The Battery Backup Unit (BBU) provides emergency dc power for Honeywell Total Distributed Control (TDC 2000) if:
1. AC line supply is at zero volts +/- 3 volts for more than 0.5 milliseconds.
2. AC line voltage zero crossings are more than 12 milliseconds apart.
3. DC system power decreases below 21.0 volts.

Switching to battery power is automatic when the above conditions occur. Return to normal operation is also automatic for the first two conditions. For the third, the manual reset button must be pressed. Provisions for a remote reset button is provided.

When the power comes on again, the BBU will automatically switch off. A three amp fuse protects the ac line monitor, and a 50 amp circuit breaker protects the BBU output line.