Can lightning strike your fibre optic cable?

A severe storm crosses the area at night. The next morning the signal on a buried route has dropped, and in the chamber the cable’s armour, at a single point, is visibly scorched. Was it lightning? And did the protection specified in the design work, or was there none?

The instinctive answer — it’s fibre optic, glass doesn’t conduct, lightning doesn’t concern it — is only true for part of the cables actually installed. Recommendation ITU-T K.47, edition 3.0 of 29 May 2012, still in force, gives a procedure for protecting lines that use metallic components against direct lightning flashes: “a procedure to protect telecommunication lines using metallic components against direct lightning flashes to the line itself or to the structures that the line enters.” Among the families it covers is the fibre optic cable itself: “Optical fibre cable: Cable with optical fibres, which may also have metallic components such as an inner strength member and/or an outer metallic sheath.” The question is not whether the cable is fibre, but whether it also carries metal.

Zero, but not for every cable

For an entirely dielectric optical fibre cable, K.47 is unambiguous, § 9.2: “A dielectric optical fibre cable is not directly struck by lightning, as there is no metallic elements to conduct the lightning current. Therefore, the use of such cable is an effective method of avoiding a lightning strike and provides a zero protection factor”, Kp = 0. No conductive path to collect anything.

An armoured cable — outer metallic sheath for mechanical protection, moisture barrier, or localisation — is a different case. Table 2 of K.47 assigns armoured optical fibre cables a withstand current (Iw) and a protection factor across three classes: Class I, 105 kA, Kp = 0.04; Class II, 80 kA, Kp = 0.10; Class III, 55 kA, Kp = 0.24. It is the same principle already seen in a shared trench with a power cable: steel armour or a steel strength member behaves, electrically, as a conductor, not as fibre. The rodent-resistant cable read yesterday — corrugated armour against gnawing — is exactly this case: a mechanical choice that changes, as a side effect, the route’s electrical exposure.

The number no cable datasheet writes on its own

Risk is calculated: the expected annual number of damages is Nd = Ng · Ad · Kp, where Ad is the line’s collection area and Ng the ground flash density, § 3.2.3: “Average number of lightning flashes to earth per square kilometre per year.” Ng is not invented by the specification: it comes from a map or a lightning-location service external to the project, or from the proxy allowed at § 6.1: “If the value of the ground flash density is not available, it can be assessed from the number of days with thunderstorms per year (Td), as Ng = 0.1 Td” — a meteorological statistic, not a cable value.

The calculated risk is compared against a tolerable risk, assessed under ITU-T K.72. Where the estimate is uncertain, K.47 suggests a reference value — “this Recommendation suggests adopting RTd = 2 × 10⁻⁴ as a reference value” — which leads to a cap of one direct-lightning damage “at each ten and fifteen year intervals for aerial and buried lines, respectively”: once every ten years for aerial, once every fifteen for buried. A proposed threshold, not an imposed one: the client may — indeed the Recommendation says should — revise it for their own case.

The measures that lower the risk class

Three levers, each with a number behind it. Burying an aerial section reduces exposure: Table 5 gives the protection factor for different soil resistivities, from 0.19 at 50 Ω·m to 0.70 at 2000 Ω·m — the more resistive the soil, the less burial protects. A guard-wire above a buried cable sits at intermediate depth between the cable and ground level: K.47 gives an example, § 10.2.1, “if the cable to be protected is installed at 0.7 m below the earth surface, the guard-wire shall be installed at about 0.35 m depth.” And electrical continuity, § 9.1: “the metallic elements of the telecommunication cable shall be continuous along the length of the line, which means that they shall be connected across all splices, regenerators, etc.”, bonded “to the equipotential bonding bar at the ends of the cable.” An exception is allowed: “In some specific cases it may be allowed to provide insulating joints in optical fibre cables with metallic components, provided that the insulation is properly dimensioned” — an insulating joint instead of continuity, but only if properly sized, not by omission.

Where the answer is written — and why it is not written in one place

None of these facts lives on its own:

The cable datasheet: metallic elements or none, test class, withstand current.
The area’s ground flash density (Ng, or thunderstorm-days Td): an external meteorological database, not a site file.
Soil resistivity along the route: the geotechnical survey done for the design — the same one needed for earth potential rise near power lines.
The guard-wire’s depth, where fitted: if it is written anywhere, it sits in the as-built installation record, not in the executive design that precedes the trench.
Electrical continuity across every splice: splicing records and closure datasheets, checked splice by splice, not declared once for the whole route.
Earthing resistance at the bonding bar: the acceptance certificate for the entry point at the exchange or cabinet — a value with a date and an instrument, not just earthed written on the specification.

Six places. The day an armoured section is struck, was it protected? is only answered by putting them together.

What we do not know

Compliance with K.47 is voluntary: the text says so itself, “Compliance with this Recommendation is voluntary.” Appendix I, which gives the formula for expected loss L = np·t / (nt·8760) and the typical values used above, states explicitly: “This appendix does not form an integral part of this Recommendation” — the same wording applies to Appendix II, on guard-wires. Annexes A and B, on sheath breakdown current and the sand-box test, have a different status: “This annex forms an integral part of this Recommendation.” We have not read ITU-T K.72 in full, referenced for tolerable risk, nor K.46, referenced for earthing resistance: we do not know whether they contain further thresholds, and we neither assert nor rule this out. IEC 62305-2, cited for the broader risk framework, is a paid standard we have not read. We neither assert nor rule out a specific Italian obligation on lightning protection for fibre networks: we have not verified this against a national primary source.

The two axes, applied to a lightning strike

First axis: the cable class with its withstand current, the Ng or Td for the area, the guard-wire depth where fitted, continuity verified at every splice, earthing resistance at the bonding bar — lines written into the specification we check and acceptance points with a record and a measured value, not cable protected against lightning as a declaration.

Second axis: that number, repeated over the years at the same earthing points, does not stay a single certificate in a drawer. With CSIDIA, the group’s other company, earthing measurements, splicing records and the guard-wire as-built flow into a single network map, because useful diagnosis is precisely the comparison between successive measurements referred to the same bar, the same splice, the same route: a value that climbs from one acceptance test to the next says more than a single reading within threshold. On that map an AI flags the drift and the crew closes the intervention — on-premise, without deep integration into the client’s network, or dedicated cloud with a data centre in Italy, always with shared management. It applies to operators and industrial networks as much as to a data centre with a single aerial feed.

From the site survey, at no cost, we bring the list relevant to your route: cable class declared or not, Ng for the area found or not, guard-wire in the as-built or absent, earthing certificate dated or never carried out — including the boxes left blank. Talk to a technician: the chamber with the scorched armour happens, and the question of what was written down beforehand should not go unanswered.