For fire warning and overheat warning purposes, the detectors in use are,
· Sensor responder type.
· Infra-red type.
· Smoke detectors.
Detectors may be used separately, or together in a combined fire warning and engine overheat system. Special detectors are used for detecting smoke, particularly in compartments which are not accessible in flight.
Thermal Switch Fire Detection System
These detectors are normally situated at points most likely to be affected by fire,such as an engine breather outlet or hot air ducting. The type most commonly used is a switch, the contacts of which are actuated by the differential expansion of dissimilar metals. The thermal switch is a unit type and called a spot or point detector.
A thermal switch fire detection system is a circuit in which one or more thermal switches are connected in an electrical circuit which also has a warning light and an aural alarm to warn the flight crew that an overheat condition exists in a particular area. If more than one thermal switch is in the circuit, the switches are connected in parallel.
This makes sure that if any one switch closes a warning is given. In some circuits the detectors are connected between two wiring loops, either of which may be supplied through a magnetic circuit breaker. A short circuit in the energized loop results in operation of the magnetic circuit breaker and the supply is then routed to the second loop. This prevents a false indication of fire. The system can withstand one fault, either an electrical open circuit or a short to ground without sounding a false alarm. A double fault must exist before a false fire warning can occur.
Switches operating at different temperature settings may look identical but, when doing maintenance or replacing the switches its vital to identify by part number in the Illustrated Parts Catalogue (IPC). The unit type is more suitable for localized overheat detection and the ‘continuous loop’ type better for fire detection.
