A detector two hundred metres long.

Picture two steel conductors twisted around each other, separated by a plastic that stands the normal temperature of the place and softens at a set temperature. While nothing is heating up, the two conductors are apart and the cable is a supervised open circuit. If at any point along the run the temperature reaches that value, the plastic gives way, the conductors touch and the circuit closes. That is the alarm.

What is notable is where it happens: at any point. There are no detectors spread out with gaps between them; the detector is the whole cable, metre by metre. And the unit watching it can also say how many metres from the start the contact happened, by measuring the resistance up to the point where they touched. That turns two hundred metres of conveyor belt into a particular location instead of «there is a fire on the conveyor».

It does not detect smoke and does not claim to: it detects heat, and so it detects later than a well fitted smoke system. The right question is not whether it detects sooner or later. It is whether there is anything else in that place that can detect anything at all, because very often there is not.

Conveyor belts. The risk is in the roller that seizes and heats the rubber, and it is on the move along hundreds of metres. A detector on the roof of the unit is not going to find out; a cable run along the conveyor is, and it also says which section.

Tunnels and car parks. Long ceilings, draughts, exhaust gases and vehicles. The smoke a point detector needs is carried off by the ventilation or produced by a lorry starting up; the heat of a burning car is not mistaken for anything. It is the classic place for linear detection, and it usually goes with splitting the response into sections.

The first is the temperature of the cable, and it is the decision that ruins the most installations. The cable is made in several actuation values, and you have to choose one far enough above the maximum temperature that place reaches on a summer day with the machine at full tilt. A cable chosen for a mild industrial unit, put over a conveyor in a hot process, is going to give an alarm in August. And a cable chosen too high out of fear of that is going to detect when it is no longer any use.

The second is where it runs. It is not laid «around the area»: it is laid against the risk, which on a conveyor is the line of rollers, on a tray is alongside the cables, and in a car park is the route and the height the project says. With its fixings, respecting the bend radius, without tension and without resting it on anything that gets hot for another reason.

And the fourth, the one nobody warns you about: when the cable acts, the section that reacted is ruined. The plastic has given way and that part does not go back to how it was. You have to cut out the affected piece, joint in new cable and check the loop again. It is cheap and it is quick, but it has to be allowed for: a spare reel in the store, proper joints and somebody who knows how to make them. Otherwise a small outbreak leaves the area with no detection for weeks.

What makes it reliable is that it has no electronics inside. A cable with two conductors and a plastic does not get dirty, does not lose its configuration, does not run out of battery and does not age like a sensor. The unit watching it supervises the circuit end to end, so a cut cable gives a fault and not silence. In places where anything else breaks down, that is the main reason for choosing it.

It goes off when it should not for three reasons, and all three are project reasons. A badly chosen actuation temperature, which is the first by a long way. Cable resting against something that gets hot and is not a fire: a steam pipe, a motor, a lamp, a metal roof in the sun. And mechanical damage, which does not give a false alarm but does give a fault: a cable run over by a forklift, crushed by racking or cut by a trade that did not know what that cable was.

It is not a substitute for smoke detection in a building with people in it. In an office, a corridor or a room, detecting heat means detecting when the fire is already formed, and that is not acceptable where anybody has to be evacuated. Linear detection covers runs and enclosures, not occupancy.

We say no when somebody wants it because it comes out cheaper than what is called for. It happens in big industrial units: a cable across the roof instead of a proper detection project. It lights up the same, it passes commissioning the same, and it moves the whole risk onto the client without the client finding out. That is the line we do not cross, and it is the same one the area sets out.

In a car park, on a conveyor or in a chamber, linear detection is usually there to do two things: raise the alert, and give the order to a water spray installation or to a deluge that wets that section. The order is ours. The cable, the zone unit, the logic that decides which section, the interlock that stops the conveyor, the alerts and the signal to the building's panel: all of that is electronics, and we do it.

The pipework, the nozzles, the valve, the flow calculation and the pressure set that pushes it, no. That is mechanical installation with another licence and another signature, and it goes with a different contractor. The same for sprinklers, dry risers, hose reels and tanks. We do not do it, and we say so on day one.

The day to day is not much: looking at the unit and noting down what is left out of service, which counts for more here than on a panel. And there are two more things that cost nothing and save the installation. The first is looking at the cable. It is a detector you can see: a run trodden on, crushed by racking, hanging loose or covered by stock is a finding anybody can make on a walk round the unit. The second is giving notice before any work in that area, because the trade that does not know what that cable is will cut it.

Repairing a section that has acted can be done by the client's own maintenance team if they have been trained and have the material, and it is almost always worth it: it is hours of difference. What is not touched from inside is the zone configuration or the interlocks with the machinery. And the inspection schedule is still compulsory and still costs money every year.