Fire Safety of Ester-Filled Transformers

A guide for fire engineers, specifiers and insurers: what K-class ester fluids change in the fire risk of a transformer, which risks remain, and which standards apply – in tunnels, underground stations, buildings and substations.

Updated 12 November 2026 · Published by IsoTec

Key facts at a glance
Why transformers catch fire
What K-class fluids change
What K-class fluids do not change
Standards and guidelines
Where ester-filled transformers are used for fire safety
Questions a fire strategy should answer
How IsoTec supports fire engineers
All articles on transformer fire safety

Key facts at a glance

Transformer fires are rare but severe. Serious fires are estimated at about 0.06–0.1 % per transformer service year – roughly 2.4–4 % of transformers over a 40-year life.
Most fires start outside the main tank. Bushings and cable boxes cause an estimated 70–80 % of transformer fires below 300 kV, on-load tap changers 10–15 %.
Ester fluids are K-class. Their fire point is above 300 °C (IEC 61039); mineral oil is O-class with a fire point of about 170 °C.
Esters need far more heat to ignite. Heating 1,000 litres from 80 °C to the fire point takes about 430–476 MJ for esters and about 147 MJ for mineral oil (calculated without heat losses).
The fire load is only moderately lower. Net calorific value: synthetic ester 31.6 MJ/kg, natural ester 37.5 MJ/kg, mineral oil 46 MJ/kg. The main benefit is resistance to ignition, not a small fire load.
Standards reward K-class fluids with reduced clearances and simpler protection – IEC 61936-1, FM Global Data Sheet 5-4 and NEC 450.23 – usually combined with tank strength, pressure relief and fast electrical protection.
Residual risks remain: arc faults with tank rupture and spray release, bushings with their own mineral oil, external fires, smoke, spills and firewater.

Why transformers catch fire

A transformer fire needs the same three elements as any fire: fuel, an ignition source and oxygen. The fuel is mainly the insulating liquid, supported by cellulose insulation, wood, gaskets and cable insulation. The ignition source is almost always an electrical fault: an arc inside the tank, a failing bushing, an overheated connection – or a fire outside the transformer.

A typical event chain looks like this:

Insulation breaks down and an arc forms in the liquid.
The arc decomposes liquid into gas; the pressure in the tank rises within milliseconds.
If the protection does not clear the fault fast enough, the tank, a cover or a bushing turret fails.
Hot liquid is released – as a spray, a jet or a pool.
The arc or hot parts ignite the released liquid; the fire spreads to the surroundings.

The fluid choice acts at steps 4 and 5. Protection, tank design and component choice act at steps 1 to 3.

What K-class fluids change

Aspect	Mineral oil (O1)	Natural ester (K2)	Synthetic ester (K3)
Flash point	about 150 °C	about 316 °C	about 260 °C
Fire point	about 170 °C	about 360 °C	about 316 °C
Net calorific value	46.0 MJ/kg	37.5 MJ/kg	31.6 MJ/kg
Energy to heat 1,000 L from 80 °C to fire point	about 147 MJ	about 476 MJ	about 430 MJ
Treatment in IEC 61936-1, FM Global, NEC	Baseline requirements	Reduced clearances and protection possible	Reduced clearances and protection possible

Typical values from published data (IChemE Hazards 31, 2021); individual products differ. For a pool of liquid to keep burning, its surface must reach the fire point. With a fire point above 300 °C, a small ignition source – a spark, a short arc, a cable fire nearby – is far less likely to start a self-sustaining pool fire. This is why codes allow K-class transformers closer to buildings and inside them.

More: K-class transformer fluids explained and ester vs mineral oil vs dry-type.

What K-class fluids do not change

Arc faults still happen. An ester does not prevent insulation failure, gas generation or tank rupture.
Sprays can ignite below the fire point. Liquid released under pressure as a fine mist can ignite even if its fire point is above 300 °C – as long as an ignition source such as the arc is present.
Components may contain their own mineral oil. Oil-impregnated paper (OIP) bushings, some tap changers and cable boxes are separate oil volumes.
External fires can heat the transformer. A long, intense fire nearby can eventually bring the liquid to its fire point.
Smoke, spills and firewater still need to be planned for.

The full list with controls: residual fire risks of ester-filled transformers.

