The "Institute of Electrical and Electronics Engineers" (IEEE) Let-Through Voltage rating is based on a test that subjects a Surge Protector to a 6,000 volt spike. The rating equates to the amount of excess voltage that reaches connected equipment. The lower the number, the better the performance of the Surge Protector is. Underwriter"s Laboratory"s UL1449 surge protection safety standard uses these ratings to help users gauge performance. UL"s best Transient Voltage Surge Suppressor (TVSS) Let-Through Voltage rating is "300V". Status Indicator LED"s: Provides a quick visual indication of your unit"s status. Transformer Block Spacing: Connect bulky transformer block plugs without covering up other outlets.