This mains tester is suitable for testing AC voltages from 100 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. It has a length of 140mm and a blade tip of 2.9mm.