Heathrow's power outage - a one-off?

Last Friday, one of the world's largest transport hubs came to a halt for 18 hours after a fire broke out at one of its substations in North Hyde. Since the incident, National Grid's CEO responded to accusations that Heathrow bosses were forced to shut down the airport due to a lack of power, confirming that the two other substations were operational and fully capable of powering the airport. Why, then, was the airport not able to connect the power, and why did it take so long to return to ...