A Steel Mast as Air-Termination and Down-Conductor per DSTU EN 62305-3

When a mast is already part of the lightning protection

The standard calls a conductive part that was installed for another purpose, but can act as a part of the lightning protection system (LPS) or alongside it, a natural component (cl. 3.15).

Conditions (cl. 5.1.3): the part is conductive, stays permanently in its designated place and is not altered; other natural components only supplement the LPS. The clause is written about buildings, while free-standing masts are named among rod air-terminations (cl. 5.2.1), so for a mast this is an analogy.

Air-termination: wall, coating and tip

A rod is usually fitted at the top, but the wall of the upper section can also end up at the strike point. For a mast or a multi-sided shaft the standard gives no direct rule: cl. 5.2.5 and Table 3 are written for sheet metal covering a building and for metal pipes, so a designer takes them by analogy.

For sheet metal, cl. 5.2.5 a sets three conditions: reliable electrical contact between the parts, a thickness not below the Table 3 value, and no insulating covering; thin protective paint or asphalt (1 mm) and PVC (0.5 mm) do not count as insulators (note 2). The values are the same for LPS classes I–IV. The note assigns t′ to sheet only, and a shaft is a tube, so the reference stays t: 4 mm for steel. The thinnest upper section is compared with it; whether a puncture would be dangerous (cables and equipment inside) is for the design to decide.

For a rod tip Table 6 gives solid round hot-dip galvanised steel of 176 mm², about 15 mm in diameter (our own calculation). Where mechanical stress, such as wind load, is not critical, a rod 9.5 mm in diameter and 1 m long is permitted. A blunt tip is useful (informative cl. E.5.2.4.1).

DSTU EN 62305-3:2021, cl. 5.2.5, Table 3, abridged (the standard also lists titanium, lead and zinc). t prevents puncture; t′ is for sheet only, where puncture, local overheating or ignition does not matter.

The shaft as a down-conductor: joints matter more than cross-section

Clause 5.3.5 a permits a metal structure to serve as a down-conductor if the continuity between its parts is reliable (cl. 5.5.3) and its dimensions are not less than Table 6 gives: for hot-dip galvanised steel, 50 mm² (strip, round or stranded conductor). An insulating covering does no harm (note 1). Clause 5.3.5, like 5.1.3, is written about parts of buildings (structures), so for a mast this too is reading by analogy; down-conductors of separate masts in an isolated LPS are addressed directly in cl. 5.3.2.

Our own calculation, π × (D − t) × t: a Ø219 × 4 mm tube has about 2 700 mm², Ø100 × 3 mm about 910 mm², that is 18 to 54 times more than 50 mm². For a multi-sided shaft, roughly perimeter times thickness.

So the weak point is the joints between sections and the flanges, where the current passes from one part to the next. Clause 5.5.3 says to keep joints to a minimum and make them reliable, for example brazing, welding, bolts or self-tapping screws.

The standard has no rules for slip joints or shaft flanges. Two informative passages come close: bolted joints in steel roof trusses (E.4.3.7) and joints between sections of metal parapet cladding (E.5.2.5). In the first, bolts tightened for mechanical strength are grounds to regard the parts as connected, thin paint is pierced by the current, and cleaned seating surfaces plus a weld of about 50 mm improve contact; in the second, conductive connectors are needed at the joints where continuity between the sections is not reliable. Both concern buildings; for a mast it is only an analogy.

A free-standing mast next to a structure

A mast placed so that the lightning current path does not touch the structure gives an isolated external LPS (cl. 3.3). The standard advises considering such a system when thermal or explosive effects at the strike point or on the conductors could endanger the structure or its contents: a flammable roof or walls, an explosion and fire risk zone (cl. 5.1.2).

A non-metal mast needs at least one down-conductor, a metal mast needs no extra ones, stretched wires need one per supporting structure (cl. 5.3.2). In an isolated system equipotential bonding is made at ground level only (informative E.6.2.1).

