When there is a fire condition, the APU fire handle switch on the engine and APU fire control panel unlocks. When you pull the APU fire warning handle switch up, a signal goes to these components:
APU generator control unit - opens the APU generator breaker
APU control unit - shuts down the APU.
When you turn the fire warning handle switch right or left, 28v dc goes to the APU extinguisher bottle. This causes the APU fire extinguisher bottle squib to release the extinguishing agent.
You get the same results when you use the APU remote control panel P28. When you pull the fire handle switch on the P28 panel, a signal goes to these components:
APU generator control unit - opens the APU generator breaker
APU electronic control unit - shuts down the APU.
The bottle discharge switch also arms.
The bottle discharge switch sends 28v dc to the APU extinguisher bottle. This causes the APU extinguisher bottle squib to release the extinguishing agent.
When the APU fire extinguisher bottle pressure is less than 250 psi, the pressure switch makes the amber APU BOTTLE DISCHARGED light on the engine and APU fire control panel come on.
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