National Grid must urgently explain why it failed to prioritise the supply of electricity to key parts of Britain during Friday’s blackouts, the government’s former infrastructure tsar has said.

The demand by Lord Adonis, former chair of the government’s National Infrastructure Commission, came as it emerged that the company had been concerned for more than a year about how its system responded to power shortages.

Almost one million people in England and Wales lost electricity on Friday after the failure of a gas-fired power station in Bedfordshire and several units at an offshore windfarm in the Humber.

The result was chaos. Traffic lights stopped working, and key hubs including Newcastle airport and Ipswich hospital were hit. There was major disruption to the country’s railways and London underground during the busy Friday night commute.

“Why did this happen now?” Adonis asked. “This is the middle of August, not the end of November. Why were key transport installations affected when they should have been protected from any but the most extreme power breakdowns? National Grid haven’t been able to answer either of those two questions. They need to provide answers and improvements in both respects.”

The failure of the two generators saw a loss of 5% of the grid’s electricity – some 1.4 gigawatts. This was less than the 1.8 gigawatts lost in 2008 when two different generators failed. Then, about 500,000 people lost power, but major infrastructure systems were unaffected.

“This is a big wake-up call for National Grid,” Adonis said. “Their resilience is below par. They need to improve their processes for ensuring the resilience of national transport and medical installations in the event of power shortages.”

The failure of the two generators on Friday caused a frequency drop in the electricity system which, while sizable, should not have troubled it unduly, according to experts.

“The frequency went down to less than 49Hz, so certain parts of the network started to disconnect automatically, causing the power cuts until the system came back into balance and the frequency stabilised,” said Phil Hewitt, a director at energy consultancy EnAppSys. “We would have expected the system to cope with this size of loss of generation but the double hit may have destabilised it.”

National Grid has to maintain the system close to 50Hz to keep it stable. But industry sources said that, the frequency had previously fallen to dangerously low levels three times in the last 12 weeks.

The company said: “Following the event, the other generators on the network responded to the loss by increasing their output as expected. However, due to the scale of the generation losses this was not sufficient, and to protect the network and ensure that restoration to normal operation could be completed as quickly as possible, a backup protection system was triggered which disconnects selected demand across GB.”

How this backup system functioned is now expected to form part of the investigation into what went wrong on Friday.

The failure of the two power stations in 2008, and the resulting fluctuation in energy frequency on the grid, caused some smaller generators – of which there are around 50,000 across the UK – to detect the disturbance and shut down in a safety operation that the energy industry refers to as “loss of mains protection”.

A National Grid presentation in March suggested the system might be “too sensitive,” causing smaller generators to shut down prematurely when disturbances were detected, leading to blackouts. In a July 2018 presentation, the National Grid claimed such widespread shutdowns were a risk that occurred once every 12 years.

The company’s director of operations, Duncan Burt, said its safety protection systems had worked well to keep the grid safe.

But he conceded that the industry needed to examine whether it was set up correctly to have “minimal impact” on people’s daily lives.

David Hunter, an analyst at Schneider Electric, told the BBC that the UK’s transition to clean energy could also be creating “greater stresses” on the system because energy such as wind power is less effective as a “shock absorber” compared to nuclear or gas turbines when there are shifts in supply and demand on the grid.