A picture of the bus ducts after the "significant heat event."

A forensic report on this summer's power outage at Seattle's Fisher Plaza technology complex cites a failure of insulation inside a bus duct -- a metal housing that contains thick strips for conducting electricity -- as the likely cause of the incident that took dozens of Web sites offline for as much as a day or more.

The report by Power Science Engineering Inc. of Shoreline recommends steps including routine maintenance and monitoring of the electrical equipment to prevent such incidents in the future. However, Fisher Plaza officials say they conducted regular maintenance of the area -- including infrared and thermal scans and physical inspection of the equipment -- inside the facility.

Read the full report here: PDF, 12 pages.

Fisher Plaza officials distributed the report to tenants of the facility along with a memo referring to the incident as a "significant heat event," without explicitly characterizing it as a fire. The facility has been shifting away from generator power, back to electricity supplied by Seattle City Light, according to the memo. It notes that the facility is expected to be back to "normal operating status" by Nov. 1.

The incident in early July at Fisher Plaza East exposed a big gamble that many tech companies take by limiting themselves to a single location when setting up their online infrastructure. Sites including Allrecipes.com, Microsoft Bing Travel, Authorize.net and many others were taken offline as a result of the outage.

The forensic report notes that failure of one bus duct also caused extensive damage to another bus duct nearby. Although it doesn't appear that separating the two would have prevented the incident, the report notes that damage and impact could have been reduced if the bus ducts weren't positioned next to one another.

Cheryl Mauer, Fisher Plaza's general manager, said via phone today that the new design inside the facility separates the two bus ducts to avoid that issue in the future.

The report also recommends the use of cable systems at critical locations, noting that they "can be a better option ... because the cable systems use insulated cables instead of bus bars." Depending on the situation inside a particular facility, the merits of cable systems vs. bus bars can be a subject of debate in the industry.

Separately, the report concludes that it's unlikely that the incident was triggered by a sprinkler system accidentally turning on. Instead, the sprinkler system appears to have been activated by the heat that came from the failure inside the bus duct. The report also says it's unlikely that the incident was caused by a fluorescent light fixture falling on the bus duct. That fixture appears to have fallen as a result of the "significant heat" incident.

For the record, several TechFlash readers who looked at a fire investigator's photos of the scene, posted on the site a few days afterward, were spot on in their assessment of the situation in the comments on that post. We'd encourage additional commentary and analysis from people who read the full forensic report linked above.

Update: Rich Miller at Data Center Knowledge offers an informative analysis of the forensic report, including this: "Investigators said such failures are usually prompted by a “stimulus” such as movement of the duct or sudden changes in large loads, but that none of those conditions existed at the time of the failure at 11:11 p.m. on July 2. Power Science concluded that the insulation wore down over time due to thermal and mechanical stresses at a bend in the duct."