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This paper reviews and discusses the mechanical loads that overhead lines experience as defined by applicable codes and the strength requirements for insulators that support the lines as described in ANSI, CSA, and IEC standards. Consideration of the probable occurrence of extreme weather conditions requires the insulator strength to be such that irreversible damage to the insulator does not occur at the maximum load considered. The inherent variation of the mechanical strength of the insulator dielectric material is discussed as a factor for its ultimate strength, and as a basis for limiting line loads below the damage limit for the insulator. Time-load tests suggest that reasonable damage limits for ceramic and composite insulator dielectric materials can be established. As discussed in a companion paper, the assignment of damage limits for specific insulators depends not only on the inherent mechanical strength of the dielectric, but also on the standard upon which the mechanical strength of the insulator is rated. Common line design requirements, coupled with insulator manufacturers' recommendations that the maximum load not exceed the routine proof test load each insulator is subjected to has, in general, given good field experience.