Ester vs Mineral Oil vs Dry-Type Transformers: Fire Risk Compared
Three technologies compete in locations where fire safety matters. Each removes some fire scenarios and keeps others.
Updated 12 November 2026 · Published by IsoTec
Comparison
| Mineral oil | Natural ester | Synthetic ester | Dry-type (cast resin) | |
|---|---|---|---|---|
| Classification | O1 (IEC 61039) | K2 (IEC 61039) | K3 (IEC 61039) | Fire behaviour class F0 or F1 (IEC 60076-11) |
| Fire point | about 170 °C | about 360 °C | about 316 °C | – |
| Liquid pool fire | Possible from small ignition sources | Unlikely | Unlikely | No liquid |
| Spray fire during an arc | Possible, keeps burning | Possible while the arc burns | Possible while the arc burns | No liquid |
| Main combustible material | Oil, cellulose | Ester, cellulose | Ester, cellulose | Resin, insulation |
| Smoke | Dense smoke | Smoke, no halogen acid gases | Smoke, no halogen acid gases | Smoke from resin; F1 requires reduced smoke and toxic emissions |
| Containment | Required | Required | Required | Not required |
| Humidity, dust, salt | Sealed tank | Sealed tank | Sealed tank | Depends on environmental class |
| Overload capability | Good | Good | Good | Limited |
| Ratings | All | Up to transmission voltages | Up to transmission voltages | Mainly distribution and medium power |
Typical properties; products and designs differ.
Mineral oil
Mineral oil is the reference technology: inexpensive, well known and available for every rating. With a fire point of about 170 °C, it can be ignited by relatively small sources and sustains pool fires. This is why codes require large clearances, fire walls or vaults for mineral-oil transformers near buildings and why it is usually avoided underground and indoors.
Ester-filled transformers
Natural and synthetic esters keep the advantages of a liquid-filled transformer – compact design, high overload capability, insensitivity to humidity and dirt, all ratings – and raise the fire point above 300 °C. Codes reward this with reduced clearances and simpler protection. The remaining fire scenarios are mainly high-energy arcs with tank rupture and components with their own fuel. More: residual fire risks.
Dry-type transformers
Cast-resin dry-type transformers contain no liquid, so pool and spray fires and spills do not occur. IEC 60076-11 defines fire behaviour classes: F0 for installations without special fire risk and F1 for installations with a fire hazard, where restricted flammability and minimised emission of toxic substances and opaque smoke are required. The resin and insulation are still combustible. Dry-type units are sensitive to humidity, condensation and pollution, have limited overload capability and are larger for the same rating. Manufacturers offer dry-type power transformers up to around 145 kV.
When which technology fits
- Dry-type: clean, dry indoor rooms; lower and medium ratings; where no liquid at all is accepted.
- Ester-filled: humid, dusty or salty environments; higher ratings; high overload requirements; compact rooms; underground substations; outdoor units close to buildings.
- Mineral oil: outdoor substations with sufficient clearance, where fire and environmental requirements allow it.
Existing transformers: retrofilling
A mineral-oil transformer in a critical location can often be converted by retrofilling with ester. The residual mineral oil lowers the fire point of the mixture, so drainage, flushing and the target residual content must be planned and the fire point verified afterwards. Compatibility of materials and the effect on cooling are checked in advance. More: synthetic ester guide.
IsoTec ester fluids
IsoTec SE 300 (synthetic ester, K3) and IsoTec NE 350 (natural ester, K2) are K-class fluids for new transformers and retrofills. See the transformer fire safety guide.
Frequently asked questions
Is a dry-type transformer always the safest choice?
Not always. It avoids liquid fires and spills, but it is combustible, sensitive to humidity and dirt and has limited overload capacity. In harsh environments or for higher ratings an ester-filled transformer can be the better overall solution.
What do F0 and F1 mean?
They are fire behaviour classes for dry-type transformers in IEC 60076-11. F1 applies where there is a fire hazard and requires restricted flammability and minimised emission of toxic substances and opaque smoke.
Can I retrofill a mineral-oil transformer with ester to reduce fire risk?
Often yes. The residual mineral oil must be kept low and the fire point checked after the retrofill. Compatibility and cooling are verified beforehand.
Related articles
- Fire Safety of Ester-Filled Transformers
- K-Class Transformer Fluids: IEC 61039 Explained
- Transformers in Tunnels and Underground Stations
- Indoor Transformer Fire Protection: IEC 61936-1, FM Global and NEC
Sources and further reading
- Reid, J.: The Use of Ester based Transformer Liquids for Reduced Fire Risk and Lower Costs – IChemE Hazards 31 (2021)
- Transformer Magazine: Modern power transformers for underground substations (2020)
- Petersen, A.: Risk Equals Probability Times Consequences – T&D World (2014)
- FM Approvals Class 3990: Examination Standard for Liquid-Insulated Transformers (2022)
Choosing the right technology?
We help you compare ester-filled options with your fire, space and environmental requirements. IsoTec products are supplied to customers outside Germany.