France is where the standard 230 V to 110 V transformer selection question gets interesting. Because French low-voltage installations are predominantly TT earthed, the question “should the secondary be earthed?” has a different answer than in a British or German TN installation. This guide explains what a 230V to 110V transformer has to satisfy on a French site, and why the RCD conversation is not the same as elsewhere in Europe.
It is written for electrical designers, maintenance engineers and buyers integrating imported US equipment or building a permanent 110 V supply in France. For the French-language version, see Transformateur 230V 110V.
A step-down transformer reduces 230 V AC to 110 V AC. It is a passive magnetic device: two windings on a shared core, with the turns ratio fixing the voltage ratio. Nothing in the device regulates the output.
Three points drive the engineering:
France operates a 230 V / 50 Hz supply at 50 Hz, but the dominant earthing arrangement for low-voltage installations is TT: the supply neutral is earthed at the source, and the installation’s exposed conductive parts are connected to a local earth electrode that is electrically independent of the supply neutral.
The consequence is straightforward: in a TT system, fault current returns through the earth itself, so earth fault protection depends entirely on a residual current device. That is why NF C 15-100 requires residual current protection of at most 30 mA on accessible socket circuits.
Now apply that to a transformer with isolated windings. The secondary becomes a separate, floating system. A fault from the 110 V secondary to earth does not return through the primary conductors — it returns through the earth electrode system. The primary RCD cannot see it. This is the point that catches out French installations again and again.
NF C 15-100 is the French installation standard and the reference that a French inspector or insurer will actually work from. The points most relevant to a step-down transformer installation:
| Requirement area | What NF C 15-100 expects | Impact on the transformer |
|---|---|---|
| Residual current protection | 30 mA maximum on socket circuits accessible to ordinary persons | Primary feed protected as any other circuit; secondary needs its own scheme if required |
| Earthing | Earth electrode with defined resistance, bonded exposed conductive parts | Enclosure and terminal box bonded to the earth bar — always |
| Overcurrent protection | Device rated for the conductor and the load | Size the primary protection for the transformer’s full-load current × 1.25 |
| Sockets and plugs | Type E (2P+T) with earth pin | Match primary plugging; do not rely on adapters for permanent installations |
| Voltage drop | Limited across the installation | Account for the transformer’s own regulation plus cable drop |
Three legitimate configurations exist on French sites. Pick by purpose, not by habit.
| Configuración | Secondary treatment | When it is right |
|---|---|---|
| Earthed secondary (TT-compatible) | Secondary neutral bonded to the installation earth at one point | Permanent 110 V distribution feeding ordinary equipment; allows secondary RCD protection to function |
| Protective separation | Secondary left isolated (floating) | Test benches, laboratories, diagnostic work where a first fault should not trip anything |
| Auto-transformer | No separate secondary earth exists | Non-critical loads where separation is not required by the application |
If the secondary is left floating in continuous production use, a single insulation fault will go undetected until a second fault appears — at which point the protective device may not operate as intended. That is an acceptable trade-off on a bench; it is not acceptable for a permanent 110 V supply feeding a machine.
The detailed method, including how to verify earth continuity and how to test the fault path at commissioning, is set out in how to ground a transformer secondary. Final connection must be made by a qualified electrician working to NF C 15-100.
| Aplicación | French designation | What it covers |
|---|---|---|
| General requirements | IEC 61558-1 and NF EN IEC 61558-1 | Base standard: construction, tests, marking |
| Isolating transformers | NF EN IEC 61558-2-4 | Separate windings for general applications |
| Safety isolating transformers | NF EN IEC 61558-2-6 | Additional output voltage and current limits |
| Auto-transformers | NF EN IEC 61558-2-13 | Step-up and step-down auto configurations |
| CE marking | Directives 2014/35/EU (LVD) and 2014/30/EU (EMC) | EU declaration of conformity |
Ask for the declaration of conformity for the specific model on the order. A generic certificate covering a product family is common in export trade and does not satisfy a French technical review, particularly when the ErP regulation requires published no-load loss figures for small transformers.
