Class "A" & Class "B" Fire Alarm

[ed downey]

Can anyone help me with the definition of a Class "A" supervision for the fire alarm signalling network and a Class "B" supervision for the initiating and notification device circuits.
My assumption is that for a Class "A" they want addressable fire alrm circuit devices and for Class "B" (Which I'am assuming are the strobes & horns) they just want to know that the circuit is operating.
Any further clarification would be appreciated.
-Ed

 

[ron]

Re: Class "A" & Class "B" Fire Alarm

Class A and B circuits apply to both initiating (pull stations, smoke detectors, etc) and indicating devices (horns, speakers, strobes).
Both types are specifically detailed in NFPA 72. There are huge cost differences between the two. Essentially Class A is a loop, and Class B is run out to the last device. There are many other requirements about redundancies between the two sides of a Class A loop.
Many paragraphs in the Fire Alarm Code (NFPA 72) about the differences.
Anything specifically you need?

 

[roger]

Re: Class "A" & Class "B" Fire Alarm

Ron is correct, Class A, think "ring circuit".

There are allowances for some "spurs" as far as conduit is concerned, (in and out series wiring in the same conduit) but the number of devices are limited in these "spurs"

As Ron said, there is quite a lot to the particulars.

Roger

 

[ed downey]

Re: Class "A" & Class "B" Fire Alarm

Ron & Roger,

Thanks For The Quick Response. I am Just Looking At It From A Budgetary Cost At This Point And I Read This In The Narrative And It Jumped Out At Me. Sounds Like The Class A System Is More Expensive. I Will Pursue This Further With Local Fire Alarm Contractors.

Thanks Again For Your Input.
-Ed

 

[ron]

Re: Class "A" & Class "B" Fire Alarm

Ed,
It is more expensive, but the redundancy might be worth it.
If your doing a school or office building where most systems are installed because they are required, it might not be worth the $$$.
If you are doing a datacenter or telco equipment building, where the lack of downtime is worth more company revenue per minute (or per second), it is worth it.

 

[Ed MacLaren]

Re: Class "A" & Class "B" Fire Alarm

This "clip" from a well-known manufacturer's instruction manual, for a very basic (non-addressable) system, might help illustrate what the others have explained.

In Class A wiring, the circuit is powered from both ends. One open wire will not disable any of the devices, but will initiate a trouble signal.

I believe it is normally the Municipality's Building Code that determines when Class A wiring is required.

In some jurasdiction's the two "sides" of the loop are not even permitted to be in the same conduit or cable.

Ed

 

[ron]

Re: Class "A" & Class "B" Fire Alarm

No municipality that I know of requires Class A circuits. It is a designers choice.