Automatic Operations; Overloads (Without Transfer); Figure 47 Current-Versus-Time Curves Of Overload Capacity - Emerson Liebert NXL 250 kVA Operation And Maintenance Manual

2.16

Automatic Operations

The Liebert NXL UPS system is designed to function while unattended by an operator. The system
control logic monitors the performance of the UPS, the availability of power sources and the current
required by the critical load.
The system control logic:
• Determines what overload conditions can be sustained without a transfer to bypass.
• Initiates an automatic transfer to bypass to sustain an overload or when selected UPS faults
occur.
• Can initiate an automatic retransfer to the UPS after an overload has been cleared.
• Initiates an automatic transfer to bypass and emergency module shutdown when specified UPS
faults occur.

2.16.1 Overloads (Without Transfer)

The UPS system is capable of sustaining full output voltage (±2% of the nominal voltage) for overload
conditions that remain within (under) the current versus time curve of overload capacity (Figure 47).
Note that the time scale is not linear.
For high current demands of short duration (momentary overloads) the critical load is supplied
simultaneously by both the UPS system and the bypass line. Whenever an overload condition occurs,
you should determine the cause of the overload. If an overload condition exceeds the overload
capacity, the UPS system initiates an automatic load transfer to the bypass line.
For overloads above the Input Current Limit, a DC source, such as a battery system or a generation
source, must be available. The Input Current limit has a default setting of 125% rated output current.

Figure 47 Current-versus-time curves of overload capacity

85min
60min
10 min
1 min
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Liebert NXL 52
Operation