The switching-impulse strength phase-to-phase and phase-to-earth has been investigated on a power transformer dummy to provide data for the design of its external clearances. The dummy was equipped with 420 kV bushings, but also simulations were used to obtain smaller or larger distances to earth. The clearances were varied between 2 m and 6 m. The influence of rain is included. The report offers design curves for power transformer clearances covering the range of rated switching-impulse withstand voltages up to 1550 kV, phase-to-earth, and up 2400 kV, phase-to-phase.