The UK’s National Grid is investigating the root cause of two generator failures at the country’s Little Barford gas-fired power station in Bedfordshire and Hornsea offshore wind farm, which began operating in June after near-simultaneous failures at both sites left 900,000 people in parts of Wales and England without power for nine hours.

Both facilities went offline at approximately 4:54pm local time, leaving hundreds of thousands stuck in gridlocked traffic after traffic lights ceased working, or stranded on trains after railways stopped operating during peak-hour commuting times. Power was restored by 6:30pm in some areas, with most trains running by 9:30pm, according to a report by The Guardian, which labelled the blackout is the largest in the country in a decade.

Hospitals were also affected. A back-up generator at Ipswich hospital took 15 minutes to come online, although a national health services spokesperson reported that patients weren’t negatively affected during that period.

The UK’s energy regulator, Ofgem wants answers, and has demanded an “urgent detailed report” from the country’s gird operator, National Grid, with the threat of action should the report not lay bare the root causes of the outage.

The operator issued a statement early on Saturday, with a spokesperson noting “The root cause of yesterday’s issue was not with our system but was a rare and unusual event, the almost simultaneous loss of two large generators, one gas and one offshore wind, at 4.54pm. We are still working with the generators to understand what caused the generation to be lost.”

“Even though these events are outside of our control, we have plans in place to respond and the system operated as planned by disconnecting an isolated portion of electricity demand.”

“We appreciate the inconvenience caused but this action allows the system to protect itself and limit the fall in frequency, allowing for power to be quickly restored. By 6.30pm, all demand was restored by the distribution network operators. The system is operating normally.”

The blackout is another major outage in a spate of international power failures making global headlines within the last month, with recent blackouts in IndonesiaVenezuela, the US states of Hawaii and New York – the latter experiencing two blackouts within a week of each other.

Watch the video below for a compilation of first-hand footage from those affected. Video courtesy of Youtube news channel Facts Express: