I've got an edwards QS4 fire alarm, I've got an adressible loop going around the building, 2 wires leaving and 2 wires returning. Can I tap 2 wires off the loop at any point to feed an adressible pull station, but not run 2 wires back onto the loop. Thanks for any input.
The 2 back are what create the Class A system. To the best of my knowledge, no you can't. You need the 2 going back. If the 2 going out get broken the Class A will still function because it is "double end fed".
Depending on how far you need the circuit extension, and the size room the device is in, the NFPA 72 (FA Code) would allow you to take two cables (2 wires in each plus shield) and run to the new device. This would effectively give you the two there and the two back.
You have some limitations in this scheme if they are in the same raceway or bundle because there would need to be some separation if the area served was large.
I have limited knowledge of addressable fire alarm systems. My question is aren't two wire class B system initiating device circuits more popular than the class A system? What are the advantages of the class A system?
Class A is generally not code required, but offers better reliability during certain faults and open circuit conditions.
Why not just identify your"in" and "out" pairs and then run 2 pair to the new pull station. Then splice them accordingly at the junction.
T-taps are allowed on Simplex addressable systems. I'm not sure if there is a maximum number of T-taps per Mapnet zone. If you remove a head or a base you do get notification of that trouble. T-tapping is a nice feature for the troubleshooter because when you have to remove a base for replacement you don't end up with up to 256 missing heads. These are initiating circuits and NOT signal circuits. I've had lots of experience on 4100, 4100U panels and on hardwired in lots of buildings, installation and repair.
Dave
Over here in the Uk where we use addressable alarm systems, we are only permitted to take t taps off a system if it is wired in a four core and ground cable. You are not permitted to use a 2 core cable as it will not provide the recommended monitoring and re feeding of the loop if there is a fault and the isolators operate. With the systems we use, you can get an interface which you put on the loop and then wire a conventional zone with 2 cores to you r chosen area, and then it is monitored via the interface.
Nick