For small server room with a few racks, an enterprise-class UPS such as the NX or NXe series might be a little excessive. Liebert has a series of small rack-mountable UPS ranging from 500VA - 3000VA works very well for these situations. For >3kVA UPS, it does not require hard-wiring. You can plug these units directly into your BS-1363 (13A) socket using conventional C14 cables.
Liebert Rack-Mountable Network UPS decision tree
GXT3 vs PSI: On-Line vs Line-Interactive
Liebert carries 2 main topology of UPS, On-Line vs Line-Interactive, and this is the major differences between the GXT3 and PSI series of UPS. GXT3 is a On-Line UPS and PSI is a Line-Interactive UPS.
Liebert PSI - Line-Interactive UPS
The battery to AC power converter (inverter) is always connected to the output of the UPS. Battery charging is done during times when the input AC power is normal when the input AC fails, transfer switch opens and then the inverter starts functioning to provide power to load immediately.
Advantages of Line-Interactive UPS
- Bi-directional inverter is always active charging the battery or powering the load
- Good overload capability
- Lower investment compare to On-Line UPS
Disadvantages of Line-Intearctive UPS
- No frequency regulation
- Narrow input voltage and frequency window may cause frequent charge / discharge battery cycle
Liebert GXT3 - On-Line UPS
In this type of UPS, the system always remains on battery, whether the mains A/C is present or not. When the mains A/C is present, it provides power to DC supply of inverter section as well as charges the battery simultaneously. When mains A/C is not present, it will run the connected load till the battery has a recommended dischargeable level.
Advantage of On-Line UPS
- Higher load voltage availability: Output MTBF = 5-10X other topologies
- Continuous duty rated inverter: allows extended battery times
Disadvantage of On-Line UPS
- Larger initial investment
PSI vs PSI-XR: Short vs Extended Backup time
PSI-XR is the series of Line-Interactive UPS which offers extended backup time by adding external battery cabinets.
| PSI battery runtime chart |
PSI-XR battery runtime chart |
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As you can see from the charts above, at full-load, a PSI-XR configuration is capabable of delivering a backup time of over an hour where as PSI is only capable of delivery backup time of just over 3 minutes.
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For more information about the UPS discussed in this article, please visit their product page:
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