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The measurement of the frequency response on power and distribution transformers is a diagnostic test technique for detecting winding displacement and deformation (among other mechanical and electrical failures), which are among the most important causes of mechanical failure in transformers. There are two different methods to carry out the measurement of frequency response: sweep frequency response analysis - SFRA and low voltage impulse - LVI. SFRA presents some important advantages over LVI such as: higher signal to noise ratio, bigger repeatability and reproducibility and less requirements regarding measurement equipment. An important disadvantage of the SFRA with respect to LVI is the time required for the measurement. The necessary time for performing an SFRA test (typically several minutes) is related to the bandwidth and number of spot frequencies, which are not universally defined. This paper presents an approach for determining a reliable number of spot frequencies in order to make a further diagnosis.