(1) If the cabin pressure drops to a pressure equivalent to 4260 + 0/-150 m (14000 + 0/- 500 ft.), the altitude pressure switch (16WR) closes. This operates the emergency-oxygen supply system. When the altitude pressure switch (16WR) closes, 28 V DC is supplied to the oxygen power relay (10WR) from the busbar (401PP) through the time delay relays (8WR) and (9WR). The circuit is protected by the circuit breaker (1WR). The busbar (801XP) supplies 115 V AC to the electrical latch assemblies of the emergency oxygen containers through the circuit breakers (4WR thru 7WR).
 **ON A/C 052-055, 057-057,
 (1) If the cabin pressure drops to a pressure equivalent to 4260 + 0/-150 m (14000 + 0/- 500 ft.) with the AHS ON pushbutton switch in the normal position, the altitude pressure switch (16WR) closes. This operates the emergency-oxygen supply system. When the AHS ON pushbutton switch is in on position the system is activated at a pressure equivalent to 4870 + 0/- 150 m (16000 + 0/-500 ft.). When the altitude pressure switch (16WR) closes, 28 V DC is supplied to the oxygen power relay (10WR) from the busbar (801PP) through the time delay relays (8WR) and (9WR). The circuit is protected by the circuit breaker (1WR). The busbar (801XP) supplies 115 V AC to the electrical latch assemblies of the emergency oxygen containers through the circuit breakers (4WR thru 7WR).

R **ON A/C 001-049, 051-099, 101-149, 151-199, 201-299, 301-399,
 (2) Each electrical latch assembly is supplied from two separate 115 V AC circuits. Each circuit supplies 115 V AC to the electrical latch assemblies on opposite sides of the cabin. The circuits make the crossover in the cabin in front and behind the impact area of uncontained engine rotor failure. The relay 25WR, at the rear of the cabin, closes in 6 seconds . This operates the intact circuit if one circuit is damaged. This configuration makes sure that there is a 100 % supply of 115 V AC to the emergency oxygen containers. This occurs if an engine rotor failure damage a circuit.
R **ON A/C 001-049, 051-099, 101-105, 151-199, 201-299,
 (3) The sequence of operation is as follows.
 (a)
 The oxygen power relay (10WR) is energised.

 (b)
 The emergency-oxygen container doors open and the oxygen masks fall out (REF 35-21-00).

 (c)
 The indicator light SYS ON (12WR) comes on.

 (d)
 The passenger information relay (19WR) operates and the taped announcement is transmitted over the passenger address system.

 (e)
 The time delay relays (8WR and 9WR) operate 30 seconds after the operation of the altitude pressure switch (16WR). They switch off the supply to the oxygen power relay (10WR).

 (f)
 If a failure of the time delay relays (8WR and 9WR) occurs, the FAULT indicator light on the TMR RESET pushbutton switch (13WR) comes on.

 (g)
 When the TMR RESET pushbutton switch (13WR) is pressed, the indicator light SYS ON (12WR) goes off. The ON indicator light (on the TMR RESET pushbutton switch 13WR) comes on while the TMR RESET pushbutton switch is pressed.

 (h)
 When you push the PRE REC ANN STOP pushbutton switch on the FAP 120RH the taped announcements stop.

R **ON A/C 106-149, 301-399,
 (3) The sequence of operation is as follows.
 (a)
 The oxygen power relay (10WR) is energised.

 (b)
 The emergency-oxygen container doors open and the oxygen masks fall out (REF 35-21-00).

 (c)
 The indicator light SYS ON (12WR) comes on.

 (d)
 The time delay relays (8WR and 9WR) operate 30 seconds after the operation of the altitude pressure switch (16WR). They switch off the supply to the oxygen power relay (10WR).

 (e)
 If a failure of the time delay relays (8WR and 9WR) occurs, the FAULT indicator light on the TMR RESET pushbutton switch (13WR) comes on.

 (f)
 When the TMR RESET pushbutton switch (13WR) is pressed, the indicator light SYS ON (12WR), goes off and the taped announcement stops. The ON indicator light (on the TMR RESET pushbutton switch 13WR) comes on while the TMR RESET pushbutton switch is pressed.

R **ON A/C 001-049, 051-099, 101-149, 151-199, 201-299, 301-399,
 B. Manual Operation The MASK MAN ON pushbutton switch (14WR) operates the emergency oxygen system when pressed. The sequence of operation will then follow the automatic operation sequence but the altitude pressure switch (16WR) does not operate.
 C. Door Malfunction You can open the door of the emergency oxygen container manually, if a malfunction of the electrical latch occurs (REF 35-21-00).
 6. Ground Test
___________ **ON A/C 001-049, 051-061, 101-105, 151-199, 201-207,

 To make sure that the doors of the emergency oxygen containers open, do a periodic ground test. To check that the electrical latches operate correctly, press the MASK MAN ON pushbutton switch 14WR (REF 35-23-00). The oxygen masks do not fall out, if you push the left hand side of the test lever on the door assembly of the emergency containers. Push the test lever until the over-center spring keeps the lever at 90 degrees to the door surface. This lets the doors open a few degrees when you do the ground test. After the test, close the doors and push the test lever back to the initial position.
　
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