Troubleshoot electrical circuits safely and easily. Unique handheld device determines the presence or absence of AC voltage in insulated wires, wall receptacles, fuses, switches or any other voltage-carrying electrical system. Simply touch the plastic probe tip to a connection point or move it along an insulated wire. If AC voltage is present, the LED light in the probe tip will glow bright red and an audible beeping sound will be heard. UL-approved. Two AAA alkaline batteries included.