What Is an Isolation Transformer? Galvanic Isolation, Uses and VA Sizing
What an isolation transformer does and how it protects; galvanic isolation, difference from autotransformers and control transformers, medical and repair bench use, VA and fuse sizing.
An isolation transformer has no conductive connection between its input and output windings and is usually 1:1 (230 V / 230 V). Because the output circuit is galvanically isolated from the mains and from earth, a person touching one output conductor offers no path to earth. Its requirements are set out in IEC 61558-2-4.
> Related Category: Browse models by power and voltage in Isolation Transformers and Control Transformers.
1. How does an isolation transformer protect?
On the mains, neutral is earthed at the substation, so a person touching line while standing on the ground completes a circuit. Neither end of an isolation transformer's secondary is connected to earth. Touching one secondary conductor gives current no return path and no dangerous current flows.
The protection holds when:
- The secondary circuit is not earthed and not joined to another circuit.
- For protective separation one transformer preferably feeds one appliance.
- Secondary leads are short and undamaged; long runs raise capacitive leakage to earth.
- Touching both secondary ends still gives a shock; the transformer protects only against shock via earth.
Since the secondary has no earth reference, an RCD on the secondary side does not trip on a first fault; systems that must keep running use an insulation monitoring device.
2. Transformer types compared
| Type | Windings | Isolation | Use |
|---|---|---|---|
| Isolation transformer | Separate primary and secondary, reinforced insulation | Yes | Protective separation, medical, measurement, repair bench |
| Safety isolating transformer | Separate windings, output up to 50 V AC | Yes | 12–24 V lighting, wet rooms, toys (SELV) |
| Control transformer | Separate windings, basic insulation | Yes (basic) | Panel control circuits 400 / 230 / 24 V; tolerant of coil inrush |
| Autotransformer (incl. variac) | Single winding, common terminal | None | Voltage conversion and adjustment; no shock protection |
An autotransformer or variac output is conductively tied to the mains and cannot replace an isolation transformer on a repair bench.
3. Applications
- Medical locations: operating theatres and intensive care use a medical IT system per IEC 60364-7-710; an isolation transformer with an insulation monitor keeps supply on at the first earth fault.
- Electronics repair and test benches: protects the technician on mains-connected circuits (switch-mode supplies, televisions) and prevents short circuits when measuring with an earthed oscilloscope. See the Oscilloscope and Probe Guide.
- Wet and conductive locations: supplying hand tools inside boilers, metal tanks and on sites.
- Noise-sensitive equipment: an electrostatic screen between windings reduces common-mode mains noise for audio, measurement and laboratory systems.
- Boats and marinas: breaks the earth link between shore supply and hull to prevent galvanic corrosion.
4. Choosing the VA rating
- Add the apparent power (VA) of the loads. If only watts are known, divide by power factor: `VA = W / cos φ`.
- Add a 25% margin; a transformer should not run at full load continuously.
- Allow for inrush on motor and rectifier loads; go one size up.
| Load | Suggested rating |
|---|---|
| Soldering station, instruments (150 W) | 250 VA |
| Repair bench, television (400 W) | 630 VA |
| Computers and lab equipment (800 W) | 1,000–1,600 VA |
| Hand tools (1,500 W) | 2,500 VA |
Example: a 600 W load at cos φ 0.8 → `600 / 0.8 = 750 VA`; with 25% margin 940 VA → a 1,000 VA transformer.
5. Connection and protection
- Primary fuse: on energizing, a transformer draws a brief magnetizing inrush of 10–20 times rated current. Use a time-lag (T) fuse or a D-curve breaker on the primary; a fast fuse blows at every switch-on.
- Secondary fuse: sized for overload from the secondary rated current, `I = VA / U`. For 1,000 VA at 230 V that is 4.3 A → 4 A or 5 A.
- Earthing: bond the transformer frame and any screen winding to protective earth; do not earth the secondary terminals.
- A transformer neither regulates voltage nor supplies power during an outage; those need a regulator and a UPS. See Online vs Line-Interactive UPS.
- No-load output voltage sits 3–10% above nominal; this is normal.
Frequently asked questions
What is an isolation transformer used for?
It separates the load galvanically from mains and earth. Touching one output conductor creates no path through earth, removing that shock risk; it also reduces mains noise.
Should the output of an isolation transformer be earthed?
No. Earthing one secondary terminal removes the isolation and turns the output into an ordinary mains line. Only the frame and screen winding are earthed.
What is the difference between an isolation transformer and an autotransformer?
An isolation transformer has separate primary and secondary windings with no conductive link. An autotransformer has one winding; its output is tied directly to the mains and gives no shock protection.
Does an RCD work after an isolation transformer?
An RCD on the secondary does not trip on a first earth fault, because current cannot return through earth. Where fault monitoring is needed, use an insulation monitoring device.
What VA rating do I need?
Add 25% to the total VA of the loads. 1,000 VA suits a 600 W load at cos φ 0.8; choose one size up for motor loads because of inrush.