As far as we know, until the end of the nineteenth century there were neither relayers nor relay protection as such. But electric power and electrical engineering already existed at that time. And short circuits obviously happened periodically. What then protected the electrical equipment from electrical damage?
The content of this note reveals the questions of how and why the Fourier transform known to all power engineers works. Here we tried to refrain from using a complex conceptual apparatus, all the calculations used should be clear to anyone who has somehow mastered the initial level of the university course in higher mathematics. For all others, it is possible that the material presented is intuitive.