In short. An isolation transformer has no electrical connection between primary and secondary. It creates a separately derived supply with its own neutral and earth reference, blocks much common-mode noise, and limits how faults travel. It does not regulate voltage.
What it does
- Separates two circuits electrically.
- Re-creates a neutral at the secondary, bonded to earth at one point.
- Reduces common-mode noise, more with an electrostatic screen.
- Can change voltage level if the ratio is not 1:1.
- Limits ground-fault current paths in the secondary.
Key parameters
| Parameter | Why it matters |
|---|---|
| kVA rating | Load plus margin; harmonic loads need extra thermal capacity |
| Impedance | Sets voltage drop and fault current |
| Inrush current | Magnetising surge on energising |
| Screen | Noise attenuation between windings |
| Vector group | Neutral and harmonic behaviour |
| Losses | Heat and efficiency |
Earthing
Bond the secondary neutral to earth at a single point, close to the transformer. Keep the earth conductor short. See earthing and neutral grounding and earthing architecture.
Nonlinear loads
Rectifier and drive loads cause harmonic currents that heat the transformer. Consider a K-rated or derated design. See harmonics.
Common mistakes
- Bonding the neutral at more than one place.
- Ignoring inrush when sizing the breaker.
- Expecting voltage regulation.
- Skipping the screen earth connection.
Compare with stabilisers and ultra-isolation units.
Illustrative example
A machine builder’s controller mis-reads signals due to ground noise. An isolation transformer with a single-point neutral bond at its secondary removes the shared earth path, and the noise falls.
Inrush and protection
When a transformer is energised, the core can saturate and draw a magnetising current of several times rated current for a few cycles. Breakers and fuses must be selected or set so they do not trip on this inrush, while still protecting against real faults. Manufacturers publish inrush multiples; use them.
Impedance and fault level
Secondary fault current ≈ Rated current ÷ Per-unit impedance
A 100 kVA, 415 V transformer with 4 percent impedance has a rated current near 139 A and a secondary fault current in the range of 3,500 A, ignoring source impedance. Use this to check breaker ratings downstream. A higher impedance limits fault current but increases voltage drop under load.
Winding connection and harmonics
| Connection | Behaviour |
|---|---|
| Delta primary, star secondary | Traps triplen harmonic currents in the delta; gives a neutral at the secondary |
| Star-star | Neutral current and third harmonics flow; less common for isolation |
| Zig-zag | Neutral current reduction |
Selecting a transformer for nonlinear loads
- Estimate harmonic current content of loads.
- Choose a harmonic-rated design or derate a standard one.
- Check temperature rise at the expected harmonic loading.
- Consider the neutral conductor size.
Testing and maintenance
- Insulation resistance and winding resistance on installation.
- Thermography at load.
- Check tightness of connections.
- For oil-filled units, oil tests as per policy.
Siting
Allow cooling space and protect from moisture and dust. In warm climates, check the ambient rating, because transformer capacity falls as ambient rises.
The master engineering question
Does the problem come from a shared path between circuits that isolation can break?
What to send us
Send load details and earthing layout. Send your application details.
Questions engineers ask
Does it clean harmonics?
No. It may limit some, but needs mitigation for harmonics.
Do I need a screen?
For noise-sensitive loads, yes.
What about inrush?
Plan for magnetising inrush in protection settings.
Is it a safety device?
It reduces some risk if installed with correct earthing.
Values are illustrative. Design earthing per site and codes.