In short. Placing a stabiliser upstream of a UPS corrects slow voltage drift before it reaches the UPS, reducing battery use. The UPS then covers short breaks and cleans the supply. This suits sites with poor utility voltage and sensitive loads, but bypass and failure paths need care.
Chain
Utility → stabiliser → UPS → critical load. Non-critical loads such as motors may take a separate stabilised branch without the UPS.
Why this order
- UPS input window is narrow compared with weak grids.
- Drift handled by the stabiliser keeps the battery for real outages.
- Battery life improves with fewer discharges. See batteries.
Design points
| Point | Detail |
|---|---|
| Stabiliser rating | UPS input kVA including charger and harmonic margin |
| Bypass of stabiliser | So it can be serviced without dropping the UPS |
| UPS bypass source | Uses stabilised or direct supply by design |
| Harmonics | UPS input current distortion; check stabiliser thermal rating |
| Fault path | Single stabiliser failure should not drop the load |
Adding a generator
Add an ATS and generator upstream or at the board. See ATS/AMF/DG and UPS and DG.
Failure modes
- Stabiliser fails: bypass or protection logic must keep the UPS fed.
- UPS fails: static bypass supplies stabilised or raw power.
- Both fail: define the fallback.
Compare with stabiliser vs UPS.
Illustrative example
A remote office with voltage between 340 and 440 V puts a servo stabiliser ahead of its online UPS. Battery discharges fall sharply because the UPS no longer sees out-of-window voltage; the batteries last longer.
Sizing chain
- Critical load: for example 30 kW, at 0.9 power factor, about 33.3 kVA.
- UPS: choose rating with margin, say 40 kVA.
- UPS input with charger and efficiency: roughly 36 to 40 kW depending on battery state.
- Stabiliser: sized for UPS input plus any other loads on the same branch, with low-input derating and inrush margin.
All values are illustrative. Use the UPS maker’s input data, which includes charger current at the worst state of charge.
Bypass arrangements
| Arrangement | Benefit | Risk |
|---|---|---|
| Bypass around the stabiliser only | Service the stabiliser without dropping the UPS | Raw supply reaches UPS input |
| UPS static bypass fed from stabilised supply | Load sees regulated supply on bypass | Stabiliser failure affects both paths |
| UPS static bypass fed directly | Independent path | Load sees raw supply on bypass |
Choose on the basis of what the load can tolerate when the UPS is on bypass.
Battery benefit estimation
Estimate the number of UPS battery discharges caused by out-of-window voltage events from the recorded data. If many per month, the battery life gain from an upstream stabiliser can be considerable. If few, the case is weak.
Monitoring
Monitor input and output voltage at both devices, UPS battery events and the stabiliser status. Alarm each, so operators can tell which device acted.
Maintenance coordination
Plan combined maintenance windows, so that the stabiliser and UPS are not both on bypass at the same time unless the plant has accepted that risk in writing.
The master engineering question
Is the UPS spending its battery on drift that a stabiliser could remove?
What to send us
Send supply logs and UPS data. Send your application details.
Questions engineers ask
Is it overkill?
Not on weak supplies; unnecessary on good ones.
Can the stabiliser slow transfers?
It adds response time; design bypass paths with this in mind.
Do I need a bypass around the stabiliser?
For critical loads, yes.
What about heat?
Both add losses; plan cooling.
Values are illustrative. Design needs site data.