Most of your questions can be answered by answering the question of "Will it work?" Will it work using Class A or Class B wiring systems, or AC or DC, or backup batteries are questions that go under the name of "Listed for Use".
If the fire alarm system is "Listed for Use" to be able to use a Class A circuit, whether the panel is a conventional fire alarm system or an addressable system, then Class A can be used.
Then again, an addressable fire alarm system is really a conventional Fire Detection and Alarm System - with an added means of addressable communications. Once you understand a "conventional" fire alarm system, understanding an "addressable" fire alarm system becomes easier.
Whether the fire alarm system is a conventional system or an addressable system, the system still has to be "Listed for Use".
When trying to understand why Class A or Class B is used, and before getting into how the fire alarm engineers design a system, first is a brief explanation of the overall design of a fire alarm system.
Listed for Use
Before they are allowed to sell anything to be used in a fire alarm system, manufacturers are required to have the system Listed for Use. The words "Listed for Use" is a whole process that the manufacturers have to go through.
The manufacturers have to design, and then build a package containing a complete fire alarm system. This system has to include everything they intend to sell in that package.
- The control panel, including:
- Battery backup
- Initiating Device Circuit (IDC) inputs
- Notification Appliance Circuit (NAC) outputs
- Signaling Line Circuit (SLC) inputs/outputs
- Relay contact outputs
- Auxiliary Power Supplies
- Smoke and heat detectors
- Pull stations or Manual Call Points (MCPs)
- Input modules
- Output modules
- Supervised output modules
- Wire types, including the maximum length, the capacitance allowed, the wire size, etc.
The manufacturers then send the entire package, with maximum wire that can be used and the maximum number of devices that can be used, to a non-biased, third party, nationally known, testing laboratory like UL, ULC, FM, CE, CCC, etc.
Example: UL stands for Underwriters Laboratories (they are a group of Insurance Companies who test equipment in their facilities). Their motivation is not for the equipment to pass the test; their motivation is for the insurance companies to not have to pay out insurance claims because unsafe equipment injures a person or burns down a building.
They test the equipment package as a whole. They want to make sure all parts of the system work together.
If the system is safe and won't otherwise cause a fire or electrocution, and also in the case of a fire alarm system, will detect fires and alarm people, then, and only then, will the testing laboratory place the fire alarm system on their "List" of things that can be used.
That's how a fire alarm system becomes "Listed for Use".
Fire Alarm System Designers
When considering installed fire alarm systems, the designers that lay out the fire alarm systems for individual buildings don't design fire alarm systems from scratch. Instead, they use the package that was Listed for Use from the manufacturer, and modify the complete system so can to be used in the building.
The designers can rest assured that, as long as they follow the design laid out in the manufacturer's installation manual, and the installation sheets that come with the devices, the fire alarm system will perform as it has been tested and then listed.
Designed - Installed - Tested - Maintained
Using methods that have already been tested to work with the panel, all fire alarm systems make sure the devices are connected. Addressable devices even have the ability to sometimes tell the panel that they are not working correctly.
If the manufacturer says in their installation manual and installation sheets that Class A and Class B circuits can be used, then, with approval of the Authority Having Jurisdiction (AHJ, which includes the fire marshal, the site commander, the owner's representative, the insurance company's representative, etc.), Class A or Class B circuits may be used in the installation.
Almost all fire alarm systems made nowadays use power from the public utility, which is AC, and inside the fire alarm panel's power supply, convert that AC to DC. It's the DC that is backed up by the battery. If the manufacturer shows a backup battery in the fire alarm panel, without any added converter, then the backup battery will work as it is shown in the manual.
As long as the equipment, including wiring, isn't failing on its own, if the installed fire alarm system is designed, installed, tested, and maintained exactly as the manufacturer shows in its installation manual and installation sheets, the fire alarm system will work.