AN FRANCISCO — Pacific Gas and Electric Co. crews Tuesday afternoon were investigating the cause of a power outage that darkened the homes of an estimated 6,700 customers in San Francisco’s Outer Richmond District for about three hours. Power was restored to nearly all of the customers by 2:10 p.m., according to PG&E spokesman Joe Molica. The outage began at about 11:15 a.m. According to Molica, three power lines went down within a four to five-block radius in the area of Balboa Street and Geary Boulevard. Investigators’ early assessment was equipment failure and not weather, he said.