Lightning Protection Earthing: Type A and Type B, Electrode Length

Two earthing types in DSTU EN 62305-3

The standard describes two ways to lay out the earth electrodes (clause 5.4.2). In a type A arrangement each down-conductor gets its own horizontal or vertical electrodes outside the structure, and there must be at least two in total (clause 5.4.2.1). Foundation electrodes that do not close into a loop also count as type A.

Type B is an external ring conductor that touches the soil along at least 80 % of its length, or a foundation electrode closed into a loop (clause 5.4.2.2). A ring that touches the soil along less than 80 % is still classed as type A (Annex E, clause E.5.4.2.1).

For bare rock the standard recommends type B only (clause 5.4.3). It gives the same advice for structures with many electronic systems or a high fire risk.

Type A against type B

Annex E gives the earth-termination system three jobs: to carry the lightning current into the ground, to equalise potentials between down-conductors and to control potentials near conductive walls. Foundation and ring electrodes of type B do all three, while radial type A electrodes and deep vertical ones do not meet the equipotential requirement (clause E.5.4.3.1).

Source: DSTU EN 62305-3:2021, clauses 5.4.2, 5.4.3, Annex E, clause E.5.4.2.

Electrode length and soil resistivity

The standard gives the minimum length l1 as a graph (Fig. 3; approximate values are in the table at the end of this section) of soil resistivity ρ and protection class. For classes III and IV it is 5 m at any ρ. For classes I and II it is also 5 m while ρ is low, and then it grows almost linearly.

A vertical or inclined electrode may be half as long: 0.5 l1. For a combined earth electrode the horizontal and vertical lengths are added together (clause 5.4.2.1).

The minimum lengths may be ignored if the earthing resistance of the whole system is below 10 Ω (measured at a frequency other than mains, to filter out interference). If the lengths cannot be met, type B is used. Lengthening the electrodes lowers the resistance in practice up to 60 m, and in soil above 3000 Ω·m the standard recommends type B or soil-improving compounds (clause 5.4.2.1, notes 1–3).

In a type B arrangement the mean radius r_e of the area enclosed by the ring must be no less than l1. If it is smaller, add horizontal electrodes of length l1 − r_e or vertical ones of length (l1 − r_e)/2, joined to the ring where the down-conductors arrive (clause 5.4.2.2).

Values read from the graph (Fig. 3, DSTU EN 62305-3:2021), accuracy about ±1 m; a design uses the graph in the standard. l1 is the length of a horizontal electrode.

Materials, concrete and corrosion

The standard gives minimum electrode sizes in Table 7; they do not depend on the protection class. The sizes for hot-dip galvanised steel are in the table at the end of this section.

Underground, an electrode corrodes. Galvanised steel can corrode in clay or damp soil, and a high chloride content makes it worse (Table 5). A galvanic link with copper can destroy it as well.

Stranded conductors of galvanised steel are not recommended in the ground: they corrode more readily, especially where they leave the soil or concrete. Galvanised steel in concrete must not run out of the concrete into the soil.

A foundation electrode goes into concrete with at least 50 mm of cover, which gives acceptable corrosion protection (clause E.5.4.3.2). Backfill the trench so that ash, coal and building rubble do not touch the electrodes. Where the soil is in doubt, corrosion is judged from experience on neighbouring sites with similar soil.

Source: DSTU EN 62305-3:2021, Table 7. Galvanised steel lattice plates, copper and other forms: see the standard’s table.

Frost, drying and measurements

Burial depth and electrode type are chosen to limit freezing and drying of the soil around the electrode. The top part of a vertical electrode within the frost depth is treated as ineffective, so 0.5 m is added to the design length of each vertical electrode (clause 5.4.3; E.5.4.1).

Earth resistance changes with the seasons, so the earthing is tested after installation and then followed up. How, when and with what records is described in Lightning Protection Inspection and Maintenance. Each electrode is measured separately with the test joint opened.

The recommended overall earthing resistance is 10 Ω, but the standard stresses that correct equipotential bonding matters more than the figure (Annex E, clause E.5.4.1).

How this differs from pole earthing under the PUE

This article is about dispersing lightning current from an air terminal. Earthing of metal lighting and line poles under the PUE is a separate subject. The permitted resistance there depends on network voltage, location and soil resistivity; those rules are covered in Earthing of Metal Lighting Poles and 0.4–10 kV Overhead Lines and Earthing of Overhead Line Poles Above 1 kV under PUE:2026.

How the pole itself acts as a down-conductor, and why its joints matter more than its cross-section, is explained in Steel Mast as Air Terminal and Down-Conductor. How to choose the structure and assemble the system is in Lightning Protection for Masts and Towers.

What to send for selecting a lightning rod

The POLYGONAL company makes lightning rods and air terminals on multi-sided poles and masts 5–40 m high; ready-made options are listed in the catalogue Lightning rods 6–40 m. The lightning protection design for a building and the earthing scheme are done by the project’s designer, and installation by a licensed construction partner company.

To select a pole, send the height and site, the number of lightning rods, the protection class if already set, and soil data if you have it. Leave blank what you do not know, and we will clarify.