The Lisle LED Continuity Tester, No. 26330, checks the ability of wires, fuses, switches or other electrical circuits to carry current. To use, clip the lead to the end of the wire in question and probe the other end of the wire. If the bulb lights up, continuity exists. If the bulb fails to light, the circuit is open. This tester is not for use on live circuits, but incidental contact with 12V supply will not burn out the LED bulb.