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Burrs occurring on electrical steel laminations are likely to increase the losses of transformer cores; however, it is still difficult to quantify this effect. Artificial burrs were applied to a three-phase distribution transformer core in a controlled manner. When large burrs, short circuiting 66 laminations together, were applied to the core, total loss at 1.8 T rose by almost 100%. From eddy current analysis, localized losses were estimated to increase to 1500 W/kg at 1.8 T for 66 laminations electrically connected by burrs. Additionally, various burr locations were investigated and it was found that it does not matter whether the burrs placed on one limb are located opposite each other, forming a rectangular affected region, or are shifted, forming a skewed shape. For burrs affecting a low number of laminations the calculated and measured losses agreed whereas, for large burr regions, calculated values were higher due to the approximations made.