In short. Combining a servo stabiliser with an isolation transformer gives regulated voltage and an isolated, clean reference. Order, sizing for transformer inrush, earthing of the secondary and protection need deliberate design.
Why combine
- Stabiliser holds voltage steady.
- Transformer isolates, reduces common-mode noise and re-creates a neutral.
- Together they protect precision equipment on poor supplies.
Order of equipment
Supply → stabiliser → isolation transformer → load is common, so the transformer sees regulated voltage and its output is isolated. Some designs place the transformer first; the choice affects fault paths and transformer sizing. Decide by study, not habit.
| Arrangement | Consideration |
|---|---|
| Stabiliser then transformer | Transformer gets regulated input; output isolated from stabiliser |
| Transformer then stabiliser | Stabiliser works on the isolated source; earthing of the secondary matters |
Sizing and inrush
Transformers draw a large magnetising inrush at energisation. Size upstream protection and the stabiliser accordingly. Add the transformer losses to the heat budget.
Earthing
Bond the secondary neutral at one point and keep the earth path short. See earthing architecture and neutral grounding.
Protection
- Overcurrent on both sides.
- Earth-fault arrangement for the isolated system.
- Temperature protection of the transformer.
Illustrative example
A precision measurement lab places a servo stabiliser feeding a screened isolation transformer. A short earth bond and a clean reference solve measurement noise and the voltage swing.
Sizing the combination
Size the stabiliser for the load kVA with the starting and low-input allowances. Size the isolation transformer for the same load kVA plus harmonic margin. The transformer is a series element, so its impedance adds to the voltage drop; include it in the check of output voltage under load.
Transformer voltage drop (%) ≈ Load fraction × Transformer impedance (%)
For a transformer with 4 percent impedance at full load, the drop is about 4 percent, roughly 16 V on a 415 V system, and the stabiliser upstream should hold the transformer input high enough to deliver the target output.
Inrush management
A transformer energised at an unfavourable point on the voltage wave can draw an inrush of many times its rated current for a few cycles. Upstream protection must tolerate this. On a generator, inrush can cause a brief voltage dip. Some systems use sequential start or controlled energisation.
Earthing design checklist
- One neutral-earth bond at the transformer secondary only.
- No downstream neutral-earth bonds.
- Screen earthed as the maker specifies.
- Earth conductor short and of adequate size.
- Earth resistance measured and recorded.
Acceptance tests
| Test | Purpose |
|---|---|
| Insulation resistance | Winding and cable condition |
| Output voltage at load steps | Regulation including transformer drop |
| Neutral-earth voltage | Earthing quality |
| Inrush observation | Protection coordination |
| Bypass and changeover | Continuity during service |
Operating notes
Record thermal behaviour in the first weeks. Combined losses can raise room temperature. Include the cooling in the design. For precision loads, keep a log of measurement noise before and after, so the benefit of the investment is visible.
The master engineering question
Which weakness is the load protected against: voltage level, noise, earth faults, or all three?
What to send us
Send equipment details and earthing layout. Send your application details. See also stabiliser vs isolation.
Questions engineers ask
Does the order matter?
Yes, for protection, inrush and earthing.
Add losses?
Yes, include heat and efficiency.
Can I add a UPS?
Yes, as a separate branch for no-break loads.
Is it needed everywhere?
No, only where the problem justifies it.
Values are illustrative. Design needs site study.