Standards and guidelines

Document	What it covers	Relevance
IEC 61039	Classification of insulating liquids by fire point (O, K, L) and net calorific value (1, 2, 3)	Defines “K-class”
IEC 61936-1 / EN 61936-1	Power installations above 1 kV AC, including fire protection of transformers	Clearances, fire walls, indoor rooms by liquid class
FM Global Data Sheet 5-4	Loss prevention for transformers at FM-insured sites	Separation and protection; recognises FM Approved transformers
FM Approvals 6933 / 3990	Certification of transformer fluids (fire point ≥ 300 °C) and of complete transformers	Basis for “FM Approved”
NFPA 70 (NEC), Art. 450.23	Less-flammable liquid-insulated transformers in the US	Indoor installation without a vault under conditions
IEEE C57.156	Tank rupture mitigation for liquid-immersed transformers	Arc energy, tank strength, protection
CIGRE TB 537 and TB 886	Transformer fire safety practices; fire risk management in substations	Statistics, risk assessment, mitigation
IEC 60076-11	Dry-type transformers, fire behaviour classes F0 and F1	Comparison with dry-type
NFPA 130, NFPA 502, TSI SRT	Fire and life safety of rail systems, road tunnels and EU railway tunnels	Context for tunnel installations

Requirements depend on the edition and national adoption. Always check the version that applies to your project.

Where ester-filled transformers are used for fire safety

Tunnels, metro systems and underground stations – traction substations, auxiliary supplies, ventilation
Substations inside buildings – basements, hospitals, high-rise buildings, shopping centres, data centers
Wind turbines and offshore platforms, where space is tight and access is difficult
Industrial plants with high fire loads or strict insurer requirements
Existing mineral-oil transformers in critical locations, by retrofilling with ester

Questions a fire strategy should answer

Which fire scenarios are credible for this transformer – internal arc, bushing, cable box, tap changer, external fire?
What is the liquid volume, its class (IEC 61039) and its calorific value – and does any component contain a different liquid?
How fast does the protection clear an internal fault, and is there a pressure relief or sudden pressure relay?
Which clearances, fire-rated walls or rooms does the applicable standard or insurer require?
Where does released liquid and firewater go?
How do smoke and heat affect escape routes and adjacent equipment?
Which residual risk is accepted – and is it as low as reasonably practicable?

A structured approach: transformer fire risk assessment.

How IsoTec supports fire engineers

Fire-relevant data for our ester fluids: flash point, fire point, IEC 61039 class, safety data sheet
IsoTec SE 300 – synthetic ester, fire point 316 °C (ISO 2592), class K3
IsoTec NE 350 – natural ester, fire point above 350 °C, class K2
Fluid analysis and support with retrofilling, including verification of the fire point after a retrofill

All articles on transformer fire safety

Frequently asked questions

Are ester-filled transformers fire-proof?

No. Esters are combustible, but with fire points above 300 °C they are much harder to ignite than mineral oil and are classified as less flammable (K-class). Residual risks such as arc faults with spray release remain.

What is the fire point of transformer esters?

Typically about 316 °C for synthetic esters and about 360 °C for natural esters, compared with about 170 °C for mineral oil. Check the data sheet of the specific fluid.

What does K-class mean?

Under IEC 61039, K-class insulating liquids have a fire point above 300 °C. A digit adds the net calorific value: K1 at least 42 MJ/kg, K2 below 42 MJ/kg, K3 below 32 MJ/kg.

How likely is a transformer fire?

Published estimates put serious transformer fires at about 0.06–0.1 % per transformer service year, or roughly 2.4–4 % over a 40-year life. Most data come from mineral-oil transformers.

Can ester-filled transformers be installed indoors?

Yes. IEC 61936-1, FM Global and the NEC allow K-class or less-flammable liquid transformers indoors with less demanding measures than mineral oil – for example fire-rated rooms or sprinklers, liquid confinement and fast protection.

Is a dry-type transformer safer than an ester-filled one?

It depends on the scenario. Dry-type units contain no liquid, so there is no pool or spray fire, but their resin and insulation are combustible. Ester-filled units offer higher ratings, overload capacity and resilience to humidity and dirt.

Do ester transformers need oil containment?

Yes. Leaks, tank rupture and firewater still have to be contained according to local rules and the applicable standard.

Can an existing mineral-oil transformer be made K-class?

Often yes, by retrofilling with ester. The residual mineral oil lowers the fire point of the mixture, so the procedure must be planned and the fire point verified afterwards.

Where does IsoTec supply?

IsoTec products are supplied to customers outside Germany.

Sources and further reading

Working on a fire strategy with transformers?

Send us your questions about fluid classes, residual risks or fire data – our ester specialists answer them. IsoTec products are supplied to customers outside Germany.