A Lightning Rod for a House: What the Standard Requires

What a house lightning rod consists of

An external lightning protection system intercepts a direct strike, carries the current from the point of impact to the ground and disperses it in the soil without fire or dangerous sparking (DSTU EN 62305-3:2021, cl. 5.1.1). It is usually described in three parts: an air terminal, down-conductors and an earth-termination system. How these terms relate to each other is explained in the article on terminology.

The air terminal can be a rod, a stretched wire or a mesh on the roof. Down-conductors run from it down the walls, and the earth electrodes lie in the ground around the house. All of it is designed together: the dimensions of each part depend on the protection class.

The system is usually fixed straight to the house. It is set apart from the house when heating at the point of impact could damage the structure or its contents, typically with a combustible roof or combustible walls (cl. 5.1.2).

Which protection class is needed

The standard has four classes of lightning protection system, I to IV. The class sets the rolling sphere radius used for the zone, the mesh size, the spacing of down-conductors and the minimum length of earth electrodes (cl. 4.1).

The class is not picked by eye. It comes from a risk assessment under DSTU EN 62305-2, risk management (cl. 4.1; the title of part 2 is in Annex NA). The assessment is made by a lightning protection designer, because it takes into account the size of the house, its use and what is kept inside.

As a guide, here is what each class puts into the external system (Tables 2 and 4 of DSTU EN 62305-3:2021).

The sphere radius and mesh values are maximum values; the down-conductor spacings in Table 4 of the standard are described as typical recommended distances.

The roof air terminal: rod or mesh

Air terminals are placed by one or more of three methods: protection angle, rolling sphere and mesh (cl. 5.2.2). On a house roof these can be rods and a mesh. How the sphere method works and why the radius depends on the class is shown in the article on protection zone calculation.

The rod height is found by calculation so that the whole roof and everything on it (chimney, aerial, vent stack) sits inside the protected zone. You can estimate the protection radius for a class in the calculator. There is no universal height «for a house».

If the roof is combustible, air terminal conductors must not lie right against it. For such cases Annex E advises keeping at least 0.1 m between the system conductors and combustible materials (cl. E.5.1.1). Easily ignited parts of the house must not touch the system components or lie directly under a metal roof used as the air terminal (cl. 5.2.4).

Down-conductors: how many and where

A system fixed to the house needs at least two down-conductors. They are spread around the perimeter, preferably at equal intervals, with one at each corner of the house where possible (cl. 5.3.3).

Typical spacings depend on the class and are given in the table above. So a long wall may need more down-conductors than one per corner.

If a sufficient distance from the down-conductor to combustible material cannot be assured, the cross-section of a steel down-conductor must be at least 100 mm² (cl. 5.3.4).

Near down-conductors the touch voltage can endanger life even when the system is installed correctly. For this reason Section 8 of the standard names the conditions under which the risk is accepted, for example when nobody is within 3 m of the down-conductors in normal use (cl. 8.1).

Earthing: type A and type B

The standard describes two ways to lay the earth electrodes (cl. 5.4.2). Type A is separate horizontal or vertical electrodes at each down-conductor, at least two in all. Type B is a ring electrode around the house that touches the soil along at least 80 % of its length, or a foundation electrode forming a closed loop.

Type A has a minimum electrode length by class. It may be disregarded if the earth resistance, measured at a frequency other than the power frequency, is below 10 ohms (cl. 5.4.2.1). Where the lengths cannot be met, type B is used.

Annex E names 10 ohms as the recommended total resistance and advises choosing the depth and type of electrodes so that corrosion, drying and frost in the soil affect them less. In frost, the first half metre of a vertical electrode is not counted (cl. E.5.4.1).

The general PUE requirements for earthing poles are collected in the article on pole earthing.

Inspection and maintenance

The system is inspected after installation, then periodically, and also after alterations or repairs, or when the house is known to have been struck (cl. 7.3). For classes I and II Annex E recommends a visual check every year and a full one every 2 years; for III and IV a visual check every 2 years and a full one every 4 years (Table E.2).

The inspection looks at corrosion of the air terminal, conductors and joints and measures the earth resistance. More on intervals, checks and records is in the article on lightning protection inspection.

What not to do yourself

Work on the roof and the choice of class are a specialist’s job. A mistake in the protection zone or in the joints leaves a system that exists on paper: it stands, but does not protect. Annex E advises entrusting the inspection to a lightning protection specialist (cl. E.7.1).

So you can work out the scheme and prepare the input data yourself. The zone calculation, choice of cross-sections, joining of conductors and measuring of earth resistance are better left to people with the right to do it and the instruments.

When a separate mast makes sense

A free-standing lightning rod on a metal mast is considered when fixing the system to the house is unsafe, for example under a combustible roof. For an isolated system with a metal mast, extra down-conductors are not needed (cl. 5.3.2 a). This is covered in detail in the article on a steel mast as air terminal and down-conductor.

The POLYGONAL company manufactures lightning rods on multi-sided bent poles 5–40 m tall. Installation is carried out by a licensed construction partner company. We make roof air terminals to the customer’s drawings or technical specification. To select a pole, send the height of the house, the size of the plot and the wind region.