Hello all, I'm currently designing all the structure, equipment and grounding for a 230-115kV terminal substation, and for our dead end connections consisting porcelain ball & socket type suspension insulators we are given the number of insulators required for each voltage level. (i.e. 14 for 230kV and 9 for 115kV) On what basis do they determine that 'X' number of insulators are needed for a voltage level of 'Y'? Is it the dry arcing distance? Is it the creepage distance? Is it a combination of multiple factors, or does one particular factor mostly dominate over the others? Also I've often heard that there is a rule of thumb through which one can guess at the voltage level of a given transmission line by counting the number of insulators in a given insulator string, is this true? Thanks again!