LIGHTNING, a bird strike or a stray branch falling on to a main power transmission line may have been responsible for power cuts that blacked out almost half of the land mass of Scotland.

Investigators are also ­examining whether Wednesday night's loss of power to 205,000 homes may have been the result of a flashover, when power comes out of the conductor and is earthed. Vandalism and metal theft have been ruled out.

A helicopter and ground crews were all involved as SSE sought to understand the cause of the power cut, which is thought to be unprecedented in terms of the size of the area it affected.

The 55-mile stretch of power lines between Blackhillock substation near Keith in Moray and Knocknagael south of Inverness was quickly identified as the likely location of the fault that caused the outage.

But SSE said it could not give a detailed explanation of why this had led to the blackouts across the north of Scotland at 8.30pm.

Industry sources are surprised, as back-up protection would normally be expected to ensure a quick restoration of power.

The failure spread across Moray, the Highlands from Fort William to Caithness, Skye, the Western Isles and Orkney.

However, more than half of homes were reconnected by 10.30pm, with 95,000 customers still off, but this dropped to 25,000 by 11pm and all were restored by 12.30am.

Inverness was off for less than half an hour, while in parts of Skye it was an hour-and-a-half and for some homes on the Black Isle, just 20 miles from Inverness, it was two-and-a-half hours.

The national grid is like a huge system of water pipes that must be kept at the same pressure. Minute by minute it has to balance out the electricity being supplied to the system -by generators such as power stations hydro schemes and wind farms -with the fluctuating levels of demand from customers.

An SSE spokesman explained: "The grid network is technologically advanced, so if it thinks it is in distress or there is a fault, it will trip pretty much as your house would trip if something is wrong. It does this to protect equipment."

He said all the lines were ­inter-connected, so the lines around the rest of Highlands and Islands had also tripped out because this advanced circuitry technology "had warned something was wrong. But the warning equipment on the rest of the grid had not reacted in this way, so hadn't tripped."

First Minister Alex Salmond, who was in Stornoway at the time, chaired a meeting of the government's resilience committee. He said the plans in place had made sure there was no major problem.