relay then closes and completes the circuit to the warning  light to give a visual fire warning. 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 9-2 , 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. When this happens, the coil 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.  Thus, arcing at the sensitive relay contacts is eliminated. Optical sensors, often referred to as “flame detectors,”  are designed to alarm when they detect the presence of  prominent, specific radiation emissions from hydrocarbon  flames. The two types of optical sensors available are infrared  (IR) and ultraviolet, based on the specific emission wave  lengths they are designed to detect. IR-based optical flame detectors are used primarily on light  turboprop aircraft and helicopter engines. These sensors have proven to be very dependable and economical for the  relatively benign environments of these applications. Radiation emitted by the fire crosses the airspace between  the fire and the detector and impinges on the detector front  face and window. The window allows a broad spectrum  of radiation to pass into the detector where it impinges  on the face of the sensing device filter. The filter allows  only radiation in a tight waveband centered around 4.3  micrometers in the IR to pass on to the radiation-sensitive  surface of the sensing device. The radiation striking the  sensing device minutely raises its temperature causing small  thermoelectric voltages to be generated. These voltages are  fed to an amplifier whose output is connected to various  analytical electronic processing circuits. The processing  electronics is tailored exactly to the time signature of all  known hydrocarbon flame sources and ignores false alarm  sources, such as incandescent lights and sunlight. Alarm  sensitivity level is accurately controlled by a digital circuit.  A typical warning system is illustrated in  Figure 9-3. When the fire has been extinguished or the critical temperature  lowered, 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. The Fenwal system uses a slender inconel tube packed with  thermally sensitive eutectic salt and a nickel wire center  conductor.  [Figure 9-4]  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  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. A continuous-loop detector, or sensing system, permits  more complete coverage of a fire hazard area than any  of the spot-type temperature detectors. Continuous-loop  systems are versions of the thermal switch system. They are  overheat systems, heat-sensitive units that complete electrical  circuits at a certain temperature. There is no rate-of-heat- rise sensitivity in a continuous-loop system. Two widely  used types of continuous-loop systems are the Kidde and  the Fenwal systems. This text briefly discusses the Fenwal  system, while the Kidde system is discussed more in-depth. Large commercial aircraft almost exclusively use continuous  thermal sensing elements for powerplant protection, since  these systems offer superior detection performance and  coverage, and they have the proven ruggedness to survive in  the harsh environment of modern turbofan engines. Pneumatic detectors are based on the principles of gas laws.  The sensing element consists of a closed helium-filled tube  connected at one end to a responder assembly. As the element  is heated, the gas pressure inside the tube increases until the  alarm threshold is reached. At this point, an internal switch  closes and reports an alarm to the cockpit. The pneumatic  detector integrity pressure switch opens and triggers the  fault alarm if the pneumatic detector losses pressure, as in  the case of a leak. Pneumatic Thermal Fire Detection Continuous-Loop Detector Systems Optical Fire Detection Systems Infrared Optical Fire Protection Principle of Operation Fenwal Continuous-Loop System 9-4