Under what condition does the alarm system's back-up battery engage?

• A. When the detector is triggered
• B. When the main siren sounds
• C. When the temperature rises
• D. When power failure occurs

Answer: (D)

Key idea: the backup power is designed to take over when the main power source fails. In most alarm systems, the control panel runs on mains power and has a rechargeable backup battery. If the power supply is lost, the system automatically switches to the battery to keep the panel, sensors, siren, and communications active. This protection is about maintaining operation during a power outage, not about events like a detector being triggered, the siren sounding, or a temperature change. The backup battery ensures continued monitoring and alerting until power is restored or the battery is replaced.