R **ON A/C 106-149, 229-299, 401-499,
 The 28VDC essential busbar 801PP and the 115VAC normal busbar 103XP supply the Emergency Power Supply Units (EPSUs). The EPSUs convert 28VDC into 6VDC to operate the lights in the system. A battery in each EPSU is charged continuously when the essential busbar is energized and the circuit breakers are closed. Each of the four EPSUs supplies electrical power to different emergency lights and signs. The emergency lights and signs are always supplied with power, even if the aircraft has crashed and the fuselage has broken. This is because the four EPSUs are installed in different places in the cabin.
 **ON A/C 159-199, 210-228, 301-302,
 The 28VDC essential busbar 801PP and the 115VAC normal busbar 103XP supply the Emergency Power Supply Units (EPSUs). The EPSUs convert 28VDC into 6VDC to operate the lights in the normal emergency light system and supply 115VAC to operate the marking system of the emergency escape path. A battery in each EPSU is charged continously when the essential busbar is energized and the circuit breakers are closed. Each of the EPSUs supplies electrical power to different emergency lights and signs. The emergency lights and signs are always supplied with power, even if the aircraft has crashed and the fuselage has broken. This is because the EPSUs are installed in different places in the cabin.
 **ON A/C 303-399,
 The 28VDC essential busbar 801PP and the 115VAC normal busbar 103XP supply the Emergency Power Supply Units (EPSUs). The EPSUs convert 28VDC into 6VDC to operate the lights in the system. A battery in each EPSU is charged continuously when the essential busbar is energized and the circuit breakers are closed. Each of the six EPSUs supplies electrical power to different emergency lights and signs. The emergency lights and signs are always supplied with power, even if the aircraft has crashed and the fuselage has broken. This is because the six EPSUs are installed in different places in the cabin.
R 1EFF : 106-149, 159-199, 210-299, 301-399, 1 33-51-00Page A1 1401-499, 1Aug 01/05 1 1 1CES 1 **ON A/C 051-099,

 Post SB 33-1041 For A/C 051-099,
 The 28VDC essential busbar 801PP and the 115VAC normal busbar 103XP supply the Emergency Power Supply Units (EPSUs). The EPSUs 10WL thru 13WL convert 28VDC into 6VDC to operate the lights in the normal emergency light system and the marking system of the emergency escape path. A battery in each EPSU is charged continously when the essential busbar is energized and the circuit breakers are closed. Each of the EPSUs supplies electrical power to different emergency lights and signs. The emergency lights and signs are always supplied with power, even if the aircraft has crashed and the fuselage has broken. This is because the four EPSUs are installed in different places in the cabin.
 **ON A/C 001-049, 101-105, 151-158, 201-209,
 The EMER EXIT LT switch 4WL on the overhead panel 25VU controls the emergency lighting system. Self-monitoring circuit breakers 1WL, 2WL and 3WL protect the system from too much current flow. The EMER EXIT LT switch 4WL has three positions, OFF, ARM and ON. Each of these positions gives a discrete ground signal to all EPSUs and power supply units. The EPSUs monitor the correct voltage of the AC and DC busbars. The power supply units monitor the correct voltage of the DC busbars. When the voltage on these busbars falls below a specified level, the emergency lighting automatically comes on. This is when the EMER EXIT LT switch 4WL is in the ARM position. When only the AC normal busbar fails:
 -
the 28VDC essential busbar supplies the emergency lighting system through the 28VDC/6VDC converter in each EPSU,

 -
the marking system of the emergency escape path stays in the off

 condition.
 When the 28VDC essential busbar fails:

 -
the internal batteries in the EPSUs and the power supply units supply the emergency lighting system,

 -
the marking system of the emergency escape path is in the on condition.

 When the EMER EXIT LT switch 4WL is in the ON position:

 -
the converters or the internal batteries in the EPSUs and the inverters or the internal batteries in the power supply units supply the emergency lighting system.

 When the LIGHT EMER pushbutton switch and its integral light on the forward attendant panel 120RH are on:

 -
the converters or the internal batteries in the EPSUs and the inverters or the internal batteries in the power supply units supply the emergency lighting system.

R 1EFF : 001-049, 051-099, 101-105, 151-158, 1 33-51-00Page A2 1201-209, 1Aug 01/05 1 1 1CES 1 An open or short circuit in the related control lines causes the emergency lighting to come on. The internal test program in the EPSUs finds defects in the load circuits.
　
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