The total number of thermocouples used in individual  detector circuits depends on the size of the fire zones and the  total circuit resistance, which usually does not exceed 5 ohms.  As shown in  Figure 17-2,  the circuit has two resistors. The  resistor connected across the slave relay terminals absorbs the  coil’s self-induced voltage to prevent arcing across the points  of the sensitive relay. The contacts of the sensitive relay  are so fragile that they burn, or weld, if arcing is permitted. When the sensitive relay opens, the circuit to the slave relay  is interrupted and the magnetic field around its coil collapses.  The coil then gets a voltage through self-induction but, with  the resistor across the coil terminals, there is a path for any  current flow as a result of this voltage, eliminating arcing at  the sensitive relay contacts. Transport aircraft almost exclusively use continuous thermal  sensing elements for powerplant and wheel well protection.  These systems offer superior detection performance and  coverage, and they have the proven ruggedness to survive in  the harsh environment of modern turbofan engines. A continuous-loop detector or sensing system permits more  complete coverage of a fire hazard area than any of the  spot-type temperature detectors. Two widely used types of  continuous-loop systems are the thermistor type detectors,  such as the Kidde and the Fenwal systems, and the pneumatic  pressure detector, such as the Lingberg system. (Lindberg  system is also known as Systron-Donner and, more recently,  Meggitt Safety Systems.)  The Fenwal system uses a slender Inconel tube packed with  thermally sensitive eutectic salt and a nickel wire center  conductor.  [Figure 17-3]  Lengths of these sensing elements  are connected in series to a control unit. The elements may  be of equal or varying length and of the same or different  The resistance of a sensor varies inversely as it is heated; as  sensor temperature is increased, its resistance decreases. Each  sensor is composed of two wires embedded in thermistor  material that is encased in a heavy wall inconel tube for high  strength at elevated temperatures. The electrical connectors  at each end of the sensor are ceramic insulated. The inconel  tubes are shrouded in a perforated stainless-steel tube and  supported by Teflon-impregnated asbestos bushings at  intervals. The shroud protects the sensor from breakage due  to vibration, abrasion against airplane structure, and damage  from maintenance activity. In the Kidde continuous-loop system, two wires are  imbedded in an inconel tube filled with a thermistor core  material.  [Figure 17-4]  Two electrical conductors go  through the length of the core. One conductor has a ground  connection to the tube, and the other conductor connects to  the fire detection control unit. As the temperature of the core  increases, electrical resistance to the ground decreases. The  fire detection control unit monitors this resistance. If the  resistance decreases to the overheat set point, an overheat  indication occurs in the flight deck. Typically, a 10-second  time delay is incorporated for the overheat indication. If the  resistance decreases more to the fire set point, a fire warning  occurs. When the fire or overheat condition is gone, the  resistance of the core material increases to the reset point and  the flight deck indications disappear. The rate of change of  resistance identifies an electrical short or a fire. The resistance  decreases more quickly with an electrical short than with  a fire. In some aircraft, in addition to fire and overheat  detection, the Kidde continuous-loop system can supply  nacelle temperature data to the airplane condition monitoring  function of the aircraft in-flight monitoring system (AIMS). temperature settings. The control unit, operating directly from  the power source, impresses a small voltage on the sensing  elements. When an overheat condition occurs at any point  along the element length, the resistance of the eutectic salt  within the sensing element drops sharply, causing current  to flow between the outer sheath and the center conductor.  This current flow is sensed by the control unit, which  produces a signal to actuate the output relay and activate the  alarms. When the fire has been extinguished or the critical  temperature lowered below the set point, the Fenwal system  automatically returns to standby alert, ready to detect any  subsequent fire or overheat condition. The Fenwal system  may be wired to employ a loop circuit. In this case, should  an open circuit occur, the system still signals fire or overheat.  If multiple open circuits occur, only that section between  breaks becomes inoperative. Kidde System Continuous-Loop Systems Fenwal System Sensing Element Figure 17-3.  Fenwal sensing element. 17-4 Inconel tube Eutectic salt packing Nickel wire conductor center