| Construcción | Separation | La mejor opción |
|---|---|---|
| Auto-transformer | None — shared winding | Heating, lighting, simple motor loads |
| Isolating transformer | Full galvanic separation | Machine control, test benches, noise-sensitive loads |
| Safety isolating transformer | Full, with limited output | Portable tools, wet or confined environments |
| Toroidal | Model dependent | Audio, medical, low stray-field requirements |
For continuous duty in an enclosure, the DT/ST range from 100 VA to 10 kVA provides the standard building blocks: transformer DT/ST series 100VA–10KVA. Mixing an auto-transformer into a job that later needs separation is one of the more expensive mistakes in this field, because it forces a re-design of the protection scheme rather than a component swap.
Transformer ratings are in VA. On resistive loads, watts and VA are close; on motors, pumps and switched-mode supplies the power factor is below 1 and the required VA is higher. Add inrush on top:
| Aplicación | Rated power | Recommended transformer |
|---|---|---|
| Bench control cabinet | 600 VA | 900 VA |
| Portable tool circuit in a workshop | 1,500 W | 3,000 VA |
| Imported US machine control | 1,000 VA | 1,500 VA |
| Motor load with hard start | 1,500 W | 3,000–4,000 VA |
En DT-3000VA step-up step-down transformer y el Transformador elevador y reductor ST-3000VA cover the majority of French workshop and light-industrial cases. For small control loads, a DT/ST series unit in the 500 VA–1 kVA range is usually sufficient.
The export range is set out at Johsun step-up and step-down transformer supplier. For custom windings, alternate terminal arrangements or OEM branding, see ODM / OEM.
| Error | Why it hurts in France | Fix |
|---|---|---|
| Assuming the primary RCD covers the 110 V circuit | In TT with isolated windings, secondary faults return through earth and are invisible to the primary RCD | Design secondary protection explicitly |
| Leaving the secondary floating in permanent use | First fault goes undetected; no automatic disconnection | Reference the secondary at one point, or install permanent insulation monitoring |
| Confusing watts with VA | Undersized transformer, thermal trips | Use the VA figure with a real margin |
| Relying on a plug adapter for a permanent installation | No earth continuity guarantee | Wire the primary properly to a Type E socket or permanently |
| 60 Hz equipment on the 50 Hz grid | Speed loss and overheating | Check frequency dependence before specifying |
On a French site, the transformer itself is rarely the hard part. The protection concept is. Confirm the earthing arrangement, decide deliberately whether the secondary is earthed or floating, size in VA with margin for inrush, and require conformity documentation for the exact model.
Send the voltage, frequency, load profile and earthing arrangement and we will come back with the appropriate type: Contacto.
Not always, but in permanent installations it usually should be. In a TT system, referencing the secondary at a single point to the installation earth is what allows a residual current device to detect a secondary earth fault. In bench and laboratory use, a floating secondary is a legitimate and deliberate choice.
Because the transformer has isolated windings. The secondary fault current returns through the earth electrode system, not through the primary current transformer, so the differential sees no imbalance. This is a fundamental property of isolated secondaries, not a device fault.
The IEC 61558 series in its French implementation, NF EN IEC 61558 — part 2-4 for isolating transformers, 2-6 for safety isolating transformers, 2-13 for auto-transformers — plus CE marking under the Low Voltage and EMC Directives. NF C 15-100 governs installation.
Often yes, by feeding the secondary winding and taking the output from the primary. Confirm in the datasheet that the manufacturer explicitly permits reverse operation before relying on it.
Size at 70–80 % of rated VA. Add 20 % on resistive loads, a factor of 2 on motor loads, and 30–50 % on switched-mode supply loads to cover inrush current.
For non-critical loads where separation is not required — heating, lighting, simple motor loads — yes, and it is more efficient. Where separation is required by the application or by risk assessment, it is not an option.
