B737-600/700/800 notes. [Mr Meljoe Ferreira].
Chapter 26 FIRE PROTECTION
*** Updated 23 Nov 2020 ***
The Fire protection system monitors the airplane for these conditions:-
D] Pneumatic duct leaks.
The Airplane has these fire/overheat detection systems:-
A] Lavatory smoke detection.
B] Engine fire detection.
C] APU fire detection.
D] Wheel well fire detection.
E] Wing/Body overheat detection.
The Airplane has these fire extinguishing systems:-A] Lavatory fire extinguishing bottles.B] Engine fire-extinguishing bottles.C] APU fire extinguishing bottle.D] Portable fire extinguishers.
Fire warn and master caution lights [P7].
Overheat/Fire protection panel [P8].
Major components of the Engine fire detection system:-
A] Engine overheat/fire detectors.
B] Engine and APU fire detection control module [M279].
C] Overheat/fire protection panel.
There are four fire detection loop assemblies in each engine.Each assembly has two detector elements, two gas charged sense tubes and a support tube. One is loop A and the other is loop B. each loop has four detectors.
The detector elements have these components:-
A] Overheat pressure switch.
B] Fire pressure switch.
C] Fault pressure switch.
E] Terminal stud.
F] A stainless steel, gas charged tube.
Locations of the Engine detector elements:-Two on the upper fan case section.Two on the lower fan case section.Two on the left core section.Two on the right core section.If one loop fails, the entire loop assembly has to be replaced.
The three pressure switches sense for these conditions:-
Fault [loss of gas pressure].
Gas pressure in the sense tube holds the FAULT pressure switch in the normally closed position. The other two pressure switches close when the gas pressure increases because of an overheat or fire condition.The overheat and fire signals go to the Engine and APU fire detection control module. This module supplies overheat or fire indication in the flight compartment.If the pressure in the sense tube decreases, the FAULT switch opens, This causes a large change in resistance which sends the fault signal to the Engine and APU fire detection control module. This module sets the fault condition and the related FAULT AREA light comes on.
The Engine and APU fire detection control module is located in E2-2.
There are five amber FAULT AREA lights on the front of the unit.
Engine 1 has a light for each detector loop.
Engine 2 has a light for each detector loop.
APU has one light for its detector loop.
Three red FAULT DISPLAY lights are below the FAULT AREA lights, these are light emitting diodes [LEDs].The FAULT/INOP TEST switch is below the FAULT DISPLAY placard.Under normal conditions, all front panel lights are off, when a fault condition occurs, the related FAULT AREA light comes on . The FAULT DISPLAY lights show a fault code.
FAULT/INOP TEST:- Press and hold for 5 seconds, Test passes if all 8 lights are on.
The Overheat detector switch on the overhead/fire protection panel lets the crew select the mode of operation.
Each switch has these positions:-
NORMAL:- loop select logic make loop A and loop B agree before the alarm condition operates [AND logic].
A:- loop select logic makes loop A the only sense authority [OR logic].
B:- loop select logic makes loop B the only sense authority [OR logic].
Results of an overheat condition:-A] Related P8 Engine fire handle unlocks.B] P8 ENG OVHT light comes on.C] P7 MASTER CAUTION lights come on.D] P7 OVHT/DET annunciator comes on.
Results of a Fire condition:-
A] Related P8 Engine fire handle light comes on and handle unlocks.
B] P7 FIRE WARN lights come on.
C] Aural warning unit gives the bell sound.
D] The overheat indications stay on.
The two tests you can do on the engine fire protection system are OVHT/FIRE and FAULT/INOP.
Each test can be done from the Flight compartment. You can also do the FAULT/INOP test at the Engine and APU fire detection control module.
A] All FAULT AREA lights on the Engine and APU fire detection control module come on.
B] The Engine and APU fire detection control module fault display lights come on after 5 seconds.
C] The P7 MASTER CAUTION and OVHT/DET annunciator lights come on.
D] P8 FAULT light comes on.
E] P8 APU DET INOP light comes on.
Indications of a good OVHT/FIRE test:-A] P7 FIRE WARN lights on the glareshield comes on.B] P8 ENG1, ENG2 and APU fire handles come on.C] P8 WHEEL WELL light comes on [115vac power is necessary].D] Aural warning unit supplies a bell sound.E] APU ground control panel operates the horn and light.F] P8 ENG1 and ENG2 OVERHEAT lights come on.G] P7 MASTER CAUTION lights come on.H] P7 OVHT/DET annunciator light comes on.
