What caused the power outage in Spain and Portugal? Reason behind the blackout explained, was it a cyber-attack - have outages happened before?

Houses, offices, trains, traffic lights and even the Madrid open tennis tournament were all hit. The Spanish and Portuguese governments as well as network operators scrambled to understand the problem and raced to fix it.

Red Eléctrica de España (REE), Spain’s electric network, said Spain and Portugal were hit by “el cero” – the zero. Its Portuguese counterpart, Redes Energéticas Nacionais (REN), said the outage started at 11:33am Western European summer time.

By mid-afternoon the Spanish operator, which is partly state-owned, said that it had started to recover voltage in the north, south and west of the Iberian peninsula. The recovery process could only be carried out gradually, to avoid overloading parts of the grid as each generator connects.

The Portuguese prime minister, Luís Montenegro, said that the issue originated in Spain. Portugal’s REN said a “rare atmospheric phenomenon” had caused a severe imbalance in temperatures that led to the widespread shutdowns.

REN said: “Due to extreme temperature variations in the interior of Spain, there were anomalous oscillations in the very high voltage lines (400 kV), a phenomenon known as ‘induced atmospheric vibration’. These oscillations caused synchronisation failures between the electrical systems, leading to successive disturbances across the interconnected European network.”

The risks posed to electrical systems by big variations in atmospheric temperatures are well known in the industry, even if it is rare for problems to manifest on this scale. Georg Zachmann, a senior fellow at Bruegel, a Brussels thinktank, said the system had suffered “cascading disconnections of power plants” – including one in France – when the frequency of the grid dropped below the European standard of 50Hz.

It appears that vibrations in the power lines - which normally create the 'hum' you can hear when electricity passes through - became so bad that the entire network broke down. REE's system operations chief Eduardo Prieto told reporters the loss of power supply was beyond the extent that European systems are designed to handle and caused the Spanish and French grids to disconnect, which in turn led to the collapse of the Spanish electricity network. "As the result of this disconnection and the serious imbalance of band generation that is in our electrical system, the electrical system collapsed," he said in a news conference.

The European Council president, António Costa, who was Portugal’s prime minister from 2015 to 2024, said “there is no evidence that it was a cyber-attack”, but cautioned that the ultimate cause was still unclear. Senior European Commission vice-president Teresa Ribera also told Spain’s Radio 5 that there was no evidence of a deliberate act having caused the outage.

However, Spain’s national security council was convened on Monday to assess the outage. Portugal’s prime minister said it was too early to say for sure what caused the blackout.

Engelaar said such a widespread failure was “extremely unusual”. However, there have been previous examples. In 2003 a problem with a hydroelectric power line between Italy and Switzerland caused a huge outage across Italy for about 12 hours. A 2006 German power overload caused outages as far away as Portugal and Morocco.