Residual Fire Risks of Ester-Filled Transformers – a Risk Register for Fire Engineers

K-class ester fluids remove much of the pool-fire risk of mineral oil. They do not remove every fire scenario. This register lists what remains, why it remains and how it is controlled.

Updated 12 November 2026 · Published by IsoTec

Short answer

Replacing mineral oil with a K-class ester makes it much less likely that released liquid is ignited by a small source or keeps burning as a pool. What remains are mainly scenarios with a strong, persistent ignition source – a high-energy arc during a tank rupture, a long external fire – and components that bring their own fuel, such as bushings with mineral oil. Smoke, spills and firewater also remain to be managed.

Risk register

Scenario	Typical cause	Effect of K-class fluid	Residual risk	Main controls
Internal arc, tank rupture	Insulation breakdown, flashover in a bushing turret	Released liquid is unlikely to keep burning once the arc is cleared	Spray or mist may ignite while the arc persists; pressure wave; liquid release	Fast protection, pressure relief, sudden pressure relay, tank strength, containment
Bushing failure	Ageing, moisture, partial discharge in oil-impregnated paper (OIP) bushings	None for the bushing’s own mineral oil	Bushing fire, burning debris, ignition of surroundings	Dry bushings (RIP/RIS), bushing monitoring, condition-based replacement
Cable box or termination failure	Poor jointing, overheating, compound- or oil-filled boxes	Depends on the filling of the box	Fire in the cable box, fire spreading along cables	Air-insulated or plug-in terminations, cable fire stopping
On-load tap changer	Contact wear, arcing in the diverter compartment	Only if the tap changer compartment also contains the K-class liquid	Tap changer fire or rupture	Vacuum-type tap changer, same K-class liquid, monitoring
External fire exposure	Fire in adjacent equipment, cables or vehicles	Much more heat is needed to reach the fire point	Long, intense exposure can heat the liquid; tank may fail from pressure	Clearances, fire walls, water spray or sprinklers, limiting nearby fire loads
Leak and ignition	Gasket failure, mechanical damage	Small sources are unlikely to ignite a pool	Contaminated area; ignition if heated by another fire	Containment, inspection, drainage design
Mixed or contaminated liquid	Retrofill residues, top-up with the wrong fluid	Fire point of the mixture is lower	Transformer may no longer be K-class	Fire point check after retrofill or top-up, clear labelling
Solid insulation and other combustibles	Cellulose, wood, plastics, cable insulation	None	Additional fire load in a fully developed fire	Compartmentation, suppression
Smoke and combustion gases	Any fire	Esters contain no halogens, so no halogen acid gases form	Smoke, carbon monoxide and soot, especially in enclosed spaces	Detection, smoke control, protected escape routes
Firefighting near live equipment	Energised equipment during a fire	None	Electric shock, re-ignition	Disconnection and earthing procedures, signage, training
Firewater and environmental release	Spills, contaminated firewater	Biodegradability does not remove containment obligations	Contamination of soil, drains or water	Bund for liquid volume plus firewater and rain, separator suitable for esters

Why sprays are the key residual risk

Released under pressure, a liquid breaks up into droplets. A fine mist can ignite at temperatures far below the flash point of the bulk liquid. A research review for the UK Health and Safety Executive notes that fine sprays of heavy oils with flash points above 300 °C can still ignite. During an internal fault, the arc is a strong ignition source right at the release point.

What changes with esters is what happens next. In published spray tests at 2 bar, a mineral oil mist kept burning for up to 49 seconds after the flame was removed, while a synthetic ester mist went out within at most 7 seconds. In practice this means: clear the arc fast, prevent the tank from rupturing, and make sure released liquid cannot reach other ignition sources. More: tank rupture and internal arc faults.

The component trap: bushings and cable boxes

Statistics compiled by the convener of the CIGRE working group on transformer fire safety attribute 70–80 % of transformer fires below 300 kV to bushings and cable boxes. An ester-filled transformer with oil-impregnated paper (OIP) bushings still carries mineral oil in each of these bushings – right at the most frequent failure point. Specifying dry bushings and suitable cable terminations is often the single most effective step after choosing the fluid. More: bushings, tap changers and cable boxes.

A bow-tie view

A bow-tie diagram is a clear way to present these risks. The central event is loss of containment of insulating liquid.

Threats (left): internal arc, bushing failure, tap changer failure, mechanical damage, external fire.
Preventive barriers: design and testing, condition monitoring (dissolved gas analysis, bushing monitoring), fast protection, tank strength and pressure relief.
Mitigating barriers (right): K-class fluid, containment and flame traps, fire-rated separation, detection and suppression, smoke control.
Consequences: fire, smoke, damage to adjacent assets, loss of supply, environmental release.

Data to request from the fluid supplier

Flash point and fire point, with the test method (ISO 2592 / ASTM D92, Cleveland open cup)
IEC 61039 class including the calorific value digit (e.g. K2, K3)
Certification or listing, if required (FM Approvals Class 6933, listing for NEC 450.23)
Safety data sheet with combustion products and extinguishing media
Effect of mineral oil contamination on the fire point
Density, for fire load and containment calculations

Arguing that residual risk is as low as reasonably practicable

In the UK, risks must be reduced “as low as reasonably practicable” (ALARP). For transformers, a typical line of argument follows the hierarchy of controls: avoid the hazard (location), substitute (K-class fluid or dry-type), engineer it out (protection, tank, dry bushings, containment, fire-rated separation), then manage what remains (monitoring, maintenance, emergency procedures). A measure can only be rejected if its cost is grossly disproportionate to the risk reduction it brings. More: transformer fire risk assessment.

Frequently asked questions

What is the main residual fire risk of an ester-filled transformer?

A high-energy internal arc that ruptures the tank and releases liquid as a spray while the arc is still burning. Fast protection, pressure relief and tank strength are the main controls.

Can an ester mist ignite?

Yes. Fine sprays of high-flash-point liquids can ignite when an ignition source is present. In published tests, ester mists went out quickly once the flame was removed, unlike mineral oil mists.

Do bushings contain mineral oil in an ester-filled transformer?

Oil-impregnated paper (OIP) bushings have their own sealed mineral oil filling. Resin-impregnated (RIP/RIS) bushings are dry alternatives.

Do esters produce toxic smoke?

Any burning organic material produces smoke, carbon monoxide and soot. Esters consist of carbon, hydrogen and oxygen, so no halogen acid gases are formed.

Sources and further reading

Need fire data for your risk register?

We provide fire-relevant data for our ester fluids and discuss scenarios with your fire engineering team. IsoTec products are supplied to customers outside Germany.