A fire alarm company operations manager recently contacted me about a combination fire alarm system and the use of a single signaling line circuit (SLC) for both fire and intrusion or burglar alarm functions.
“The company I work for installed an addressable combination burglar/fire alarm system. Our installers on the job ran a single addressable loop to which they attached addressable monitoring input modules. A local inspector failed the system on final inspection because ‘the fire alarm system does not meet NFPA 72 requirements,’” he wrote. “The inspector stated that he’d prefer we use different systems for security and fire, but at minimum we’ll need two separate addressable loops to do the job with a single alarm panel. Can you explain the proper way to do this so I can pass it on to my installers?”
In this edition of “Fire Side Chat,” we’ll talk about this fire alarm dealer’s situation, including how his technicians might solve it.
Is the Fire Alarm Inspector Right?
Technically, the fire alarm inspector is correct, you cannot use a single SLC for both fire and burglar alarm functions. There are several reasons for this and two methods I can suggest to correct it, all of which involve the issues surrounding the use of combination fire alarm systems (see sidebar).
First and foremost, the burglar alarm function must not interfere with the fire alarm function. According to Section 126.96.36.199, NFPA 72, 2010, “Operation of a non-fire system function(s) originating within a connected non-fire system shall not interfere with the required operation of the fire alarm system, unless otherwise permitted by this Code.”
Second, the fire alarm function must take precedence above the burglar alarm system. For example, if the perpetrator of a crime happens to start a fire outside a building and the combination fire (fire/burg) alarm system detects his entry a second or so before the fire, the fire alarm portion of the system must do its thing without a moment’s hesitation.
At this point, the inspector appears to be concerned about the issue of interference between fire and burglar alarm addressable devices, which in the case of the writer are directly connected to the same SLC as the fire alarm MIMs devices. In a word, if an addressable security module fails, the MIMs must not in any way interfere with the ongoing operation of the SLC and the fire alarm panel to which it connects.
How to Solve the Problem at Hand
There are two possible ways to solve the immediate problem without pulling a second SLC. I’ll briefly touch on each of them and I invite anyone who needs additional details to contact me at www.alcolombo.net.
First, if the writer’s installers happened to use a four-conductor fire power limited (FPL) cable (riser or plenum), and if the fire/security MIMs modules in use require only two conductors for both data and power, then it’s possible to employ the remaining two conductors for the security MIMs.
I mention this because it’s standard operating procedure for some fire alarm engineer/designers to automatically specify four-conductor FPL cable for the SLC as a matter of course. In support of this practice: “Wire is cheap, but labor will break you.”
The second method of correction involves the use of something called “isolation modules.” Isolation modules, which connect between SLC and one or more security MIMs modules, are designed to “isolate” or insulate a lone branch circuit with a fault from all the others downstream on the SLC.
Make Use of Isolation Modules
The use of isolation modules is of utmost importance in a combination fire alarm system where addressable burglar alarm devices are employed. The integrity of fire MIMs is maintained by disconnecting faulted security MIMs when a short circuit occurs in a nonfire circuit.
“In Class B/Style 4 configuration, one AML-770 loop isolator is used on each detector branch. A short circuit condition on one branch will isolate that branch, leaving the remaining branches in operation. A maximum of 25 detectors may be connected to each branch” (Digital Security Controls [DSC], Concord, Ontario, Canada).
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