A FAULTY electronic relay in a single substation was responsible for a blackout which affected most of northern Scotland, according to the Finance Secretary.

More than 200,000 properties in the Highlands and Islands were affected at the height of the disruption last Wednesday.

The outage left engineers mystified, with "windy weather, lightning strike, pollution or equipment failures" cited at the time as possible causes by provider Scottish & Southern Electric Power Distribution (SSEPD).

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The fault has now been identified as a faulty electronic relay at the Knocknagael substation near Inverness which caused circuit breakers to open across the entire network, Finance Secretary John Swinney told MSPs.

Mr Swinney "emphatically" dismissed Tory claims that Scotland's "over-reliance on wind turbines may have contributed to grid instability".

Speaking at Holyrood, Mr Swinney said: "It is believed the relay malfunctioned just before the outage last Wednesday, and circuit breakers identified a potential fault in the main network.

"These circuit breakers opened to protect the system supplying the north and west of the country from more protracted and significant damage.

"SSEPD have reviewed the events, modified systems and are confident this will prevent a recurrence of the same problem affecting the wider network in the future."

Highland SNP MSP Rob Gibson said: "That is the first explanation we have had in any detail on it."

North East Scotland Conservative MSP Alex Johnstone said: "Some individuals with engineering experience have suggested that over-reliance on wind turbines may have contributed to grid instability."

Mr Swinney said: "I can emphatically say to Mr Johnstone, engineering or no engineering experience, that the comments we have had about the involvement about wind turbines are utterly misplaced in the analysis of this incident."