I have two questions regarding Class A/B Fire Alarm wiring.
- Is Class A for addressable systems and Class B for conventional systems or can they be either way?
- When are Class A required and when are Class B required?
Thanks!
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I don’t know about the second question, but Class A/Class B is only for conventional systems as far as I know: Class A has one pair of wires that go from a zone on the panel to the device(s), & then another pair that go from the device(s) back to that zone. Class B is the same except it omits the second pair of wires. Class A does not need an EOL resistor, Class B does.
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That is actually incorrect. The difference between Class A and Class B wiring is whether or not the circuit returns to the panel, conventional or addressable. Class A returns to the panel using a different route than it took from the panel. Class B does not, and thus has an end of line resistor on the last device on conventional circuits.
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Alright. I still got the 2nd & 3rd parts correct though. Addressable systems do not need EOL resistors on the initiating devices however as the panel looks for the devices themselves: thus if one or more of them go missing the panel assumes there must be a break in system wiring somewhere & throws a trouble.
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@HenBasket is correct. Class A wiring creates a full loop back to the panel, class B just has an EOL resistor.
Benefit of Class A is the ability to continue having a functional system despite a single break in the line. A trouble will still go out if the line is cut, but devices will continue to work.
Where Class A is used varies from one jurisdiction to another, however I’ve frequently seen it in healthcare, government, and some schools.
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In the future please refrain from answering questions you do not 100% know the answer to, in an effort to be the first to answer the question.
This can be detrimental when a professional is getting incomplete or incorrect information from an amateur.
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I don’t think anyone here will be able to describe the wiring classes better than Douglas Krantz already has.
The only ones you are concerned with for device wiring are Class A, B, and X.
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That’s correct in the case of a cut.
Another key part of Class A is isolator modules.
In the case of a short (this can happen if someone accidentally screws/drills/nails/etc. into the wires), the isolator modules can disconnect the shorted portion of the loop. The devices between the isolator modules will be nonfunctional but the rest of the loop will remain functional.
Some manufacturers offer peripheral devices with built-in isolator modules.
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I appreciate all the replies, very informative!
So if I’m understanding correctly from the above, then Class B wiring cannot be used in addressable systems, is that right?
Thanks
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No: I’m pretty sure it can be according to HenBasket & Zach: it’s just that addressable systems don’t use EOL resistors (on the initiating devices at least).
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Class B can be used for addressable systems. (unless the AHJ or project specifications require class A.)
Even if not required, class A is a good idea if the building has areas that will undergo frequent refurbishment.
Example: a mall. Tenants come and go, and that means that the tenant spaces get refurbished frequently as part of the new tenant’s fit-out.
In such a building, consider using class A with isolator modules strategically placed so that a shorted loop in one tenant space won’t impair anything in the rest of the mall.
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Which is also known as Class X
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Not necessarily, at least, somewhat.
Usually, when you read fire alarm manuals, you will sometimes see things listed as “Class A (Style Z)” or “Class B (Style Y)”. Those “Style” classifications are more or less deprecated, and circuits and pathways are nominally listed as Class A, Class B, Class C, Class D, Class E, Class N, and Class X.
Essentially, they handle supervision in different ways (and in the case of Class D, not at all), but Class X wiring is wired the same as Class A, with a redundant path back to the panel, but with fault isolation that helps protect against short circuits.
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Ah ok understood, and in that note… can Class A wiring be used in conventional systems?
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can Class A wiring be used in conventional systems?
Yes, if the panel supports it (check the manual/datasheet).
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Class B adressable loops don’t need an EOL resistor at the end, but you still need ISO modules.
We just did this recently when we upgraded an apartment to fully addressable devices. You take the old conventional Class B circuit, and put an ISO at the beginning, and take the EOL out from the last device.
But certain panels (I work with Kidde VS and VM systems) have addressable modules with conventional circuits that require an EOL resistor, like the CT1 and CT2 module.
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