Fire Zones N/O?

[heffneil]

I am wiring up some heat sensors in my garage, can someone explain to me how in the world it makes sense for a Normally Open circuit for Fire Zones? What happens if there is a fire between the wire and one of the devices.  I guess it would throw a trouble but thats not the same as a FIRE result.  I would think you want N/C and if a device burns up or the wire burns up it would open and trip? What am I not understanding logically here?
 
Thanks!

 

[ano]

You wire sensors like this with "firewire" which shouldn't burn up.  I think its very naive to think the failure of a wire will be an open circuit.  You have two (or more) conductors separated by a plastic, what makes you think a fire wouldn't melt the plastic so the conductors touch?  Hopefully you have a variety of heat sensors, and smoke sensors.  Most sensors have a rate-of-rise feature, so they should trigger long before flames burn the wire. The goal is the alarm trips before all the wires burn up.

 

[heffneil]

Yeah I was thinking if a fire occurred between the sensors in a way that didn't trip the sensor.  I think youre right if the jacketing melts - it would short the wires initially? 

 

[RAL]

Yeah I was thinking if a fire occurred between the sensors in a way that didn't trip the sensor.  I think youre right if the jacketing melts - it would short the wires initially? 

 
The  possibility for shorts or opens is why you need EOL resistors at the end of the zone wiring.   That way, even with N.O. contacts in the sensors, the panel is able to tell that the circuit is ok.  

 

[ano]

The  possibility for shorts or opens is why you need EOL resistors at the end of the zone wiring.   That way, even with N.O. contacts in the sensors, the panel is able to tell that the circuit is ok.  

You do need EOL resistors, no doubt, but the reality is if the wire did burn through you would only get a trouble indication.  Your alarm company would know, they would send you an email, but that isn't much help if your house is burning down.  Use fire-proof wire which is RED, provide some redundancy, and you'll be O.K.  Sensors and wiring is cheap. It never hurts to have twice as many sensors as you think you need.
 
If your alarm company gets a fault on one fire zone, then gets an fire alarm on another zone, they will take it very seriously.

 

[RAL]

You do need EOL resistors, no doubt, but the reality is if the wire did burn through you would only get a trouble indication.  Your alarm company would know, they would send you an email, but that isn't much help if your house is burning down.  Use fire-proof wire which is RED, provide some redundancy, and you'll be O.K.  Sensors and wiring is cheap. It never hurts to have twice as many sensors as you think you need.
 
If your alarm company gets a fault on one fire zone, then gets an fire alarm on another zone, they will take it very seriously.

 
 
You'll get no argument from me about using fire-rated cable (and it's not necessarily red).  FPL (or better) is a must.  EOL resistors help protect against many of the other things that can cause a failure besides a fire burning through the cable.

 

[Del91574]

Let's clear up some fallacies.
 
FPL is not "fire rated" cable. The rating refers to the generation of smoke and how fast flames travel on it, not how well it would survive in a fire situation. That is an entirely different ballgame that involves a rating called CI or CIC cable(or even MI) and even then, it's only a 2 hour rating (I won't bring up the cables that are listed for longer, whole other discussion)
 
FPL will and can burn through and not generate a trouble condition or even an alarm. Has happened many times before, which is why CI cable exists.
 
The purpose of the EOLR is as stated, supervise the circuit for a single open or short (or device removal). It does not guarantee the loop as being viable if more than one fault exists. That's why there's different wiring methods and isolation devices out there.
 
As RAL alluded to, fire alarm is generally has FPL (or higher) rating. Riser is not needed in residential, and plenum does not mean that it can be run through ductwork. Also, fire cabling does not need to be red, again, as RAL said. It can even be run in romex or THHN, but the wiring methods change to be the same as HV.

There are also listed substitutes for FPL and a hierarchy to what supersedes what.

 

[heffneil]

Thanks for all the insight guys.  I appreciate everyone's expertise and experience.  I was told by my insurance inspector I should put heat sensors in the garage - which I am going to do and also heat sensors in my attic.  I think smokes would be better and my attic is part of the building envelope since it is sealed not vented.  I am ok with heat sensors but assume the smoke would catch something faster (maybe I will put a couple near the AC units)

 

[Del91574]

Unless the attic is finished space designed for habitation, do not install smoke detectors there.
 
If the concern is the HVAC, then install a duct detector in the supply.

 

[heffneil]

Yeah I think someone here said it isn't about life safety as much as it is for property damage.  There are a lot of junctions and whatnot in the attic along with the AC units. Just makes me nervous that something could go wrong up there and I wouldn't know.  I kinda wish I put smoke detectors because as the old adage says Smoke Before Fire.  It is a sealed attic.  I haven't installed them yet so I might change to a smoke up there - not sure.

 

[ano]

Yeah I think someone here said it isn't about life safety as much as it is for property damage.  There are a lot of junctions and whatnot in the attic along with the AC units. Just makes me nervous that something could go wrong up there and I wouldn't know.  I kinda wish I put smoke detectors because as the old adage says Smoke Before Fire.  It is a sealed attic.  I haven't installed them yet so I might change to a smoke up there - not sure.

You won't easily find UL smoke detectors that can take attic temps, but you should install heat and rate-of-rise sensors up there (designed for attics).  Also a dusty environment and smoke alarms are not a good combination.) We have a gas furnace up there, so I installed heat and RoR sensors as close as allowed to this furnace.  You are correct, many fires start in the attic, so you do need protection. and the most common cause of attic fires? Electrical wiring.
 
Where I live, fire sprinklers are required in your house or apartment,  but they are still relatively uncommon in attics, but that is starting to change.

 

[Del91574]

It's not about UL listing, it's the definition of conditioned space, which an attic can be, if it's completely finished and has temperature control. You can, however, use an air sampling detector, but you're not going to like the price.
 
NFPA 72:
 
11.8.3.5
Specific Location Requirements. The installation of smoke
alarms and smoke detectors shall comply with the following
requirements:
(1) Smoke alarms and smoke detectors shall not be located where
ambient conditions, including humidity and temperature, are outside
the limits specified by the manufacturer's published instructions.
(2) Smoke alarms and smoke detectors shall not be located within
unfinished attics or garages or in other spaces where temperatures can
fall below 4°C (40°F) or exceed 38°C (100°F)