Form 4B is a typical connection to the public utility, with a secondary power supply that is developed using a motor generator or rectifier, with a 4-hour battery backup supply across the generator/rectifier circuit. Power inverters or converters then take the AC power and convert it to DC to power the box circuits.

The 4-hour supply makes Form 4B different from the other two forms and has a much lower time requirement for the float storage batteries than Form 4A.

NOTE: This is an unformatted excerpt from our online fire protection training library.