The APU detector loop has three sense elements. Each element has thee pressure components:-
A] Fire pressure switch.
B] Fault pressure switch.
D] Terminal stud.
E] stainless steel gas filled tube.
These are the locations of the three sense elements:-
A] Upper APU compartment.
B] Lower APU compartment
These are the results of an APU fire condition:-
A] APU fire handle lights come on.
B] APU fire handle unlocks.
C] Aural warning unit gives the bell sound.
D] APU automatic shutdown.
E] APU remote horn and light operate.
These are the flight compartment indications of an APU fire condition:-
A] Fire handle light on P8.
B] Fire warn light on P7.
C] Aural warning unit gives a bell sound.
The APU ground control panel supplies the external indications of an APU fire. A horn and a red light operate on the APU ground control panel. The horn sound and the red light operate alternately at a one time per second rate.
To stop the horn and bell sounds and cause the light to operate continuously push one of these buttons.
A] BELL CUTOUT push-button switch on P8.
B] FIRE WARN lighted push-button switch on P7.
C] HORN CUTOUT push-button switch on P28.
These are the fire extinguishing systems on the airplane:-
A] Engine fire extinguishing system.
B] APU fire extinguishing system.
C] Lavatory fire extinguishing system.
D] Portable fire extinguishers.
These are the components of the Engine fire extinguishing system:-
A] Engine and APU fire control unit.
B] Two fire extinguishing bottles.
C] Engine fire extinguishing ports.
When there is an overheat or fire condition, the fire warning switch for that engine unlocks. You can also use the warning switch override under the handle to unlock the fire warning switch. When you pull the switch up, the engine systems are shutdown and isolated for safety.
When you turn the Engine 1 [left] fire warning switch clockwise, the extinguisher bottle squib 1 on the R bottle operates. This breaks a seal and lets the halon gas in the R bottle to go to engine 1. If you turn the same engine fire warning switch counterclockwise, the extinguisher bottle squib 1 on the L bottle operates. This sends the halon gas in the L bottle to go to engine 1. When an engine fire extinguisher bottle is empty, the related BOTTLE DISCHARGED light on the overheat/fire protection panel comes on.
The Engine 2 [right] fire extinguishing system operates the same as engine 1. The difference is that the engine 2 fire handle switch causes the discharge cartridge 2 on the L or R bottle to operate and break a seal.
Each Fire extinguisher bottle has two extinguisher bottle squibs. The extinguisher bottle squib 1 for each bottle sends halon to engine 1.The engine 1 fire switch controls it.
The extinguisher bottle squib 2 for each bottle sends halon gas to engine 2. The engine 2 fire warning switch controls it.
The Engine fire extinguisher bottles are spherical. Each bottle contains halon and nitrogen at a pressure of 800 psi and a temperature of 70 degF [21 degC].
These are the components of each engine fire extinguishing bottle:-
A] Pressure gage and switch.
B] Safety relief port.
C] Two electrical connections.
D] Two discharge ports with squibs.
The two engine fire extinguisher bottles are in the upper aft left corner of the left main wheel well.
If the bottle temperature increases to 266 degF [130 degC], the safety relief port ruptures, this releases the halon gas into the wheel well.
The purpose of the squib is to break the diaphragm seal and start the release of halon gas from the bottle.
A squib is an explosive device that operates electrically, the nitrogen pressure in the bottle pushes the halon through the discharge port.
MAKE SURE THERE ARE CAPS OR SHUNT PLUGS INSTALLED ON THE SQUIBS WHEN YOU TOUCH OR MOVE THE SQUIBS. THE SQUIB IS AN EXPLOSIVE DEVICE AND CAN CAUSE INJURY IF IT IS ACCIDENTALLY FIRED.
These are the components of the Engine and APU fire protection panel:-
A] Engine and APU fire warning switches.
B] OVHT DET switches
C] FAULT/INOP test switches.
D] OVHT/FIRE test switches.
E] EXT TEST switch
F] Bell cutout switch.
These are the displays on the overheat/fire protection panel:-
A] Wheel well fire warning light, comes on for a wheel well fire condition.
B] ENG OVERHEAT light comes on for an engine overheat condition.
C] Fire warning switch lights come on for an engine fire condition.
D] Bottle discharge lights come on when an extinguisher bottle is discharged.
