Heathrow Airport Transformer Fire – Incident Analysis and Technical Insights

In the early hours of Friday, March 21, 2025, a massive fire broke out at an electrical substation near London’s Heathrow Airport, triggering a complete power outage at Europe’s busiest aviation hub​.

• The blaze occurred at the North Hyde substation in Hayes, about 5 km from the airport, and involved a large high-voltage transformer filled with 25,000 liters of oil​
• Heathrow Airport’s management quickly declared a major power emergency, announcing an unprecedented full-day closure of the airport to ensure safety​
• This led to the cancellation or diversion of over 1,300 flights, affecting more than 200,000 passengers worldwide​
• Airlines and authorities immediately activated emergency plans as firefighters battled the inferno through the night.

Analysis: Fire origin leads to Power Transformer

Investigators are focusing on the transformer at the North Hyde substation as the point of failure. While the exact cause remains under investigation, early forensic analysis by engineers points to a catastrophic internal fault in the transformer that led to an explosion and fire​.

Several potential failure mechanisms are being considered:

• Internal insulation failure & arcing: 

Most likely, an electrical fault occurred inside the transformer’s windings or connections, causing a severe arc flash (high-current spark). Expert analysis indicates the fault would have produced intense arcing that “ignited the oil as it was ejected forcefully from the transformer,” leading to a large oil-fed fireball​. Once the mineral oil (used for insulation and cooling) caught fire, it resulted in an intense blaze that was very difficult to extinguish. Any internal arc in a sealed oil-filled tank also causes a rapid buildup of pressure. In such scenarios, transformers are designed with rupture vents, but a high-energy arc can still rupture the tank violently if not isolated in time.

• On-load tap changer (OLTC) malfunction: 

Investigators will examine the transformer's tap changer, if present. The OLTC is a mechanism that adjusts the transformer's output voltage under load. It’s a complex moving part immersed in oil. A Fellow of the IET (Institution of Engineering and Technology) noted that if the OLTC was operating at the time, a fault in this switchgear could be a trigger for the incident​. OLTC failures have caused transformer fires in the past, as a malfunction can create an arc or loosen contacts inside the tank.

• Oil degradation or leaks: 

Maintenance records and oil sample tests will be scrutinized. Over time, transformer oil can degrade (reducing its insulating strength) or slowly leak. Low oil level exposing parts of the winding can lead to overheating and arcing in air. Investigators will look for any signs that “periodic oil tests had shown signs of leakage” or unusually high moisture or gas content in the oil, which might indicate a developing fault​. Deteriorated oil or insulation paper (due to age, heat, or moisture ingress) can lose dielectric strength, potentially resulting in an internal short circuit.

• Overloading or thermal stress: 

Another line of inquiry is whether the transformer was subject to sustained overloading or abnormal electrical stress. Transformers at critical nodes like Heathrow are usually rated for heavy loads, but unexpected load spikes or cumulative thermal stress could weaken insulation. A transient voltage surge or fault on the network could also precipitate a failure. 

• Cooling system failure: 

Large power transformers often rely on oil circulation and radiator fans or pumps (ODAF/ONAF cooling) to dissipate heat. If the cooling system failed (for example, loss of cooling pumps or fans) unnoticed, hotspots could develop. Over hours or days, such overheating can damage insulation. 

• Protection system issues: 

Every high-voltage transformer is guarded by protective relays (such as differential relays and the Buchholz relay for oil-filled units) that should detect faults and isolate the transformer quickly. Investigators will likely check if the Buchholz relay triggered and whether it did so in time. A possibility is that the fault escalated so rapidly (a high-energy arc causing a tank rupture) that even fast protection could not prevent the fire, though it did cut off supply to limit further damage.In this case, the electrical protection did operate, as evidenced by the power cut. However, there are questions about timing and whether the primary protection tripped instantaneously or a slower backup relay acted after a delay. If an internal fault wasn’t cleared in milliseconds, the arc could have persisted long enough to ignite the oil. Industry experts noted that “electrical protection on that transformer should be almost instant; it may have failed, with slower backup protection operating”​

In summary, the leading hypothesis is an internal equipment failure – potentially an insulation breakdown or switching component fault – that caused a high-energy arc and transformer oil ignition.

