Down Conductors in Lightning Protection: How Many, How Far Apart, Where to Run Them

How many down conductors and at what spacing

A building with a non-isolated external lightning protection system (LPS) needs at least two down conductors. They are spread around the perimeter, subject to architectural and practical limits, ideally at equal intervals (DSTU EN 62305-3:2021, cl. 5.3.3). For a non-isolated LPS, one down conductor is not enough.

The number follows from the typical spacing between neighbouring down conductors, which depends on the LPS class set by the risk assessment under DSTU EN 62305-2.

DSTU EN 62305-3:2021, cl. 5.3.3, Table 4. Typical recommended values; the design sets the actual number.

How they are placed on a building

A down conductor should go at every corner of the building where possible (cl. 5.3.3). The informative Annex E (cl. E.5.3.3) makes an exception for an exposed corner: the conductor can be omitted if the corner is within half the Table 4 spacing of both neighbouring conductors, or within a quarter of one. Internal corners are ignored.

The spacing may deviate by ±20 % as long as the average matches Table 4 (cl. E.5.3.1). If a facade cannot take a conductor, compensating down conductors go on other facades, at no less than a third of the Table 4 spacing. In an enclosed courtyard with a perimeter over 30 m, down conductors are needed too.

On large flat buildings whose dimensions are four times the down-conductor spacing, the informative annex recommends extra internal down conductors roughly every 40 m (cl. E.5.3.4.2). If the separation distance to internal equipment comes out too large, more down conductors are added (cl. E.5.3.4.1).

The conductor is laid as the direct continuation of the air-termination conductors: straight, vertical and by the shortest path to earth. Loops are avoided; where unavoidable, the distance between two points of the loop and the conductor length between them must satisfy the separation distance (cl. 5.3.4, cl. 6.3).

What not to do:

• do not run a down conductor inside a gutter or downpipe, even insulated, because moisture corrodes it quickly (cl. 5.3.4);
• avoid embedding it directly in external plaster, which can be damaged by thermal expansion and discolour through chemical reactions (informative cl. E.5.3.4.2);
• do not treat a pipe carrying a flammable or explosive mixture as a natural down conductor if its flange joints are non-metallic or not electrically bonded (cl. 5.3.5).

If the wall is combustible and heating of the conductor by lightning current would endanger it, keep more than 0.1 m between conductor and wall (the fixings may touch the wall). Where that gap cannot be assured, a steel down conductor needs a cross-section of at least 100 mm² (cl. 5.3.4, E.5.1.1).

Natural down conductors: frame, reinforcement, facade

A metal structure can stand in for a separate conductor if electrical continuity between its parts is reliable and its dimensions are no smaller than Table 6 gives for ordinary down conductors (cl. 5.3.5 a).

Reinforcement in concrete works if the frame is electrically continuous; for precast concrete, connection points between reinforcement elements are provided (cl. 5.3.5 b). An interconnected steel frame also qualifies, and ring conductors are then not required (cl. 5.3.5 c). Facade elements, profile rails and metal structures under facades qualify if their size meets the conductor requirements, the sheet or tube is 0.5 mm or thicker, and vertical conductivity meets cl. 5.5.3 (cl. 5.3.5 d).

A metal gutter or downpipe that meets these requirements can serve as a down conductor too (E.5.3.5). The annex advises using natural down conductors as far as possible to increase the number of parallel current paths. If continuity is in doubt, ordinary down conductors are installed.

The test joint

A test joint is fitted on every down conductor, except natural down conductors bonded to foundation earth electrodes (cl. 5.3.6). It is opened only with a tool and stays closed in normal use. It simplifies electrical testing and measurement, including earthing resistance and continuity of the connections (informative cl. E.5.3.6).

How the system is checked after installation, and how often, is covered in the article on lightning protection inspection.

Fixing the conductors

Informative Table E.1 gives recommended distances between fixing points. For tape, flexible and stranded round conductors, vertical runs are fixed every 1000 mm from ground to 20 m and every 500 mm from 20 m up. Horizontal runs: 1000 mm on horizontal surfaces and 500 mm on vertical ones. For solid round conductors the spacing is 1000 mm in every case.

DSTU EN 62305-3:2021, informative Annex E, cl. E.5.2.4.2, Table E.1. A recommendation, not a requirement.

Mast and shaft: how they differ from a building

The Table 4 rules were written for buildings. On a free-standing mast used as an air terminal, the shaft itself acts as the down conductor. For an isolated LPS the standard says metal masts need no extra down conductors (cl. 5.3.2); non-metal masts need one each.

This is reading by analogy: for a shaft with its cross-section, section joints and flanges the standard gives no direct rules. What designers check is set out in the article on the steel mast as air terminal and down conductor. The protection zone and LPS class for a mast are set separately: see protection zone calculation and the lightning rod selection guide.

The POLYGONAL company manufactures and designs lightning rods on polygonal shafts from 5 to 40 m tall. This article does not replace a building’s lightning protection design: the designer does that under DSTU EN 62305, and installation is carried out by a licensed partner construction company. Do not work on electrical connections at height yourself.

What to send for lightning rod selection

If you need a lightning rod on a shaft, send the shaft height, the type of site, the wind zone, any extra equipment (lighting, antennas) and the size of the area to protect. Leave blank what you do not know; we will clarify. Terms are collected in Lightning rod, air terminal, lightning protection.