Lightning Risk Assessment — is protection required?
The risk assessment of IEC 62305-2: the risk of losing human life (R1), of losing a service to
the public (R2), of losing cultural heritage (R3) and of economic loss (R4), each compared with
the tolerable risk of Table 4. Answer in plain language — what the building is, what is
around it, what comes into it and who is inside — and the tool picks the coefficients out
of the standard's own tables and shows you which row it used. The collection area comes from an
outline you draw or type, as A.2.1 permits, instead of forcing the building into a rectangle.
Sets the environmental factor CE of the lines, Table A.4.
2 The site — where it is, and what stands on it
In the order the corners go round. Three or more.
Decimal degrees, or degrees minutes seconds. A place name goes through OpenStreetMap.
snap
Corner coordinates, rows of blocks, georeference
Decimal degrees, the form Google Maps takes: latitude, longitude.
For a ground-mounted PV plant the assessed structure is what the plant covers: the blocks set
the extent, and the outline follows it. Each block is drawn to scale so the drawing in the
report is the plant, not a sketch.
With an origin set, the corner table can be typed and read as latitude and longitude.
Distances are computed on the local tangent plane about that origin.
How exposed this place is
NG = 0,1 × TD. The standard gives this for temperate regions only.
Only these two turn a quoted density into a checkable one — IEC 62858
Formula (1). Leave them empty and the check is reported as not made, never as passed.
Lightning density around the site
Flash-rate climatology from NASA GIBS, centred on the site. Its units are its own:
flashes per km² per year of total lightning, in-cloud and cloud-to-ground
together, while NG in IEC 62305-2 A.1 counts flashes to ground.
Read it for the picture and the order of magnitude. The figure that goes into the
assessment is your national annex map, or the thunderstorm-day route above, or this
one once you have applied a cloud-to-ground fraction and said where it came from.
Drawing the MV station on the plan does not answer this step, which is why it is still here:
the line-borne components RU, RV, RW and RZ are
typically half of R1 — in the standard’s own office example RV alone is 50 %
— and they depend on whether the cable is buried or overhead, whether its screen is bonded,
and what the equipment withstands. None of that is on a drawing.
Every buried or overhead service that enters brings a surge with it. A structure with no external
line at all is a case in itself — remove both rows and only the flashes to the structure are
counted.
4 The zones, and who is in them
A zone is a part of the site with one kind of floor, one fire risk and one group of people —
the yard outside, the offices, the archive, the server room. Pick the nearest description and the
coefficients follow; open adjust the details in any zone to overrule them.
The free plan assesses one zone; several zones are a PRO feature.
Leave blank to use the sum of the zones.
Building, contents, internal systems and animals together. Needed for R4 only.
5 What protection is already there
Probability PB, Table B.2.
Probability PEB, Table B.7. An LPS to IEC 62305-3 includes this.
Probability PSPD, Table B.3.
Table B.7 gives 0,005 to 0,001 for SPDs better than LPL I and no single value.
Table B.3 gives 0,005 to 0,001 for SPDs better than LPL I and no single value.
KS1 = 0,12 × w, equation (B.5), with w the mesh width in metres.
The risk, against the tolerable risk
Figure 1 — site layout and lightning collection area
The drawing the collection area was measured from, as it goes into the report. A line drawing,
not the imagery: what matters here is the geometry that produced AD.
Where the risk comes from
What would bring it below the tolerable risk
Is the protection worth its cost — Annex D
R4 does not decide anything on its own: the standard compares the annual cost of the loss with the
annual cost of preventing it. Enter the rates and the price of the measures.
The derivation, step by step
Where every number came from
Every input that is not a coefficient of the standard: the flash density, the coordinates, the
imagery the outline was traced over, the station records that were counted. With the licence,
the method, the period and the distance, so a reviewer can go and check each one.
The tables these numbers came from
Validation against the worked examples of the standard
Annex E of IEC 62305-2 works three cases through and prints every intermediate quantity. The engine
in this page is run against all three, in your browser, every time this page loads.
Scope and limits
This is the risk assessment of IEC 62305-2. It says whether protection is required and which
components dominate; it does not design an air-termination system, a down-conductor system or an
earth termination — that is IEC 62305-3 — and it does not select SPDs, which is
IEC 62305-4 and IEC 61643.
The ground flash density is never assumed. Take it from the flash density map of your national
annex or from a lightning location network, and record which one in the report.
The withstand voltage UW is equipment data. Tables B.8 and B.9 are tabulated at 1,
1,5, 2,5, 4 and 6 kV; use the rated impulse withstand voltage of the equipment actually
installed.
The loss values of Annex C are typical mean values. Where a national annex or the client sets
different ones, overrule them — the standard says the values are typical, not mandatory.
The collection area of a strongly concave outline is over-stated by the offset band, because
the band overlaps itself in a notch. That over-states the risk, which is the safe direction; for
a re-entrant plan, assess the wings as separate structures if the margin matters.
A hazardous-area classification (the explosion rows of Table C.5) is never chosen for you.