There are mainly two opinions about the mechanism of propagation of local arc along a polluted dielectric surface: electrical breakdown process or intense thermal ionization process ahead of the arc that causes the arc propagating. The key point is to determine whether the electric field can reach the breakdown strength of the air gap ahead of the arc roots. Although the electric break characteristic of the pure air gap had been investigated comprehensively

there was still lack of research on the characteristic of air gap along a dielectric surface. This paper devotes to investigate the difference of the dielectric strength between pure air gap and air gap along a dielectric surface as well as the influence of altitude. The effects of different dielectric surface conditions on the breakdown voltage are also studied including dry-clean

dry-polluted

nature-moistened and wet polluted. The experimental results show that the existence of dielectric surface would reduce the breakdown strength of air gap

and the shorter the gap distance

the greater of the influence. Also

the breakdown strength of the air gap along a dielectric surface would decrease with the increase of altitude

but the influence of altitude is smaller than that on the breakdown strength of pure air gap. Comparing with the measured results of electric strength ahead of arc roots

it is found that the electrical breakdown process ahead of the partial arcs is reasonable to occur and should be taken into consideration when modeling the polluted flashover process.