Human error was responsible for a data center electrical overload that knocked the Cisco Systems web site offline for about three hours Wednesday afternoon, the company said last night. The outage came as Cisco released four security advisories about vulnerabilities in its networking products, and left system administrators unable to access Cisco’s support resources. The company was able to alert users through blogs.cisco.com, which is hosted on an another network.

“The issue occurred during preventative maintenance of one of our data centers when a human error caused an electrical overload on the systems,” Cisco said in an update. “This caused Cisco.com and other applications to go down. Because of the severity of the overload, the redundancy measures in some of the applications and power systems were impacted as well, though the system did shut down as designed to protect the people and the equipment.” The outage can be clearly seen on this performance chart for www.cisco.com:

A dynamically updating chart of the Cisco.com web site performance is available. Netcraft offers a web site performance monitoring service that provides similar charts, along with e-mail alerts when an outage occurs.