The problem was arcing across the insulators because of dust or moisture. Here is where this is discussed: E. R. Stauffacher, “Short-time Current Carrying Capacity of Copper Wire,” General Electric Review, Vol 31, No 6, June 1928
Actually, Onderdonk was considering the copper wires that supported the poles and insulators of high-voltage transmission lines. He wanted to determine the minimum wire size necessary to carry a short-circuit for a time period long enough for the circuit protection devices to kick in. His...
First, the formula is incorrect. The " 33*Time " should be under the square root radical. Second the denominator 234-Ta should be 234+Ta. The two primary assumptions are (a) no cooling, not from convection, conduction, nor radiation (Thus rapid heating assumption), and (b) time is the time TO...