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Smoke detectors play a crucial role in saving lives by notifying you of smoke indoors. Here's how they work to keep you safe. Steven Puetzer / Getty ImagesBattery-operated smoke detectors are standalone devices. However, if you have AC-powered smoke detectors and live in a home built in the past 10 years in the U.S., they are likely interconnected. This wiring ensures that if one alarm is triggered, all alarms in the house will sound off. For instance, even if a fire begins in the basement, those sleeping upstairs will hear the alarm because of this intercommunication feature – all alarms sound simultaneously.
Modern AC-powered smoke detectors come with three wires: black, white, and red. The black wire carries 120 volts AC, the white wire serves as the neutral, and the red wire is used for interconnection. All alarms are connected to the same circuit from the fuse box and are typically wired using standard three-way switch wiring (see How Three-Way Switches Work for more details – this setup involves black, white, and red wires enclosed in a Romex casing). Electricians connect the alarms by running the red wire from one alarm to the next, enabling them to communicate.
When any alarm detects a fire, it sends a 9-volt signal through the red wire. Any alarm receiving this 9-volt signal on the red wire will immediately start sounding. Most alarms can handle up to a dozen units connected through the same red wire. It's a simple yet highly effective system.