E] FAULT light comes on to show a fault condition in the engine fire detection system.
F] APU DET INOP light comes on to show a fault condition in the APU fire detection system.
G] EXT TEST lights come on after you do a successful squib continuity test.
These are the controls on the Overheat/fire protection panel:-
A] Fire warning switches show fire indication and control fire extinguisher squibs.
B] TEST switch lets you to do two tests. One test is for the fault detection circuits. The other test is for the fire detector loops.
C] OVHT DET switches let you select the mode of operation, loop A or loop B, you may also select NORMAL for both loops.
D] EXT TEST switch lets you do a check of the extinguisher bottle squibs for continuity.
When you pull the switch up, these functions occur:-
A] Engine fuel shut off valve closes.
B] Thrust reverser control power goes off.
C] Spar fuel valve closes.
D] Power to the engine driven pump low pressure warning system goes off.
E] Bleed air valves closes.
F] Hydraulic shut off valve closes.
The fire extinguishing system receives power from the 28vdc hot battery bus.
The Engine and APU fire extinguisher test has these components:-
A] Three position toggle switch, spring loaded to the neutral position.
B] L bottle test light.
C] R bottle test light.
D] APU bottle test light.
When you set the EXT TEST switch to the 1 position, a test of these squibs occurs:-
A] L bottle number 1 squib.
B] R bottle number 1 squib.
C] APU bottle squib.
When you set the EXT TEST switch to the 2 position, a test of these squibs occurs:-
A] L bottle number 2 squib.
B] R bottle number 2 squib.
C] APU bottle squib.
The green test lights come on to show a successful test.
When the EXT TEST is carried out, The voltage regulator in the overhead fire protection panel decreases the voltage to a level that will not cause a squib explosion.
When you operate the APU fire extinguishing system, it fills the APU compartment with halon gas to extinguish the fire.
The APU fire extinguishing system has these components:-
A] Fire Extinguisher bottle.
B] Overheat/Fire protection panel.
C] APU ground control panel.
When there is a fire condition, the APU fire warning switch unlocks, you can also use the warning switch override under the handle to unlock the switch. When you pull the APU fire warning switch up, the APU systems are shutdown and isolated. Turn the APU fire warning switch clockwise or counter-clockwise, this causes the APU fire extinguisher bottle to discharge halon gas. You can also use the APU ground control panel to operate the APU fire extinguisher bottle.
When the APU fire extinguisher bottle is empty, the APU BOTTLE DISCHARGED light on the overheat/fire protection panel comes on.
The APU DET INOP light and MASTER CAUTION lights come on if the APU fire detection circuits do not operate.
The APU extinguisher bottle is spherical in shape. The bottle contains halon under nitrogen pressure of 800 psi at a temperature of 70degF [21degC].
These are the components of APU fire extinguisher bottle:-
A] Pressure gage and switch.
B] Safety relief port.
C] One electrical connection.
D] One discharge port with squib.
The APU fire bottle is in section 48 horizontal stabilizer accessory compartment.
The bottle releases halon when you pull up and turn the APU fire warning switch in any direction. There is a squib explosion, which breaks open a diaphragm on the bottle. The halon gas flows from the bottle to the APU compartment. If the bottle temperature increases to 266 degF [130 degC] or approximately 1800 psi, the safety relief port breaks open. This blows out the disc and discharges the halon into the accessory compartment.
The APU ground control panel on ground gives you visual and aural indications of an APU fire and lets you extinguish it.
The APU ground control panel has these components:-
A] Red fire warning light.
B] Fire horn.
C] Horn cutout switch.
D] APU fire control handle.
E] Bottle discharge switch.
When thee is a fire in the APU, the horn and light operate alternately and the APU stops.
When you push the horn cutout button, the horn sounds stops and the red light stays on continuously.
When you pull the APU fire control handle, the APU system stops and isolates from other systems.
When you pull down the APU fire control handle, you also arm the bottle discharge toggle switch. You push the toggle switch to cause the APU bottle squib explosion.
When you pull the APU fire warning handle switch up, a signal goes to these components:-
A] APU generator control unit to open the APU generator breaker.
B] APU electronic control unit to shutdown the APU systems.
C] The P28 APU ground panel bottle discharge switch also arms [If P28 handle is pulled].
The bottle discharge switch on the p28 panel uses break away lockwire. This wire protects against accidental operation of the fire extinguisher squib. Broken wire shows switch movement and possible squib operation.
