IEC 62305-3: the lightning protection system and its inspection
Part 3 is where lightning protection becomes physical: the air terminations, down conductors and earthing that intercept a strike and carry it safely to ground, the classes that grade how strong that system is, and the periodic inspection that proves it still works years after install.
IEC 62305-3 is the part of the standard that turns a protection decision into hardware.
Part 2 decides whether a structure needs lightning protection and how strong it must be. Part 3 is where that protection is designed, built, classified and, just as importantly, inspected over its whole life. It also carries the human side of the standard: protecting people from the touch and step voltages a strike can create near a building. It sets what a lightning protection inspection covers and how its interval is chosen, and a later section gives the US and Australian rules under NFPA 780-2026 and AS 1768:2021.
The protection it describes is the lightning protection system, almost always shortened to LPS. An LPS has two halves. The external part intercepts a direct strike and carries its current safely to earth. The internal part stops the very high voltage that current creates from arcing across to metalwork, pipes and wiring inside the building. This guide walks through both halves, the three ways air terminations are positioned, the four classes that grade how strong a system is, the protection of life near down conductors, the role of earthing, and the periodic inspection and report that keep an installed system valid.
The physical system, and the protection of life
Part 3 has two jobs that are easy to treat as one but are worth separating. The first is to protect the structure: to catch a strike at a controlled point and conduct it to ground so it does not start a fire, blow apart masonry or punch through a roof. The second is to protect people: a strike sends a large current down the structure and into the soil, and for a fraction of a second the conductors and the ground around them sit at a dangerous voltage. Someone touching a down conductor, or simply standing near where the current enters the earth, can be hurt by that voltage even though the strike never touched them.
A well-designed LPS answers both at once. The same conductors that carry the current to ground are positioned, bonded and, where needed, insulated so that the structure survives the strike and nobody nearby is exposed to a hazardous touch or step voltage. The rest of this guide takes the two halves of the system in turn, then turns to keeping them sound.
Catching the strike and leading it to ground
The external lightning protection system has three parts that work as one chain: catch the strike, route the current down, and dissipate it into the soil. A weakness anywhere in the chain undoes the whole.
Where the strike is caught
Rods, masts, catenary wires or a mesh of conductors placed on the roof and exposed edges so that a strike attaches to the system rather than to the structure itself. Its job is to give lightning a preferred, controlled point to hit. Get the positioning wrong and a strike can bypass it and reach an unprotected corner.
Where the current travels
Conductors that carry the captured current from the air termination down to the earth termination. They are spread around the perimeter so the current divides between several parallel paths rather than concentrating in one, which lowers the voltage on each and reduces the magnetic field inside. Closer spacing means a safer, quieter descent.
Where the current goes
The electrodes, rings or foundation earth that dissipate the current into the soil. This is where the energy finally leaves the building, so a low and stable earth resistance matters: the faster the current spreads away, the smaller the rise in potential and the lower the hazard to anyone nearby.
The three ways to position the air termination
IEC 62305-3 gives three methods for deciding where air terminations go, so no part of the structure is left exposed. They can be combined on one building, and each tightens as the LPS class becomes more demanding.
Rolling sphere
Picture a sphere of a fixed radius rolled over and around the structure. Wherever it touches, a strike could land, so air terminations must be placed to keep the sphere off every vulnerable surface. The radius shrinks for higher protection, which is the same as saying the protection must catch weaker, closer strikes that a larger sphere would have rolled straight past. It is the general method and works on any shape.
Mesh
A grid of conductors laid across a roof so that any strike is caught by the nearest mesh wire and shared between the conductors around it. It suits flat and gently pitched roofs. The grid is made finer for a higher class, so a more demanding system has a tighter mesh and less roof area between conductors.
Protection angle
A rod or mast protects a cone of space beneath it, bounded by an angle from the vertical. Anything inside the cone is considered shielded. The angle narrows with height and with a higher protection class, which is why tall masts protect a smaller footprint than their height alone suggests. It suits simple shapes and individual rooftop items.
The exact sphere radius, mesh dimension and angle for each class are set out in tables in the standard itself. The principle to carry away is that they all move in the same direction: a more demanding LPS class uses a smaller sphere, a finer mesh and a narrower angle, because it has to catch a wider range of strikes.
