Pakistan’s generators produced more power than was required on Monday, causing voltage fluctuations that culminated in a system collapse that plunged 220 million people into darkness, an internal government document reviewed by Reuters showed.

Complete grid failures are rare, and operators of modern grids count local shocks from integration of renewable energy as their primary challenge. But the blackout in Pakistan on Monday was its second near-complete grid failure and the third in South Asia in three months.

The grid’s failure plunged 220 million people into darkness for a whole day and disrupted commercial activity as outages also hit internet and mobile services.

The blackout was triggered by the power grid’s frequency rising to 50.75 hertz (hz) early on Monday, causing severe voltage fluctuations in transmission lines in the south, according to the internal note. A frequency over 50 hz indicates the power generated exceeds demand, while a frequency under 50 hz points to supply falling short of demand.

Grid operators attempt to keep the frequency of the grid stable at 50 hz, with deviations over 0.05 hz typically considered abnormal. The frequency of the grid was already 50.30 hz moments before the incident, according to the note.

The severe frequency fluctuations in the transmission lines caused it to trip, Sajjad Akthar, general manager at state-run National Transmission and Distribution Company wrote in the note drafted on Tuesday. “Transmission lines tripped, which resulted in isolation of north and south system,” Akthar said in the note.

Pakistan’s energy ministry did not respond to a request for comment. The note did not mention why supply overshot demand.