An interim report indicates that the exact origins of the fire that prompted Heathrow Airport’s shutdown and disrupted countless global journeys “remain unclear.”

On March 21, Heathrow ceased all flight operations for a significant part of the day due to a fire at a nearby electrical substation that ignited the evening prior, leading to a power failure at the airport.

The final report from the National Energy System Operator (Neso) is expected to be submitted to the government by the end of June. In the interim, the grid operator stated that power was restored to Heathrow’s terminals approximately seven hours before flights were able to resume.

Heathrow Airport expressed its appreciation for the interim findings and expressed hope that the final report would shed light on the fire’s cause.

An initial investigation into the fire was conducted by the Metropolitan Police’s counter-terrorism unit. However, Neso reported that detectives found “no evidence to suggest” any suspicious activity related to the incident.

The resultant power failure and subsequent airport closure impacted over 270,000 journeys.

In response, Energy Secretary Ed Miliband promptly ordered a thorough investigation into the fire to ensure it does not “happen again” and requested that Neso present preliminary findings within six weeks.

On Thursday, Neso released a comprehensive timeline, indicating that the fire at the North Hyde substation began at 23:21 GMT on March 20 and caused “a simultaneous loss of connection.”

According to the report, one of the three supergrid transformers, essential for adjusting voltage levels for distributed electricity, became disconnected and caught fire.

Fintan Slye, Neso’s chief executive, emphasized the necessity of learning the correct lessons from this situation to avert future occurrences and to effectively handle them when they do arise.

Neso noted that the power outage affected 66,919 commercial and domestic customers, including Heathrow Airport.

A significant incident was declared by the Metropolitan Police at 00:42, and by 01:11 on March 22, the decision was made to close the airport.

The restoration of power to Heathrow’s terminals occurred around seven hours prior to the resumption of flights late on March 21, according to Neso.

Heathrow depends on three electrical substations and maintains emergency backup power supplies, which are primarily meant to keep essential safety systems operational, such as runway lights.

The chief executive of Heathrow previously clarified that the shutdown resulted not from a lack of electricity but from delays in transitioning from the damaged substation to the other operational substations.

Criticism was directed at airport officials for their decision to halt operations despite claims of sufficient power availability during the substation fire.

Neso’s interim report noted that electricity flow to all four of Heathrow’s passenger terminals was restored by 10:56 on March 21, with power returned to the “wider Heathrow Airport Limited network” by 14:23.

Following this, “a period of safety checking” took place to verify that “safety-critical systems were fully operational before allowing passengers back into the airport.”

Flights resumed after 18:00 and continued through the night to help Heathrow return to full capacity.

In a statement, Heathrow remarked: “Gaining further insights into the fire’s initiation and the subsequent impact on the two transformers can enhance the resilience of the UK’s energy grid going forward.”