IBM NeXtScale System Planning And Implementation Manual page 59

As shown in Figure 3-5 on page 38, the following LEDs are on each power
supply:
AC power LED
When this LED is lit (green), it indicates that AC power is supplied to the
power supply.
DC power LED
When this LED is lit (green), it indicates that DC power is supplied from the
power supply to the chassis midplane.
Fault LED
When this LED is lit (yellow), it indicates that there is a fault with the power
supply.
Removing a power supply: To maintain proper system cooling, do not
operate the IBM NeXtScale n1200 Enclosure without a power supply (or
power supply filler) in every power supply bay. Install a power supply within 1
minute of the removal of a power supply.
Chapter 3. IBM NeXtScale n1200 Enclosure
41