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.
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.
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.
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.
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:
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.
How often these happen depends on three things: the protection level the system was built to, the environment it sits in, and the consequences of it failing. A harsh, corrosive or high-consequence site is inspected more often than a benign one. A yearly cycle is common in practice, but the interval is set by the standard against those factors rather than fixed at a single number. On top of the periodic cycle, a system is always inspected at three points: after it is first installed, after any significant change to the structure or its services, and after a known lightning strike, because a strike can damage the very system meant to protect against the next one.
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.
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.