What is a lightning protection system?

Every lightning protection system has two parts that work together, and it helps to keep them distinct because they defend against different things.

The external LPS catches the strike and gets rid of it. Its job is to give the lightning current a deliberate, low-impedance path so it does not find its own through the structure, its occupants or its wiring. It does this with three components in series: an air-termination system that intercepts the strike, down conductors that carry the current down the outside of the structure, and an earth-termination system that disperses it safely into the ground.

The internal LPS stops dangerous sparking inside. When a huge current flows down the external system, it raises the voltage of everything connected to it and can flash over to nearby metalwork or services, a real fire and shock hazard. The internal LPS prevents this in two ways: equipotential bonding, which ties the LPS, structural metalwork, pipes and incoming services together so they rise and fall in voltage together rather than sparking between each other, and a maintained separation distance, which keeps the LPS far enough from internal conductors that a spark cannot bridge the gap.

Knowing the six parts above is not the same as being able to specify them. IEC 62305-3:2024 (Ed.3) gives clause 5.5 to the components themselves, and the requirement there is stricter than most specifications assume. It names the parts one by one: conductors and air-termination rods, earth electrodes and their seals, the inspection houses over them, connection components, conductor fasteners, isolating spark gaps, earthing enhancing compounds, lightning strike event counters, and the components used in isolated systems. Every one of them has to take the electromagnetic forces a lightning current brings, and the stresses the design expects, undamaged. The standard says how that is proved: use components tested successfully against whichever part of the IEC 62561 series applies (5.5.1).

That single sentence is what separates a compliant LPS from an assembly of parts that merely look right. A conductor of the correct material and cross-section, joined by a clamp that was never tested to IEC 62561, is not a compliant system. The same clause pins the rest of the chain. Components are made from one of the materials Table 6 lists, which covers copper, hot galvanized steel, steel with electro-deposited copper, stainless steel, aluminium and lead. Another material is allowed where it matches those on mechanical strength, electrical performance and resistance to corrosion. A conductor fastener has to meet IEC 62561-4 (5.5.2). An insulating part, a stand-off or an insulated down conductor, has to meet IEC TS 62561-8 (5.5.4).

Connections carry a requirement that depends on the class, which is easy to miss when ordering. The number of connections along a conductor shall be kept to a minimum, and connection components are selected by their tested class under IEC 62561-1: class H for LPS class I and II, and class N for LPS class III and IV (5.5.3). So the class the risk assessment arrived at does not only set the geometry of the system, it reaches all the way down to which clamp is acceptable on the shelf.

The single most important thing to understand about a lightning protection system is that you do not start by choosing one. You start by assessing the risk. Part 2 of IEC 62305 models the structure, its surroundings and the services connected to it, computes how often a strike is likely to cause real harm, and compares that against the level of risk that can be tolerated. Only if the computed risk is too high is an LPS specified at all.

The assessment does not just answer yes or no. It tells you which protection measures bring the risk back into line and how efficient they must be, which is exactly what sets the LPS class. A modest structure may need only a class III or IV system, or none; a data centre or petrochemical plant may need class I plus coordinated surge protection. This is why two similar-looking buildings can warrant very different systems. The risk, not the appearance, decides. For a worked, clause-by-clause walk-through, see how an IEC 62305 assessment works.

An LPS protects nothing on paper. Once designed to the right class, it has to be installed correctly and then stay in that condition for the life of the structure. Lightning protection is exposed to weather, corrosion and mechanical damage, and buildings change. A new rooftop plant, an extension or a severed conductor can quietly undermine a system that passed on commissioning.

For that reason IEC 62305-3 requires an LPS to be inspected and tested periodically, with the interval set by the class, the environment and the importance of the structure. Inspection checks the physical integrity of air terminations, down conductors and bonds; testing confirms the earth resistance is still within limits. An LPS is only valid while that inspection and testing remain current, which is why maintenance is part of the system, not an afterthought.

One point of confusion is worth settling. IEC 62305 specifies a conventional lightning protection system: passive air terminations (rods, mesh, catenary wires) positioned by the rolling-sphere, mesh and protection-angle methods. The protection it claims is backed by the standard's geometry and risk method.

Early streamer emission (ESE) air terminals are a separate approach, claiming a larger protected volume from a single mast. ESE is covered by national standards such as the French NF C 17-102, not by IEC 62305, and its enhanced range is contested. If a project is specified to IEC 62305, its LPS is the conventional system described above; ESE is a different method that should not be conflated with it. For the detail and where each applies, see ESE air terminals and IEC 62305.