We are going to do building work in an occupied building, what happens to the detection?

The second thing is dust. The detectors in the area are covered so they do not get dirty, and every one covered goes on a list with its address. That list is the only reason a covered detector ever gets uncovered again: without it they turn up three years later with the cover still on, at an inspection, if you are lucky. And people have to be told: the owner, the health and safety service, the alarm receiving centre if it is connected and the insurer when the works are of any size.

And the third thing is what is left when the works finish, which is almost always more than it looks. A new mezzanine creates a volume under the slab that the ceiling detector no longer sees; a partition splits a zone in two; a suspended ceiling changes the useful height; a door taken out breaks a separation between compartments that the design took for granted. None of those four things shows up as a fault on the control panel. Any change of compartmentation or of layout means reviewing the spread of detectors, the definition of the zones, the labelling and the drawings. The compartmentation itself —partitions, sealing of service penetrations, fire-resisting doors— is passive protection and another trade carries it out: we say what breaks in the detection when that changes, and we seal the penetrations we open ourselves.