B737 Overheat Fire Protection Panel

Overheat Detector (OVHT DET) Switch

• NORMAL: Detection loop A and loop B are active.
• A: Detection loop A is active.
• B: Detection loop B is active.

Fire Warning BELL CUTOUT Switch

• Push:
  • Extinguishes both master FIRE WARN lights
  • Silences the fire warning bell
  • Silences the remote APU fire warning horn (on the ground only)
  • Resets the system for additional warnings

Extinguisher (EXT) TEST Switch

• Spring-loaded to center:
  • 1: Tests discharge squib continuity of extinguisher bottles for engine 1 and APU
  • 2: Tests discharge squib continuity of extinguisher bottles for engine 2 and APU

FAULT/INOP and OVHT/FIRE TEST Switch

• Spring-loaded to center:
  • FAULT/INOP: Tests fault detection circuits for both engines and the APU
  • OVHT/FIRE: Tests overheat and fire detection loops on both engines, APU, and main wheel well
  • Note: See Fire and Overheat System Tests in Section 20

APU Fire Switch

• Illuminated (red):
  • Indicates fire in APU
  • Unlocks APU fire switch
  • Note: Master FIRE WARN lights illuminate, fire warning bell sounds (in flight and on the ground), and in the main wheel well, the APU fire warning horn sounds and APU fire warning light flashes (on the ground only).
• In (normal position): Mechanically locked if no fire signal
• Up:
  • Arms APU fire extinguisher bottle squib
  • Closes fuel shutoff valve, APU bleed air valve, and APU inlet door
  • Trips generator control relay and breaker
  • Allows APU fire switch to rotate
• Rotate (left or right): Discharges APU fire bottle

Engine Fire Switch

• Illuminated (red):
  • Indicates fire in the related engine
  • Unlocks the related engine fire switch
  • Note: Master FIRE WARN lights illuminate and fire warning bell sounds.
• In (normal position): Mechanically locked if no fire signal
• Up:
  • Arms one discharge squib on each engine fire extinguisher
  • Closes fuel, hydraulic shutoff, and engine bleed air valves
  • Disables thrust reverser
  • Trips generator control relay and breaker
  • Deactivates engine-driven hydraulic pump LOW PRESSURE light
  • Allows engine fire switch to rotate
• Rotate (left or right): Discharges the related fire bottle

Fire Switch Override

• Push: Unlocks the fire switch.

Engine (ENG) OVERHEAT Light

• Illuminated (amber): Indicates overheat in the related engine.
  • Note: MASTER CAUTION and OVHT/DET system annunciator lights illuminate.

FAULT Light

• Illuminated (amber):
  • With the OVHT DET switch in NORMAL: Indicates both detector loops for an engine have failed.
  • With the OVHT DET switch in A or B: Indicates the selected loop for an engine has failed.
  • Note: MASTER CAUTION and OVHT/DET system annunciator lights do not illuminate.

WHEEL WELL Fire Warning Light

• Illuminated (red): Indicates fire in the main gear wheel well.
  • Note: Master FIRE WARN lights illuminate and fire warning bell sounds.

Engine BOTTLE DISCHARGED Light

• Illuminated (amber): Indicates the related fire extinguisher bottle has discharged or pressure is low.

APU Detector Inoperative (DET INOP) Light

• Illuminated (amber): Indicates the APU detector loop has failed.
• Note: MASTER CAUTION and OVHT/DET system annunciator lights illuminate.

APU BOTTLE DISCHARGED Light

• Illuminated (amber) – indicates APU extinguisher bottle has discharged or pressure is low.

Extinguisher Test (EXT TEST) Lights

• Illuminated (green) – EXT TEST switch is positioned to 1 or 2 and circuit continuity is normal.