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The revision of the relevant recommendations allows to introduce new definitions of lightning impulse parameters. Experimental results have shown that the breakdown voltage of insulating materials depends on the frequency of the oscillations or overshoot near the peak and that this behaviour can be described by a test voltage factor k. Different procedures to evaluate the test voltage and the time parameters of a lightning impulse voltage are described. Filtering methods leads to very good results in determining the test voltage, but at the moment differences exists in the determination of the time parameters of a lightning impulse. The time to peak of a switching impulse can also be determined more accurate by a procedure similar to lightning impulses. The new definitions seems not to influence the results of impulse tests on insulation systems but they contribute very much to the reproducibility of the tests results.