Symptom
On a utility outage or a test, the controller, HMI, radio, or network switch lost power although a UPS was installed to carry them; the UPS shows a battery or bypass alarm; the runtime displayed is far below what was expected; the UPS transferred and then dropped the load after seconds.
The short answer
UPS Not Carrying Load
A UPS that did not carry its load on an outage has a battery that could not deliver, a transfer that did not happen, or a load that was not on it. Batteries age out in three to five years and fail silently unless tested under load; a UPS that has gone to bypass after an overload or an internal fault passes utility power straight through and shows a status nobody read; an overload beyond the UPS rating causes it to drop the load or go to bypass; an output breaker can be tripped; and loads are often plugged into the surge-only receptacles or a strip that is not on the UPS output at all. The UPS event log says what happened, a load test under a real input loss proves the recovery, and a schedule of battery replacement, monthly self-tests, an annual load test, and a status contact wired to the SCADA keeps it from recurring.
Key points
- Read the UPS log first; it records the transfer, the battery state, the overload, and the bypass.
- Batteries last three to five years and fail without warning unless tested under load.
- A UPS in bypass is a power strip; the status has to be read or alarmed.
- Confirm which receptacles the loads are on; surge-only outlets are not on battery.
- Test by pulling the input with the load on, on a schedule, and record the runtime.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Battery aged out or failed | The battery alarm or a replace-battery indicator; the runtime on battery collapses in seconds on a test. Replace the battery set; note the date. |
| Battery never connected | A new or a serviced UPS with the internal battery connector still unplugged for shipping; the UPS runs on utility and shows a battery fault. Connect it. |
| UPS in bypass | The status shows bypass, manual or automatic, after an overload or a fault. Clear the fault, return to normal mode, and find what caused the bypass. |
| Overload beyond the UPS rating | The load percentage on the display near or above one hundred; the UPS went to bypass or shut down on transfer. Reduce the load or install a larger UPS. |
| Output breaker tripped | The output breaker on the UPS rear panel; the loads dead while the UPS runs. Reset after finding the overload. |
| Loads not on the UPS output | The controller plugged into a surge-only receptacle, a wall outlet, or a strip fed from utility. Trace every critical load to the UPS battery-backed output. |
| Failed self-test or internal fault ignored | The log shows failed tests or faults for months. Service or replace the UPS; alarm the status. |
| Temperature | A UPS in a hot panel or enclosure has half the battery life and derated capacity. Measure the temperature; relocate or cool. |
| Runtime expectation wrong | The UPS was sized for minutes and the outage was hours; the generator did not start. Size the runtime for the generator start with margin, and alarm on battery so someone responds. |
After the fact
- 1
Read the log
The UPS event log, through its display or its management interface: the time of the input loss, the transfer, the battery voltage, the load, the bypass, the shutdown.
- 2
Read the status
Battery condition, mode, load percentage, alarms, the battery installation date if recorded.
- 3
Trace the loads
Which receptacles the controller, the switch, the radio, and the HMI are actually on; the battery-backed outlets against the surge-only ones.
- 4
Test
With the loads on the UPS, pull the input plug or open the input breaker and time the runtime to the low-battery alarm. Compare with the expected runtime.
- 5
Fix
Battery, mode, load, or wiring per the finding; then test again.
What the log says
| Log entry | Meaning | Fix |
|---|---|---|
| Input loss, on battery, low battery within seconds, shutdown | Battery failed | Replace the battery set |
| Input loss, no transfer, output lost | UPS was in bypass, or the battery was disconnected | Return to normal mode; connect the battery; find the bypass cause |
| Input loss, on battery, overload, bypass or shutdown | Load exceeds the rating on battery | Reduce load or a larger UPS |
| Input loss, on battery, runtime normal, loads still died | Loads not on the UPS output, or the output breaker | Trace the loads; check the breaker |
| Self-test failed, repeatedly, before the event | The UPS told everyone for months | Alarm the status; service the UPS |
| On battery for hours, then shutdown | The outage outlasted the runtime; the generator did not start or does not exist | Runtime sized for the generator; the generator problem |
Testing properly
A UPS self-test switches to battery for seconds and reports; it detects a dead battery and misses a weak one. The test that matters is a load test: the real loads on the UPS, the input removed, and the time to the low-battery alarm measured and recorded. Done annually, and after every battery replacement, it produces a runtime history that shows the battery aging, and it is scheduled when the process can tolerate the controls going down if the test fails, with the manual procedure ready. A UPS that has never been load tested has an unknown runtime.
Maintenance
- Battery replacement on a schedule of three to five years by the manufacturer guidance and the temperature, with the date on a label.
- Monthly self-test with the result read; annual load test with the runtime recorded.
- Status alarmed to the SCADA: on battery, low battery, bypass, fault, replace battery.
- The UPS in a location within its temperature rating, with airflow.
- A load list on the UPS so additions are checked against its rating and its runtime.
- A replacement plan for the UPS itself at the end of its service life, typically eight to ten years.
Frequently asked questions
- How long should the UPS run?
- Long enough for the generator to start and the transfer to complete with margin, or for a clean shutdown, or for the outage the utility decides to ride through where there is no generator. Fifteen to thirty minutes is common for controls with a generator; hours for a remote site radio and controller with no generator, which is a larger battery than most UPS units carry.
- The UPS runtime display says twenty minutes. Can I trust it?
- It is an estimate from the battery voltage and the load, and it is optimistic on an aged battery that collapses under load. Trust the load test, not the display.
- Should the UPS be on the generator circuit?
- Yes: the UPS input from the generator-backed circuit, so that when the generator picks up, the UPS recharges and returns to normal, and its runtime is spent only on the transfer gap. A UPS on a circuit the generator does not feed runs until its battery is gone.
- Is a UPS enough for a remote site?
- For the controller and the radio through short outages, yes, sized for hours rather than minutes. For the pumps, no; the site needs a generator or a portable connection, and the UPS keeps the site reporting the outage while the pumps are down.
Related topics
- UPS for Control PanelsKeeping the controller, the radio, and the SCADA server alive through an outage: AC and DC UPS types, what to put on the UPS and what not to, sizing for runtime and inrush, the battery as a maintenance item, the alarms a UPS must provide, and what happens when the UPS is the thing that fails.
- Generator Transfer ProblemsThe generator started and something still went wrong: no transfer, controls reset during the gap, drives faulted, pumps ran backward, or the retransfer caused a second outage. The sequence and its timers, neutral and rotation, and the monthly test.
- Loss of Control VoltageControl voltage that drops out for a moment and comes back, resetting controllers and dropping contactors: utility sags when a large load starts, a transfer switch operation, a loose neutral, or an overloaded control transformer. Catching it with a recorder.
- Lift Station Generator OperationStandby power at a lift station: what the transfer switch does, how the controls ride through the transfer, staggered pump restarts, what SCADA should see, load testing, and the portable generator connection.
- Server FailureHow SCADA servers fail and what to do when one does: failure modes from a full disk to a dead hypervisor host, what the plant experiences with and without redundancy, the recovery sequence from restore to alarm verification, and the monitoring that warns.
- No Control PowerThe control power path from the disconnect to the last relay, and how to work along it when the panel is dead: transformer and fuses, control breaker, emergency stop and safety relay, DC supply and distribution, what trips them, and design that makes it easy.
Direct contact
Have a controls question?
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