panel P11 circuit breakers are closed.
DUCT LEAK DET LEFT 115 Vac
DUCT LEAK DET RIGHT 115 Vac
DUCT LEAK CONT 28 Vdc
(2)
The 115 Vac is fed directly to the control card in P50, where it is transformed to 18 volt peak to peak, and applied to the detector elements. The 28 Vdc goes to the power supply card in P50, where it is converted to regulated 5 Vdc, 19 Vdc, and 20 Vdc. These voltages go to the control card for the microprocessor control and monitor circuits.
(3)
When a bleed air duct leak occurs, the sensors monitoring the duct, sense the change in temperature. When a temperature of 255°F is sensed by the dual loop detector, the appropriate amber duct leak light on P5 lights. An EICAS message L ro R BLD DUCT LEAK is also displayed. As soon as the alarm is sensed, an auto test procedure (controlled by the microprocessor on the control card) begins. The system is checked at 20 millisecond intervals. If the alarm exists 12 out of 15 of these cycles, the alarm is considered valid.
(4)
If the auto test procedure identifies an open or short circuited sensor element, it reconfigures the system to an operable state. The configuration of the sensor loops is controlled by the microprocessor and configuration relays. The alarm is cancelled, but the defective sensor fault code is stored in nonvolatile memory, and DUCT LEAK BITE message is recorded on the maintenance page of EICAS.
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(5)
If the auto test procedure identifies multiple faults, no operable state exists. The appropriate amber duct leak light on P5 will remain on, and L or R BLD DUCT LEAK message will display on EICAS. The fault codes for the multiple faults will be stored in nonvolatile memory, and DUCT LEAK BITE message will be recorded on the maintenance page of EICAS.
(6)
Should the system lose all DC power or if the control circuitry fails then the system reverts to a "fail-safe" mode of operation. In this mode, all relays are relaxed and all chips are reset. Any overheats or fires will light the warning lights. The system must still have the three AC voltage sources applied.
B. BITE
____
(1) The system has both hardware and software built in test features. A description of BITE follows, and a placard with BITE operating instructions is posted in the cover of P50.
(a)
Pressing the DUCT LEAK test switch on the misc test panel (P61), tests all detectors for continuity and faults. All flight compartment displays of duct leak will display if at least one loop in each zone is operating correctly. Any faults detected during the test will be stored in nonvolatile memory for recall as required. The DUCT LEAK BITE message will also be recorded on the maintenance page of EICAS.
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