(1) the detector circuit, (2) the alarm circuit, and (3) the test  circuit. These circuits are shown in  Figure 9-2 . power system and thermal switches that control operation of  the light(s). These thermal switches are heat-sensitive units  that complete electrical circuits at a certain temperature.  They are connected in parallel with each other, but in series  with the indicator lights  [Figure 9-1] . If the temperature  rises above a set value in any one section of the circuit, the  thermal switch closes, completing the light circuit to indicate  a fire or overheat condition. No set number of thermal switches is required; the exact  number usually is determined by the aircraft manufacturer.  On some installations, all the thermal detectors are connected  to one light; others may have a separate thermal switch for  each indicator light. The relay box contains two relays, the sensitive relay and the  slave relay, and the thermal test unit. Such a box may contain  from one to eight identical circuits, depending on the number  of potential fire zones. The relays control the warning lights.  In turn, the thermocouples control the operation of the relays.  The circuit consists of several thermocouples in series with  each other and with the sensitive relay. Some warning lights are push-to-test lights. The bulb is tested  by pushing it in to check an auxiliary test circuit. The circuit  shown in  Figure 9-1  includes a test relay. With the relay  contact in the position shown, there are two possible paths  for current flow from the switches to the light. This is an  additional safety feature. Energizing the test relay completes  a series circuit and checks all the wiring and the light bulb.  Thermocouple leads are made from a variety of metals,  depending on the maximum temperature to which they are  exposed. Iron and constantan, or copper and constantan,  are common for CHT measurement. Chromel and alumel  are used for turbine EGT thermocouples. The point where  these metals are joined and exposed to the heat of a fire  is called a hot junction. There is also a reference junction  enclosed in a dead air space between two insulation blocks.  A metal cage surrounds the thermocouple to give mechanical  protection without hindering the free movement of air to the  hot junction. Thermocouple leads are designed for a specific  installation and may not be altered, if they are altered they  will no longer be calibrated specific probe and instrument  combination. Also included in the circuit shown in  Figure 9-1  is a  dimming relay. By energizing the dimming relay, the circuit  is altered to include a resistor in series with the light. In  some installations, several circuits are wired through the  dimming relay, and all the warning lights may be dimmed  at the same time. If the temperature rises rapidly, the thermocouple produces  a voltage because of the temperature difference between the  reference junction and the hot junction. If both junctions  are heated at the same rate, no voltage results. In the engine  compartment, there is a normal, gradual rise in temperature  from engine operation; because it is gradual, both junctions  heat at the same rate and no warning signal is given. The thermocouple fire warning system operates on an  entirely different principle than the thermal switch system.  A thermocouple depends on the rate of temperature rise and  does not give a warning when an engine slowly overheats  or a short circuit develops. The system consists of a relay  box, warning lights, and thermocouples. The wiring system  of these units may be divided into the following circuits:  If there is a fire, however, the hot junction heats more rapidly  than the reference junction. The ensuing voltage causes a  current to flow within the detector circuit. Any time the  current is greater than 4 milliamperes (0.004 ampere), the  sensitive relay closes. This completes a circuit from the  aircraft power system to the coil of the slave relay. The slave  Figure 9-1.  Thermal switch fire circuit. Figure 9-2.  Thermocouple fire warning circuit.  Thermocouple Systems Indicator light R Dimming relay Resister Test switch Thermal switches Test relay Detector  circuit Alarm circuit Test circuit Thermocouples Sensitive relay Slave relay Heater Test switch 9-3