The Gardner Bender low voltage tester is used to test all circuits with the power on. The case contains a 24V 3W filament fuse that glows when voltage is present. It tests 4.5 To 24 Volts, is UL Listed and ideal for testing fuses, grounds, shorts, contacts, diodes and all circuit components in model railroads, airplanes, boats, and in automotive circuitry. It has a 30 inch test lead with an alligator clip at the end.