A central battery transfer switch continuously monitors utility AC and, on failure, switches the emergency bus to battery DC inside the 10-second window NFPA 101 permits — Sage does it in under 10 ms. Here's what's happening inside.
What a transfer switch does
A central battery transfer switch sits between the AC utility input and the DC emergency bus. In normal operation it draws from utility AC, charges the battery, and feeds conditioned power to the DC bus. When utility voltage drops below a threshold (typically 80% of nominal, per UL 924), the transfer switch instantly disconnects from utility and connects the battery directly to the DC emergency bus.
How fast
NFPA 101 §7.9.2.1 permits up to 10 seconds for emergency illumination to reach required levels after AC loss; UL 924 tests transfer operation within the same envelope. Sage central battery transfer switches activate in <10 ms via solid-state switching — well inside the allowable window and imperceptible to occupants. Extra margin matters when an egress event happens during a re-strike on the normal-lighting circuit.
Self-tests
Per NFPA 101, transfer switches and emergency power systems must be tested monthly (short functional test) and annually (full 90-minute discharge test). Sage systems log every self-test automatically and — with Sage Live™ — deliver the log to designated facility staff without a technician walking the building.
Failure modes and protection
- AC undervoltage — detected, transfer to battery
- AC overvoltage — detected, disconnect and log
- Battery undervoltage during emergency — system still operates until 85% voltage floor, then graceful shutdown with alert
- Charger fault — detected and alerted, emergency capability retained until next discharge