This part of IEC 60099 covers metal-oxide surge arresters with external series gap (externally gapped line arresters (EGLA) that are applied on overhead transmission and distribution lines, only to protect insulator assemblies from lightning-caused flashovers. This standard defines surge arresters to protect the insulator assembly from lightning-caused over-voltages only. Therefore, and since the metal-oxide resistors are not permanently connected to the line, the following items are not considered for this standard: " switching impulse spark-over voltage; " residual voltage at steep current and switching current impulse; " thermal stability; " long-duration current impulse withstand duty; " power-frequency voltage versus time characteristics of an arrester; " disconnector test; " aging duties by power-frequency voltage. Considering the particular design concept and the special application on overhead transmission and distribution lines, some unique requirements and tests are introduced, such as the verification test for coordination between insulator withstand and EGLA protective level, the follow current interrupting test, mechanical load tests, etc. Designs with the EGLA's external series gap installed in parallel to an insulator are not covered by this standard.