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Straits of Mackinac submarine transmission cables

Current activity: July 2020 - A replacement project is underway to repair the damaged cables. Read more here:

American Transmission Co. owns and operates two 138-kilovolt transmission circuits that electrically connect the Upper Peninsula of Michigan to lower Michigan via overhead and submarine electric cables. These two electric transmission circuits each tripped offline 22 seconds apart Sunday evening, April 1, 2018. Through monitoring the following morning, it was determined that two of the submarine cables were releasing some dielectric insulating fluid. ATC notified the National Emergency Response Hotline, which then engaged the appropriate federal, state and local agencies, and the Coast Guard became the lead agency overseeing the response. A Unified Command, comprised of the Michigan Department of Environmental Quality, county emergency managers, local tribes, the National Oceanographic and Atmospheric Administration, U.S Fish and Wildlife, Environmental Protection Agency and ATC, is overseeing the response and mitigating any risks to the environment.

With unfavorable weather conditions hindering ATC’s efforts to pinpoint the area of the release, the company elected to permanently shut down the two cables. Less than 600 gallons of fluid were released, and given its dilution in the Straits and currents, the Coast Guard has reported low risk to the public, fisheries and wildlife. The Coast Guard also is leading an investigation into the possibility that a passing vessel caused the damage to the electric cables. ATC maintained electric system reliability in the region via alternate electric circuits.

STATUS: ATC completed its recovery efforts for this incident on Sunday, April 29, 2018. See the Underwater Inspection section below. As of May 1, one circuit between the U.P. and lower Michigan has been restored. Read the news release.

Event response

When two electric circuits tripped offline on Sunday evening, April 1, 2018, ATC dispatched crews to inspect the overhead elements of the transmission circuits in Point La Barbe in St. Ignace and at the McGulpin Riser Station in Mackinaw City, which showed no visible signs of damage. Overnight, ATC monitored the six submarine transmission cables, which contain an insulating dielectric fluid, and subsequently determined two of the cables may be releasing some fluid. ATC reduced pressure on the system to minimize the fluid release as personnel worked to locate the compromised section of the two submarine cables, starting with a portion of the cable that is on land.

ATC notified the Michigan Department of Environmental Quality and the National Emergency Response Center of the suspected fluid release. Through this notification to the National Emergency Response Center, an Incident Command System was initiated and led by the U.S. Coast Guard, which brought together the appropriate state, federal and tribal entities to become the Incident Management Team.

ATC launched its Spill Response Plan, which is prepared according to federal requirements.

With unfavorable weather conditions hindering the investigation to pinpoint the area of the underwater fluid release, ATC notified the Incident Management Team, which included several agencies and tribes, that it would halt the flow of insulating fluid to the two cables, rendering them permanently inoperable.

STATUS: ATC completed its recovery efforts for this event on Sunday, April 29, 2018. With the severed cables capped and set back in place on the lake bed, ATC completed the extraction of dielectric fluid from the cables.

Underwater inspection

ATC completed underwater inspections of all six ATC cables using a remotely operated underwater vehicle (ROV) which captures visual imagery of the cables.

Capping severed cables

ATC reviewed the visual data of all six cables. The photos below confirm that two cables were completely severed. ATC capped the severed ends of the two damaged cables to mitigate environmental risks. One by one, the severed ends of each cable were lifted to the surface, placed on a barge and caps installed. Then, the cables were placed back in position. Concrete anchor mats are being placed on top of the severed ends to keep them in place. We are developing a longer-term plan to remove the damaged cables from the straits. No timeline has been set.

Visit the Coast Guard News site for more information.

Event management

The U.S. Coast Guard is serving as the lead agency and operates a Joint Information Center, 906-748-0737, or email:

The Unified Command conducted air, marine and on-shore observations with trained spotters to detect any signs of sheen from the fluid release or ongoing activities. The Coast Guard reported no sheen and detected no indication of pollution.

A Coast Guard marine science technician and an environmental quality analyst for Michigan’s Department of Environmental Quality inspected the Straits by boat and did not observe any signs of pollution. Another Coast Guard marine science technician was on board flights over the Straits and saw no signs of a sheen.

The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, Michigan Department of Environmental Quality, and a Michigan Department of Natural Resources wildlife biologist also surveyed the area on the water and from the shore to look for signs of pollution or affected wildlife. No impacts to the environment or wildlife have been identified.

To report affected wildlife, please call the U.S. Department of Agriculture at 517-336-1928.

About the dielectric insulating fluid

Submarine transmission cables of this vintage typically include a dielectric fluid that acts as insulator to the copper conductors of electricity in the submarine cables. The fluid is housed in a vessel measuring approximately 3/4 of one inch at the center of the cable.

The fluid is primarily comprised of mineral oil, a thin and light substance. Samples of the fluid were sent for laboratory analysis with results reported to the Michigan Department of Environmental Quality.

In total, less than 600 gallons of dielectric fluid were released. ATC valved-off the two damaged cables, eliminating further fluid release by closing them from the fluid supply. ATC extracted and safely disposed of 625 gallons of dielectric fluid from the damaged cables.

Impacts to environment, public

The Unified Command reported that given the dilution of the dielectric fluid in the Straits and the mobility of fish, there is a low risk to fisheries and wildlife. According to the Unified Command, the greatest threat is to waterfowl or shore birds that may come in contact with the product floating on the surface, but no impacts to fisheries or waterfowl have been detected to date.

The Coast Guard says that due to the inaccessibility of the shorelines from ice, there is a low-risk threat to the public since they are unable to come in contact with the fluid. The dilution of the fluid and distance from the bridge provide additional protection from the product. There are two water intakes in the general vicinity, but given the dilution of the fluid and the distance of the intakes from the source of the discharge, there is no significant threat to drinking water, according to the Coast Guard.

Cause investigation

The Coast Guard announced Friday, April 6, that it initiated a marine casualty investigation to determine whether a marine vessel was the cause of the damage to the cables.

About the cables and circuits

ATC’s two transmission circuits across the Straits are composed of two overhead transmission lines that transition to six submarine cables. Three cables together make up one transmission circuit.

The six cables enter the Straits on the north at ATC’s Point Le Barbe Substation in St. Ignace and run about 4 miles to the ITC’s McGulpin Riser Station in Mackinaw City. Four of the cables were installed in 1975; and two cables were installed in 1991/1992. The cables were originally installed by Edison Sault Electric Co., and ATC took ownership and operation in 2001.

The cables, which are 3 ¼ inches in diameter, sit on the lake bed about 150 to 200 feet apart. ATC last conducted a submarine survey of the cables in 2016, and cable testing in 2017. All six cables were in good condition. ATC monitors the lake currents, and based our 2016 study of lake currents, the cables have not moved significantly since 2008.

Electric system reliability

There is no electrical connection operating between the Upper Peninsula and lower Michigan at this time. For the short-term, the system can operate reliably this way given anticipated electric load requirements.

ATC has inspected all six submarine cables. That inspection confirmed that two cables were severed. One cable was displaced from its original position and shows no signs of damage. Three cables are in good condition.

ATC is testing three cables to reestablish a circuit as soon as possible to help ensure electric system reliability to the Upper Peninsula and the northern portion of lower Michigan.

ATC continues to coordinate short- and long-term reliability solutions with Midcontinent Independent System Operator and Midwest Reliability Organization.

About ATC

ATC is a transmission-only electric utility that owns and operates the electric transmission grid in Upper Michigan, Wisconsin, Minnesota and Illinois. It owns and operates more than 9,600 miles of transmission lines and 554 substations.

Last Updated: July 13, 2020