To keep the mast electrically apart from metal parts of the structure, a separation distance (in the Ukrainian text, роздільча відстань) is kept: s = (ki / km) · kc · l (cl. 6.3). Here ki depends on the class (0.04 to 0.08), km is 1 for air, kc is an indicative value from Table 12 (1 for a single down-conductor in an isolated LPS, 0.66 for two, 0.44 for three or more), and l is the length along the mast from the point considered to the nearest equipotential bond or earthing. A worked example is in Lightning Protection of Masts and Towers.

What zone a mast protects, and up to what height above the reference plane of the protected area (roof or ground) the protective angle method applies (20, 30, 45 and 60 m for classes I–IV; cl. 5.2.2, Table 2, Figure 1), is covered in Calculating a Lightning Rod’s Protection Zone.

Galvanised steel, copper and corrosion

Table 5 lists hot-dip galvanised steel as suitable in open air and in concrete; corrosion grows with a high chloride content, and a galvanic couple with copper can destroy the zinc. Notes d and e: galvanised steel in concrete should not come out of it into the soil; in coastal areas where groundwater may be salty it is better not to let it touch the steel reinforcement.

The informative annex (E.5.6.2.2) adds practical advice: avoid joints between dissimilar metals or protect them; never put copper parts above galvanised or aluminium parts that are not protected against corrosion, because copper sheds fine particles that attack zinc even without contact; join steel to copper in air without welding only after full tinning or under a continuous moisture-proof coating; near the sea make all conductor joints welded or sealed.

Earthing the mast and measuring it

The standard recommends an earthing resistance of up to 10 Ω, measured at low frequency, where possible (cl. 5.4.1): a recommendation, not a hard limit. The earth-termination is type A (outside electrodes, at least two) or type B (ring or foundation electrode, cl. 5.4.2). For type A, however, cl. 5.4.2.1 ties 10 Ω to electrode length: with a resistance below 10 Ω the minimum lengths of Figure 3 may be disregarded. The lengths themselves and PUE norms are in the article Earthing of Poles.

A test joint goes on every down-conductor (cl. 5.3.6), except a natural one connected to a foundation earth electrode; it opens only with a tool. It makes measuring the earth resistance easier (E.5.3.6).

A separate lightning rod beside a switchyard: what the PUE add

Under the Rules for the Arrangement of Electrical Installations (PUE), 2026 edition, cl. 4.2.162, open switchyards (in the PUE, ВРУ) of 15.75 to 750 kV must be protected against direct lightning strikes, with regard to the technological features of the facility and DSTU EN 62305. The exception is open 35 kV substations with transformers of up to 1.6 MVA each and open 3–20 kV substations: no such protection is provided. If lightning rods may not be placed on the switchyard structures, protection must be provided by separately installed lightning rods (cl. 4.2.165).

A free-standing lightning rod with a separate earth electrode must have an earthing resistance of no more than 10 Ω. The distance from its electrode to the switchyard earth electrode must be at least 5 m and more than 0.2·R, and the air distance from the rod itself to earthed structures and equipment at least 5 m and more than 0.12·R + 0.1·H (R: earthing resistance of the rod, Ω; H: height of the earthed structure above ground, m). If these distances are not kept or the resistance exceeds 10 Ω, the rod must be connected to the switchyard earth electrode, and its earthing must meet cl. 4.2.163. These are design requirements; the designer takes final values from the current PUE text.

What to agree with the designer

Questions worth settling in the design before the mast is ordered:

• drawings of joints and flanges: how the sections connect, where there are bolts and where welds, whether the contact surfaces are cleaned (for orientation: E.4.2.2.2.5, item q, and E.4.3.7);
• wall thickness and cross-section at the thinnest points compared with Tables 3 and 6;
• who measures continuity after installation and where it is recorded; separately, access to test joints and hidden parts for periodic checks (E.4.2.2.2.5, items n, p);
• corrosion zones and fastener material, plus a heating calculation if the dimensions or materials of Tables 5–7 are departed from (E.4.2.3.2).

For the status of the standard series see Lightning Protection Norms; our lightning rods and air-terminations are on the page Lightning Rods.