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Stress concentrations due to the rather abrupt variation of the cross-section at the transitions from core to shed have an important influence on the elastic material stresses and the allowable loading of ceramic insulators. For the user the effect is of immediate relevance only for the non-quasi-static dynamic bending case, which is the prevailing short-circuit loading of the post insulators of a substation. It can be described in terms of local stress-concentration factors. The paper reports on a special Finite-Element calculation method allowing to determine the required sets of concentration factors of insulator types with least discretisation- and computing effort possible. It thus becomes feasible to determine the static local stress maxima between sheds along the insulator and to derive from their maximum value together with the rated value of Minimum Breaking Load MBL the value of allowable material stress inherently assigned to the insulator by the manufacturer. It may be named Minimum Breaking Stress MBS. Since rates dynamic material strength as not lower than static, a general dynamic strength analysis can be performed. Exemplary results from static and dynamic studies on a standard type 110 kV post insulator are presented, that are supported by static test results. The studies show that the dynamic cross-sectional stress distribution differs from the static case and results in higher stress-concentration factors than the respective static. Yet, test experience have shown that the slow dynamic response of the insulator may compensate fast loading effects as regards its strength. Yet, this particular influence requires more and closer study to come to a simplified determination of the Equivalent Static Loads (ESL) for the dynamic short-circuit case.