1 Scope
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
overvoltages only. Therefore, and since the metal-oxide resistors are not permanently
connected to the line, the following items are not considered for this standard:
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• switching impulse sparkover voltage;
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• residual voltage at steep current and switching current impulse;
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• thermal stability;
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• long-duration current impulse withstand duty;
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• power-frequency voltage versus time characteristics of an arrester;
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• disconnector test;
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• 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.