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"The First Alert SA521CN-3ST (Model SA520B) Wireless Interconnect Integrated Smoke Alarm links the alarms throughout your home. This 120VAC hardwired wireless detector with battery backup integrates single alarms into a system. By adding Wireless Interconnect battery powered alarms elsewhere, when one alarm sounds, all alarms will be notified. Easily link alarms with the press of a button. The RF Interconnect technology continues to provide a reliable and secure radio frequency communication between Wireless Interconnect alarms. This technology features 915 MHz frequency with 65,000 security codes, and three (3) channel frequency hopping. The unit is compatible with an assortment of First Alert wireless alarms, which includes the SA501CN-3ST (SA501B), SA501CN2-3ST (SA501B2), SA511B, SA511CN2-3ST, SA520B, SA521CN-3ST, CO511B. SCO501CN-3ST (SCO500B), SCO501CN2-3ST. Photoelectric smoke sensor reduces nuisance alarms. Easy access battery drawer: no need to remove an alarm from the ceiling to change battery. Includes 10-year limited warranty. Meets UL standards. Note: This product is intended for non-professional do it yourself installation. If you would like information about a professionally installed and monitored system, please contact First Alert Professional at 1-800-921-6025. First Alert Professional is not affiliated with BRK Brands, Inc. "