Dielectric strength of multi-shields is a key factor determining the upper limit of static insulation performance for high-voltage vacuum interrupters. We experimentally studied the lightning impulse breakdown (BD) characteristics of typical shield vacuum gaps in series under different distances and diameters. Combined with the simulation

the key electric field parameters affecting the BD characteristics were determined. On these bases

a method considering the area effect was proposed for evaluating the BD voltage. The results indicate that the maximum field strength at cathode Ecmax is an important factor influencing the BD voltage of shield gaps. With the decrease of Ecmax

the 50% BD voltage U50 increases exponentially. Moreover

due to the non-uniform voltage distribution

the dielectric strength of high-voltage gap is critical to ensuring the coordinated voltage withstand of series gaps

reducing its Ecmax leads to a more pronounced insulation improvement. In addition

the shield gaps with large diameters exhibit a significant BD area effect. When the diameter exceeds 80 mm

the U50 will drop by more than 10%. Based on the obtained Ecmax-U50 relationship

the quantitative evaluation of BD voltage can be achieved by introducing a correction for the area effect. Validated by experiments

the evaluation error is within 3%

effectively addressing the overestimation of BD voltage for large diameters. The results can provide a theoretical basis for the insulation design in high-voltage vacuum interrupters.