The Santronics AC Detection Sensor is a hand held 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 determines defective grounds, reverse polarity, and induced voltage. 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 cherry red.The orange Ultimate AC Sensor operates very much like the AC Sensor 3115, the choice of OSHA. The Ultimate AC Sensor has a new feature, an audio alert! The tip will glow cherry red and a sound will play when voltage is detected. Another important feature of the Ultimate AC is that the audio alert reacts only to an AC voltage field, and will not beep when it comes in contact with an innocuous static field.There is no ON/OFF switch on Santronics AC Detection Sensors. Switches can wear out over time or even malfunction, but Santronics Sensors have no switch which means they are always ready for use.