Transformer Protection and Fire Mitigation – Performance in this Incident

Transformers and substations are equipped with multiple layers of protection and safety mechanisms which should prevent fatal events. The Heathrow substation fire is a case study in how those measures can be tested by an extreme scenario:

• Relay Protection: 

The substation was protected by relay systems that detected the fault and disconnected the high-voltage supply. However, as noted earlier, the speed of protection is crucial. A differential relay should trip a transformer in a fraction of a second if an internal short occurs. If the primary relay was slow or failed, a backup would trip after a brief delay. It’s not public yet which relay operated, but the transformer clearly did not survive the fault. Protection tripping would save the grid from broader outages (which it did), but it cannot always save the transformer itself once a major internal fault begins. Investigators will analyze relay logs to see if any issues occurred in the protection system’s operation.

• Buchholz Relay: 

As mentioned, an oil-filled transformer typically has a Buchholz relay (a gas- and oil-flow sensor) between the main tank and the conservator (oil expansion tank). In the event of a minor internal arc or slow-developing fault, the Buchholz relay will detect gas bubbles or oil movement and trigger an alarm or trip. In a major sudden fault, a surge of oil can flip the relay and instantly trip the circuit breakers feeding the transformer. In theory, the Buchholz should have tripped as soon as the internal rupture started, and it likely did – but by that point, the fault had already created an arc and ignited oil. The relay’s action cut off electrical supply, yet the fire was self-sustaining (burning oil can continue to burn without electrical input, once ignited). So, while the Buchholz (and other relays) helped isolate the substation from the rest of the grid (preventing a larger blackout), they could not stop the fire in progress.

• Fire Suppression Systems: 

Large utility transformers often have automatic fire suppression, such as a water deluge system or foam sprayers that activate if a fire is detected (via heat or flame sensors). It has been reported that the transformer had a deluge system, but “it clearly did not work” effectively in this case​. It’s possible the explosion damaged the suppression piping, or the system was overwhelmed by the size of the blaze (25,000 liters of oil burning is extremely intense). Fire suppression in outdoor substations is challenging – many utilities rely on manual firefighting response with specialized foam for oil fires, rather than automatic systems, especially if the equipment is outdoors. One takeaway is that even with on-site deluge systems, a transformer fire of this magnitude may not be quickly containable; thus, prevention (via protective tripping and maintenance) is far more effective than trying to extinguish a fire after it starts.

• Physical Barriers: 

As discussed, the substation lacked a firewall between the two main transformers​. Industry best practices generally recommend a fire/blast wall or adequate separation to localize any single transformer fire. The absence of a sturdy firewall meant the “backup” transformer was knocked out alongside the failed one, eliminating the redundancy that could have shortened the outage. 

In conclusion, the protective systems did isolate the fault electrically as designed, but the transformer still experienced a destructive failure that protections alone couldn’t mitigate. This incident will likely lead to improvements such as faster acting protection schemes, enhanced fire mitigation and reviews of whether automatic shutoff valves or deluge triggers worked properly. It serves as a stark reminder that even with multiple layers of protection, high-voltage equipment can still fail dramatically, so contingency plans must assume the worst-case scenario.

How Oktogrid could have helped

Considering the potential failure mechanisms, here is how Oktogrid's monitoring solution could have helped to detect the potential failure and possibly prevent the fatal destruction of the transformer: 

• Winding insulation failure & arcing / inter-turn failure: 

How could Oktogrid track this:

• Partial Discharges

Insulation failures can be predicted by monitoring and assessing partial discharges. Deteriorating insulation (both paper and oil) can cause partial discharges which can eventually result in failure. Partial discharges create ultrasonic emissions which can be measured with the acoustic sensor of Oktogrid's Data Collector. AI driven models are then used to flag those partial discharges. With that information, additional tests can be performed to assess the condition of the insulation or targeted corrective actions can be taken to address insulation degradation.

• Mechanical Vibrations

Winding insulation failure leads to uneven magnetic forces, electrical arcing, and thermal expansion, all of which cause mechanical stress inside the transformer. This results in abnormal vibration patterns, which can be detected by Oktogrid's Data Collector to reveal early signs of inter-turn faults or structural loosening.

• Thermal performance and cooling efficiency

To monitor the above, Oktogrid's measurements include:

• Hotspot Temperature
• Top-oil Temperature
• Cooling efficiency

With Oktogrid's Data Collector direct surface temperature measurements along with electromagnetic field analysis are used to determine transformer top-oil and hotspot temperature via thermal modeling. This makes it possible to track transformer thermal performance and whether the transformer is being cooled adequately.

