We aim to determine the applicability of standard microprocessor units of relay protection and automatics for obtaining a digital signal of currents for their subsequent mathematical processing with the purpose of identifying internal faults of electrical equipment. Mathematical processing of experimental data (time series) based on regression analysis approximation in orthogonal basis was carried out. To that end, the weight coefficients of basis functions obtained by the least squares method were compared in terms of multidimensional space vectors, corresponding to the coordinates of this space. The investigated signal was a data set obtained by field experiments conducted with an induction motor, which assumed the possibility of creating artificial internal damage. Experimental data were obtained using two devices with different sampling rates and quantization levels. The first set of data was obtained using a 12-bit PCI board of analog-to-digital converter for installation in a National Instruments 6024E PC at a sampling rate of 10 kHz. The second set of data was obtained using a standard block of microprocessor relay protection and automation at a sampling rate of 2.4 kHz. Indicators for the presence of internal damage to the rotor circuit of an induction motor, which reduces the motor energy characteristics without affecting its operation significantly, were determined. The indicators of the damaged and undamaged state differed by a factor of five. The proposed method for selecting the diagnostic sign of internal damage of electrical equipment of electrical installations detects a 3% deviation in their parameters from the normal state, ignoring the presence of electrical/mechanical load. The diagnostic sign was established to behave similarly in the presence of internal damage, in both sets of signals under study. Thus, the possibility of obtaining a digital signal of acceptable accuracy from standard relay protection units for ensuring reliable identification of internal faults of electrical equipment of electrical installations is confirmed.