A fire panel talks to every detector on its loop by address and each one answers with its own reading, and the weekly test prints the lot. Address 47 was the detector in the ceiling of screen 6, which became half of screen 5 nine years ago. It answers every week, from a ceiling that is not there, and its reading goes up when it is dark.
I hold the fire log for a cinema, which means the weekly test is mine.
How the loop works.
Every detector, sounder and call point has an address on a two wire loop. The panel polls each address in turn and the device answers with its status and, for a smoke detector, its chamber reading, which is a number that rises when there is smoke in it. The panel prints the roll call on the weekly test and I sign it.
When a device is removed its address is deleted from the panel, and if it is not, the panel reports the address as missing, every week, until somebody fixes it.
Address 47 was the smoke detector in the middle of screen 6's ceiling.
Screen 6 was knocked through into screen 5 in 2015 to make one bigger room. The wall came down, the ceiling came out, and the loop was rewired: screen 6's devices were removed, their addresses deleted, and the new room's devices commissioned with new addresses. I have the commissioning certificate.
Address 47 answers the roll call every week.
It reports as a smoke detector, healthy, with a chamber reading.
Not a missing device. A missing device is what you get when an address is left on the panel with nothing on it, and that is a fault I have cleared a hundred times. This is an address that was deleted, answering.
What has been done.
The panel was replaced in 2019. The new panel was programmed from the new drawings by the alarm company, with the devices that exist, and it found 47 on the loop on its first poll and added it as an unconfigured device, which is a thing panels do when they hear an answer from an address they were not told about.
The loop has been walked with a tester that reads each device's address at the device. Every device on the loop is where the drawings say, with the address the drawings say. There is no device with address 47. There is nowhere for one to be: the ceiling it was in is gone and the space is the upper part of screen 5's auditorium, air, with the new ceiling six metres higher.
The alarm engineer measured the loop's current. Every device draws a little standing current and the total should be the sum of the devices. It is the sum of the devices plus one smoke detector's worth.
The chamber reading.
A smoke detector's reading sits at a baseline in clean air and rises with smoke, and the panel logs it so a slowly dirtying detector can be caught before it false alarms.
Address 47's reading is at baseline during the day.
In the evening, when the lights go down in screen 5 for a show, it rises. Not to alarm. To about a third of the way, the reading you get from a detector in a room with a little haze in it, and it falls back when the lights come up.
Screen 5 has no haze. It is a smoke free building and the detectors that exist in that room, the real ones, sit at baseline through every show.
Something at the height of the old ceiling, in the dark, in the air above the audience, reads as a light haze to a detector that was removed nine years ago.
What is done about it.
Address 47 is on the panel as an unconfigured device, which is a permitted state, and the fire officer has seen it on the printout twice and asked once and accepted the explanation that it is a legacy address that does not affect the cause and effect.
That is true. It cannot raise the alarm because it is not configured to, and it cannot suppress one. It just answers.
What I do.
I run the test on a Tuesday morning and I sign the printout.
The one thing I have changed is that I do not do the test with the house lights off in screen 5.
The test involves activating a call point in each zone and confirming the sounders, and I used to do screen 5's with the room dark, because that is when the sounders' strobes are easiest to check.
I do it with the lights up now, and the reason is a Tuesday in 2022 when I did it dark and looked up, as you look up in a dark cinema, at the space where the old ceiling was, and there was a small red light up there, at the height of the old ceiling, in the middle of the air, blinking once every few seconds the way a smoke detector's indicator blinks when it is polled.