Cause of outage which shut airport ‘remains unknown’

The root cause of a fire which led to the shutdown of Heathrow Airport and affected hundreds of thousands of journeys worldwide “remains unknown”, an interim report says.

Heathrow was closed to all flights for much of 21 March after a fire at a nearby electrical substation, which started the previous night, caused a power outage at the airport.

The National Energy System Operator’s (Neso’s) final report to the government is due by the end of June, but the grid operator said power was restored to Heathrow’s terminals seven hours before flights resumed.

Heathrow Airport said it welcomed the interim findings and that it hoped the final report would provide answers on what caused the fire.

The Metropolitan Police’s counter-terrorism unit carried out an initial investigation into the fire, but Neso noted that detectives found there “no evidence to suggest” the incident was suspicious.

The power outage and subsequent closure of Heathrow led to more than 270,000 journeys being affected.

Energy Secretary Ed Miliband shortly after ordered an urgent investigation into the fire to prevent it “from ever happening again”, and Neso to provide its initial findings within six weeks.

On Thursday, Neso published a detailed timeline saying the fire at the North Hyde substation started at 23:21 GMT on 20 March and resulted in a “simultaneous loss of connection”.

The report stated that one of three supergrid transformers – which enable voltage to be stepped up or down so electricity can be efficiently distributed – became disconnected and caught fire.

Neso’s chief executive Fintan Slye said: “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.”

The power outage impacted 66,919 domestic and commercial customers, including Heathrow Airport, Neso said.

A major incident was declared by the Metropolitan Police at 00:42 and Heathrow took the decision at 01:11 the following morning to close the airport.

Power was restored to Heathrow’s terminals around seven hours before flights resumed late on 21 March, Neso said.

Heathrow relies on three electricity substations, and also has emergency back-up power supplies – but these are only to keep safety systems running, such as runway lights.

Heathrow’s chief executive has previously said the shutdown was caused not by a lack of power, but by the time it took to switch from the damaged substation’s supply to the other operational substations.

Airport bosses were criticised for their decision to shut Heathrow down following the claim that it had enough power despite the substation fire.

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

Neso said this was followed by “a period of safety checking” to ensure “safety critical systems were fully operational prior to passengers arriving at the airport”

Flights resumed after 18:00 and ran through the night to allow Heathrow to get back up to full capacity.

Heathrow said in a statement: “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.”

Miliband said: “We now await the full report to understand what happened and learn lessons to strengthen UK energy resilience and protect our critical national infrastructure.”