Section 125.26 - Line Insulators(1)Material and Marking. Insulators used to support transmission conductors shall be of porcelain, made by the wet process, or other material which can be expected to give equally good results in respect to mechanical and electrical performance and durability. Identification of maker, year of manufacture, and strength rating should be marked on each unit. Marking shall be so applied as not to reduce the electrical or mechanical strength of the insulator.(2)Electrical Strength of Insulators in Strain Position. Where insulators are used in a strain position, they shall have not less electrical strength than the insulators generally used on the line, when under the mechanical stresses imposed by the loadings specified in 220 CMR 125.24.(3)Ratio of Flashover to Puncture Voltage. The dry flashover voltage of insulators shall be not more than 75% of their puncture voltage at a frequency of 60 Hertz.(4)Selection of Insulators. In selecting the amount of insulation to be used for any nominal voltage between conductors, consideration should be given to the following factors: (a) Power frequency over voltages(f) Effect of structure on electrical strength(g) Maintenance requirements