The Lower cargo compartment smoke detection system gives a warning in the Flight deck if there is smoke in a lower cargo compartment, The warnings are for the forward and aft cargo compartments.
The Forward lower cargo compartment has four smoke detectors and the aft lower cargo compartment has six. The smoke detectors are in the ceiling of the cargo compartment.
One smoke detector cargo electronics unit is in the ceiling of each cargo compartment inboard of the cargo door.
These are the components in the Flight deck that have an interface with the lower cargo compartment smoke detection system:-
A] Cargo smoke detection and fire suppression module.
B] Aural warning unit.
C] Fire warning lights on P7.
These are the functions of the lower cargo compartment smoke detection components on the cargo smoke detection and fire suppression module:-
A] DETECTOR FAULT amber light is on if one or more of the detectors has a power failure.
B] A three position [A,NORMAL,B] DET SELECT SWITCH, one for each cargo compartment. NORM position lets both detectors or with one detector failed sense smoke and give fire alarm. A or B lets the selected detector sense smoke and give fire alarm.
C] TEST push button switch, does a test of cargo smoke detectors and extinguishing system.
D] FWD, AFT red cargo fire warning switch light is on if smoke is detected in the related cargo compartment.
Smoke detection in the forward or aft cargo compartment puts on the two fire lights on the P7 panel and operates the fire bell in the aural warning unit.
If there is smoke in the lower cargo compartment, the smoke detector electronics unit causes a cargo fire warning alarm. The Fire warning bell operates and the fire lights come on, The FWD or AFT cargo fire warning light on the cargo smoke detection and fire suppression module comes on.
The smoke detector electronics unit has built-in test equipment [BITE]. The BITE does power up and periodic tests of the system.
You can also use the cargo fire test switch to do a test of the system.
The smoke detectors have these components:-
A] One electrical connector.
B] Four shock mounts.
C] Smoke detection chamber [Internal].
D] Electronic circuit.
The smoke detectors are in pans in the ceiling of the Forward lower cargo compartment and in the Aft lower cargo compartment. Both detectors are interchangeable.
The smoke detectors use photoelectric cells to detect smoke.
The smoke detector is an LRU. There is no scheduled maintenance of them.
If BITE Fails, replace the unit.
The Cargo electronic units have these components:-
A] Two electrical connectors [at the back of unit].
B] Two attachment brackets.
C] 16 red light emitting diodes [LEDs].
D] Lamp test switch.
E] System test switch.
F] Electronic circuits.
G] Translucent fiberglass cover with eight fasteners.
Six bolts attach the unit to the structure.
Loop A, Detectors 1,2 & 3.
Loop B, Detectors 1,2 & 3.
As the detectors 4 to 8 not present, there are not to be considered, but will illuminate on test.
Cargo Electronic unit BITE instructions:-
Properly functioning smoke detectors and unused positions respond by illuminating the corresponding indicator.
If the forward cargo compartment with four smoke detectors [2 loop A and 2 loop B], only lights 1A,1B,2A and 2B connect to the smoke detectors. The other lights are active and connect externally by a jumper cable loop.
In the aft cargo compartment with six smoke detectors [3 loop A and 3 loop B], only lights 1A,1B,1C,2A,2B & 2C connect to the smoke detectors. The other lights are active and connect externally by a jumper cable loop.
Smoke detection test [CEU]:-
LAMP TEST:- If any of the 16 red lights do not come on, change the CEU.
PRESS TO TEST:- All 16 red lights come on, test passed. At the same time, on P8-75, 2 EXT FWD, AFT green lights come on, FWD and AFT red switch lights come on, DISCH amber light comes on. If during test, the red light does not come on , change smoke detector or check Jumper cable. At the same time, on P8-75, the DETECTOR FAULT amber light is on.
These are the alarm indications:-
A] FWD or AFT red fire light comes on [Cargo smoke detection & fire suppression module P8-75].
B] FIRE lights come on [P7].
C] FIRE Bell operates.
If the detector does not operate, the DETECTOR FAULT amber light comes on for a fault indication.
Smoke detection test [P8-75]:-
TEST, 2 EXT FWD, AFT green lights come on, FWD and AFT red switch lights come on, DISCH amber light comes on, FIRE red light come on [P7] and bell rings. At the same time, on CEU, all 16 red lights come on. If FWD or AFT red switch light does not come on select A or B or DET SELECT switch and do the test again. A red light on the CEU not on shows you which detector is failed or a problem with external Jumper loop.
