Electrical connector Diaphragm Gland nut Bottle (REF) Discharge port Ground lug Squib Discharge assembly L E F T Disch 1 2 L E F T Disch 1 2 ENG BTL 1 DISCH ENG BTL 2 DISCH Engine fire warning light Quick release fastener (4) Engine fire switch Engine fire override switch Electrical connector Engine bottle discharge lights Figure 9-26.  Fire switch.  Figure 9-25.  Squib or cartridge. APU Fire Detection and Extinguishing  System APU Fire Warning The APU fire protection system is similar in design to engine  fire protection systems, but there are some differences.  The APU is often operated with no personnel in the flight  deck and; the APU fire protection system can operate in  an unattended mode on the ground with the engines not  running. If there is an APU fire in the unattended mode, the  fire extinguisher discharges automatically. The APU operates  in the attended mode when at least one engine is running. If  there is an APU fire in this mode, the crew discharges the  bottle manually. Fire switches are located on the cargo fire/ engine control panel and the service and APU shutdown  panel located outside the airplane on the nose landing gear.  [Figure 9-28] If there is an APU fire, the APU fire detection system gives  fire warnings and automatically stops the APU. The APU fire  warning light comes on to identify the correct fire switch to  use to extinguish the fire. The fire switch solenoid releases  the switch so that it can be pulled up. If the APU is running,  9-18