Abstract: Over time, buried cables lose their dielectric strength because of aging and water treeing. Many methods have been devised to determine the proper time to replace cables before failure occurs. Applying superimposed AC upon DC to cable insulation with the intent of evaluating breakdown strength is a possible cable testing method. However, there is little experience with the use of this type voltage and the affect it has on cable insulation. This research studies the breakdown strength of XLPE insulation under combined AC and DC voltage. Based on past research of paper-oil insulation, the breakdown strength is expected to follow the arctangent and arccotangent curves for the pulsation parameter \"p\".