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Installing the Control Unit
■ the carrier has a total of more than 45 MLX-20L telephones or 34-
button analog multiline telephones installed
Unit Loads for Key or Behind Switch Mode
In a Key or Behind Switch system with four or fewer modules, no calculation is
needed. The power supply (model 391A1) generally supports four modules
of any type in Key or Behind Switch mode.
Auxiliary Power Units
The power supply provides 54 unit loads to each carrier. If the unit load
requirement for a carrier exceeds 54, an auxiliary power unit is needed to
allow that carrier to support an additional 27 unit loads.
CAUTION:
Running the system with more than 54 unit loads per carrier may not
appear to do harm. However, this can cause the system to
malfunction, thereby creating “no trouble found” situations, such as
malfunctioning LEDs on multiline telephones, or power unit failure.
Any station connected to the modules in the last two slots receives power
from the auxiliary power unit instead of from the power supply.
If an auxiliary power unit is required, complete instructions are provided later
in this chapter, as the fourth step under “Installing the Processor.”
Unit Loads
2-19