The glycerin-impregnated multi-layer polymer-film insulation is often used as the high voltage insulator for the secondary turn of LTD（Linear Transformer Driver）

and the layer number of film for this insulator can be more than several hundreds and even a thousand. As the development of insulating materials

polymer films of tens of micrometers of high intrinsic breakdown strength become cheap and easily available

for example

the electric strength of PP（polypropylene） film used in high voltage capacitors has reached 500 V/μm（500 k V/mm or 5 MV/cm） or higher. As a result

it is feasible that technical improvement for the LTD’s multi-layer film insulation makes it possible to more compact insulators of lower inductance. Consequently

we derived a Weibull-type distribution to apply to the prediction of breakdown probability for multi-layer film insulation by semi-empirical analysis. The distribution parameters can be obtained from a smaller scale samples of multi-layer film insulation

and the prediction of breakdown probability for larger scale samples can be fulfilled using a simplified computation method. The electric strength reduces and breakdown probability increases with the increase of film layers. However

the rate of electric strength reduction and breakdown probability augmentation becomes slow with the increase of film layers. An example indicates that for an insulation of N times n layers

this method is valid for n less than 10 with N unlimited. For N=100

if breakdown test is performed every 20 layers

then the breakdown probability for the insulation of 1000 layers can be computed with n=10

helpful in the development of new insulators.