Energy Secretary Ed Miliband has ordered the National Energy System Operator (NESO) to “urgently investigate” the power outage caused by a substation fire that shut Heathrow Airport on Friday.

The investigation by the body that operates Britain’s electricity grid would build a “clear picture of the circumstances surrounding this incident” and the UK’s “energy resilience more broadly” to prevent it “from ever happening again”, the Department for Energy Security and Net Zero said.

“We are determined to properly understand what happened and what lessons need to be learned,” Miliband said.

The fire at the North Hyde substation in west London that supplies power to the airport led to thousands of cancelled flights and stranded passengers across the world.

Miliband said he has commissioned the investigation to “understand any wider lessons to be learned on energy resilience for critical national infrastructure, both now and in the future”.

NESO is expected to report to the power regulator Ofgem and the Department for Energy Security and Net Zero in six weeks with its initial findings.

Heathrow closed in the early hours of Friday morning after the fire.

Nearly 1,400 flights were disrupted by the closure on Friday, according to air traffic website flightradar24.com. Around 120 flights were diverted elsewhere.

Heathrow Airport said it was “open and fully operational” on Saturday morning, but the chaos has raised questions about the resilience of the major transport hub.

On Saturday, more than 30 flights due to depart from the airport were cancelled and more than 15 were delayed, according to Heathrow’s live departure board.

It also showed that more than 70 flights expected to arrive at the airport had been cancelled, including from Doha, Riyadh, Dubai, Manchester, and Newcastle Upon Tyne.

Substations are designed to produce, convert, and distribute electricity at suitable voltage levels. Heathrow uses three electricity substations, each with a backup.

There are also backup diesel generators, and uninterruptible battery-powered supplies which provide enough power to keep safety critical systems such as aircraft landing systems running.

However, when the fire broke out the substation, it was out of action, along with its backup.

Heathrow’s main fall-back was the two remaining substations, but the airport’s CEO, Thomas Woldbye, told the BBC that it “takes time” to “switch them”.