Calculation of the protected volume by Early Streamer Emission (ESE) Air Terminals: two valid ways with identical results

Lightning protection in buildings is crucial to avoid personal, material and economic damage. An Early Streamer Emission air terminal is a high-tech device that responds to the approach of a lightning strike, anticipating its capture of other elements within its protection zone, in order to conduct the discharge current safely to the earthing. To fulfil their functions, they must be installed after calculating their protection radius in such a way as to protect the desired elements, an operation that can be carried out using two different methods but with identical results: those proposed by the UNE21186 standard and by the Technical Building Code (CTE). However, an erroneous interpretation of the latter can be dangerous.

The current Technical Building Code (CTE) includes since its publication in 2006 a section (SUA 8) on lightning protection, with a basic risk calculation and a series of installation instructions. It also includes a method for calculating the volume protected by a Early Streamer Emission (ESE) air terminal. It is a graphical method and in this it differs from the way it is calculated in the international standard for this type of lightning arrester (UNE21186 – Lightning Protection: Lightning Protection with Early Streamer Emission lightning protection systems), where an analytical method is proposed by means of a formula.

However, the results are identical for any lightning arrester, height and level of protection. There is no one protection radius according to the Technical Building Code (CTE) and a different one according to the UNE21186 standard, despite the fact that it appears in a lot of commercial documentation and measurements. This confusion comes from the misinterpretation of the Technical Building Code (CTE). This misinterpretation, however, is dangerous, as it assumes that the lightning arrester protection is maximum at roof level, which is not true, and can therefore leave particularly vulnerable areas unprotected.

What the Technical Building Code (CTE) says

The confusion about the apparent differences between the two methods has its origin in point B.1.1.2 of SUA 8 of the Technical Building Code (CTE), which states:

a) Under the horizontal plane located 5 metres below the point, the protected volume is that of a sphere whose centre is located on the vertical of a point at a distance D and whose radius is:

R = D+ΔL


R is the radius of the sphere in metres defining the protected area.

This R is confused with the radius of protection when, as explained, it is the radius of the sphere that defines the protected volume. One looks only at the formula, ignoring the sentence that introduces it and the one that follows it in part b:

b) above this plane, the protected volume is that of a cone defined by the pick-up point and the circle of intersection between this plane and the sphere.

Moreover, this R is taken as the radius for any height as if the protected volume were a cylinder around the axis of the lightning arrester. Of course, this is not the case. In point b) it is clear that above h=5m the protected volume is a cone. And, as stated in point a), above that plane it is a sphere.

What the UNE21186 standard says

The standard UNE21186 – Lightning protection: Early Streamer Emission lightning protection systems specifies an analytical method for the calculation of the protected volume using the following formula:




The result of applying this formula is identical to the graphic method of the Technical Building Code (CTE), with a single exception: the standard considers that objects less than 2 metres (in height) from the lightning conductor are not protected, but the Technical Building Code (CTE) does not mention this indication.

According to the UNE21186 standard, in addition, the calculation of the protection radius cannot be applied to advance times greater than 60µs, even if a higher result has been obtained in the laboratory.

What radius can an ESE air terminal protect? A graphic example

The protection radius of a PDC / an ESE air terminal depends on its advance time, the Protection Level required by the structure and the height of the point in relation to the object to be protected. For height differences of less than 5 metres, the protection radius decreases highly rapidly, which is why special attention must be paid to the protection of objects on the roof and the corners of the building.

In the following example, the risk calculation has determined that the building requires Protection Level 3 (D = 45m), and the distance to the furthest point is 57 metres. According to the misinterpretation, a lightning arrester of ΔL = R-D = 57-45 = 13 m would be required. Therefore, a lightning rod of Δt = 15µs would be sufficient.

But this is not what the Technical Building Code (CTE) specifies. The correct way to make the calculation by graphical methods is this:

“sphere whose centre is located at the vertical of the tip at a distance D and whose radius is: R = D + ΔL”

In the following figure it is clear that with the 15µs arrester the building is not protected, an arrester with a longer advance time, for example, 30µs, is required:

The coverage of the lightning conductor is, as it is stated in the Technical Building Code (CTE), the circumferential section to the ground for h>5m, and a cone above this plane.

At Aplicaciones Tecnológicas we always recommend installing the air terminal so that it is 6 metres above the roof, in order to protect the objects on the roof and the corners of the roof, which are the most likely to be hit by lightning. As can be seen in the previous illustration and in the following table, with a difference of fewer than 5 metres the protection radius decreases a lot, and from that height onwards the increase is not highly significant.


Aplicaciones Tecnológicas S.A.’s DAT CONTROLER® air terminals are equipped with the latest technology in Early Streamer Emission devices. In addition, within the Smart Earthing and Lightning Solutions product line, the smart lightning arrester DAT CONTROLER® REMOTE has self-diagnosis of the head and communication via IoT of the results, to supervise the correct operation of the equipment remotely.

DAT CONTROLER® REMOTE is a part of the Smart Lightning and Earthing Solutions range, a line that applies cutting-edge smart technology and achieves complete optimisation of LPSs by integrating algorithms and Big Data in automated control systems.

If you want to know more about the Lightning arrester with Early Streamer Emission (ESE) air terminals and the calculation of protection levels, you can contact us at this link.

If you wish, you can also attend any of our free online lightning protection trainings on our webinars page.