In an old fire alarm system in an apartment building, with 4 wires to the initiating devices, someone disconnected the last two heat detectors on the farthest zone (disconnected behind a functioning horn in the hall) and installed an end-of-line resistor above a functioning smoke detector in the hall, so the system didn't show a trouble.
It was picked up on an annual inspection when the last two devices weren't on the circuit. The normal annual test probably doesn't include pulling all the devices, so it may have been this way for many years.
I traced the circuit, removed the incorrect end-of-line resistor and reconnected the heat detectors with their existing end-of-line resistor and the circuit was fine.
My assumption is that a different person disconnected the last detectors than the person who installed the extra end-of-line resistor or both things would be in the same box.
What are your thoughts about why someone would do this?
On a different note, my favourite tool is now a telephone line toner. This system was installed many years ago and most of the conduits and #500 wiremold contained 8 wires (for 2 zones). All the wires were brown and blue. The boxes were too small and the wires were short or pulled through the boxes without a splice. It would have been a lot harder to trace the wires without a toner. I wonder what else a toner can be used for.
I can imagine a few different scenarios, some of which involve only one person. What matters is that all scenarios boil down to two causes: either there was a problem that was 'fixed' by disconnecting the devices, or there was a deliberate reason the system was altered.
In their UL testing, the primary concern is that there be no false alarms. With that in mind, I'd pay particular attention to the disconnected section of the system: Is there any reason that there may have been false alarms? For example, could the afternoon sun have caused a higher ambient, causing the heat detectors to trip? Or, does a megger test suggest any damage to the wires themselves?
Otherwise, there are many reasons a system might be altered. Changes in tenancy, secretive behaviour by someone, preparation for arson, etc.
It's even possible that the two 'disconnected' devices were not disconnected at all. That is, someone may have tried to add the devices to the system, and to have been unsuccessful because they didn't know enough to remove that first resistor.
The way the devices were disconnected was by identifying the wires with tape so they could easily be reconnected with the proper polarity - not that polarity is an issue with those heat detectors. It looked like someone with easy access to coloured tape did it.
The extra eolr was installed by someone who had access to resistors, possibly one that was removed during the original installation and left in the panel, or a new resistor.
The heat detectors were old-school with the pin in solder - one shot style. They were not rate of rise and are in dark hallways away from windows. I didn't find any faults on the last part of the circuit but I wasn't the first to look at it. There must have been a fault or it wouldn't have been disconnected.
I expect anyone working on a fire alarm system to not leave it partly disconnected. In whose mind is that okay? If someone died in a fire, the only upside would be that you might be re-trained to cut hair in prison.