CHICAGO, AUG. 5 -- An explosion and fire today at an electrical substation caused the second power outage in a week on the city's West Side, cutting off electricity temporarily to about 25,000 customers, officials said.

Unlike last week's outage, which for some customers lasted more than two days, power today was restored in less than eight hours.

The fire was reported just after 9 a.m. at a substation that Commonwealth Edison had used to supply electricity to customers hit by last weekend's outage, an Edison spokeswoman said. Last weekend's outage led to sporadic looting on the West Side, which contains some of the city's poorest neighborhoods.

"It was a powerful and serious explosion," said Edison spokesman Steve Solomon. An operator at the site heard a transformer "trip," or shut off because of a malfunction, then heard an explosion and ran outside and saw the fire, said company spokeswoman Margaret Winters.

The cause of the fire is under investigation. Winters at first said sabotage was possible, but later said it was unlikely.

About 25,000 customers in a 30-square-block area were affected, plus some customers on the eastern edge of the suburb of Oak Park, Winters said.

Temperatures were in the mid-70s, as they were during the July 28 outage.

The earlier outage, which came at 10 p.m. Saturday, was caused by a fire in a malfunctioning generator. It affected about 40,000 customers.

Mayor Richard M. Daley and other city officials toured the newly blacked-out area today, and a Daley spokesman said the city had launched several initiatives to help residents cope.

The Streets and Sanitation Department put up stop signs where street lights were out, said spokesman Terry Levin.

"We also have food inspectors out there to make sure that no spoiled food gets sold," he said. "We have also been in contact with all the nursing homes in the area to make sure that there's no problem with electricity."