Edison Sault provides electric service on Mackinac Island to approximately 600 year round customers, and this number grows to approximately 900 during the peak summer tourist season at this heavily-visited resort area. The major customer load consists mainly of electric space and water heating, air conditioning, and lighting. Summer peak power demand has grown steadily over the years. In 2000, the summer load served exceeded 10 megawatts.

Mackinac Island is presently served by two special purpose, three-phase, power supply circuits, utilizing specially fabricated submarine cables. These cables operate at 13,200 volts and are installed across the Straits of Mackinac for more than three miles from the mainland at St. Ignace to the Island. Each circuit uses individual cables with a 25,000-volt insulation system for added margin. The cables are also covered with a stranded steel wire, concentric neutral, armor jacket to protect them from possible damage should a ship anchor in the Straits. One circuit was installed in 1983 and the second in 1988.

From July 22 to July 29, 2000, Mackinac Island customers experienced intermittent power supply outages caused by multiple failures of the land-based portion of the power supply cables, primarily on the Island. It was an unfortunate and inconvenient situation for Edison Sault’s customers and an embarrassing one for the Company. Edison Sault has prided itself on providing economic, quality and reliable electric service to its customers on Mackinac Island. Over the years, the Company’s reliability of service on the Island has actually been about 41% better than for the rest of its service territory, reflecting in part, the Company’s major investment in serving this unique part of its service territory. The last significant problems on the Island were seven years ago, in 1993, when high loads and several equipment failures resulted in several power outages in May and June. In response, a variety of facilities upgrades and improvements were made.

This summer’s power outage presented Edison Sault with one of the most demanding challenges in the Company’s history. Edison Sault personnel responded promptly at the onset on Saturday, July 22, and throughout the outage period, with its repair crews working around the clock along with crews from other utilities and outside contractors to effect repairs and restore service to customers.

Edison Sault focused on maintaining at least one power supply circuit to Mackinac Island during the initial outage period, but by Monday, July 24, multiple cable failures made it impossible to configure even one power supply circuit to serve Mackinac Island customers. Contact was made with Consumers Power Company, Detroit Edison Company, Wisconsin Electric Power Company, and one of the cable suppliers (Kerite Corporation) early on Monday for assistance with locating and repairing failed cable as well as for testing the cables. This multi-company repair effort began on July 24 and continued throughout the outage period with maximum resources (numbering over 40 personnel at times) committed to restoring the power supply to the Island.

In order to restore service to customers, Edison Sault and Mackinac Island officials worked to locate portable diesel generators to supply the Island with electric power. By Wednesday, July 26, suitable diesel generators had been located, and activity was in full gear to move them to the Island. The first generators began supplying the Island on Saturday, July 29. In the interim, Edison Sault began replacing all the land-based power supply cable on Mackinac Island and at St. Ignace. This work was completed on Monday, August 1, and the power supply system was switched back to the submarine cables on Tuesday, August 2.

At present, all temporary repair work has been completed, and full power has been restored to Mackinac Island, although the submarine cables are known to be in a stressed condition. As a precaution, Edison Sault has continued to ask its Island customers to reduce their electrical usage. The diesel generators, along with their operator, are being kept on the Island to continue to provide a backup power supply. The diesels will not be released until the Company is satisfied that the cable systems supplying Mackinac Island with electric power will continue to do so reliably.

Three independent consultants have reviewed the circumstances surrounding the failure of the submarine cables and concluded that the likely cause was thermal breakdown of the cable insulation in those portions of the submarine cables installed on land. The breakdown is believed to have been caused by magnetic heating of the steel armor wires induced by current flow through the cables and due to the proximity of the cables to each other. A common contributing factor was the excessively dry soil surrounding the portions of the cables installed on land that reduced the ability of the soil to dissipate heat generated by the cables. The dry soil condition was particularly prevalent near the lake shore due to the unusually low lake level experienced for the last few years.

Edison Sault intends to replace all of the electric power supply system submarine cable facilities from St. Ignace to the Pat Chambers Switching Station on the Island as quickly as possible. Company officials have been working with its consulting engineer to design a new power supply system, and a specification for new submarine cables was released on August 22. Edison Sault plans to install two new submarine cable circuits by the end of 2001, with one circuit being installed by the end of 2000 if delivery and installation can be expedited. The Company is also examining a number of related issues to improve the distribution system on Mackinac Island and will be working with Island officials to discuss the scope and schedule for the improvements.

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