Problems on this page:
UPS running on battery with mains present
Hits OEE: Availability
Symptoms: The UPS shows it is on battery even though the mains supply is on, and the battery runtime counts down.
Input breaker or fuse open
Check the UPS input breaker and fuses, and find why they opened before resetting.
Input voltage or frequency outside the UPS window
Measure the input; a generator or a weak supply may be outside the window the UPS accepts.
Rectifier fault
Check the UPS alarm log and call the service provider for a rectifier fault.
Loose input connection
Have the input terminals checked with the UPS isolated by trained staff.
Prevention: UPS alarm log reviewed weekly, and alarms sent to someone who acts on them.
Battery runtime shorter than expected
Hits OEE: Availability
Symptoms: A test or a real outage shows the UPS runs on battery for much less time than it should.
Batteries near the end of their life
Run a battery test and replace the string if the capacity has dropped, since cells age even when unused.
Battery room too warm
Keep the batteries at the temperature the maker specifies, because heat shortens battery life.
Load has grown since the UPS was sized
Measure the load and remove non-essential equipment or plan more capacity.
One weak cell in the string
Measure each block voltage and internal resistance and replace weak blocks.
Prevention: Battery tested on a fixed interval and battery age tracked against the maker life.
Battery fault on self-test
Hits OEE: Availability
Symptoms: The UPS raises a battery fault or replace battery alarm after its automatic test.
Weak or failed battery block
Measure block voltages and internal resistance and replace the failed blocks.
Loose or corroded battery terminal
Have the terminals cleaned and tightened by trained staff; batteries can deliver very high currents.
Battery breaker or fuse open
Check the battery breaker and fuses.
Charger fault
Check the charging voltage and current against the maker settings.
Prevention: Battery connections inspected and torque-checked on the maintenance plan.
UPS switched to bypass
Hits OEE: Availability
Symptoms: The UPS has transferred the load to bypass, so the load is no longer protected.
Overload on the output
Measure the load and move non-essential equipment off the UPS.
Inverter fault
Read the alarm log and call the service provider.
Over temperature in the UPS
Check the cooling fans, filters and room temperature.
Manual bypass left on after maintenance
Return the UPS to normal mode following the maker procedure and record it.
Prevention: Bypass status shown on the plant alarm system, not only on the UPS screen.
UPS overheating
Hits OEE: Availability
Symptoms: Over temperature alarms appear, fans run loud or the UPS reduces output.
Cooling fans failed
Replace failed fans; most have a service life and should be changed on schedule.
Air filters or vents blocked
Clean the filters and keep the vents clear.
Room cooling failed or undersized
Check the room air conditioning and its capacity for the heat the UPS gives off.
UPS running near full load
Measure the load and reduce it or add capacity.
Prevention: Fans and filters on the planned maintenance list with the maker intervals.
Load drops during transfer
Hits OEE: Availability
Symptoms: Equipment on the UPS resets or shuts down when the UPS switches between modes.
Static switch fault
Have the static switch tested by the service provider.
Bypass supply not in sync with the inverter
Check the bypass supply voltage and frequency, especially when a generator feeds it.
Load sensitive to even short breaks
Check the ride-through of the affected equipment and the UPS transfer settings.
Wrong configuration after a firmware change
Compare the UPS settings with the commissioning record.
Prevention: Transfer tested during planned maintenance with the critical loads watched.
Overload alarm
Hits OEE: Availability
Symptoms: The UPS raises an overload alarm, sometimes only when certain equipment starts.
Equipment added to the UPS over time
List what is connected and move non-critical loads to normal supply.
Inrush current from motors, heaters or printers
Keep motors, heaters and laser printers off the UPS output.
Phases unevenly loaded on a three phase UPS
Measure each phase and rebalance the loads.
Capacity setting wrong
Check the rated capacity setting with the service provider.
Prevention: A list of connected loads kept with the UPS and updated with every change.
Batteries not charging
Hits OEE: Availability
Symptoms: Battery voltage stays low after an outage, or the UPS shows a charger alarm.
Charger fault
Check the charger output voltage and current and call the service provider if it is out.
Battery breaker open
Check the battery breaker or fuse.
Float voltage set wrong for the battery type
Check the charger settings against the battery maker data.
Temperature compensation sensor faulty
Check the battery temperature sensor reading against a thermometer.
Prevention: Battery voltage and charging current checked after every outage.
UPS alarms not reaching anyone
Hits OEE: Availability
Symptoms: The UPS was on battery or bypass for hours before anyone noticed.
Network or monitoring card fault
Check the monitoring card and its network connection.
Alarm contacts not wired to the plant alarm system
Wire the key alarms to the control room or building management system.
Email or message settings out of date
Update the alarm recipients and send a test alarm.
Alarm silenced and forgotten
Agree who acknowledges UPS alarms and record each one.
Prevention: A test alarm sent from the UPS every quarter.
Guidance only. Always follow lockout/tagout and your site's safe-work procedures, and verify against OEM manuals before acting.
Related equipment guides: transformer · switchgear · PLC · servo drive · chiller
Stop the same fault coming back
Recurring UPS system stops usually trace to a cause you cannot see by hand. The partner we recommend is Fabrico: it tracks OEE and downtime live from PLCs, IoT sensors or AI cameras; where cameras are fitted, the video of a stop sits next to its machine data so the cause can be reviewed; and a technician opens the work order from the stop in the same system. Data is hosted in the AWS EU region, and Fabrico and the company behind it hold ISO/IEC 27001, ISO/IEC 20000-1 and ISO 9001.
Book a Fabrico demo
The directory stays free.
Related tools: full troubleshooting directory · OEE calculator · downtime cost · MTBF / MTTR · glossary
Methods that cut recurring stops: the six big losses · root cause analysis · preventive vs predictive maintenance · TPM · SMED & changeover