K-Class Transformer Fluids: IEC 61039 Classes O, K, L and K1–K3 Explained
“K-class” is the term behind most fire safety relaxations for liquid-filled transformers. What the classification says, how it is tested and what it means for the fire load.
Updated 12 November 2026 · Published by IsoTec
The classification
IEC 61039 classifies insulating liquids by two properties. The letter describes the fire point, the digit the net calorific value – the energy released when the liquid burns.
| Letter | Fire point | Digit | Net calorific value |
|---|---|---|---|
| O | ≤ 300 °C | 1 | ≥ 42 MJ/kg |
| K | > 300 °C | 2 | < 42 MJ/kg |
| L | No measurable fire point | 3 | < 32 MJ/kg |
A higher digit means less energy available to sustain a fire. The fire point is the lowest temperature at which the liquid keeps burning for at least 5 seconds after ignition.
Typical transformer liquids
| Liquid | Flash point | Fire point | Net calorific value | Class |
|---|---|---|---|---|
| Mineral oil | about 150 °C | about 170 °C | 46.0 MJ/kg | O1 |
| Natural ester | about 316 °C | about 360 °C | 37.5 MJ/kg | K2 |
| Synthetic ester | about 260 °C | about 316 °C | 31.6 MJ/kg | K3 |
Typical values from published data; individual products differ. High molecular weight hydrocarbons can also reach a fire point above 300 °C; as hydrocarbons they have a high calorific value. Always use the values of the specific fluid.
Fire load: a worked example
The fire load of the liquid is its mass times its net calorific value. For 1,000 litres, with typical densities:
| Liquid | Typical density | Mass | Fire load |
|---|---|---|---|
| Mineral oil | 0.88 kg/l | 880 kg | about 40.5 GJ |
| Natural ester | 0.92 kg/l | 920 kg | about 34.5 GJ |
| Synthetic ester | 0.97 kg/l | 970 kg | about 30.7 GJ |
The fire load of an ester is about 15–25 % lower than that of mineral oil – a real but moderate difference. The decisive advantage is that esters are much harder to ignite and do not easily sustain a pool fire. Solid insulation such as cellulose adds to the fire load in every case.
Energy needed to reach the fire point
Heating 1,000 litres from an operating temperature of 80 °C to the fire point takes, without heat losses, about 147 MJ for mineral oil, about 430 MJ for synthetic ester and about 476 MJ for natural ester. An ester therefore needs roughly three times as much heat before a pool fire can sustain itself.
What published tests show
- Flame on the liquid surface: under an oxy-acetylene torch, a mineral oil surface began to flash after about 3 minutes and burned after about 4 minutes; a synthetic ester did not ignite after 70 minutes.
- Spray at 2 bar: a mineral oil mist kept burning for up to 49 seconds after the flame was removed; a synthetic ester mist went out within at most 7 seconds.
- Transformer exposed to fire: a distribution transformer filled with 365 kg of synthetic ester was exposed to burning wood cribs for about 70 minutes. Internal temperatures peaked at about 204 °C, the tank did not leak and the transformer still worked afterwards.
Source: IChemE Hazards 31 (2021). Tests show the behaviour of specific fluids under specific conditions; they do not replace a risk assessment.
Test methods
| Property | Method | Note |
|---|---|---|
| Flash point and fire point | ISO 2592 / ASTM D92 (Cleveland open cup) | Basis for the IEC 61039 letter and for FM Approvals Class 6933 |
| Flash point, closed cup | ISO 2719 / ASTM D93 (Pensky-Martens) | Gives lower values – do not compare with open cup |
| Net calorific value | Bomb calorimeter (net value calculated from the gross value) | Basis for the IEC 61039 digit |
| Auto-ignition temperature | ASTM E659 / DIN 51794 | Not part of the classification, but useful for hot-surface scenarios |
Approvals and listings
- FM Approvals Class 6933 certifies transformer fluids with a fire point of at least 300 °C (average of three ASTM D92 tests), including surveillance of production. Toxicity is not evaluated.
- FM Approvals Class 3990 certifies complete transformers with such a fluid plus tank strength, pressure relief and fault protection.
- NEC 450.23 in the United States requires a listed less-flammable liquid with a fire point of at least 300 °C.
- Product specifications such as IEC 61099 (synthetic esters) and IEC 62770 (natural esters) define the properties of unused fluids.
Mixing changes the class
Mineral oil has a much lower fire point. Even a small residue – after a retrofill or a top-up with the wrong fluid – lowers the fire point of the mixture. FM Approvals states that contamination with uncertified fluids voids the certification of that lot. After a retrofill, the fire point should be measured and the transformer labelled with the liquid it contains. More: residual fire risks.
IsoTec K-class fluids
IsoTec SE 300 is a synthetic ester with a fire point of 316 °C (ISO 2592), class K3. IsoTec NE 350 is a natural ester with a fire point above 350 °C, class K2. Background: synthetic ester transformer fluid and natural ester transformer fluid.
Frequently asked questions
What is the difference between K2 and K3?
Both have a fire point above 300 °C. The digit describes the net calorific value: K2 below 42 MJ/kg (typical for natural esters), K3 below 32 MJ/kg (typical for synthetic esters).
Is K-class the same as “less flammable”?
In practice the criterion is the same – a fire point above 300 °C. “Less flammable” is the term used by FM Global and the NEC, which additionally require certification or listing.
Why is the flash point lower than the fire point?
At the flash point the vapours ignite only briefly. The fire point is the higher temperature at which enough vapour is produced to keep the liquid burning.
Does an ester have no fire load?
No. An ester burns once it has reached its fire point. Its fire load is only about 15–25 % lower than that of mineral oil; the main benefit is the much higher resistance to ignition.
Need certified fire data for your project?
We provide flash point, fire point and IEC 61039 class of our ester fluids. IsoTec products are supplied to customers outside Germany.