The countdown nobody could see

Four Point Nine Nine Nine Seconds

At two o'clock on the afternoon of Friday 8 January 2021, the 400 kV substation at Ernestinovo (flat winter farmland in eastern Croatia, not far from the Serbian border) was doing something unremarkable. Inside it a busbar coupler, the switch that joins the two halves of a substation, carried 1,736 amperes, as it had since one o'clock. Calm weather. Nothing out of service. The synchronous grid of Continental Europe was having an ordinary Friday: 396.5 GW of load, Portugal to Turkey, one machine turning at fifty cycles a second.

Then the number climbed. At 14:00:59 the coupler hit 1,931 A, crossed its alarm limit of 1,920 A, and raised a permanent alarm in the regional control centre. Seven seconds later it fell back below the limit and settled into a fidget between 1,830 A and 1,920 A. The operators got on the phone: regional control centre to remote, remote to national. They were not idle and they were not confused. They had a fix and they knew what it was: move the 400 kV Ernestinovo–Pécs 2 line onto busbar 1 and the coupler would be relieved. The discussion ran a little over three minutes.

At 14:04:16 the current touched 1,922 A and the alarm went again.