Bonding and separation distance
Carrying a strike to ground is only half the problem. While the current is flowing, the external system sits at a very high potential relative to everything around it. If a metal pipe, a cable tray or a piece of structural steel runs close to a down conductor, that potential can jump the gap as a spark. Inside a building that holds fuel, gas, dust or sensitive electronics, such a spark is exactly the event the whole system exists to prevent.
The internal LPS handles this in two complementary ways. Equipotential bonding ties the external system, the structural metalwork, the incoming services and the earthing together so they rise and fall in potential as one. If everything is at the same voltage, there is no gap for a spark to cross. Where a service cannot be bonded directly, a surge protective device bridges it during the strike and opens again afterwards. Separation distance handles the metalwork that must stay electrically isolated: the standard sets a minimum distance that keeps an unbonded metal part far enough from the LPS that the potential cannot arc across. Keep that distance, or bond the part. Doing neither is where dangerous sparking begins. The separation distance calculation guide sets out how the distance is worked out, and the free separation distance calculator checks a measured gap against it.
Classes I to IV, and how they follow from the risk assessment
An LPS comes in four classes. They are not four products to choose between on taste; the class is decided for you by the protection level the Part 2 risk assessment arrived at.
The Part 2 risk assessment ends by choosing a lightning protection level, LPL I to IV, that brings the computed risk below what is tolerable. That level maps straight onto an LPS class: LPL I needs a Class I system, and so on down to LPL IV and Class IV. The class is therefore an output of the risk work, not a separate decision. What changes between classes is how hard the system tries.
Class I is the strongest
It uses the smallest rolling sphere, the finest mesh, the closest spacing of down conductors and the largest separation distance. It intercepts the widest range of strike currents, so it is specified where the consequences of a miss are most severe, for example high-risk industrial or life-critical sites.
Class IV is the least demanding
A larger sphere, a coarser mesh and fewer down conductors. It is specified where the risk assessment shows a lighter system is enough to bring the risk into line. A higher class is not better in the abstract; it is only correct where the risk calls for it.
Because the class carries through into the geometry of the whole system, an inspection cannot be judged against a generic checklist. It has to be judged against the class the structure was designed to, which is why the original design and protection level travel with the system for its whole life.
Touch and step voltages near down conductors
When a strike runs down a conductor, the conductor briefly sits at a high voltage and the ground around the earth electrode does too. A person touching or standing beside the conductor can be exposed to a touch voltage, between their hand and their feet, or a step voltage, between two feet a stride apart on ground that is at different potentials. In places where people gather close to down conductors, an entrance, a walkway, a seating area, this hazard has to be managed deliberately rather than left to chance.
IEC 62305-3 gives several recognised measures, used alone or in combination:
Insulation
Cover the exposed down conductor in insulating material so the dangerous potential cannot reach a hand resting on it.
Physical restriction
Keep people away from the conductor with barriers, fencing or landscaping, so nobody is within reach of it during a strike.
Equipotentialisation
A meshed earthing arrangement near the surface so the ground rises in potential evenly. If the soil under both feet moves together, the step voltage between them stays small.
Warning signs
Where the hazard cannot be fully removed, clear signage tells people not to shelter against a down conductor during a storm.
Earth resistance and why it is read every time
The earth termination is where the strike energy finally leaves the structure, so the quality of the earthing decides how cleanly the whole system performs. The figure that matters most is the earth resistance: how easily the electrode lets current spread away into the soil. A low, stable resistance means the current dissipates quickly, the rise in potential at the electrode stays modest, and the side-flashing and step-voltage hazards near it stay small. A high resistance does the opposite, holding the structure at a higher potential for longer.
The reason earth resistance is measured at every inspection, rather than once at install, is that it drifts. Electrodes corrode, joints loosen, soil dries out in a long summer or is disturbed by construction nearby, and any of these can push the resistance up without anyone noticing. A single good reading on the day of handover is no guarantee of a good reading three years later. Trending the value across inspections is what catches a slow degradation before it becomes a real loss of protection.
Why periodic inspection exists
A lightning protection system is correct on the day it is signed off and then spends years exposed to weather, corrosion and the ordinary churn of building work. A bond works loose. A down conductor corrodes where it meets the ground. A new rooftop plant unit is installed without extending the air termination over it. A renovation breaches a separation distance behind a wall. None of these announces itself, and any of them can quietly leave a structure less protected than its paperwork claims. The system fails silently, and the failure is only discovered when a strike finds the gap.
