Shown here is a visual of how the UL 1449 test is performed. It starts by applying a 3 kA surge and measuring the VPR to receive a benchmark voltage protection level. Then, it continues to surge the device at the declared kA rating in three sets of five with one minute between each surge and thirty minutes between each set of five. In the case of Littelfuse SPDs, this is at 20 kA; the highest threshold that UL will test to. At the end, measure the VPR again with a 3 kA surge; the VPR must be within 10% of the original benchmark.