Clamping voltage is the maximum voltage allowed to pass through the circuitry to the connected equipment when tested with the UL test surge. 330 volts is the lowest rating allowed by the UL and the rating must be stated on the unit. The lower the number, the better. Other allowed ratings are 400, 600, 800, and 1000. Some manufacturers use a rating called “let through voltage” to present appearances of superior performance, but UL does not validate this terminology.