The City of Los Angeles was plunged into darkness yesterday when a Department of Water and Power crew inadvertantly cut a power cable, triggering a series of events that led to the shutdown of several power plants. The blackout affected major parts of Los Angeles, the San Fernando Valley, and lasted about two hours. The blackout cut off Los Angeles workers from cell service and the Internet, and interrupted the Microsoft Professional Developers Conference being held at the Los Angeles Convention Center. A few web sites hosted in the Los Angeles area also reported failures as their backup power systems failed to trigger. The Los Angeles power grid, like the rest of the U.S., relies on a complex series of load balancing systems which can fail if demand exceeds the capacity of transmission lines. Those systems also automatically shuts down to prevent overload of transmission plants, and in this case were triggered when the DWP mistake threatened to overload a transmission station.