A lot of attempts are made to calculate the breakdown voltage of typical electrode structures

which is important for the design of external insulation

but in fact the influence of floating metals should often be analyzed in high voltage transmission and transformation projects. Therefore

3 kinds of streamer inception criteria including Raether-Meek criterion

critical charge criterion and photoionization criterion are adopted in this paper to calcu-late the breakdown voltage of short sphere-plane-sphere air gap with the consideration of floating metals

and the power-frequency withstand voltage tests are carried out for the comparison. The results indicate that the calculated values are in good accordance with the experimental results of short sphere-plane-sphere air gap in slightly uneven electric field and all of the relative errors and the mean absolute percentage errors are less than 7.5% and 4%respectively; the accuracy of the calculated results based on photoionization criterion is the highest with the mean absolute percentage errors of 2.5%. The study verifies the validity and generalization values of streamer inception criteria within a certain range

and contributes to explaining the discharge mechanism of complex air gap in slightly uneven electric field.