Form 4A is a typical connection to the public utility, with a secondary power supply that is developed using a motor generator or rectifier with a 24-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 24-hour supply makes Form 4A different from the other two forms. 

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