Seeing that something is heating up before there is a fire.

The digital cable is a long switch. Two conductors separated by a plastic that gives way at a particular temperature: until that value is reached nothing happens, and when it is reached the conductors touch and there is an alarm at that point. It is simple, robust and very reliable, and it is what you fit when what you need is to know that somewhere along the run a limit has been passed.

The analogue cable works another way: instead of waiting for a value, it measures. The unit interrogates the cable continuously and knows what temperature every part of the run is at, so it can do two things the digital one cannot. One is to warn when the temperature is rising fast even though it is still a long way from the limit. The other is to show a trend: this section is ten degrees above last week.

Translated into what happens to a client: with the digital one you find out that there is a problem. With the analogue one you find out that there is going to be one. And in the installations where this firm is strong, that difference is not a nuance, it is the whole product.

On a conveyor belt the fire starts in a bearing that seizes or in a roller that stops turning and begins to rub the rubber. That goes on for a good while before there is anything like a flame: first there is a hot spot, then it smells, then it smokes, and in the end the belt burns and the fire travels with it. An analogue cable run along the conveyor shows that hot spot while it is still a mechanical maintenance problem, and says which section it is in.

In waste and recycling plants the mechanism is different and the effect the same: a pile of material self-heats from the inside, or something goes in that should not —a lithium battery, an aerosol— and starts to heat up in the middle of tonnes of fuel. What is needed there is not a smoke detector, which would be giving alarms all day because of the dust: it is knowing which part of the pit or the line is hotter than normal.

The decision that rules is still the temperature. On the digital one it is the cable's actuation temperature, and it has to be chosen above the real maximum of the place on the worst day of the year with the process at full tilt. On the analogue one the choice is different and finer: you have to know the normal temperature of the run, because what gets configured is an absolute threshold and a rate of rise, and that requires knowing how that place behaves across a full shift.

The route follows the risk, not what is convenient. On a conveyor that is the line of rollers, and in some cases more than one run has to be laid because the belt return has a risk of its own. With its fixings, its bend radius and without resting it against anything that gets hot for another reason: a pipe, a motor, a steel sheet in the sun.

What makes it reliable is that inside the area there is only cable, and the unit supervises the circuit end to end: a cut cable gives a fault, not silence. And the analogue one adds reliability of another kind: because it measures instead of waiting, it lets you tell «this is rising because it is August and the unit is at forty degrees» from «this is rising because something is rubbing». That distinction is what lets you have useful alerts in a place where a fixed threshold would give alarms every summer.

It goes off when it should not for the same three reasons as always, and all three are project reasons: a badly chosen threshold because nobody measured the real temperature of the place, cable resting against a heat source that is not a fire, and mechanical damage, which gives a fault. In waste plants there is a fourth that belongs to them: the cable shifted by the loader or buried where it should not be. That is not a system fault, it is an installation that has changed, and it is fixed with an inspection round and not with the equipment.

It does not cover occupancy. In a building with people who have to evacuate, detecting heat is detecting late, and what is called for is smoke detection with its panel. Linear detection protects runs, machines and enclosures; it does not replace the building's system, it completes it.

And there is one particular case where we say no, and in these industries it happens: fitting analogue linear detection and putting nobody in charge of looking at what it measures. All the value of measuring instead of waiting is in somebody seeing that a section has been hotter for three days and sending a fitter. If the early alert does not reach maintenance, all that is left is a system more expensive than a digital one that does the same thing.

Our work here is the electronics and the decision: the cable, the unit, the thresholds, the zoning, the alert to whoever has to go and look, the interlock that stops the conveyor or the line, and the signal to the building's panel so that what happens in the unit is seen where everything is watched. And the maintenance of all of that.

The water is not ours. In these installations there is usually water spray over the conveyor behind the detection, a deluge over the pit or a monitor, and that is installed by a licensed mechanical installer, with their flow calculation, their pressure set, their pipework and their signature. The same goes for sprinklers, dry risers, hose reels and tanks: another trade.

This is operated from maintenance more than from security, and that is what is particular about it. The person who has to learn to read it is the mechanical maintenance manager, because the early alerts are for him: a section of conveyor that has been hotter for days is a bearing to change at the next shutdown, not an emergency. Used well, the system ends up saving unplanned stoppages, and that is a benefit that was not in the fire budget.

What is not touched from inside are the configured thresholds and rates of rise, the zoning, or the interlocks with the machinery. And repairing a damaged section can be done by the client's own team if they have been trained and have the material, which in a plant with a loading shovel is worth doing.

The two free routines that save the installation: looking at the cable on the inspection round —trodden on, buried, hanging, covered— and giving notice before any work in that area. And the inspection schedule, which is compulsory, has its cost every year and it is worth knowing what that is before signing.