Commissioned by the UK Department for Energy Security and Net Zero, and Ofgem, NESO has published its six-week interim report, which focuses on establishing the timeline and sequence of events of the power outage, before a final report is due at the end of June 2025.

Heathrow Airport was closed to all flights for the majority of 21 March after a fire at the nearby North Hyde 275 kV electrical substation caused a power outage at the airport.  

According to the report, at 23:21 GMT on 20 March, one of three supergrid transformers disconnected and caught fire at the substation in Hayes, west London. Shortly after, the adjacent transformer also tripped, resulting in the simultaneous loss of connection to the remaining transformer.  

The consequence was the loss of all supply from the substation, including 66,919 domestic and commercial Scottish and Southern Electricity Networks (SSEN) Distribution customers losing power, and the complete loss of power to part of Heathrow Airport’s private internal electrical distribution network.

London Fire Brigade confirmed it received the first call about a fire at the North Hyde site at 23:22, with the first fire appliance arriving at 23:28. Nine additional fire appliances were requested to safely tackle the fire, and a major incident was declared by the Metropolitan Police at 00:42 the following morning. Heathrow took the decision to close the airport at 01:11.

In order to tackle the fire, London Fire Brigade worked with National Grid Electricity Transmission and SSEN Distribution to ensure power was isolated on the substation site and enable parts of the site to be re-energised whilst firefighting actions continued.  

London Fire Brigade didn’t confirm cessation of firefighting until 26 March, five days and 18 hours after it first arrived on site.  

The Metropolitan Police’s counter-terrorism unit carried out an initial investigation into the fire, but NESO notes that detectives found there was ‘no evidence to suggest that the incident was suspicious in nature’.  

A forensic fire investigation to establish details of its root cause and spread is now being undertaken by London Fire Brigade.  

The power outage and subsequent closure of Heathrow led to more than 270,000 journeys being affected. According to the report, power was restored to Heathrow’s substations by 06:25 using circuits from another nearby substation, and the flow of electricity to all four of Heathrow’s passenger terminals was restarted by 10:56. The report says the re-energisation of the entire airport was fully completed by 14:23.  

However, flights did not resume until after 18:00 that day, once safety checks were completed.  

Heathrow's Chief Executive Thomas Woldbye had previously said the shutdown was caused by the time it took to switch from the damaged substation’s supply to the other operational substations. Due to Heathrow’s ‘size and operational complexity’, managers decided to close the airport on safety grounds while this took place.

The report confirms that the restoration of power to the airport was followed by ‘a period of safety checking’ to ensure ‘safety critical systems were fully operational prior to passengers arriving at the airport’.

Fintan Slye, Chief Executive, NESO, comments: ‘It is important that the right lessons are learnt from this incident to prevent future instances where possible and to manage them effectively when they do occur.’

Heathrow Airport said: ‘Further clarity on how the fire started and why two transformers were subsequently impacted can help ensure greater resilience for the UK's energy grid moving forward.’

NESO anticipates that the final report will contain findings and recommendations relating to the resilience of energy infrastructure, the response and restoration of energy infrastructure including cross-sector incident management, and the resilience of critical national infrastructure to energy disruption.