General Order 64-A
Section IV
Strength Requirements For All Classes Of Lines
44 Safety Factors
The safety specified in these rules are expressed as the ration of the ultimate strength of the material to the maximum stress that will exist in it under the working conditions to which it will be subjected with the loadings specified in Rule 43, except that the safety factors fro design of structural steel, towers, poles and cross arms shall be based on the ration of the yield point to the maximum working stress.