IEC 62305-3 answers this with a regime of periodic inspection and testing. There are two depths of check. A visual inspection confirms that the system is intact, that nothing is corroded, loose, disconnected or obscured by new construction, and that no recent change to the building has compromised it. A fuller complete inspection adds measurement and testing: continuity along the conductors, the earth resistance reading, and a check that bonding and separation distances still hold. The two alternate through the maintenance cycle, with the lighter visual check between the deeper ones.
Clause 7.4 sets out when an inspection is needed rather than how often. Inspections should be made during construction, to check components that will be concealed and become inaccessible; after the LPS is installed; periodically, on an interval set by what kind of structure is being protected; after alterations or repairs; and once a strike on the building is known about. That last one matters more than it sounds: a strike can damage the very system meant to protect against the next one, often in ways nobody can see from the ground.
Inspection and testing intervals
The intervals below come from clause D.7.2.2, in Annex D, which is informative. They are frequencies the standard says can be selected, not durations it requires. The binding rule is the one in IEC 62305-3:2024 (Ed.3) clause 7.4: what kind of structure is being protected is what sets the periodic interval.
1 year visual, 3 years complete
Annex D offers a visual inspection every year and a complete inspection, electrical testing included, every three years. Drop below one visual check a year and the complete inspection moves to every two years, so skipping the light check shortens the deep one. (D.7.2.2)
6 months visual, 1 year complete
Where the LPS sees severe weather excursions or extreme conditions, for example flexible bonding straps in high wind, SPDs on pipelines, outdoor cable bonding and earthing systems, Annex D puts visual inspection at six-month intervals with a complete inspection and testing once a year. (D.7.2.2)
Yearly visual and yearly testing
Structures with a risk of explosion, the ones Annex C sets extra requirements for, get a visual inspection every year, with the installation electrically tested on the same yearly cycle. Clause C.5 also points at IEC 60079-17 for further inspection guidance. (D.7.2.2, C.5)
Complete inspection every year
A building holding sensitive internal systems, an office block, a commercial building, or anywhere large numbers of people gather, can warrant a complete inspection every year, electrical testing included. The internal protection measures, bonding included, can be assessed at the same time. (D.7.2.2)
The 14 or 15 month cycle
Annex D allows testing on a 14-month or 15-month cycle instead of a strict yearly one, so that earth testing lands at different times of year and reveals seasonal variation. It is the deliberate answer to a site where rainfall or temperature swings move the earth resistance reading. (D.7.2.2)
Five factors, not a class
Five things set the interval: what the building is used for and what damage to it would cost, the materials the LPS components are made of, the surface they are fixed to, the local environment, and the soil. Corrosive atmospheres, freeze and thaw cycles, heat extremes, corrosive soils and large swings in soil moisture all shorten it. (D.7.2.2)
Two triggers sit outside the cycle entirely. Inspect the LPS after any significant alteration or repair to the building it protects, and after a strike on it that is known or suspected. A strike can be spotted by eye, counted by a lightning strike counter, reported from nearby, or read off a location network that meets IEC 62858. And where one inspection to the next shows the measured earth resistance moving further than the design allowed for, rising in particular, the standard says to look at improving the earthing. (D.7.2.2)
Lightning protection inspection under NFPA 780 and AS 1768
The intervals above are IEC guidance. A system built to NFPA 780 in the US or AS 1768 in Australia is inspected to that standard's own rules, which differ in both period and status.
