Polish energy officials say human error was to blame for a massive outage at Europe’s largest brown coal power plant of Belchatow last month and a separate fire there just days later, according to the state PAP news agency.

Poland needed snap energy imports May 17 when 10 of Belchatow’s 11 units suddenly went out, and the outage was felt across Europe’s grid. Some 3.6 gigawatts of energy were lost and only the newest, 858-megawatt unit was operating.

The problem was fixed that day and in the following days the units were gradually restarted.

Belchatow plant, fed by an adjacent open-pit lignite mine, supplies over 20% of Poland’s energy, serving some 11 million households.

The head of Poland’s energy grid PSE, Eryk Klossowski, told a special parliament commission late Tuesday that the outage was caused by an employee who was installing automatic equipment at a power switch. That led to a chain of events that automatically switched off Belchatow’s units, Klossowski told the commission, according to PAP.