Officials say there was no explosion or fire, but a transformer near 13th and Lombard streets went offline Saturday afternoon and caused an emergency shutdown.
The emergency system, PECO says, then dispensed the fire suppressing foam.
Officials say there was a flash momentarily causing an audible popping sound. They say that's what people heard, not an explosion.
The foam from the fire-suppression system is not toxic and firefighters were able to clean it up after a couple hours.