Energy company SSE has said the incident, one of the largest power cuts to hit Scotland, was most probably caused by lightning, a bird strike or debris hitting electricity lines.
The energy giant insisted its network was “reliable 99.9 per cent of the time”, but it is yet to determine what caused the fault.
Questions have been asked about how one fault could knock out such a large area of the country.
SSE says that to ensure quick recovery from a significant outage, large parts of the network can shut down to protect the infrastructure from further damage.
Among the areas affected were Fort William, Moray, Caith ness and Orkney. The Western Isles and Skye were also hit by the outage, which started at about 8:30pm on Wednesday.
Engineers have been concentrating their efforts on a 55-mile stretch of cable line between Moray and south of Inverness. An SSE spokesman said: “Engineers have completed aerial and ground inspections of around 90km of overhead power lines between Blackhillock and Knocknagael substations following the rare and unexpected power failure in the north of Scotland which briefly affected 205,000 homes late on Wednesday evening.
“Although no obvious damage has been found to date, our teams will continue their technical analysis and close-up inspections of equipment in the area over the coming days and weeks if necessary.
“However, early indications point to what is known as a ‘transient’ fault which, although it leaves no obvious damage to a power line, will cause a brief interruption and is usually caused by an object striking the line.
“Examples could include debris, lightning, birds or a failure within a specific piece of equipment.”
The blackout affected street lights and train services, while shops and bars were forced to shut as they were plunged into darkness. Radio services monitoring the shipping distress channel were affected for a period, while mobile phone masts stopped transmitting.
Highland league football matches were abandoned as floodlights went out.
First Minister Alex Salmond has chaired emergency meetings of the Scottish Government’s resilience committee following the blackout.
A spokesman said the SSE had reported that it had visually inspected some 90km of lines where the fault is believed to have occurred, but that as yet no visible damage or signs of the fault had been identified.