Lightning protection for telecom sites

How often a structure is struck depends on its collection area, the patch of surrounding ground from which it intercepts strikes that would otherwise have reached the earth nearby. That area is not set by the footprint alone; it grows strongly with height. A tall mast reaches up into the path of strikes that would have landed well away from its base, so it draws them in from a much wider circle than a low building of the same plan covers. The taller the structure, the larger that circle and the more strikes it gathers in a year.

This is why a telecom tower behaves so differently from the buildings near it. A mast that stands tens of metres above flat ground, or a tower on top of an already tall building, can be struck many times more often than the low structures around it, which sit in its shadow. The assessment starts from this: the height of the tower drives the expected number of dangerous events, and that number is the first input that pushes a telecom site toward needing protection.

For how the expected number of strikes is built from the structure and its surroundings, see the IEC 62305-2 risk method, and for the damage and loss model behind it see what is IEC 62305.

IEC 62305-2:2024 gives every assessment two outputs. The combined risk R is about injury to people and physical damage. The frequency of damage F covers the availability of the internal systems. The 2024 edition names three kinds of loss, L1 to L3: injury to human beings, physical damage to the structure and its contents, and failure of internal systems. The 2010 edition counted economic loss as a fourth kind, which the 2024 edition no longer lists. A telecom site turns the usual order around. Nobody lives in an equipment cabin, and riggers climb the mast only on planned visits, so the people side of R rarely sets the answer.

What a carrier loses in a strike is coverage. A base station that drops off air takes calls, mobile data, emergency calls and the backhaul for any sites chained behind it down with it. That is why the frequency of damage F is normally what settles a telecom study. The 2024 edition says F can cover pure economic loss and loss of service (clause 5.3). How often a strike would put the site off air is the count that the cost of a truck roll to a remote hilltop, the lost traffic and any credits owed to tower tenants all hang on. R still has to be calculated and still has to pass, because riggers and technicians do work on the mast and in the cabin. A hospital sits at the other end: there, harm to patients decides almost everything.

IEC 62305-2:2024 (Ed.3) clause 5.2 groups what a strike can do into four causes of damage: shock to people from a direct flash (D1D) or from touch and step voltages (D1T), physical damage such as fire or mechanical destruction (D2), and the failure of electrical and electronic systems caused by the lightning electromagnetic pulse, or LEMP (D3). In a cabin full of radio units, baseband and DC power plant, D3 is the cause of damage that decides the outcome, so the protection is planned around it.

The mechanism is direct and severe. A strike to the mast drives a large current down to earth a few metres from the cabin. That current radiates a strong electromagnetic field, induces surges on the antenna feeders and the cables running into the cabin, and momentarily lifts the local earth potential. Electronics that were never touched can fail from any of these, because their tolerance is a small fraction of what the strike delivers.

The assessment gives credit for the measures that break those paths. Power surge protective devices at the mains intake and the site distribution board, and coaxial and data protectors where the feeders and copper lines enter, take the surge off the conductors before it reaches the radios. Earthing kits bond the feeder screens to the mast and again at the cabin entry, so current on the screens goes to earth outside instead of travelling in. Feeders run down the inside face of the mast and in a closed, earthed metal tray pick up less of the field. And a lightning protection zone (LPZ) plan treats the cabin as an inner zone, with every conductor protected where it crosses the wall, so the equipment sits in a much quieter electrical environment than the mast outside. IEC 62305-4 describes how these measures combine against LEMP, and IEC 62305-2 reflects them as a lower probability of internal-system failure.

For the device classes and how they are graded, see SPD types under IEC 62305. Carriers also apply the ITU-T K-series recommendations on protecting telecommunication lines and equipment; they sit beside the lightning standards rather than replacing them.

When lightning strikes the tower, the current it pushes into the ground briefly raises the local earth potential by a large amount. If the tower, the equipment, the cabin and the incoming lines are not all tied to one earthing system, that rise shows up as a dangerous voltage difference between them, and the surge looks for a way across, often straight through the equipment that bridges two differently referenced earths. A telecom site survives a strike by making sure there is no such difference to exploit.

The answer is a single bonded earthing system with a low impedance to true earth. The tower base, a ring earth around it, the cabin earth bar, the cable trays, the feeder screens and every incoming service are all bonded to one earth, so when a strike lifts the potential the whole site rises and falls together and the difference across the electronics stays small. A sound tower earth is the foundation everything else builds on: the surge protective devices have a dependable reference to clamp against, the bonding has somewhere to take the current, and the zone plan has a consistent ground to work from. Get the earthing wrong and the rest of the protection has little to work with.

For how the earthing and bonding are built and why a single low-impedance system matters, see earthing under IEC 62305.

Besides the strikes its height invites, a mast site takes in surges along every metal conductor that reaches it. Most sites have a mains supply and many still have copper lines. Others reach the network over fibre, which carries no surge when the cable has no metal strength member or armour, or over microwave, whose feeders run down the mast itself. Rural and hilltop sites are usually fed by long overhead power lines across open ground, which is the worst case.

