For how long must storage batteries for emergency systems maintain total load?

The correct answer of 90 minutes reflects the standard requirement for storage batteries used in emergency systems to ensure that they can supply power reliably during an unexpected outage. This duration is crucial because emergency systems, which may include emergency lighting, alarms, and other critical functions, need to operate long enough to safeguard individuals' safety and allow for an orderly evacuation if necessary.

The 90-minute timeframe is commonly required by codes and standards related to life safety and power systems, ensuring that these systems function effectively in emergencies. This duration allows a buffer that accounts for peak demands and ensures that power is available long enough to manage the immediate risks associated with power outages.

The other durations may be less effective in supporting the required safety protocols, as they might not provide sufficient time for personnel to execute emergency plans or for systems to function at full capacity during a power failure. Thus, 90 minutes serves as an optimal balance for preparedness without overwhelming the capacity of the battery systems.