George there are several good reasons. if for some reason the loop gets broken, you'll loose the devices beyond the break in the wire, not terrible if it can be fixed asap but that hardly happens that quickly.With a class "A" system the F.a.c.p is setup to see the break in the loop and start polling the slc loop in both directions so your devices will not go missing from the loop. another reason is the eol. for some reason the as builts are never accurate in where they may be, and if you've ever been in a super market searching for the eol on a h/s circuit you'd swear to yourself you'd make the next install a class "a" system to avoid that hassle.
Wirenuts- You didn't convince me. I'm going to give you the one about the Class A being fed from both ends. Now bear in mind this would only be true if the wire were cut. If the wires were shorted your dead in the water. As for the eol, it should be on the as-built or plans as submitted. In this day and age of addressable equipment it's hard to justify the cost of a Class A system and as a result I don't see Class A specified at all. I've been an inspector for 25 years and only once and possibly twice have I seen a plan that asked for a Class A System. There are other ways of accomplishing the same level of monitoring, for example multiple zones on a Class B system. With multiple zones you could lose one zone and still have coverage with the other zones.
Last edited by George Little; 08/29/0911:12 PM. Reason: After thought.
I suspect this is one of those debates where every manufacturer will come up with some manner of logic to defend his proprietary product.
The fact is, you couldn't really have addressable detectors untill the microchip revolution came along. Sure, there were some attempts - but more than one head tripped, the coded signals became gibberish. Mind you, we really didn't have reliable electronics untill the 80's.
EOL's were a great innovation, one that -for the first time - allowed you to detect a problem.
Which to use probably depends on whose equipment you sell - though it's pretty much a design issue. A system gets large and complex enough, the EOL systems are too limited.
George to be honest with you. there are many hacks who rather than finding a fault on a service call, would move or add a resistor within the loop to "fool" the panel. the added cost. at most is a section of wire from the last device to the panel. very reasonable. I've been on many service calls to older systems where "t" tapping was an acceptable practice and that is a nightmare in itself. the entire point of having an eol is purely for supervision of the loop. if the eol is not truely at the last device then the supervision is void. In my neck of the woods Mass and especially R.I. the fire alarm codes are becoming more and more stringent. It is a requirement in the state of R.I to install all class "a" systems. including all slc , and Nac ckts. they even require the loop on the load side of any monitor module to be a 4c system 2 out and 2 return. your point about the wires being shorted is solved with a isolation module. most panels have the integral to the panel. and recomend they be installed every 25 devices. the iso module opens up between two of them if there is a short on the slc loop . for instance the loop may originate at the facp and pick up the front of a store say 15 devices, then hit an iso module. if there is a short within that section than the loop between the panel and that iso will come up missing, while the rest of the devices will remain intact. if that was a class "b" system then the entire loop would be lost, as the panel would not be able to poll the return loop. I've seen alot of different ways to install systems and as far as I can tell from installing and especially servicing the class "a" is the most ideal way and least likely to be compromised wiring method.
Thanks Wirenuts- Learn something new every day. I don't claim to be a fire alarm technician and the technology changes so fast in that field it's hard to keep up. Sounds like the big issue between Class A and Class B in the economics. The design professional will design the job as cost effective as he/she can.
The beauty of this Board is we all learn from others experiences. Nobody gets upset with being in error. (I hope)