Engineering guide
Liebert EXL S1 alarm and fault codes
Every Liebert EXL S1 message has a display ID. In Vertiv's SL-26090 guide, alarm IDs start with 2 and fault IDs start with 3: 24-012 Battery under voltage is an alarm that stops the inverter, while 32-028 Rectifier Temperature fault is a fault set at 95 °C by default. The tables below explain the codes operators see most, and which ones need a technician.
Key facts
- Rectifier overtemperature alarm: 80 °C by default (22-061)
- Rectifier temperature fault: 95 °C by default (32-028)
- Inverter overload alarm: overload timeout at 100%, transfer to bypass requested (26-031)
- CAN communication loss: no CAN messages for 10 seconds (28-008)
How the EXL S1 numbers its messages
Vertiv's EXL S1 operation and maintenance guide, SL-26090, lists every message in Appendix A in three tables: status, alarms and faults. Each line gives the component, the text on the display and a display ID such as 24-012. In those tables, every alarm ID starts with a 2 and every fault ID with a 3, so the first digit already tells you which table to look in.
Alarms (IDs starting with 2)
| Message (ID) | What Vertiv's manual says | What you can check |
|---|---|---|
| Battery under voltage (24-012) | The battery voltage is below the shutdown voltage; this warning stops the inverter. | Whether the UPS is on battery and how long the outage has lasted. |
| Battery Is Not Connected (24-064) | The battery is not connected. | Whether the battery breaker was left open after work. |
| BCB Breaker Open (24-068) | The battery circuit breaker is open. | Whether it was opened on purpose; until it is closed, there is no battery backup. |
| Bypass mains failure (21-013) | The bypass supply has failed, after a set delay. | The breaker feeding the bypass input. |
| Bypass Wrong Phase Rotation (21-083) | The bypass input phase rotation is wrong. | Whether upstream electrical work was done. Correction is electrician work. |
| Rectifier mains failure (22-015) | The rectifier input supply has failed, after a set delay. | Whether the site has an outage or a tripped feeder. |
| Wrong phase rotation (22-017) | The input line phase rotation is incorrect. | Same as the bypass case: an electrician corrects it. |
| Overtemperature Phase U-A (22-061) | Rectifier phase temperature is above its setting, 80 °C by default. | Room temperature and airflow. |
| Inverter overload (26-031) | The overload timeout has reached 100%; with the default configuration this requests a transfer to bypass. | The load on the output and anything added recently. |
| System Maint. Bypass Switch Closed (20-019) | The system maintenance bypass switch is closed. | Whether maintenance is under way; the load is on unconditioned utility power. |
| Input Air High Temperature (20-027) | The incoming air is too hot. | Room cooling and blocked intakes. |
| Fan Failure Phase U (20-210) | A fan has failed (phases U, V, W and other locations have their own IDs). | Book a fan replacement. |
| I/O Transformer Overtemperature (20-224) | An input or output transformer is too hot. | Load level and room temperature. |
| AC Ground Fault (20-133) | A ground fault has been detected on the AC side. | Do not open the cabinets or work on live parts; note the message and call a technician. |
| CAN Communication Loss (28-008) | Internal CAN messages have not been received for 10 seconds. | Do not open the cabinets or work on live parts; note the message and call a technician. |
Faults (IDs starting with 3)
| Message (ID) | What Vertiv's manual says | What you can check |
|---|---|---|
| Battery Test Failure (34-023) | The battery test has failed. | Book a string test block by block. |
| Battery Ground Fault (34-071) | The battery ground fault detector has tripped. | Do not open the cabinets or work on live parts; note the message and call a technician. |
| Back-feed protection (31-023) | A back-feed fault has been detected. | Do not open the cabinets or work on live parts; note the message and call a technician. |
| Overload (36-046) | The inverter RMS overload counter has run out. | Reduce the load before any restart. |
| Fuse Blown Phase U-A (36-080) | An inverter output fuse is open (phases V and W have their own IDs). | Do not open the cabinets or work on live parts; note the message and call a technician. |
| Rectifier Temperature fault (32-028) | Rectifier phase temperature is above its fault setting, 95 °C by default. | Room cooling first; the fault itself needs a technician. |
| Parallel cable missing (30-071) | A parallel cable is missing. | Do not open the cabinets or work on live parts; note the message and call a technician. |
| High Ambient Temperature (30-202) | The ambient temperature is too high. | Room cooling. |
When the message points to the batteries
Battery under voltage, Battery Test Failure and repeated BCB alarms usually end in a string replacement. EXL S1 sites typically fall under our high-capacity three-phase service, and blocks for Liebert cabinets are listed with the Liebert and Vertiv batteries.
New systems
The initial start-up of a new EXL S1 is done by Vertiv; we do not commission new large Vertiv systems. The tables above are for units in service.
Related pages
Common questions
What does EXL S1 code 24-012 mean?
Battery under voltage: the battery voltage is below the shutdown voltage, and the warning stops the inverter. Check whether the UPS is on battery and whether the utility is back.
How can I tell an alarm from a fault on an EXL S1?
By the first digit of the display ID in Vertiv's tables: alarm IDs start with 2, fault IDs with 3.
At what temperature does the EXL S1 rectifier alarm?
Vertiv's guide gives a default of 80 °C for the rectifier phase overtemperature alarm (22-061) and 95 °C for the rectifier temperature fault (32-028).
What does 20-019 System Maint. Bypass Switch Closed mean?
The maintenance bypass switch is closed, so the load runs on utility power without UPS conditioning or battery backup. It is expected during maintenance and should be cleared afterwards.
Can I reset an EXL S1 fault myself?
Note the code and the time first. Faults such as 36-080 Fuse Blown or 34-071 Battery Ground Fault need a technician before any restart.
Sources, checked October 4, 2026:
- Vertiv, Liebert EXL S1 operation and maintenance guide SL-26090, Appendix A, pp. 56, 57, 58, 59, 61, 62, 63, 64, 65, 66, 67
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