Inception characteristics of partial discharges developing in a short sphere-ring gap bridged by polymeric insulating surfaces are investigated using positive ramp high voltages with variable rate of rise and several detection threshold discharge currents. Inception probability distributions of surface partial discharge voltage were obtained. A pronounced effect of material relative permittivity on partial discharge inception characteristics was mainly observed for the lowest detection current employed. The lowest measured values of inception voltage and pulse current amplitude could be considered as characteristics of streamers developing adhering to the insulating surface. A good agreement between theoretical and experimental results on partial discharge inception voltage is shown to exist.