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An experimental investigation was carried out by means of a single sheet and strip tester for electrical steels to find the effect of slitting on the magnetic properties of electrical strips of various widths (from 40 to 660 mm). Individual effects have been analyzed separately (slitting only, tension, and bending) regarding their influence on the deterioration of the magnetic properties of strips during this manufacturing operation of transformers. It has been found out that at the analyzed slitting machine the main cause of deterioration was bending of strips over rollers of too small diameter. By reconstruction of the slitting machine, i.e., by increasing the diameter of most critical rollers, deterioration of the magnetic properties of strips has been considerably reduced. Measurement of the magnetic properties of factory-supplied electrical strips before and after stress-relief annealing has also shown that grain-oriented electrical steels usually have inner mechanical stresses the level of which can vary considerably, depending on the manufacturer of steels and the shipment.