The Santronics 3115 AC Voltage Sensor is a pen-like device for determining the presence or absence of 50-1000 volts AC in insulated wires, wall receptacles, fuses, junction boxes, switches, and other voltage carrying electrical systems. The AC Sensor can determine defective grounds, reverse polarity, and induced voltage. Therefore, it is not necessary to disconnect the system in question because no contact is required for operation, and current flow is not necessary to locate voltage. Simply touch the plastic tip to a connection point or move it along a wire. If AC voltage is present, the LED light in the tip will glow red.