Fire Detection and Protection — Page 292, Lesson 369

Let's pick up with continuous fire detectors. These are the ones commonly called Fire Wire, and the full name you'll see in the manuals is "Free From False Detection," abbreviated FFFD. That name tells you the whole design goal: it's built to resist giving you a false warning. The operating principle is that the sensing element has either a negative coefficient of resistance or a positive coefficient of capacitance. One particular system actually uses both. Let me unpack that. A negative coefficient of resistance means the material's resistance goes down as temperature goes up. A positive coefficient of capacitance means capacitance goes up with temperature. Either way, the physical property changes with heat, and that change is what triggers the warning. Now the construction. The element is a stainless steel tube. Inside that tube, running down the center, is a central electrode. Between the electrode and the tube is a temperature-sensitive insulating material. So you have a tube, an insulator, and a central wire — that's the basic anatomy of a fire wire element. Let's look at how the resistive type works, because that's the classic one. As temperature rises, the resistance of that insulating material decreases. That means current can leak between the central electrode and the outer tube more easily. The leakage current increases with temperature. At a predetermined level, enough current flows to operate the warning system. And here's a nice feature: if the temperature drops back below a preset value, the system automatically resets. So it's self-resetting, no manual intervention needed. Now the capacitance type. Here, an increase in temperature causes an increase in capacitance. The element is polarized by applying half-wave rectified AC from a control unit. Let me explain that. Half-wave rectified AC means only one half of the alternating current cycle is allowed through, so the element gets charged up in one direction. The element stores that charge, then discharges it as a feedback current. Once that feedback current reaches a predetermined level, it activates the aural and visual fire warnings. Aural means the sound warning, visual means the light or display indication. This capacitance system also resets itself once temperature drops below a preset level. But it has a distinct advantage over the resistive type: if a short circuit grounds the element or the system, it does not result in a false warning. That's a significant reliability point. A grounded short in a resistive system could give you a spurious fire indication; the capacitance system is immune to that failure mode. Now, how are these wires installed? They're positioned around the engine fire zones in a continuous double loop. That's important. Both loops have to detect a fire to initiate the warning. So it's a voting system — two independent sensing loops, and you need agreement from both before you get a fire warning. That's another layer of false-warning protection. The system is AC supplied, and it has the ability to continue functioning with a single wire break. So if one loop breaks, the system still works. But you get a warning of that malfunction, and it may be displayed on the fire detection panel or on the electronic system display unit. So you're told that you've lost redundancy, even though the fire detection itself still functions. Let me show you the construction so it's clear in your mind. So to summarize the key points you need to hold onto: Fire Wire is FFFD, Free From False Detection. It works on negative coefficient of resistance or positive coefficient of capacitance. The element is a stainless steel tube with a central electrode insulated by temperature-sensitive material. The resistive type triggers on leakage current at a predetermined level and auto-resets. The capacitance type triggers on feedback current from stored half-wave rectified AC, also auto-resets, and is immune to false warnings from a grounded short. The wires are installed as a continuous double loop around the engine fire zone, both loops must detect fire, the system is AC supplied, and it keeps working with a single wire break, with the malfunction shown on the fire detection panel or electronic system display unit.