3. Test circuit  2. Alarm circuit  1. Detector circuit  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 17-1]  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 is usually determined by the  aircraft manufacturer. On some installations, all the thermal  detectors are connected to one light; on others, there may be  one thermal switch for each indicator light. These circuits are shown in  Figure 17-2.  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 17-1  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. Also included in the circuit shown in  Figure 17-1  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. The thermocouple is constructed of two dissimilar metals,  such as chromel and constantan. The point at which 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. 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. 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  relay then closes and completes the circuit to the warning  light to give a visual fire warning. The thermocouple fire warning system operates on an  entirely different principle from 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:  Thermocouple System 17-3 Figure 17-2.  Thermocouple fire warning circuit. Figure 17-1.  Thermal switch fire circuit. R Resistor Indicator light Dimming relay Test switch Test relay Thermal switches Thermocouples Sensitive relay Slave relay Test switch Heater Test circuit Alarm circuit Detector circuit