You may already have them fitted and not know it.

In a detection system there are two different worlds. One is the panel: the cabinet that asks, decides and commands. The other is the elements that hang off the cable: the detectors, the call points, the sounders and the modules. There are houses that do both and houses that stick to the second. This one is one of the second sort.

Its elements are found hanging off panels from several brands, because the protocol they speak is understood by more manufacturers than its own. The practical consequence is the one that brings people to this page: plenty of installations have these detectors without anybody remembering having chosen them, and when the panel has to be extended or changed, the first thing is to know what is there.

It is found in every type of building: offices, schools, industry, hotels, care homes, retail. And it turns up in both ways of building a system, which are worth telling apart — conventional and addressable— because almost everything else depends on that.

In a conventional system the detectors are cabled by zone: all the ones in a wing on the same pair of wires. When one goes off, the panel says «zone 3» and somebody has to walk zone 3 to see which it was. It is simple, it is cheap and for an industrial unit with four zones it is enough.

In an addressable system each element has its address inside a loop that goes out and comes back. The panel says exactly which detector, it can tell you its history —whether it is getting dirty, whether it has been at the limit for months— and it carries on talking to all of them if the cable is cut at one point, because it gets there from the other side. With isolators spread out, a short circuit eats one section and not the storey.

The difference shows up twice: on the day it goes off, because in one you know the point and in the other the zone, and at every inspection. In a large building, conventional means walking; addressable means going to whatever asks for it. That is why the border between the two is not set by the budget, it is set by the size of the building and what it costs to walk it.

Optical for offices, corridors, rooms, suspended ceilings with cable inside. Heat for kitchens, car parks and plant rooms, where the smoke and steam of normal activity would set off an optical detector every week. Multi-criteria where you need the sensitivity of the optical one in an environment that is not clean: it looks at more than one thing and only believes the smoke if something else goes with it.

And where none of the three will do, you change family: flame outdoors, in very tall industrial units and in areas with air currents, because the smoke does not reach the ceiling; aspirating detection when you have to find out very early or the ceiling is twelve metres up; linear heat detection cable on conveyor belts and in service tunnels.

What a detector manufacturer brings to this is not glamorous and shows up at the inspection: chambers that get dirty slowly, electronics that move little with temperature and humidity, and a small drift over the years. It is not a catalogue argument, it is what decides how many times somebody has to go up to a six-metre ceiling.

A protocol being spoken by several manufacturers has one big advantage: when the panel is changed there is a real chance of keeping the elements and the cabling, and that turns the budget for a project into the budget for an upgrade. But it has to be checked element by element and in the installation, not on a spreadsheet: inside the same protocol family there are generations, and what works with one panel does not always work with another.

And it has a seam that is worth knowing about before it shows up: when the panel is from one house and the detector from another, odd behaviour —an element that goes missing now and then, an alert nobody understands— turns into a conversation between two technical support teams who do not know each other. That is why in that scenario the one who answers for it working has to be one party, and it is whoever built the installation.

It fits when it is already there and the head end has to be extended or changed while keeping the field equipment, and it fits when it suits you not to be tied to a single house for the elements. It does not fit if what you want is a single-brand system from end to end with one phone number for everything: that is a legitimate decision, and then the conversation is a different one.

And what is not ours, said early: we are an electronics firm. Detection, alarm, evacuation, control panel, integration with the rest of the building and maintenance, yes. The water, no — sprinklers, fire hose reels, dry risers, pump sets and tanks are another line of work, with another licence and other trades, and we do not subcontract it to look as if we did it. And if there is gas fire suppression, the line runs down the middle: what decides, counts the time, warns and releases is ours; the cylinders and the pipework leaving them belong to somebody else.

Using the system belongs to the client: silencing, resetting after checking, reading the history to know what time it started and where, and understanding a fault alert. In a conventional installation you also have to teach people to read the zone plan, because the panel is going to say a zone and somebody is going to have to walk it.

In between there is the temporary disabling of a zone, which happens every week as soon as there is work going on inside. It is taught, with a written rule and a record of who disables and who puts it back, because if it is not taught somebody is going to solve it by disconnecting something — and that leaves no trace.

What does not belong to the client: adding or moving elements, touching the programming, changing sensitivities and updating the panel. That is maintained from a licensed company, with its schedule and its signed file, and it is not a preference of ours. What is in your hands is to demand the full handover: drawings of what is there, one-page procedures and separate training for whoever operates and whoever maintains.