• On-load tap changer (OLTC) malfunction: 

To assess the condition of OLTCs, Oktogrid uses information on:

• Mechanical Vibration
• Acoustic Sound Level
• Top-oil Temperature
• Hotspot Temperature

Malfunctioning on-load tap changers can cause changes in the vibration patterns and acoustic sound level of a power transformer. Oktogrid's monitoring solution is able to detect those anomalies and provide insights about potential causes. Deteriorated contacts of OLTCs can also lead to higher temperatures in the transformer.

• Overloading and cooling system failure:

Regarding overloading and cooling system failure, Oktogrid assesses:

• Transformer Load
• Top-oil Temperature
• Hotspot Temperature

Through the combination of transformer load and temperature monitoring, Oktogrid's transformer assessment solution is able to define safe operation range of the transformer and prevent uncontrolled overloading. 

• Acoustic Sound Level
• Top-oil Temperature
• Hotspot Temperature

If the power transformer cooling systems fail, acoustic and vibration monitoring can be used to detect when oil pumps and fans are not in operation. In addition, through Oktogrid's temperature assessment safe operation range can be defined although the cooling system is not working properly.

With these measurements and AI-driven models in place, Oktogrid’s online monitoring solution enables grid operators and industrial customers to:

• Prevent outages through early fault detection
  
  
• Optimise transformer loading to maximize available capacity
  
  
• Optimise maintenance by shifting from reactive to condition-based strategies
  
  
• Optimise transformer lifetime by minimizing stress and degradation
  
  

All while improving the reliability and efficiency of critical power infrastructure.

Sources

London Fire Brigade (LFB) – Official incident report on the North Hyde substation fire and emergency response. London Fire Brigade - Fire at electrical substation - Hayes

Reuters – Coverage on Heathrow Airport's closure, the fire incident timeline, and official reactions. Reuters - Heathrow flights resume after closure causes global flight turmoil

Al Jazeera – Reporting on the infrastructure vulnerability and impact on national transport and economy. Al Jazeera - London’s Heathrow shut after power outage: Which flights were disrupted?

The Guardian – Insights into the substation fire's impact on Heathrow's operations. The Guardian - National Grid boss says Heathrow could have stayed open despite substation fire

The Times – “There was power available to keep Heathrow running, says National Grid”
https://www.thetimes.co.uk/article/national-grid-heathrow-airport-power-nfwvfftpv

Financial Times – “National Grid chief says Heathrow had 'enough power' despite fire shutdown”
https://www.ft.com/content/670758a0-6631-43c9-a828-7471ea128d5b

Wikipedia – Overview of the Hayes substation fire and its consequences. Wikipedia - Hayes substation fire

AP News – Details on the Heathrow Airport standstill due to the fire. AP News - Heathrow Airport fire: What we know about the standstill

CNN – Live updates on the Heathrow Airport shutdown and subsequent developments. CNN - Flights start landing at Heathrow Airport after shutdown leads to massive disruption

CBS News – Reporting on Heathrow Airport resuming operations after the power outage. CBS News - London's Heathrow Airport is "fully operational" after fire caused power outage, prompting flight cancellations and chaos

NPR – Coverage on Heathrow's plan to resume flights after the power outage. NPR - Heathrow to resume some flights Friday after fire cut power

Investopedia – "London's Heathrow Airport to Resume Flights After Lengthy Power Outage" https://www.investopedia.com/london-heathrow-airport-shuts-as-substation-fire-causes-power-outage-11700976

The Scottish Sun – "Terror cops lead probe into blaze which sparked Heathrow Airport carnage as govt say it could be shut for DAYS"

https://www.thescottishsun.co.uk/news/14520984/huge-fire-erupts-electrical-station-evacuated/

People Magazine – "More Than 1,300 Flights Canceled After Catastrophic Fire Shuts Down London's Heathrow Airport" https://people.com/flights-london-heathrow-airport-shut-fire-power-outage-11700982

New York Post – Reporting on Heathrow Airport's expected resumption of full operations after disruptions. New York Post - Heathrow Airport, still crippled by 'unprecedented' disruptions, expects to resume full operations by morning

Fox Business – Information on the UK's investigation into the Heathrow airport shutdown. Fox Business - UK orders investigation after Heathrow airport shutdown

The Times – "Heathrow closure: what to do if your flight has been cancelled or delayed"
https://www.thetimes.co.uk/article/heathrow-closed-what-do-flight-cancelled-20fh5c2l2

The Sun – "UK power bosses boasted that outage like the one that crippled Heathrow would happen only once every 346 years"
https://www.thesun.co.uk/news/33992262/uk-power-bosses-outage-heathrow-fire/