The country yesterday suffered the worst power outage in eight years, which, according to officials, was triggered by a trip at a substation in Ghorashal.

It disrupted industrial production, water supplies and telecommunication services in more than half the country.

Millions suffered in sweltering heat as most areas in Dhaka, Chattogram, Sylhet, and Mymensingh divisions were left without electricity for up to eight hours from 2:00pm.

Besides, there were uncertainties as to when  power would be restored as the authorities didn't issue any detailed statement clearing up things until around 6:00pm.

A spokesperson for Bangladesh Power Development Board said around 10:00pm that engineers were able to restore the electricity supply in 90 percent of the affected areas.

As of 12:30am, power disruptions were still being reported in some parts of the capital.

The power, energy, and mineral resources ministry in a statement last night said power supply started getting restored in some areas of Dhaka after 5:15pm.

Power Division Secretary Habibur Rahman said the problem began at a substation near Ghorashal when the demand for electricity suddenly dropped, causing a power surge that tripped power stations one after another.

"We will be able to come up with the exact cause very soon," he told The Daily Star.

A power trip happens when an electrical circuit gets a load beyond its capacity, according to experts.

Prominent energy expert Prof M Shamsul Alam said the National Load Dispatch Centre, which is supposed to coordinate the generation, transmission and distribution of power, often remains helpless because it is not provided with proper data by power generators and distributors.

When the dispatch centre fails to ensure coordination, a risk of grid failure arises.

If the voltage is not controlled, the electrical frequency cannot be controlled, and when the frequency rises or falls beyond a certain range, power stations trip one after another.

Also, the frequency stability should be controlled automatically, but it is done manually here, he said.

The Power Grid Company Bangladesh (PGCB) last night formed a five-member committee to find out what caused the failure and asked it to come up with its findings by Friday.

Contacted, Nasrul Hamid, state minister for power, energy and mineral resources, said, "It was a grid failure. We need a technical report to know what happened. I asked the secretary of the power division to form two separate committees in this regard."

Although the state minister assured that power supply would be restored by 7:00pm in Dhaka, vast areas of the capital were without electricity till 9:30pm.

On September 7, nearly one-third of the country was left without power for about 40-60 minutes because of a grid failure.

On November 1, 2014, the entire country suffered an outage because of a glitch in the Bheramara Power Plant.

Yesterday's outage affected production in garment factories and small and medium industries. Patients suffered in health facilities that have no generator backup, our correspondents reported.

Thousands of people couldn't make phone calls or access the internet while many ATM machines stopped working as the cell phone towers ran out of battery backups.

In the capital, long queues were formed at the petrol pumps as people sought fuel for generators while Puja celebrations were affected in many places.