The fire protection system is separated into two categories: detection and extinguishing.  Detection systems are installed in the engine; wheel well, APU, and lavatory areas. Fire extinguishing capability is available for all engines, the APU, and the lavatories.
A fire detection sensor is wrapped around each engine case. Another fire detection sensor is installed on the engine firewall. The firewalls for #1 and #3 engines are in the engine mounting struts. Due to the mounting of #2 engine in the aft fuselage, there is a single overheat sensor serving both the vertical firewall forward of the engine and the horizontal firewall above.
The electrical power source required for activation of the engine fire detection circuits is essential AC.
The fire shutoff handles are located in the pilot's overhead panel. Upon activation of an engine fire detection systems a red warning light in the fire shutoff handle comes on, a master FIRE WARN light an the glareshield in front of each pilot comes on. and a bell sounds in the cockpit. Pressing the bell cutout button near the fire shutoff handles can silence the bell. On some aircraft also by pressing either FIRE WARN cap on the glareshield, or using the RESET switch on the FE's auxiliary panel. The lights in the fire shutoff handles remain illuminated until the high temperature condition no longer exists or the fire detection system is destroyed.
Two fire extinguisher bottles charged with Freon are available for combating an engine fire. These bottles, with associated plumbing are mounted on the right side of the aft airstair's area. Each bottle has a pressure gage. Required engine fire extinguisher pressure is around 575 pounds per square inch at a bottle temperature of 70 degrees Fahrenheit. Bottle pressure will decrease approximately four PSI for every one-degree drop in bottle temperature.
A selector valve is installed for each engine to control the direction of flow of the extinguishing agent.  Pulling a fire shutoff handle arms the associated selector valve.  When the discharge button is pushed. The extinguishing agent flows through the selector valve to the appropriate engine.
Fire shutoff handles contain the warning lights for the respective engines. In addition, on some aircraft there is a master "FIRE WARN" light located on the glareshield in front of each pilot. It comes on and the bell sounds along with the illumination of the respective fire detection system light.  Pressing on either master FIRE WARN light cap will silence the bell and the FIRE WARN light goes out. The fire shutoff handles are used to actuate the fire switches. When a fire shutoff handle is pulled the following things occur:
It arms the: Bottle discharge circuit and Engine selector valve. It closes the following Fuel shutoff valve, Engine bleed air (eng.#1 & #3), Bleed air valves, (eng. #2), Wing anti-ice shutoff (eng.#1 & #3), Cowl anti-ice (eng.#2), Hydraulic supply shutoff (eng.#1 & #2), Disarms the associated "A" hydraulic pump low pressure light and trips the generator field relay after a 5 - 10 second delay to allow the valves to close.
After the fire shutoff handle is pulled, pressing the bottle discharge button releases the extinguishing agent and opens the proper engine selector valve. Once it is open and pressure has built up at the valve, the discharge button may be released.  It takes about two seconds for the pressure to build up at the valve.  The engine selector valve closes when the extinguishing agent is depleted.  The left or right bottle is selected with the bottle transfer switch.  After discharging the first bottle and selecting the other, pressing the bottle discharge button again discharges the remaining bottle and the associated bottle discharge light comes on when bottle pressure is diminished.  Ordinarily, the bottle transfer switch is positioned to the left bottle. 
Three discharge discs are incorporated in the engine fire extinguishing system.  Each bottle has a red disc, which ruptures in the event of bottle discharge due to thermal relief.  There is a single yellow disc, which is ruptured by intentional discharge of either extinguisher bottle.  These discs are located on the right aft fuselage under the #3 engine strut.
Electrical power for discharge of the engine fire extinguisher bottles comes from the battery bus.  Engine fire detection power source is ESSENTIAL AC.
Detector circuit ground fault lights monitor the integrity of the engine fire detection system on some aircraft.  They are installed on the P-6 circuit breaker panel next to the fuel dump panel.  If an electrical ground fault in any engine system occurs. Its light comes on.  This provides warning of a potential malfunction in the engine fire detection system.
The APU fire detection system is identical to the systems used on the engines.  The fire detection sensor is looped around the APU within its shroud.  The electrical power required for activation of the APU fire detection circuit comes from the battery bus.
