten years of data between 2013 and 2022, and fixes k at 1,7 for the UK.
The national foreword explains why they exist. It says the parameter values IEC proposes in Annexes B and C are recognised by IEC as possibly not appropriate in every country that uses the standard, that different values may be assigned by each national committee, and that the UK committee reviewed the relevant parts and provided UK interpretations in these national annexes. It then states that the national annexes should be used wherever and whenever the British Standard is adopted for designing lightning protection systems. That is not a footnote. It is an instruction about which numbers a UK assessment runs on.
Two UK differences that change a result
Saying the annexes carry UK interpretations is easy. These are two specific cases, each checkable in a single side by side comparison, where a UK value produces a different figure to the IEC one.
The ground strike point ratio
IEC equation (A.1) sets NSG = k NG and its NOTE 1 says a factor of 2 can be assumed when the lightning location system provider cannot supply k. Annex NG.1 fixes k at 1,7 for the UK. Since NSG feeds every dangerous event count in Annex A, a UK assessment on the national value lands below one that fell back to the IEC assumption.
Enhanced SPD credit
IEC Table B.13 gives the bonding probability PEB as 0,05, 0,02 and 0,01 for protection levels III to IV, II and I. UK Table NB.13 keeps those and adds starred enhanced rows a tenfold step lower, at 0,005, 0,002 and 0,001, with a note defining what qualifies as enhanced. Table NB.7 does the same for PSPD against sources of damage S1 and S3, where IEC Table B.7 leaves the equivalent cells blank.
Both belong in a report rather than in a spreadsheet nobody sees again. If you claim the enhanced PEB or PSPD rows, the note attached to Table NB.13 ties the claim to a specific property of the device, its voltage protection level relative to the equipment susceptibility level, so the assessment has to name the device and the level it was credited for. A figure that quietly took the enhanced row without recording why is not reproducible by the next person to open the file.
What March 2025 actually corrected
The BSI consolidated edition carries the line "Incorporating corrigendum March 2025" on its cover, which reads like a technical revision and is not one. The amendments and corrigenda table on the copyright page records exactly one entry. Dated 31 March 2025, its text affected reads: "Correction to supersession details in national foreword".
In other words, BSI corrected its own front matter about which earlier publications this standard replaces. No clause moved, no equation changed, no table value was revised. The British Standard itself was published under the authority of the Standards Policy and Strategy Committee on 28 February 2025, a month earlier.
The distinction that matters is this. That BSI corrigendum is not the IEC corrigendum COR1 to IEC 62305-2:2024, which is a separate document issued by the IEC. A page that treats the two as the same thing, or that presents the March 2025 date as the moment the UK calculation changed, is repeating an assumption rather than reading the document.
There is a neat piece of evidence for how narrow the March 2025 fix was. The standard carries a known indexing error: in the clause 4 list of symbols, the entry for PSPD points the reader to "Table B.8, Table B.9". The actual captions are Table B.7 for PSPD on the low-voltage system, Table B.8 for PSPD on the telecommunications system and Table B.9 for the factors CLD and CLI, and every reference in the body of the standard says B.7 and B.8. That off-by-one entry is still present in the BSI consolidated edition, at printed page 24, five months after the corrigendum. The correct tables are B.7 and B.8. If a tool or a reviewer cites B.8 and B.9 for PSPD, it followed the index rather than the standard.
Which edition governs, and until when
Two editions are live in the UK market at once during the transition, so the edition a figure was computed on is part of the figure.
BS EN 62305-2:2012 withdrawal
The national foreword states that BS EN IEC 62305-2:2024 supersedes BS EN 62305-2:2012, which will be withdrawn on 31 October 2027. It also supersedes PD 62305-2 Flash density map 2014, which is already withdrawn, which is why UK ground strike density now lives in Annex NG instead.
The CENELEC dates
The European foreword fixes the same transition in CENELEC terms across the member countries: the latest date for implementation at national level, dop, is 2025-10-31, and the latest date by which conflicting national standards have to be withdrawn, dow, is 2027-10-31. The UK dates are the national expression of these.
The 2024 edition is a technical revision of the 2010 second edition, and the changes are substantial enough that a 2012 result and a 2024 result are not comparable figures. If you need the change list rather than the UK cut, read what changed in IEC 62305:2024. The UK committee also advises in the national foreword that the third edition series should be implemented in full, warning that it would not be safe to implement Parts 1, 3 and 4 of the third edition without having done a risk assessment to BS EN IEC 62305-2:2024. Mixing editions across the four parts is the failure mode it is pointing at.
Why RT is not a constant, in the UK or anywhere
A great deal of writing about IEC 62305 states the tolerable risk as a fixed figure. The standard does not. 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, and the Annex A, B, C and E calculation rules and parameter values.
Keep those two permissions apart, because they run through different bodies. The one on printed page 12 is addressed to national or local regulations. What BS EN IEC 62305-2:2024 demonstrates is the separate mechanism described in its own national foreword, which says different values may be assigned by each national committee and that the UK committee did so, supplying UK parameter values through Annexes NB, NC and NG. A standards committee is not a regulator, so the existence of the UK annexes is not by itself evidence about what any UK regulation requires.
