757 AMM 飞机维护手册 仪表 CHAPTER 31 - INSTRUMENTS 2(91)

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 altitude exceedance, and autopilot (A/P) disconnect warning.
 There is a one-half second delay for A/P disconnect warnings.
 (m)  GUN 004-007, 206;
 The siren will sound for cabin altitude exceedance above 10,000
 feet (standard altitude warning selected) or 14,680 feet (high
 altitude warning selected).
 (n)  GUN 001-003, 008-205, 207-999;
 The siren will sound for cabin altitude exceedance above 10,000
 feet.
 (o)  The siren will continue until the warning condition is
 corrected. The GND PROX/CONFIG GEAR OVRD switch can be used to
 cancel the siren during a landing configuration warning.
 (p)  The SPEEDBRAKES light and level B EICAS message SPEEDBRAKE EXT
 are generated in either of the following conditions:
 1) RA >15 feet and speedbrake lever is above the trip point
 (spoilers extended) and flaps are in the landing position.
 2) 15 feet < RA < 800 feet and speedbrake lever is above the
 trip point (spoilers extended).

WEU Power Supply Function (Fig. 3)
 (a)  
The Warning Electronics Unit (WEU) power supply A receives 115 volts ac through the WARN ELEX A circuit breaker from the right unswitched ac bus. It rectifies the input ac voltage and provides +5 volts and |12 volts dc output to the warning modules.

 (b)  
During normal operation, the fault ball on the front of the WEU shows black. The presence of |12 volt dc also energizes the fault relay to remove the ground from the power supply failure discrete signal to the EICAS computers.

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115V AC
R BUS

LINE LOAD
WARNELEX A
400 HZ P11
EMI FILTER

YELLOW INDICATES FAULT
BLACK INDICATES RESET POWER SUPPLY A  PSU A FAULTRELAY
Warning Electronics Unit (WEU) Power Supply Schematic (Example)
Figure 3

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 (c)  
Each regulated power output is monitored for an under or over voltage condition. These conditions will relax the fault relay to provide a WARN ELEX failure discrete signal to the EICAS computers. The fault ball also latches to yellow to show a fault condition.

 (d)  
Pressing the RESET switch on the warning electronics unit resets the fault ball to black (normal). When the Warning Electronics Unit returns to normal, the |12 volts dc is applied to the fault relay. The relay is energized, WARN ELEX failure discrete signal to the EICAS computers is removed.

 (3)  Master Warning Module Function (Fig. 4)
　
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