What happens to fire warning protection during the initial APU startup phase?

During the initial APU startup phase, the fire warning protection is not available until AC power is established. This is because the fire detection and warning systems depend on electrical power to function. When the APU starts, it initially runs on its battery until the generator is able to provide AC power. Only after the generator is online and supplying power can the fire detection systems activate, ensuring that any potential fire is detected immediately. Understanding this sequence is critical for safety during operations involving the APU, as it highlights the importance of establishing power in order to enable essential safety features.