NFPA 72 code sections · Troubleshooting & Repairs
One fault, one zone: how much an SLC short is allowed to take out
A short circuit or open on an addressable loop can't be allowed to silence a whole building. The code limits the effect of a single fault on an initiating device circuit or signaling line circuit so that the loss does not extend beyond one zone — the designer decides where fault-isolation modules go to meet that limit, and the installer has to put them where the drawings say. A loop that passes acceptance with no isolators because nothing was shorted that day is a loop waiting to fail hard.
NFPA 72 reference
Chapter 23 — Protected Premises Fire Alarm Systems / Chapter 12
23.6.1 Performance of Initiating Device Circuits, SLCs and NACs (a single open or short shall not cause the loss of more than one zone); 12.3.6 Class X and isolation modules; annunciation zone limits per 10.18
References use NFPA 72 2019/2022 numbering — section numbers can shift between editions, so confirm in the edition your AHJ has adopted.
On the job
A tenant's contractor drives a screw through an SLC in a six-story building. All 212 devices drop off. The shop drawings showed an isolator module at each floor; the installer had 'saved' them in a box. Six isolator modules later, a repeat of the fault takes down one floor and the trouble pinpoints it.
Quick check
1. A single short on an SLC is permitted to disable at most:
2. The device that limits the spread of an SLC short is a(n):
3. Isolator placement is decided by:
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