Numerical simulations are required for composite insulators to ascertain electric field on the grading ring and end fittings, on the housing surface and at the triple point where air and housing meet end fitting. The purpose is to assess risk of erosion related to water drop corona. The situation is different for toughened glass cap & pin insulators where the characteristic to consider is visible corona inception voltage related to electric field in the air surrounding a string and to voltage grading along the string. This presentation discusses use of Finite Element Analysis to validate design of insulator strings up to complete assemblies with hardware and fittings and provides examples for AC O/H lines.