Lightning Protection of Masts and Towers: Choosing a Lightning Rod

What lightning protection consists of

• air-termination: a metal element at the top of the shaft that receives the lightning strike;
• down-conductor: a conductor that carries the current from the air-termination to the ground; often the steel shaft itself, if its section joints are electrically reliable and the cross-section meets the standard;
• earth-termination system: electrodes in the ground that disperse the current; the standard sets their shape and length and recommends a total resistance of up to 10 ohms at low frequency, where possible.

Requirements and methods for calculating protection zones are given in the EN 62305 series (in Ukraine, DSTU EN 62305). The class of the lightning protection system (LPS), I–IV, is chosen by a risk assessment (DSTU EN 62305-2; DSTU EN 62305-3:2021, cl. 4.1); the customer, the insurer or the licensing authority may also require protection.

Shafts and masts with an air-termination

Bent multi-sided shafts are convenient as the supporting structure for an air-termination: they are strong and resistant to wind and corrosion. A lightning rod on a shaft protects buildings, open areas, warehouses and equipment from direct strikes; the protection zone is calculated.

Lighting equipment can go on the same mast: a combined lightning rod that also serves as a lighting pole or a telecom tower. Lightning rods on masts are found at industrial sites, ports, substations and stadiums.

Earthing and testing

The earthing is designed to cl. 5.4 of DSTU EN 62305-3:2021. There are two types: A (electrodes at the down-conductors, at least two in all) and B (a ring electrode around the structure or a foundation loop). The minimum length of type A electrodes depends on the class and, for classes I and II, on the soil resistivity; it may be disregarded if the resistance of the whole earth-termination system, measured at a frequency other than the power frequency and its multiples, is below 10 ohms.

After installation the system is inspected (cl. 7.3). Informative Annex E (cl. E.7.2.4) advises checking continuity (above all of hidden parts) and measuring each electrode’s earth resistance on its own, with the test joint open; results go into a report. High-frequency or impulse measurements are made if needed. PUE rules are in the article on earthing of poles.

Coating and service life

Shafts are protected by hot-dip galvanising to DSTU EN ISO 1461:2024 or, by agreement with the customer, paint. Hot-dip galvanising gives corrosion resistance of at least 20 years.

What to send for a calculation

State the shaft height, the type of facility, the wind region, any extra equipment (lighting, antennas) and the size of the protected area. If you do not know something, leave it blank — we will clarify. See also lightning rods and air-termination rods, telecom towers and floodlight masts.

Example: protection radius at object height

Calculations often quote the protection radius at ground level, but the object has a height, and the zone is narrower there. In the rolling sphere method the radius at height h is R = √(H × (2r − H)) − √(h × (2r − h)), where H is the mast height and r is the sphere radius (the formula applies when H does not exceed r). The LPS classes and methods are described in Calculating a Lightning Rod’s Protection Zone.

Example: a mast H = 20 m, an object h = 8 m high (a warehouse or equipment), class II (r = 30 m, the largest sphere radius for this class, Table 2 of DSTU EN 62305-3). At ground level √(20 × 40) ≈ 28.3 m, at a height of 8 m — 28.3 − √(8 × 52) ≈ 28.3 − 20.4 = 7.9 m. The same calculation for all classes:

A single 20 m mast on level ground: a simplified illustration. The higher the level of protection (class I is the highest), the smaller the radius at equipment height (4.0 m for class I against 14.8 m for class IV), so an area usually needs several masts.

Example: spacing between two masts

In the rolling sphere method, a sphere of radius r resting on the tips of two masts of height H a distance d apart sags between them by p = r − √(r² − (d/2)²) (for d not more than 2r). An object of height h on the line between the masts is protected if h is less than H − p.

Example: masts H = 20 m, class II (r = 30 m), a warehouse 8 m high. At d = 48 m, p = 30 − √(900 − 576) = 30 − 18 = 12 m, so the sphere drops to 20 − 12 = 8 m midway and touches the object. A smaller spacing is needed:

Two masts of 20 m, class II. This checks only along the line between the masts; the lateral limits of the zone and the corners of the site come from a full calculation to DSTU EN 62305-3. For class I (r = 20 m) and d = 40 m the sphere reaches the ground, so there is no protected height above ground midway.

A mast next to a building: separation distance

If the mast stands apart from the structure (an isolated LPS), a separation distance s = ki × (kc / km) × l is needed between the down-conductor and the metal parts, installations and internal systems of the structure. Here l is the down-conductor length from the point under consideration to the nearest equipotential bonding point or earthing; ki depends on the class (0.08 for I, 0.06 for II, 0.04 for III and IV); km = 1 for air and 0.5 for concrete, brick and wood; kc depends on the number of down-conductors (approximate values, simplified approach): 1 for a single one (only in an isolated system), 0.66 for two, 0.44 for three or more; for type A earthing these values apply only if neighbouring electrodes’ resistances differ by no more than a factor of two; otherwise kc = 1 (DSTU EN 62305-3, cl. 6.3, Tables 10–12).

Example: class II, one down-conductor, a point 10 m above the earthing: s = 0.06 × (1 / 1) × 10 = 0.6 m. The coefficients and lengths for your facility come from the design.

Separate or combined mast: how to choose

A mast is chosen by what it must carry besides the air-termination. For its height and structure, see Height of a Telecom Mast or Tower. Three typical options:

General guidelines; the design sets the requirements for the facility.

From task to order: five steps

• a design organisation assesses the risk under DSTU EN 62305-2 (this gives the LPS class, I–IV) and marks building and equipment heights and distances on the plan;
• a zone calculation at the height of the tallest and most sensitive object gives the height and number of masts;
• you decide whether the mast also carries lighting or antennas and list the equipment with its mass and wind area;
• you agree the down-conductor, earth-termination and separation distance;
• you send the data to us, choose the coating (hot-dip galvanising or paint) and order manufacture: the typical lead time is 3–4 weeks if the item is not in stock. Installation on the foundation is done by a specialised organisation to the design.

Common mistakes when choosing a lightning rod

• the protection radius is taken at ground level although the object has a height;
• the mast is placed too close to the building and the separation distance is not checked;
• the steel shaft is treated as a down-conductor without checking for a reliable electrical contact between its sections and the earthing connection.