Engineering guide
XPC M90U alarm messages explained
The XPC M90U writes its alarms on the LCD as Fault!, Warning! or Pre-Alarm! followed by a short message, such as Warning! Over Load or Fault! Battery Fault. The M90U User's Manual explains each one, and its troubleshooting table says plainly that most faults and warnings have to be released by service personnel; only a few can be solved by users. The tables below separate those few from the rest.
Key facts
- Who releases most faults: service personnel; few can be solved by users
- Warning! Over Load Fail: overload; Line mode transfers to Bypass mode
- Fault! Over Load Fault: heavy overload causes a UPS fault; reduce some load
- Warning! Battery Open: battery not connected; check the battery breaker
Pre-Alarm, Warning and Fault
The prefix groups the messages. In the manual's list, the Pre-Alarm! entries report the line or the load going over range or out of balance; the Warning! entries include Over Load Fail, where Line mode transfers to Bypass mode; the Fault! entries include Over Load Fault, where a heavy overload causes a UPS fault. Write down the full message and the time before anything is reset.
Utility and input messages
| Message on the display | What the manual says | What to do |
|---|---|---|
| Line Fail | Utility lost or abnormal. | Check the utility and, if one is running, the generator. |
| Line Restore | Utility recovered to normal. | No action; note the outage time. |
| Pre-Alarm! Line Voltage Fail | Line voltage over range. | Check the upstream supply. |
| Pre-Alarm! Line Frequency Unstable | Line frequency over range. | Check the generator if one is running. |
| Warning! Turn On Abnormal | Start-up was abnormal. | Check whether the input voltage or frequency is out of range. |
| Warning! Line Phase Error | Mains phase sequence error. | Have the mains phase sequence checked; call service. |
| Warning! N Loss | Neutral loss. | Have the neutral connection checked; call service. |
Load messages
| Message on the display | What the manual says | What to do |
|---|---|---|
| Pre-Alarm! Over Load | Output load over range. | Reduce some load before it becomes a warning. |
| Pre-Alarm! Load Unbalance | Output load unbalance. | Check how loads are spread across phases. |
| Warning! Over Load | In Line mode, the loads demand more power than the UPS can supply. | Reduce some load and check the output capacity. |
| Warning! Over Load Fail | Overload; Line mode will transfer to Bypass mode. | Reduce some load; the load is on bypass meanwhile. |
| Fault! Over Load Fault | Heavy overload causes a UPS fault. | Reduce some load. |
Acting on Pre-Alarm! Over Load is the cheapest fix: shed load before the unit escalates to Over Load Fail and moves the load to raw utility power through the bypass.
Battery messages
| Message on the display | What the manual says | What to do |
|---|---|---|
| Warning! Battery Open | Battery not connected. | Check the battery breaker position with someone trained on the unit. |
| Warning! Battery voltage High | Battery voltage too high. | Call service to check the nominal battery voltage setting. |
| Warning! Battery Age Alert | Battery life expiration. | Check the battery installation date; plan replacement. |
| Fault! Battery Fault | Battery reverse. | Call service personnel. |
EPO, maintenance bypass and heat
| Message on the display | What the manual says | What to do |
|---|---|---|
| Warning! EPO Active | EPO active; check the EPO connector. | Find what triggered the EPO; a loose EPO connector can cause it. |
| Warning! Maintenance Bypass | Unit entered maintenance bypass. | Check whether maintenance is under way. |
| Fault! Over Temperature | Make sure there is space for air vents and the fan works. | Check the room temperature and that vents are clear; or call service. |
| Warning! Fan Lock | A fan is locked. | Check from outside that nothing blocks the fan inlet; or call service. |
Messages that need a technician
| Message on the display | What the manual says | What to do |
|---|---|---|
| Fault! Bus Over Voltage | DC bus voltage too high. | Call service personnel. |
| Fault! Bus Soft Start Time Out | Rectifiers could not start: low DC bus voltage. | Turn off and restart once; if it fails again, call service. |
| Fault! R Inverter Voltage Short | R phase inverter output shorted (S, T, RS, ST, TR alike). | Call service personnel. |
| Fault! Relay Fault | Inverter relay fault. | Call service personnel. |
| Fault! Line SCR Fail | Line SCR short circuit fault. | Call service personnel. |
| Warning! Charge Fail | Charger failure. | Call service personnel. |
| Warning! ID Conflict | Two power modules have the same ID. | Call service personnel. |
Apart from the single restart the manual allows on a soft start time-out, these messages are for service personnel. Our XPC technicians handle M90U module and battery work on site, and a preventive maintenance plan catches battery age and fan alerts before they turn into faults.
Related pages
Common questions
What is the difference between Pre-Alarm, Warning and Fault on an M90U?
Pre-Alarm entries report the line or load over range; Warning entries include Over Load Fail and the transfer to bypass; Fault entries include Over Load Fault, where heavy overload causes a UPS fault.
What does Warning! Over Load Fail mean?
The loads demand more power than the UPS can supply, and Line mode transfers to Bypass mode. Reduce some load.
What does Warning! Battery Open mean on the M90U?
The battery is not connected. The manual's first check is the battery breaker.
Can I restart after Fault! Bus Soft Start Time Out?
The manual allows turning the UPS off and restarting it once; if it fails again, contact service personnel.
Which M90U alarms can users solve themselves?
Only a few, per the manual: overloads by reducing load, EPO and input checks. Most faults need service personnel.
Sources, checked October 4, 2026:
- Xtreme Power Conversion M90U User's Manual, Troubleshooting, pp. 62, 62-64, 63-64
- Xtreme Power Conversion M90U User's Manual, Alarm List, p. 59
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