Investigation has yet to establish exact cause of Czech blackout, continues on several levels

The exact causes of the extensive power outage on 4 July remain unclear even a month after the incident. The blackout investigation is continuing on several levels, according to ČEPS, the transmission system operator. Results are expected within weeks to months. Extraordinary inspections of the grid have been completed, but no major faults were found. ČEPS spokesperson Lukáš Hrabal told the Czech News Agency (ČTK) in response to a query.

The massive power outage on 4 July, which lasted several hours, affected about one million customers. There are 6.3 million electricity supply points in Czechia.

“Data are currently being collected in cooperation with distribution system operators and other parties, and will be evaluated,” Hrabal said.

Other experts are also involved in investigating the incident, including in assessing the conductor in question, he said. “The results of these investigations are expected within weeks,” Hrabal noted.

ČEPS has previously announced that the main mechanical cause of the blackout was the rupture of a phase conductor, the cable that carries electricity between power plants and substations. Hrabal noted that the police are also conducting an expert assessment of the conductor, which he said is standard practice in similar incidents.

The blackout is also being investigated by an Expert Panel of the European Network of Transmission System Operators for Electricity (ENTSO-E), according to ČEPS. “A preliminary report is expected within six months of the start of the investigation, that is, by mid-January 2026, with the final report due by mid-July 2026,” Hrabal said.

In addition, a working group is being set up in Czechia under the leadership of the Czech Energy Regulatory Office (ERÚ) to assess the outage in a broader context. It will examine the role of the various parties during the outage, with its main aim being to assess possible measures to prevent similar incidents.

Following the blackout, ČEPS launched extraordinary inspections of the transmission system in addition to the investigation. These have now been completed, Hrabal said. “Thermal imaging identified findings that were non-urgent or for the record, and these will be addressed during planned outages for routine line maintenance,” Hrabal said. He added that standard inspections carried out as part of maintenance are also continuing.

According to previous reports from ČEPS, the extensive power outage in Czechia resulted from several outages occurring at the same time. It began with the rupture of a phase conductor, which took the backbone V411 line out of service. The line is used to transmit high power output from power plants and carry electricity over long distances. The Ledvice power plant unit in the Teplice District and the large Krasíkov substation in the Ústí nad Orlicí District then went offline. Due to the high load, the system operator also switched off line V208, which connects substations. The reason these events coincided remains unclear.

The system operator has ruled out the possibility that the outage was caused by an overloaded grid due to electricity flows from abroad or by damage caused by a third party. The line whose outage led to the blackout was damaged last year as well, but that did not cause any further outages at the time.