Dielectric strength (also called electrical strength) is the electric potential per unit thickness of specimen (i.e. magnitude of gradient) at which the electrical resistance of an insulator breaks down, and the material begins to conduct electricity much more readily.
Not measured for materials that are conductive to begin with.
Uses units of electric potential (voltage) per unit distance (specimen thickness), such as kV/mm or MV/m. Measurements are typically taken at 50 or 60 Hz (commercial power frequencies), and values do not vary significantly between the two frequencies.
However, the relationship between dielectric strength and specimen thickness is typically not linear, and testing samples of different thickness will yield different values. The norm is a 1mm specimen.
While the term breakdown potential is often used as a synonym for dielectric strength (such as used in the database), it can also refer to the overall voltage, without dividing by specimen thickness. When looking at values, the choice of definition becomes apparent from the units.
The main international testing standard is IEC 60243. ASTM testing standards include D149 for commercial power frequencies, D3426 for impulse waves, D3755 for direct current, and F1116 for the specific application of electrically protective footwear.