How much a line adds depends on how long it is and how it runs. A long overhead feed exposes far more ground to nearby strikes than a short buried one, and a cable with a bonded metal screen passes on less of a surge than an unscreened one. These are the inputs behind the line components of the risk. On a remote site with kilometres of overhead mains, the mains feed alone can rival the direct strikes to the mast, so the assessment needs the real length of each service, whether it runs overhead or buried, its screening, and the surge protection fitted where it enters.

In the United States the lightning protection standard is NFPA 780, and in Australia it is AS 1768. Both keep the older way of counting loss that IEC 62305-2 dropped in 2024: loss of service is a risk of its own, R2, judged against its own tolerable value, rather than part of a frequency of damage. Both also have provisions written for antennas, towers and communications services.

NFPA 780-2026. Section 4.16 requires a metal antenna mast or support on a protected structure to be tied into the protection system with main-size conductors and listed fittings, unless it already sits inside a zone of protection. The informative A.4.16 goes further and advises against using an antenna mast as a strike termination device at all: the antenna belongs inside a zone of protection, isolated from the system, with communications cables kept away from lightning conductors. Section 4.13.3 brings communication and antenna grounding electrodes into the common bonding with the lightning protection and electric service electrodes. For surges, Section 4.19.4.1 requires surge protectors on every signal, data and communications system that enters or leaves a structure, unless the service provider has already provided for lightning surges (4.19.4.1.1), and A.4.19.4.1 counts antennas as part of those services. A line that leaves one building to supply another over a run of more than 100 ft (30 m) needs protectors too (4.19.4.2). Under 4.19.4.3 each protector needs an Imax rating no lower than 10 kA 8/20 µs, and it must not be grounded through a down conductor (4.19.4.7). Power services get SPDs at, or right beside, the service equipment (4.19.2.1, 4.19.2.1.1).

Annex L, the informative risk assessment, lists telecommunications facilities among its critical facilities (L.5.3.1). L.5.3.2 says a critical facility should either have a lightning protection system to the standard or a detailed assessment under L.6, so the L.5 simplified risk assessment alone does not settle a telecom site. In the detailed method, loss of service is R2, made up of six components (L.6.5): physical damage from strikes to the structure or a service (R_B, R_V) and failure of internal systems from all four sources (R_Z, R_W, R_M, R_C), with a default tolerable value of 10⁻³ per year (Table L.6.2). Section 1.7 calls for inspection or testing every year, or at intervals the authority having jurisdiction sets. More detail is in NFPA 780 surge protection and NFPA 780 Annex L.

AS 1768:2021. Clause 3.3.3(e) treats a metal communications tower as a natural downconductor that needs no extra downconductors. Appendix I, which is informative, recommends that antennas outside the zone of protection have their structure bonded to the protection system. Where a communications tower is a main feature of the site, it recommends bonding the antennas to the tower, bonding the coaxial screens where they leave the tower base and again where they enter the building, and fitting SPDs on the coaxial lines at the point of entry (I.2.1). I.2.3 lets the earth termination of the protection system double as the radio earth, with every earthing system bonded together. Appendix G, also informative, works through a rooftop cellular base station in G.5.3 (Figure G.8): the steel tower is self-protecting and bonded to the building system, the tower and cabin are treated as one unit, the mains gets an SPD at the main switchboard and a second at the rooftop board, every other metal service is bonded and protected, and all services come into the cabin at one point. A note to G.5.2 sends telecommunications SPDs on customer cabling to AS/CA S009, and Clause B.1 points to the ITU-T K series for protecting telecommunication networks.

Case study 6 (C.3.9) is a mountaintop broadcasting site with a steel transmitting tower. Its assessment calls for no direct strike protection, given an electrically continuous tower on a proper earth, but it does call for surge protection at the service entrance. The text adds that a site like this probably also warrants secondary protection, that the antenna feeders should carry earth bonding kits, and that coaxial SPDs should be considered. Note b to Table 4.1 suggests a primary SPD with a higher rating at mountaintop telecommunication sites fed by long overhead power lines, so it lasts longer. Loss of service to the public (R2) applies to public utilities including telecommunications (B.2.2), with a tolerable value of 10⁻³ per year in Table 2.1. Clause 5.4.4 asks for inspection at least every two years, and a note to 5.4.3 lists fitting communication antennas as a change that calls for the protection to be reassessed. More detail is in AS 1768 surge protection and AS 1768 earthing.

Lumex^™ sets up a telecom site as a structure with its mast height, its zones, each incoming service with its length, routing and screening, and the protection fitted. It runs the risk assessment of IEC 62305-2:2024, NFPA 780-2026 Annex L or AS 1768:2021, whichever the site's market works to, and shows the working behind every figure. Raise the mast, add a copper line or swap an overhead feed for a buried one, and the result moves in front of you, which is how an engineer finds the one measure a marginal site needs. Every value carries the clause it came from, so an operator, a tower owner or an insurer can check the reasoning. Repeating the same model across hundreds of sites gives a network one consistent basis, much as a renewable energy portfolio of scattered structures is assessed. Lumex^™ runs and reports the assessment; the design of the earthing, bonding and surge protection stays with the engineer.

For background, read what is IEC 62305 or NFPA 780 vs IEC 62305, or see how the Lumex^™ platform takes a site from inputs to a signed report.