This indispensable household tester kit includes the most commonly used testers to test Voltage, GFCI receptacles, batteries, and USB ports. The voltage tester allows the user to verify the presence of AC voltage prior to working on an electrical installation or during troubleshooting without requiring physical contact with live electrical conductors. The included GFCI receptacle tester detects common wiring problems in standard and GFCI protected outlets. Simply plug it in and view the light pattern on the tester to determine if it's correct, open ground, open neutral, open hot, hot/gird reverse, or hot/net reverse. Tester works on 110-125 VAC receptacles only and will not indicate quality of ground. The battery tester tests common household battery types (AA, AAA, C, D, and 9V batteries) to determine existing charge. Finally, the USB tester can check USB ports for the presence of voltage and test for proper wiring and polarity. Simply plug it into a USB port and the indicator bulb will glow a certain color to tell you the condition of the USB port.