If the distance between the arrester and the device is not too great – arresters have a
protective zone of only a few meters in a distribution system and up to about sixty meters in high- and extra-high-voltage systems – this normally means that a protective level equal to the standard lightning impulse withstand voltage of the device to be protected,divided by a factor of 1.4, is adequate in protecting against lightning overvoltages.
It should, however, be kept in mind that this simplification might not be adequate for special system configurations and cases of application, or when the distance between the arrester and the device is great. Thus, the correct and standard procedure is to determine the expected overvoltages through calculations and to establish the necessary protective level of the arrester by means of insulation coordination studies. Information and instructions for this are found in the IEC publications 60071-1 and 60071-2, and recommendations for the application of surge arresters are made in IEC 60099-5.