During these tests, the red lights on the CEU are visible through the fiberglass cover.
The Lower cargo fire extinguishing system has these components:-
A] One fire extinguishing bottle.
B] Fire discharge nozzles.
The Fire extinguishing bottle contains halon 1301 fire extinguishing agent pressurized with nitrogen. Tubing connects the bottle to the discharge nozzles in the cargo compartment ceilings.
The lower cargo compartment fire extinguishing bottle is in the air conditioning distribution bay. The bottle is on the left side of the air conditioning mix chamber. It attaches to a bracket on the aft bulkhead.
The bottle has two discharge [squibs] assemblies connected to the discharge tubing. The discharge tubing sends extinguisher to the forward and aft cargo compartments.
The bottle contains halon fire extinguishing agent pressurized with nitrogen. The bottle weighs approximately 33 lbs. [15 kgs].
The fire extinguishing bottle share these components:-
A] Safety relief and fill port.
B] Two handles.
C] Pressure switch with test button.
D] Three mounting brackets.
E] Two discharge assemblies with squibs.
The pressure switch monitors the pressure in the bottle. When the bottle pressure decreases to less than 250 psi, the switch sends a signal to the flight compartment for indication.
The pressure switch test is by push of the test button on the switch. This simulates low pressure and sends a signal to the flight compartment for indication.
These are the functions of the lower cargo compartment extinguishing components on the module [P8-75]:-
A] ARMED white indication is on after you push the FWD or AFT red cargo fire warning switch light.
B] Guarded DISCH amber switch light to discharge extinguisher bottle and bottle discharge indication.
C] TEST pushbutton switch, does a test of the extinguishing system and cargo smoke detectors.
D] FWD, AFT green EXT lights are on during the extinguisher test.
Bottle discharge:- Push the forward or aft fire arm switch to arm the cargo fire extinguishing system. Push the cargo fire discharge switch for at least one second. The switch lets power from the hot battery bus go to the bottle forward or aft squib. The bottle discharges extinguishing agent to the affected cargo compartment to provide fire suppression for 60 minutes.
You can do a test of the lower cargo compartment extinguisher bottle discharge squibs and the DISCH amber light. You use a test switch on the cargo smoke detection and fire suppression panel. A test switch on the cargo electronics unit also does a test of these components.
The Lavatory smoke detection system alerts the flight crew to smoke conditions in the lavatories.
A green indicator light comes on when a smoke detector has power. When there is smoke in the lavatory, the detector red alarm indicator light comes on and the detector horn sounds. There is an optional P5 panel smoke light which illuminates.
The smoke detector assembly has these components:-
A] Alarm horn.
B] Alarm indicator.
C] Electrical connector.
D] Power indicator.
E] Interrupt switch.
F] Self test switch.
G] Smoke sensor.
The smoke detector are modular units. There is a detector in each lavatory. The smoke detector uses 28vdc from the P18 panel.
The wheel well fire detection system detects high temperatures in the main wheel well and sends an alert to the flight crew.
These are the major components of the wheel well fire detection system:-
A] Wheel well fire detector elements.
B] Compartment overheat detection control module.
C] Overheat/Fire protection panel.
Detector elements in the main wheel well supply the wheel well fire detection signal. The signal goes to the compartment overheat detection control module. This module sends the fire alarm signal to the flight compartment.
The wheel well detector elements have these components:-
A] Nickel wire.
C] Electrical connector
The detector element attaches to the ceiling of the wheel well by quick release fasteners.
The detector element is a thermistor type with resistance inversely proportional to temperature. As the element temperature rises, its resistance is lowered. If any portion of the sensing element is heated above alarm temperature, the element resistance drops sharply. The single wire in the center of the core is the power lead and the outside tube is electrically grounded. The insulating resistance of the core material decreases suddenly at the alarm temperature. Current flows through the core material to ground when the alarm temperature is sensed.
The minimum bend radius for the sensor element is one inch [2.5 cm]. If possible do not bend the element in a radius less than three inches [8 cm].
Detector element are in clamps with colour-coded bushings. The detector end brackets have collars that prevent the detector from twisting when the electrical connectors are put on.
DO NOT MAKE THE PERMITTED DENTS OR KINKS STRAIGHT. DAMAGE TO THE TUBING CAN OCCUR.