The practical consequence is the same either way. The tolerable value your UK assessment is judged against is something to establish and record for the project, from the code, the authority having jurisdiction and the specification that actually govern it, rather than a constant to paste in. This page does not tell you what those say, because that is not something the standard can tell you and not something we can source from it.
The same reasoning applies to the tolerable frequency of damage FT, which the 2024 edition introduced alongside the single risk R. If a report states either value without saying where it came from, the verdict it supports cannot be audited, because the threshold is half of the comparison.
The question BSI tells you to ask a tool
The national foreword contains an instruction aimed squarely at anyone buying or using risk assessment software. Having set out the UK specific values, it says it is important to ensure that any software package used in conjunction with the standard is specifically designed to use the national annexes in the document. That is a rare thing for a standard to say, and it is worth taking at face value.
It reduces to two questions you can put to any tool, ours included. Which edition does it compute, 2010 or 2024. And which annex values does it apply, the IEC informative annexes or the UK national ones. A tool that cannot answer both in writing cannot support a UK submission, because you would have no way to tell a UK figure from an international one after the fact.
Being straight about where Lumex sits on that: Lumex computes the IEC 62305-2:2024 method, which is the identical technical text BS EN IEC 62305-2:2024 carries, on the current edition, with every figure traceable to the clause, equation or table it came from. It ships a United Kingdom jurisdiction profile. Selecting it applies the Annex NB and NC values and the Table NC.2 loss classes, fixes k at 1,7 per Annex NG.1, and cites the national annex tables on the report in place of the IEC ones. The enhanced SPD and bonding tiers are not offered as a choice, because the national annex grants them on a condition rather than a preference. You record the device's voltage protection level, Lumex applies the enhanced tier only where that figure earns it, and the report prints the tier alongside the figure it rested on. One part is not automatic yet, and we would rather name it than let a UK badge imply an automation that is not there: the Annex NG ground strike density is still read from the map and entered by you.
What this means on a British project
Four practical consequences that follow from everything above, in the order they usually bite.
Your IEC knowledge transfers
The clauses, the risk components, the equations and the zoning rules are the IEC ones. An engineer who knows the IEC 62305-2 method already knows the BS EN IEC 62305-2 method, and a specification naming either points at the same calculation.
Cite the annex you used
Because NB and NC mirror B and C, a value can look identical while coming from a different table. Recording Table NB.13 rather than Table B.13, and the same for loss values, is what lets a reviewer reproduce the figure two years later.
Get NSG from Annex NG
A UK site has a published ground strike point density and a published k of 1,7. Falling back to the IEC assumption of a factor of 2, or to a global satellite estimate, when a national map exists is a weaker input that a reviewer can challenge.
State the edition in the report
Until 31 October 2027 both editions circulate. A report that says only BS EN 62305 leaves a reader unable to tell a 2012 result from a 2024 one, and the two are computed under different models.
Where to go from here
For the international picture this page is the UK cut of, read IEC 62305 around the world, which covers the other national adoptions and the recorded national differing practices for Germany, Greece, Italy, the Netherlands and South Africa. For the delta between the 2010 and 2024 editions that the 2027 withdrawal date is driving, read what changed in IEC 62305:2024.
New to the standard itself? Start with what is IEC 62305 for the four parts and the vocabulary, then the IEC 62305-2 risk assessment for the part that decides whether protection is needed. To see the arithmetic end to end on a worked building, read how an IEC 62305 risk is calculated. The SPD probabilities that Tables NB.7 and NB.13 govern are explained in SPD Types 1, 2 and 3.
An assessment a UK reviewer can follow
Lumex runs the IEC 62305-2:2024 risk assessment in full, which is the identical technical text BS EN IEC 62305-2:2024 publishes in the UK. It computes the risk R together with the frequency of damage F per risk zone from the structure, its surroundings and its services, compares each against the tolerable values you set, and prints the clause, equation or table behind every figure so a reviewer can retrace it rather than take it on trust. For a UK job you select the United Kingdom jurisdiction and its national annex parameters are applied for you, with the Annex NG ground strike density still entered by hand, and the report records what was used alongside everything else. See the Lumex platform.
Questions answered
What is BS EN 62305?
Is BS EN 62305 the same as IEC 62305?
What do the UK national annexes in BS EN IEC 62305-2:2024 contain?
What did the March 2025 corrigendum to BS EN IEC 62305-2:2024 change?
When is BS EN 62305-2:2012 withdrawn?
Does a UK risk assessment use different numbers to an IEC one?
Is the tolerable risk fixed at 10 to the minus 5 in the UK?
Does software need to be designed for the UK national annexes?
Lumex computes the IEC 62305-2 method and shows the working. It does not certify a structure. You may not issue or submit a Lumex output until a competent person, qualified where the structure is located, has reviewed the inputs and the result and signed it.
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, and the Annex A, B, C and E calculation rules and parameter values. Every Lumex assessment states the jurisdiction it was computed under and the values that applied.