BLOOMINGTON - Thousands of workers at State Farm Insurance Cos.' corporate headquarters got an unexpected day off Thursday after power went out for about four hours.

Between the power outages and the recent snow storm, the employees have had several surprise days off in the last two weeks.

Most of the 5,000 or so employees at the corporate office at Washington Street and Veterans Parkway were sent home for the day around 11 a.m., about an hour after the power went out. A second shift did come into work around 3 p.m., shortly after power was restored.

As of late Thursday afternoon, State Farm and Ameren Corp. had not determined the exact problem. Other customers in the area, including OSF St. Joseph Medical Center across the street, experienced a brief power outage that lasted just several seconds.

The outage stemmed from a blown "lightning arrester" at Ameren's substation on G.E. Road, said Ameren spokeswoman Natalie Hemmer. The arrester, she said, protects equipment from power surges.

Ameren is working with State Farm to determine why power did not return quickly like it did for other customers, she said. It could be a problem with State Farm's equipment, Ameren's equipment or both, she said.

"What other customers saw was a blink," Hemmer said.

For example, across the street, lights at OSF St. Joseph Medical Center flickered as power was lost for several seconds, and emergency generators were immediately activated, spokeswoman Katy O'Grady-Pyne said.

"We were on emergency generators for only eight seconds," she said.

Other nearby businesses, including Eastland Mall and Country Insurance & Financial Services, were not affected.

Meanwhile, it was a confusing scene in a darkened front lobby as workers poured out of State Farm late Thursday morning.

Some of the front doors, normally only accessible with an electronic badge, were swung open to allow the masses to exit the building.

A few others were trying to enter the building through the open doors. A lone security guard tried to answer questions from people who apparently weren't aware of the situation while also trying to keep an eye on the doors to ensure the wrong people weren't trying to enter the building.

Outside, pedestrians flowed down crosswalks as cars lined up waiting to exit. On Washington Street, cars trying to turn onto Veterans Parkway lined up back beyond the next stoplight.

State Farm policyholders, meanwhile, weren't affected by the unexpected closure, said spokeswoman Xochitl Yanez.

Most customer-related functions are handled at other State Farm offices, and those that aren't, were on Thursday, she said.

"We are making sure that we are still serving our policyholders," Yanez said.

Pantagraph reporter Paul Swiech also contributed to this story.