UL 1449 is the standard for equipment installed on the load side of AC electrical service and in low voltage AC distribution systems. To obtain a UL Listing, the suppressor must meet safety standards and pass a duty cycle test. UL conducts a Let-through Voltage test on the suppressor and assigns a Voltage protection Rating (VPR). UL1449 ratings represent a component rating, not the actual let-through voltage of the electrical distribution system. The duty cycle test is based on a 26-shot withstand test using waveforms similar to IEEE 62.41. To pass UL 1449, the SPD unit must not degrade by more than 10% from its initial Let-Through Voltage value. All UL Listed SPD equipment displays the VPR rating for each applicable protection mode.