An energy company is investigating how an underwater cable connecting the Western Isles with mainland Scotland was cut suddenly, causing a blackout that affected roughly 18,000 homes. The 20-mile (32km) cable, which runs on the sea bed from Skye to Harris, failed without warning six days ago. That forced Scottish and Southern Energy Networks to call on a diesel-fuelled power station in Stornoway to provide back-up electricity, and airlift extra diesel generators from the mainland in case Stornoway’s turbines fail. SSEN announced on Thursday that the 33,000V cable was unrepairable, adding that it could take between six months and a year before a new cable could be commissioned and laid. It believes the breach happened at a depth of over 100 metres about 15km offshore from Skye, and is now investigating whether it was cut by a trawler or another vessel. SSEN has asked for local shipping records so that it can pinpoint which vessels were in the area at the time the cable failed. Mark Rough, the company’s director of customer operations, said: “We’re acting as quickly as possible to progress this significant cable replacement project. “We’d like to reassure our customers that our well-established resilience plans are in place to maintain power supplies to local homes and businesses as we source and install the new cable.” John Cunningham, head of the energy unit at Comhairle nan Eilean Siar, the local council, said he and the council’s leader, Roddie Mackay, had met SSEN earlier on Thursday and had been reassured that the company was working hard to restore supplies. Cunningham said the delay in replacing the cable was due to the difficulty of commissioning a new subsea cable of such length at short notice. SSEN said it also needed to hire the right vessels to carry out the work, but it could install a bigger cable, with greater capacity. The cable’s failure has major implications for the islands’ windfarms and small hydro-schemes, which are community-owned. With a total capacity of 21.5MW, they export electricity to the National Grid, raising money for local communities and investors. They have rent, wages and other costs to pay, and are counting on their insurance policies to cover income losses of up to a year, Cunningham said. “The council has very ambitious green ideas,” he said. “We’re looking to be net-zero by 2035, 10 years earlier than Scotland as a whole, so it’s disappointing to be running the whole system on diesel at this point in time.”