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.