What is the principle difference between class A and class B fire alarm wiring

http://www.homecontrols.com/homecontrols/pdf/449CTE_wiring.pdf

Fire Department use "A" (material fire > water) : "B" (Electrical Fire > Non liquid suppression) : and "C" (Chemical >Foam) to describe the type of fire to expect and what to use to kill the fire.

NEC 725 defines Class I, II and III alarm systems.

Given the above, what is your question?

Hey Don, if you check my link, it answers the OP's post.

You answered it by forgetting to place yourself in the right world first. The guy that's been an auto mechanic who knows that RED is POSITIVE comes across thermocouples and doesn't know that RED is always connected to NEGATIVE.

Or the EE that learned that BLACK is NEGATIVE puts on his electrician's hat and doesn't realize that BLACK is HOT.

Ron,

You lost me but that's easy to this evening.

How did you make the leap from his generic question to the instruction pamphlet furnished by your link?

Option #2: Class B (Style B) Wiring
(Extra wires, no extra zones)
The 449CTE, CSTE, and 448CTE can be used
For every smoke detector on a class B loop, by
running all the supervisory relay outputs to the
end of the line. This way, a trouble signal will
Break the loop only at the very end, and it will
Not prevent any other smoke detector from
Signaling an alarm. No added power supervision
Unit is needed.

Option #3: Class A (Style D) Wiring
(No extra wires, no extra zones)
When Class A wiring is used, the 449CTE,
CSTE, and 448CTE can be used for every
Smoke detector on the loop without having to
Run the supervisory wires to the end of the line.
The supervisory relay output is simply connected
in series with the alarm loop at every
detector. With Class A wiring, a break in the
Alarm loop will not prevent other smoke
Detectors from signaling an alarm, since the
Loop is bi-directional. No added power supervision
Unit is needed.

If you want another link:

Class "A" & Class "B" Fire Alarm [Archive] - Mike Holt's Forum (http://forums.mikeholt.com/archive/index.php/t-58406.html)

And

Survival strategies for fire alarm systems | Crime, Law Enforcement & Corrections > Crime Prevention from AllBusiness.com (http://www.allbusiness.com/operations/disaster-preparedness/977581-4.html)

Redundancy is main benefit of Class "A". For Life Safety, extra wiring is a small price to pay for a home system. Large Buildings get involved.