Using the Klein RT250 GFCI Receptacle Tester is easy. First, insert the tester into a 3 - wire, North American 120V electrical outlet. The large backlit LCD will start showing the voltage and wiring conditions. To check the GFCI device, wait for the LCD to display the time it takes to trip. If you're dealing with a hard - to - reach outlet, use the auto - hold function. When it comes to safety, always make sure the device has a CAT II 135V safety rating. Don't try to use it on other types of outlets. For maintenance, keep the device in a dry place. Replace the 2 AAA alkaline batteries when the power is low. If you notice any damage after a drop, don't use the tester until you've had it checked by a professional.