[Hamid]

Generally the LIPL (Lightning Impuls Protection Level) shall be lower than LIWL (Lightning Impuls Withstand Level) for a certain protection against all possible conditions. About your concerned case (LIPL=1050 KV , LIWL=950 KV), that recommendation can not be fulfilled obvioslly. As you know the LIPL is related to distance protection and advised safety margin , so you must change your protection strategy by using other arrester or reduction of distance protection.

Margins are normally excellent due to the low Upl, Ups and also that most equipment at present have a high Uwl and Uws. However, depending on the electrical distance between the arrester and the protected equipment, the Upl margin is reduced and thus arresters fail to protect equipment that is not in the close vicinity of the arresters (i.e. within their protection zone).