APU Fire Protection
The APU fire detection system provides cockpit and external visual and aural warnings.  The external warnings consist of a horn located in the nose wheel well, which sounds intermittently, and a flashing red light below the APU control panel in the left wheel well.  The cockpit warnings are illumination of the master FIRE WARN lights on the glareshield, if installed, the fire warning bell rings, and a steady red light in the fire shutoff handle on the engineer's auxiliary panel illuminates.
The fire warning bell can be silenced by pressing the bell cut-out button on the pilot's overhead panel; pressing either master FIRE WARN light on the glareshield; by selecting RESET on the APU test reset switch on the auxiliary panel; or by pressing the horn cut-out button on the exterior APU control panel in the left wheel well.  Pressing any of these cutouts silences both the horn and the bell and the red light in the left wheel well changes from flashing to steady.
The APU fire extinguishing system includes a fire extinguisher battle charged with Freon, and the associated discharge line and circuits.  The fire extinguisher-bottle is mounted behind the APU control panel in the left wheel well.  When this bottle is properly charged. The pressure gauge should indicate 350 PSI at a bottle temperature of 70 degrees Fahrenheit.
The APU fire extinguishing circuit can be activated by pulling the fire shutoff handle and pressing the battle discharge button on the APU cockpit control panel or by use of the fire shutoff handle and bottle discharge button on the exterior control panel. By pulling either APU fire shutoff handle, the following things occur: Arms the discharge button, Closes the fuel valve at the tank and the fuel solenoid valve on the fuel control unit, Closes the air load control valve, After a 5-9 second delay. trips the APU generator field relay, 
After either APU fire shutoff handle has been pulled, pressing the associated APU bottle discharge button releases the extinguishing agent into the discharge line.  There is no visual indication that the extinguisher bottle has been discharged, as is the case with the engine system. The battery bus provides the electrical power required for the extinguisher circuit.  In addition, the aircraft battery switch must be on in order for the APU to run.  Without the battery switch in the on position, there is no fire protection or detection for the APU.  It will shut down automatically if the switch is inadvertently turned off.
A red thermal discharge disc is installed on the fuselage Just forward of the left wheel well.  The disc ruptures if a thermal discharge of the fire extinguisher bottle occurs.
To conduct a fire test, the Fire Test switch is held in the TEST position.  The fire bell should ring within 60 seconds.  When the detector is heated to the alarm levels the red light in the APU fire shutoff handle comes on. the master FIRE WARN lights on the glareshield illuminate.  The fire bell in the cockpit will ring and the horn in the nose wheel well will sound intermittently. And the red light on the APU panel in the left wheel well begins to flash.
There is no need to fire test an operating APU, however, the automatic fire shutdown circuit can be defeated so that this can be done.  The auto fire shutdown switch controls this feature. which is labelled ARMED/OFF.  With the switch in armed, the APU will shut down when the fire detection circuit is energized.  When the switch is in the OFF position. A fire test may be made while the APU is operating.
Since the fire test system energizes the fire detection systems it must be reset after each fire test.  This is done by momentarily holding the fire test switch in the reset position after the fire warning light has gone out.  Remember. reset must be selected twice. Once to silence the bell and once to reset the auto fire shutdown circuitry.
Wheel Well Detection
The overheat detection system for the wheel wells provides detection only.  There is no extinguishing equipment installed in the wheel wells.  Fire detection elements are installed in the top of each of the three wheel wells.  When the detector is activated by an overheat condition, both visual and aural warnings are given.  The master FIRE WARN lights on the glareshield and the wheel well fire warning light on the pilot's overhead panel illuminate. and the fire warning bell rings.  The bell can be silenced by use of any of the bell cutouts used for engine or APU fire warnings.  The wheel well light remains an until the overheat condition no longer exists. The electrical power source for the wheel well fire detection circuit is essential AC.
To test the fire detection system. A fire test switch is located on the pilot's overhead panel.  The switch is spring loaded to the center.  The sensitivity of the circuits is such that it should take no more than 60 seconds to activate all three of the fire warnings.  Holding the switch in the up position tests the detection circuits for the firewalls.  Holding the switch in the down position tests the engine sensors and the wheel well detection loops.  The wheel well warning is activated immediately upon initiation of the test.  There is a delay for the engine and firewall warnings.
Fire extinguishing devices are located in the lavatories. Heat sensitive plugs automatically release the extinguishing agent to flood the water heater or towel dispenser if a fire should occur. A temperature sensitive indicator records temperature in the critical areas.  The appropriate circle will blacken to indicate temperature reached.  There is no visual or aural warning if this system in the toilet is activated.