| Topic | NFPA 780-2026 | AS 1768:2021 |
|---|---|---|
| The interval the standard sets | Inspection or testing every year, or on the interval the authority having jurisdiction (AHJ) sets. A requirement (Section 1.7) | Inspect at least every two years, more often where location calls for it, such as a marine site with fast corrosion (5.4.4 and NOTE 1) |
| Testing | Annex D guidance: a complete inspection every 3 to 5 years, every 1 to 3 years for critical systems (D.1.1.2) | Test the system at least every two years for its whole service life (5.2) |
| Visual checks | Annex D guidance: at least once a year, twice a year where the climate swings hard (D.1.1.2) | A visual examination for integrity, corrosion, secure fixings, burning or pitting, joints, SPDs and strike counters (5.4.4) |
| Event triggers | After alterations or repairs, and after a known strike (D.1.1.1) | After alterations to the LPS or the structure, and after a strike (5.4.4) |
| Earth readings | Annex D guidance: compare ground resistance with earlier results and investigate a large change (D.1.3); test in both dry and wet seasons (D.1.1.3) | Network under 10 ohms, single electrodes may read higher; repeat tests when readings vary significantly (5.2) |
| Explosive contents | Explosives facilities: visual inspection at least every 7 months (8.9.6), electrical testing no more than 14 months apart (8.9.7) | Intervals set so the system is proven under its worst conditions, typically 12 months for a new installation up to 24 months once results justify it (J.4 and NOTE 1) |
The status of each rule matters. NFPA 780-2026 Section 1.7 sits in the body of the standard, so a yearly inspection or test is a requirement unless the AHJ sets another interval. The 2020 edition left the interval to the AHJ alone. Annex D is informative: its 3 to 5 year complete inspection is advice on how deep to go, not a second requirement. Annex D also suggests what an inspection looks at: repair, loose joints, corrosion, intact down conductors and grounding electrodes, secure fastening, changes to the structure, and SPD damage or dead status lights (D.1.2). For a working list, read the NFPA 780 inspection checklist.
AS 1768:2021 Section 5 words its intervals as recommendations, with firm rules around them. Testing has to happen at installation and through the service life (5.2), the LPS has to keep its physical and electrical characteristics (5.4.1), and every defect found has to be recorded and put right (5.4.5). The maintenance program adds earth resistance measurement and the reading of SPD status indicators (5.4.2), and the records list runs from drawings and test results to SPD indicator readings and the installer's details (5.3). An alteration or extension is a reason to reassess how well the LPS still protects, not only to inspect it (5.4.3). For how the standard fits together, read AS 1768 explained.
What is measured and recorded
A complete inspection is a structured set of checks, each producing evidence that goes into the report. Nothing is judged by eye alone where it can be measured.
Continuity
The conductors are tested end to end to confirm the current still has an unbroken path from air termination through the down conductors to earth. A corroded or fractured joint that looks fine can read as a break.
Condition and corrosion
Every component is examined for corrosion, mechanical damage, loose fixings and signs of a past strike, especially at fixings, joints and the point where conductors meet the ground.
Earth resistance
The earth termination is measured and the value recorded so it can be trended against earlier inspections, catching a slow rise before it matters.
Bonding and separation
Bonds are checked as present and sound, and the separation distance to nearby metalwork is verified against the design, since later fit-out is where it most often breaks.
Defects with photos
Every fault is documented with a photograph and a location, so the owner has unambiguous evidence and a precise list of what to repair rather than a vague note.
Match to design
The findings are checked against the original design and the protection level the risk assessment called for, confirming the system as built and as found still delivers the class it was specified to.
The inspection report and the maintenance that follows
An inspection ends in a dated report. It names the inspector, sets out the condition of each part of the system, gives the continuity and bonding results, records the earth resistance reading, lists every defect with its photograph and location, and states plainly whether the system still matches its design and protection level. Where it does not, the report lists the remedial work that brings it back into compliance, so the owner leaves with a list to act on rather than a verdict to interpret.
The report is not a one-off. Read in sequence, the reports from successive inspections are the audit trail of the system: they show that it has been maintained, how the earth resistance has trended, when defects were found and when they were fixed. That trail is what an auditor, an insurer or an authority asks for when they want proof the protection is real and not just installed. The maintenance that follows each report, repairing the defects it lists, closes the loop, so the next inspection starts from a sound system rather than carrying old faults forward.
Questions answered
What is IEC 62305-3?
How often do NFPA 780 and AS 1768 call for a lightning protection inspection?
Who should carry out an IEC 62305-3 inspection?
What are LPS classes I, II, III and IV?
How often should a lightning protection system be inspected?
What is measured during a lightning protection inspection?
How are concealed parts of a lightning protection system inspected?
What is in a lightning protection inspection report?
Does a building need a fresh inspection after a lightning strike?
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. The standard's introduction then lets national or local regulations fix
RT, the tolerable frequency of damage FT
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