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Insulators surface doped with Ti and a Mn/Ti mix were studied. The results always show an exponential relationship between resistance and the reciprocal temperature. At high voltages, a 'self-heating effect' of the insulator was observed, because the ohmic loss across the doped insulator surface is larger than the heat transport away from the insulator. The onset voltage of this effect is dependent on the temperature. It was found that the pure Ti dopant has the highest onset voltage. Therefore, alumina insulators doped with Ti have higher operating temperatures than those doped with a Mn/Ti mix. However, all the doped insulators have significantly lower flashover rates than plain insulators in the temperature range studied, from room temperature up to about 300 degrees C.