The measurement of direct current or voltage drop across an electrical insulating material to determine its resistance by the application of Ohm's law. The main purpose of electrical insulation is to isolate the energized components of a system from each other and from ground. This requires that the insulation resistance to the flow of current be as high as possible. Good insulators have resistances in the range of 10⁶ to 10²⁰ ohms, and must also have acceptable mechanical, chemical, and thermal properties.