Rechargeable power supplies for smoke alarms must recharge in ____ hours or less when connected to a ____ portion of branch circuit.

Rechargeable power supplies for smoke alarms are critical to ensure that these devices are always operational, especially during emergencies. The standard requirement dictates that these power supplies must fully recharge within a specified timeframe to ensure consistent functionality.

In the context of this question, the requirement specifies a maximum recharge time of 4 hours when connected to a switched portion of a branch circuit. This is significant because a switched circuit means that the power supply can be turned off, and thus the charging may need to occur quickly to minimize downtime for the smoke alarms.

This timeframe is established to guarantee that safety devices, such as smoke alarms, can return to full operational capability promptly after a power disruption. Quick recharging in a practical timeframe helps ensure that in case of an emergency, the alarms will be functional and ready to provide warnings.

The context for this preparedness reflects the critical nature of fire safety, where even a short period without adequate functionality can have serious consequences. Therefore, the correct answer aligns with the safety standards that prioritize rapid recharging to ensure continuous protection in residential and commercial buildings.