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Numerical analysis of UHV gapless metal-oxide surge arresters have been performed utilizing capacitive-resistive coupling transient electric field simulations. The calculations are performed in 3D and for 3-phase installations. Two types of voltage grading methods for surge arresters have been studied, with grading rings and with external grading capacitors. The results show that important advantages like a shorter and lighter arrester with improved protection performance is possible to obtain using external grading capacitors instead of grading rings.