Make sure there is at least 0.50 inch [1.5 cm] between the element and the structure except at the mounting clips.
The Compartment Overheat detection control module front panel has these components:-
A] MAINT ADV light.
B] LED display.
C] BITE instructions.
D] Four BITE control switches.
The Compartment overheat detection control module is in the electronic equipment compartment on the E1-4 rack.
For normal conditions, the MAINT ADV light is off. When the light comes in, it shows there is a fault data stored in memory. The light stays on until you correct the problem and clear the memory. Use the BITE functions to find the fault or alarm condition.
These are the functions of the BITE control switches:-
A] LOC TEST switch starts a self test.
B] MEM READ switch shows the fault code for the last item kept in memory.
C] MEM CLEAR switch erases the non volatile memory. You cannot erase current faults and alarm until corrected.
D] DISP TEST switch makes sure the control circuits operate correctly.
These are the indications of a wheel well fire:-
A] P7 Fire warn light comes on.
B] P8 Wheel well light comes on.
C] Aural warning unit gives a bell sound.
D] MAINT ADV light comes on.
Push the P7 FIRE WARN lighted push-button switch or the P8 BELL CUTOUT switch to stop the bell sound and turnoff the P7 FIRE WARN lights.
The P8 wheel well light stays on until the alarm conditions goes off.
Push the OVHT/FIRE test switch to start a test of the wheel well fire detection system.
115vac power must be on to do a test of the Wheel well fire detection system.
These are the Flight compartment indications of a Wheel well fire:-
A] P8-1 WHEEL WELL light comes on.
B] P7 FIRE WARN light comes on.
C] P9 AURAL WARNING UNIT gives a bell sound.
To stop the bell sound, push one of these switches:-
A] P8 panel BELL CUTOUT push-button.
B] P7 panel FIRE WARN light.
C] P28 APU GROUND CONTROL panel.
If you have a wheel well fire alarm, the MAINT ADV light on the compartment overheat detection control module comes on.
The construction and operation of the wing/body overheat elements are similar to the wheel well elements. The alarm temperature set points are different for the wing and body overheat detection sense elements. Elements actuate at approximately 255 degF [124 degC] for these areas in the airplane.
A] Aft cargo compartment.
B] Right and Left air condition compartment.
Elements actuate at approximately 310 degF [154 degC] for these areas of the airplane
A] Engine strut cavities.
B] Wing Leading edge.
These are the indications of a wing/body overheat condition:-
A] Related P5 wing-body overheat light comes on.
B] P7 master caution and AIR COND lights come on.
C] MAINT ADV light comes on.
Push and hold the P5 OVHT TEST switch for 5 seconds to start a test of the wing and body overheat detection system. If the test is successful, the two p5 wing-body lights and the P7 MASTER CAUTION light come on.
The Lavatory fire extinguishing system extinguishes fires under the wash basin and trash bin areas.
The Lavatory fire extinguishing system has these components:-
A] Fire extinguisher bottle.
B] Temperature sensors.
C] Temperature indicator strip.
The extinguisher bottle has two discharge ports. The ports have eutectic plugs. These plugs melt at high temperatures, When there is an overheat or fire condition, this occurs:-
A] Temperature indicators change colour.
B] The eutectic plug melts.
C] The extinguisher releases inert gas to stop the fire.
There are two types of portable fire extinguishers:-
Halon extinguishers are for electrical and flammable liquids.
Water type extinguishers are for solid combustible fires.
There are portable extinguishers in these areas:-
A] Flight Compartment.
C] Passenger compartment.
The Water-type extinguishers contain a water and anti-freeze mixture. Each water extinguisher has these ports:-
A] Trigger [discharge valve].
B] Water Cylinder.
C] Handle with internal gas cartridge.
D] Discharge nozzle.
E] Quick-release mounting strap.
Each Halon extinguisher has these parts:-
A] Pressure gage.
B] Halon cylinder.
D] Discharge nozzle.
E] Handle lock pin.
To operate the water-type extinguisher, turn the handle and push the trigger.
To operate the Halon extinguisher, remove the handle lock pin and push the handle.
After you use the water-type extinguisher, you must fill it again and replace the cartridge.
If the handle lock pin on the halon extinguisher is not there, you must weigh the extinguisher to find out if it is not full. The pressure gage is not a satisfactory check of the extinguisher.