Engineering guide
Liebert NX UPS alarm messages: what they mean and what to check
Most Liebert NX alarms fall into five groups: battery, input, bypass and transfers, overload and temperature, and parallel operation. Vertiv's NX user manual, SL-25217, gives the meaning of each message. Some clear on their own once the condition goes away, such as Unit Overload or Inverter Overtemp.; others, such as Inverter STS Fail, stay locked until power is off and need a technician.
Key facts
- Battery Low Pre-warning: about 3 minutes left at full load; settable from 3 to 60 minutes
- Unit Overload threshold: above 105% of nominal
- Return from overload bypass: load below 95% after 5 minutes
- Inverter Asynchronous: more than 6 degrees out of phase
- Bypass Overcurrent: above 135% of rating, alarm only
Where the messages come from
The Liebert NX 40 to 200 kVA shows each event in the current status area of its display and records it in the history log. Appendix A of Vertiv's user manual, SL-25217, lists every message with a description and a suggested action. The tables below group the messages operators see most, in plain words, with what can be checked safely from outside the cabinet.
Battery messages
| Message | What Vertiv's manual says | What you can check |
|---|---|---|
| Battery Overtemp. | The battery temperature is over its limit. | Battery room temperature and ventilation. |
| Replace Battery | The battery should be replaced. | Book a string test and replacement; the alarm will not clear on its own. |
| Battery Low Pre-warning | Comes before the end of discharge; Vertiv states the battery should still hold 3 minutes at full load, a time the user can set from 3 to 60 minutes. | Shut down the load in an orderly way if the utility is not coming back. |
| Battery End of Discharge | The inverter has turned off on low battery voltage. | Whether the utility supply is back; the manual's Auto start message shows the inverter restarting when it returns. |
| No Battery / Battery Reverse | The battery is not seen, or is connected with reversed polarity. | Whether the battery breaker is closed. Wiring checks are technician work. |
Input and rectifier messages
| Message | What Vertiv's manual says | What you can check |
|---|---|---|
| Mains Undervoltage | Input voltage is low, 288 V to 384 V line to line, with the load derated. | Whether other equipment on the same feed shows low voltage. |
| Mains Phase Reversed | The input phase sequence is reversed. | Whether electrical work was done upstream. Correction is electrician work. |
| Rectifier Overtemp. | The rectifier heat sink is too hot to keep running; the UPS can recover automatically. | Room temperature and ventilation. |
Bypass and transfer messages
| Message | What Vertiv's manual says | What you can check |
|---|---|---|
| Bypass Unable to Trace | The bypass voltage or frequency is outside the normal range; the amplitude limit is plus or minus 10%. It resets when the bypass returns to normal. | Whether the bypass feed is shared with a generator or a source being worked on. |
| Inverter Asynchronous | Inverter and bypass waveforms are more than 6 degrees out of phase; it resets when the condition clears. | Whether a bypass alarm came first; that one is solved first. |
| Load Impact Transfer | A large step load sent the load to bypass; the UPS should recover by itself. | Turn equipment on in sequence rather than all at once. |
| Transfer Timeout | Too many transfers in the last hour; the load stays on bypass and returns to inverter within an hour. | What is causing repeated transfers: overloads, step loads, bypass alarms. |
| Bypass Overcurrent | Bypass current is above 135% of rating; the UPS only raises the alarm. | The load on the output panel. |
Overload, temperature and fans
| Message | What Vertiv's manual says | What you can check |
|---|---|---|
| Unit Overload | The load is above 105% of nominal; the alarm resets once the overload is removed. | The load percentage per phase on the display; disconnect unnecessary load. |
| Unit Overload Timeout | The overload has lasted too long: the inverter static switch opens and the load goes to bypass. If the load falls below 95% after 5 minutes, the system returns to inverter. | Any load added just before the alarm. |
| Inverter Overtemp. | The inverter heat sink is too hot; the UPS recovers 5 minutes after the overtemperature signal clears. | High room temperature, a blocked airway, a failed fan, a long overload. |
| Fan Fault | At least one cooling fan has failed. | Book a fan replacement before the next hot day. |
Parallel systems
| Message | What Vertiv's manual says | What you can check |
|---|---|---|
| Parallel Comm. Fail | Communication between units of a parallel system has failed. | Whether every unit of the parallel system is powered on. |
| Load Sharing Fault | Units of a parallel system are not sharing the load current correctly. | Do not open the cabinets or work on live parts; note the message and call a technician. |
Messages that need a technician
| Message | What Vertiv's manual says | What you can check |
|---|---|---|
| Inverter Fault | A fault exists within the inverter. | Do not open the cabinets or work on live parts; note the message and call a technician. |
| Inverter STS Fail | A static switch on the inverter side is open or short-circuited; the fault stays locked until power is off. | Do not open the cabinets or work on live parts; note the message and call a technician. |
| Output Fuse Fail | At least one output fuse is open. | Do not open the cabinets or work on live parts; note the message and call a technician. |
| Output ground fault | An output line is shorted to ground. | Do not open the cabinets or work on live parts; note the message and call a technician. |
| Byp. Abnormal Shutdown | Both bypass and inverter voltages were unavailable: the load was interrupted. | Do not open the cabinets or work on live parts; note the message and call a technician. |
These calls are routine for the crews behind our Liebert maintenance contracts, and a fault description can also be sent through the repair request form with the message text and the history log.
What not to do
- Do not open the UPS or the battery cabinet: the battery string stays live even with its breaker open.
- Do not clear a fault repeatedly to get the inverter back; note the message first.
- Do not move the load to maintenance bypass unless you have been trained on the procedure for your site.
Related pages
Common questions
What does Unit Overload mean on a Liebert NX?
The load is above 105% of the nominal rating. The alarm resets once the overload is removed; if it lasts, the load transfers to bypass and returns to inverter when it drops below 95% after 5 minutes.
Why is my Liebert NX on bypass after a transfer timeout?
Vertiv's manual explains that too many transfers within the last hour keep the load on bypass. The UPS returns the load to inverter power within an hour; the cause of the repeated transfers still needs to be found.
Will the NX recover from an overtemperature alarm?
Yes, the manual says it recovers 5 minutes after the overtemperature signal clears. Check the room temperature, the airways, the fans and any long overload.
Which NX messages always need a technician?
Inverter Fault, Inverter STS Fail, Output Fuse Fail, Output ground fault and Byp. Abnormal Shutdown. Note the message and the history log, and do not open the cabinet.
Does a Battery Low Pre-warning mean the batteries are bad?
Not by itself. It warns that the end of discharge is near during an outage; Vertiv states the battery should still hold about 3 minutes at full load at that point. Repeated early warnings call for a battery test.
Sources, checked October 4, 2026:
- Vertiv, Liebert NX 40 to 200 kVA user manual SL-25217, Appendix A, Table 51, pp. 121, 122, 123, 124, 125
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