This Faithfull Mains Tester Screwdriver is fitted with a neon indicator in the clear plastic handle which illuminates when the point of the tester is touched on a live voltage source. This mains tester is suitable for testing AC voltages from 125 to 250 volts against earth. The tip of the tester should be placed on the suspected live source, while finger touch contact is applied to the metal stud on the top of the handle. If the circuit or terminal contact point at the tip of the tester is live, the bulb will then light up as confirmation. Note: ALWAYS test the mains tester on a known live source before use, to determine that its indicator bulb is functioning correctly prior to use. WARNING: The earthing position of the user largely determines whether the tester bulb will illuminate or not. Failure to illuminate therefore does not provide 100% confirmation that no voltage is present.