In high voltage testlng. impulse flashover voltage of xtemal insulation that has the ability of self-restoring s estimated by up-and-down method (UDM) or nterpolation method (IM), supposing that flashover oltage follows a normal distribution with mean M nd standard deviation S. Trans l.E.E of Japan
UDM is suited for estimating M. On the other hand, IM is suited for estimating S if sampling points are appropriately selected. Therefore, by combining UDM and IM, we can deslgn the testing method where estimators M and S have more adequate errors than those by UDM or IM alone Especially, when it is important to estimate the voltage corresponding to low probability of flas_hover (for example probability = 0.001, 0.01) , combined method (CM) is also suf~ciently eff ective. This paper explains two research results as follows. ( I ) To show the way to gain the estimators M and ~ and their errors (with the data obtained by CM) by maximum likelihood method. ( 2 ) To decide the parameters of UDM and IM of CM for doing efflcient test and getting reliable estimators and small errors. and to estimate the voltage corresponding to low flashover probabil-ity. To achieve these object, it is desirable to test CM by the following method. At first, getting Ml and S1 by UDM where T1 (the number of repeti-tion test times) is 30-50 and d/s (d : up-and-down voltage distance) is about l. Next, with M1 and S1, test T2 times ( T2 is greater than 20) at two sampling points (M1-1. 5S1, Ml+1 5S1) by IM. Then, CM is a good method to estimate M,S and the voltage corresponding to low flashover proba-bility also with high reliability.

The paper written in English is available upon request.