KARACHI: A major power breakdown on Friday left 90 percent of Pakistan’s largest and richest city, Karachi, without electricity and brought much of normal life to a standstill.

The sweltering and violence-plagued metropolis of 23 million people accounts for half of Pakistan’s national revenues and is home to the stock exchange, the central bank and a giant port.

The third major outage of the country’s dilapidated electricity grid within a month forced shops, hospitals and businesses to turn to back-up generators as the lights went out, including those in the provincial assembly.

However, stock market trading was not interrupted.

The private utility company running the electricity supply said high levels of humidity tripped a transmission line to cause the blackout on Friday morning.

“The tripping should not be attributed to any lack in the infrastructure, rather it was caused by high levels of humidity,” K-Electric said in a statement on Twitter, adding that power had returned to most of the city by early afternoon.

An official at the ministry of water and power said at least 90 percent of the city was affected.