Transformer Fire Risk Assessment: Method, Data and Checklist

A structured method for fire engineers, asset owners and insurers – from the scenarios to the evidence that the residual risk is acceptable.

Updated 12 November 2026 · Published by IsoTec

Seven steps

Define the objectives: life safety, protection of the building and adjacent assets, continuity of supply, environment.
Identify the scenarios: internal arc with tank rupture, bushing or cable box failure, tap changer failure, external fire, leak with ignition.
Estimate the likelihood using failure and fire statistics, adjusted for the design, fluid, protection, age and loading.
Estimate the consequences: fire size, heat radiation, smoke, structural effects, outage time, environmental release.
Identify the barriers that prevent or mitigate each scenario – a bow-tie diagram is useful here.
Evaluate against codes, insurer requirements and risk criteria; in the UK, demonstrate that risks are as low as reasonably practicable.
Document and review when the design, the loading or the surroundings change.

Likelihood: what the statistics say

Published figures give a sense of scale:

Serious transformer failures: about 0.5–2.5 % per transformer service year, on average about 1 %.
Serious transformer fires: about 0.06–0.1 % per service year – roughly 2.4–4 % of transformers over a 40-year life.
Causes of fires below 300 kV: bushings and cable boxes 70–80 %, on-load tap changers 10–15 %, other causes the rest.
Tank ruptures are rare below 245 kV and occur mainly above 300 kV.

These figures come largely from mineral-oil transformers. For an ester-filled unit, the probability that a failure turns into a sustained fire is lower; the probability of the failure itself depends on design, components, protection and maintenance.

Consequences

Fire size: liquid volume × density × net calorific value gives the fire load; containment and flame traps limit the pool area. See the worked example in K-class fluids explained.
Heat and spread: radiation to adjacent transformers, buildings and cables.
Smoke: visibility and toxicity in escape routes, especially indoors and underground.
Business impact: a transformer fire almost always destroys the unit. An insurer study of 100 loss events found average losses above US$7 million – about US$4 million where contingency plans were in place.
Environment: liquid and contaminated firewater.

Barriers and the hierarchy of controls

Level	Examples for transformers
Eliminate	Place the transformer outside the building or away from escape routes
Substitute	K-class ester fluid instead of mineral oil; dry-type where suitable
Isolate	Clearances, fire walls, fire-rated rooms, flame traps
Engineering controls	Fast protection, pressure relief, sudden pressure relays, rupture-resistant tanks, dry bushings, detection and suppression, containment
Administrative controls	Condition monitoring, dissolved gas analysis, maintenance, keeping combustibles away, emergency procedures

Evaluation and ALARP

Codes and insurer guidelines define a minimum. Beyond that, the question is whether further measures are reasonably practicable. Under the UK ALARP principle, a measure may only be rejected if its cost is grossly disproportionate to the risk reduction. Document which measures were considered, which were implemented and why others were not. A ready-made scenario list: residual fire risk register.

Site survey checklist

Rating, voltage, year of manufacture, loading history
Liquid type, volume, IEC 61039 class; liquid in tap changer, bushings and cable boxes
Bushing type (oil-impregnated paper or resin-impregnated) and condition data
Protection: differential, overcurrent, fuses, clearing times
Buchholz relay, sudden pressure relay, pressure relief device
Dissolved gas analysis history and trends
Distances to buildings, openings, escape routes and other transformers
Fire walls, room fire ratings, doors, ventilation openings
Detection and suppression systems
Containment, flame traps, drainage, separator
Combustible materials stored nearby
Access for firefighters, disconnection and earthing procedures

Frequently asked questions

How often do transformers catch fire?

Published estimates for serious transformer fires are about 0.06–0.1 % per service year, or roughly 2.4–4 % over a 40-year life, based mainly on mineral-oil units.

What causes most transformer fires?

For transformers below 300 kV, bushings and cable boxes account for an estimated 70–80 % of fires and on-load tap changers for 10–15 %.

Which guidelines cover transformer fire risk?

CIGRE Technical Brochure 537 on transformer fire safety practices, CIGRE TB 886 on fire risk management in substations, IEEE C57.156 on tank rupture mitigation, FM Global Data Sheet 5-4 and IEC 61936-1.

Sources and further reading

Preparing a transformer fire risk assessment?

We provide the fluid data and discuss scenarios with your engineers. IsoTec products are supplied to customers outside Germany.