Transformer Tank Rupture and Internal Arc Faults
The most severe transformer fire scenario starts with an arc inside the tank. How it develops, how it is prevented – and what an ester fluid changes.
Updated 12 November 2026 · Published by IsoTec
How a tank ruptures
When insulation breaks down, an arc forms in the liquid. Its energy vaporises and decomposes the liquid into gas within milliseconds. Because liquid is almost incompressible, a pressure wave travels through the tank and loads walls, covers and bushing turrets. If the arc energy is high enough – high fault current, long clearing time – the weakest part fails and liquid escapes, often as a spray.
The key variables are the arc energy, the resulting hydraulic pressure and the pressure withstand of the tank.
How often and where
According to the convener of the CIGRE working group on transformer fire safety, tank ruptures are rare below 245 kV and occur mainly above 300 kV, usually after insulation breakdown to the tank or in a bushing turret rather than from internal winding faults.
Mitigation according to IEEE C57.156
IEEE C57.156 describes measures to reduce the risk of tank rupture from internal faults in four areas:
- System design: limit fault current and clear faults fast – the arc energy grows with current and duration.
- Product design: tank strength, flexible or reinforced tank structures, turret and cover design.
- Monitoring and signalling devices: sudden pressure relays, Buchholz relays, pressure relief devices, dissolved gas analysis.
- Insulating fluid selection.
Criteria for FM Approved transformers
FM Approvals Class 3990 gives concrete criteria for transformers with certified less flammable fluids:
| Element | Requirement |
|---|---|
| Tank | At least 7 psi (48 kPa) without permanent distortion; 15 psi (rectangular) or 20 psi (cylindrical) without rupture |
| Pressure relief device | Nominal opening pressure 10 psi (69 kPa), minimum flow rate depending on rating |
| Low-current faults | Fuses acceptable up to 2,500 kVA; above that alarm contacts and sudden pressure relays or equivalent |
| High-current faults | Energy limited, primarily by current-limiting fuses (I²t limits) |
| Fluid | Certified under Class 6933, fire point at least 300 °C |
What the fluid changes – and what not
The fluid does not prevent the arc, the gas generation or the pressure wave. It changes what happens to the released liquid. A fine spray can ignite while the arc burns, even for liquids with a flash point above 300 °C. Once the arc is cleared, a K-class liquid is much less likely to keep burning: in published spray tests, a synthetic ester mist went out within at most 7 seconds after the flame was removed, while a mineral oil mist kept burning for up to 49 seconds. Fast protection and a tank that holds are therefore the first line of defence, the fluid the second.
Specification checklist
- Maximum fault current and protection clearing time at the transformer
- Tank pressure withstand and rupture-resistant design
- Pressure relief device with alarm contact; sudden pressure relay
- Bushing turrets designed for internal fault pressure; dry bushings
- K-class fluid in the main tank and in the tap changer
- Containment and flame trap for released liquid
- Online monitoring: dissolved gas analysis, bushing monitoring
Frequently asked questions
Why do transformer tanks rupture?
An internal arc decomposes the liquid into gas within milliseconds. The pressure wave can exceed the strength of the tank, a cover or a bushing turret if the arc energy is high enough.
Does an ester prevent tank rupture?
No. The fluid does not prevent the arc or the pressure rise. It reduces the probability that released liquid keeps burning once the arc is cleared.
Which standard covers tank rupture?
IEEE C57.156 is the guide for tank rupture mitigation of liquid-immersed power transformers and reactors. FM Approvals Class 3990 sets tank and pressure relief criteria for FM Approved transformers.
Related articles
- Fire Safety of Ester-Filled Transformers
- Residual Fire Risks of Ester-Filled Transformers
- Bushings, Tap Changers and Cable Boxes as Fire Sources
- Transformer Fire Risk Assessment: Method and Checklist
Sources and further reading
- IEEE C57.156-2016: Guide for Tank Rupture Mitigation of Liquid-Immersed Power Transformers and Reactors
- FM Approvals Class 3990: Examination Standard for Liquid-Insulated Transformers (2022)
- Petersen, A.: Risk Equals Probability Times Consequences – T&D World (2014)
- HSE Research Report RR980: Generation of flammable mists from high flashpoint fluids (2013)
- Reid, J.: The Use of Ester based Transformer Liquids for Reduced Fire Risk and Lower Costs – IChemE Hazards 31 (2021)
Specifying a transformer for a critical location?
We support you with K-class ester fluids and the fire data for your specification. IsoTec products are supplied to customers outside Germany.